JOURNAL OF BACTERIOLOGY, Jan. 2010, p. 264–279
0021-9193/10/$12.00 doi:10.1128/JB.01204-09
Copyright © 2010, American Society for Microbiology. All Rights Reserved.
Vol. 192, No. 1
Identification and Characterization of Noncoding Small RNAs in
Streptococcus pneumoniae Serotype 2 Strain D39䌤†
Ho-Ching Tiffany Tsui,1‡ Dhriti Mukherjee,1‡ Valerie A. Ray,1 Lok-To Sham,1
Andrew L. Feig,2 and Malcolm E. Winkler1*
Department of Biology, Indiana University—Bloomington, Bloomington, Indiana 47405,1 and Department of Chemistry,
Wayne State University, 5101 Cass Ave., Detroit, Michigan 482022
Received 5 September 2009/Accepted 12 October 2009
including pneumonia, otitis media (ear infection), sinusitis, meningitis, and septicemia (49). Pneumococcus exists as a commensal bacterium that inhabits and colonizes the nasopharynx of
up to 20 and 50% of healthy adults and children, respectively,
at any time (10). The transition from commensal bacterium to
opportunistic pathogen often occurs after a respiratory tract
infection, and invasive pneumococcal diseases result in over
1.6 million deaths annually worldwide, especially among young,
elderly, debilitated, and immunosuppressed individuals (reviewed in references 13 and 30). Clearly, S. pneumoniae has the
ability to inhabit numerous niches in the human body (31, 32),
and responses to these different environments likely play roles
in colonization and disease progression.
Complete genome sequences of several serotypes of S. pneumoniae have significantly increased our understanding of pneumococcal physiology, pathogenesis, and evolution (27, 28, 39,
70). The three highly conserved sRNAs, RNase P, tmRNA,
and scRNA, are contained in all pneumococcal genomes, but
other potential sRNAs were not identified or annotated. Bioinformatic searches failed to identify pneumococcal homologues
of RNA binding protein Hfq and endoribonuclease RNase E,
which are important in sRNA functions and RNA metabolism
in Escherichia coli (17, 40, 45, 47, 48, 65). Bioinformatic analyses indicate that Hfq homologues are also absent from several
other bacterial pathogens, including Chlamydia and Mycoplasma species, Streptococcus pyogenes, Enterococcus faecium,
Helicobacter pylori, and Campylobacter jejuni (66), some of
which are known to produce sRNAs.
The only sRNAs identified in S. pneumoniae to date were
A large number of noncoding small RNAs (sRNAs) 50 to
400 nucleotides (nt) in length have been detected and characterized recently in numerous bacterial species (reviewed in
references 3, 18, and 78). Some abundant, stable sRNAs, such
as RNase P (14), tmRNA (34), and scRNA (4.5S RNA) (23,
33), are highly conserved and play important housekeeping and
stress-related functions in RNA metabolism, protein degradation, and secretion. But most regulatory sRNAs are conserved
only among closely related species (42). Many sRNAs play key
roles in responses to stress conditions, such as iron limitation,
osmotic shock, temperature shift, stationary phase, and metabolic imbalance, in different bacterial species (3, 15, 17, 25, 26,
46, 76, 78, 79). Other sRNAs are expressed during growth or
developmental phases that are specific for particular bacterial
species (38, 64, 68, 75). In addition, sRNAs have been postulated to mediate virulence gene expression in several pathogenic bacteria and their survival in hosts (3, 6, 37, 55, 68, 73).
Little is known about RNA metabolism in Streptococcus
pneumoniae (pneumococcus), which is a major human respiratory pathogen that causes several serious invasive diseases,
* Corresponding author. Mailing address: Department of Biology,
Indiana University—Bloomington, Jordan Hall 142, Bloomington, IN
47405. Phone: (812) 856-1318. Fax: (812) 855-6705. E-mail: mwinkler
@bio.indiana.edu.
† Supplemental material for this article may be found at http://jb
.asm.org/.
‡ H.-C.T.T. and D.M. contributed equally to this work.
䌤
Published ahead of print on 23 October 2009.
264
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We report a search for small RNAs (sRNAs) in the low-GC, Gram-positive human pathogen Streptococcus
pneumoniae. Based on bioinformatic analyses by Livny et al. (J. Livny, A. Brencic, S. Lory, and M. K. Waldor,
Nucleic Acids Res. 34:3484–3493, 2006), we tested 40 candidates by Northern blotting and confirmed the
expression of nine new and one previously reported (CcnA) sRNAs in strain D39. CcnA is one of five redundant
sRNAs reported by Halfmann et al. (A. Halfmann, M. Kovacs, R. Hakenbeck, and R. Bruckner, Mol. Microbiol.
66:110–126, 2007) that are positively controlled by the CiaR response regulator. We characterized 3 of these 14
sRNAs: Spd-sr17 (144 nucleotides [nt]; decreased in stationary phase), Spd-sr37 (80 nt; strongly expressed in
all growth phases), and CcnA (93 nt; induced by competence stimulatory peptide). Spd-sr17 and CcnA likely
fold into structures containing single-stranded regions between hairpin structures, whereas Spd-sr37 forms a
base-paired structure. Primer extension mapping and ectopic expression in deletion/insertion mutants confirmed the independent expression of the three sRNAs. Microarray analyses indicated that insertion/deletion
mutants in spd-sr37 and ccnA exerted strong cis-acting effects on the transcription of adjacent genes, indicating
that these sRNA regions are also cotranscribed in operons. Deletion or overexpression of the three sRNAs did
not cause changes in growth, certain stress responses, global transcription, or virulence. Constitutive ectopic
expression of CcnA reversed some phenotypes of D39 ⌬ciaR mutants, but attempts to link CcnA to -E to comC
as a target were inconclusive in ciaRⴙ strains. These results show that S. pneumoniae, which lacks known RNA
chaperones, expresses numerous sRNAs, but three of these sRNAs do not strongly affect common phenotypes
or transcription patterns.
VOL. 192, 2010
sRNAs OF STREPTOCOCCUS PNEUMONIAE D39
MATERIALS AND METHODS
Bacterial strains and growth conditions. Strains used in this study are listed in
Table S1 of the supplemental material. Strains were grown on plates containing
trypticase soy agar II (modified; Becton-Dickinson [BD]) and 5% (vol/vol) defibrinated sheep blood (TSAII BA) and incubated at 37°C in an atmosphere of
5% CO2. For liquid cultures, strains were cultured statically in BD brain heart
infusion (BHI) broth or a chemically defined medium (CDM) (69) at 37°C in an
atmosphere of 5% CO2. The pH of the BHI broth before or after equilibration
with 5% CO2 was 7.4 or 7.1, respectively. Unless indicated otherwise, ciaR⫹
strains were inoculated from frozen stocks into 4 ml of BHI broth in 17-mm
plastic tubes, serially diluted, and grown for 10 to 16 h. Cultures with an optical
density at 620 nm (OD620) of 0.1 to 0.4 were diluted to a starting OD620 between
0.002 and 0.005 in 5 ml of BHI broth in 16-mm glass tubes. For ⌬ciaR strains,
overnight cultures were limited to 10 to 11 h, and cultures with an OD620 of 0.05
to 0.13 were diluted to an OD620 of ⬇0.001 in 5 ml of BHI broth in 16-mm glass
tubes. Culture tubes were gently inverted before the OD620 was monitored
directly using a Spectronic 20 Genesys spectrophotometer. In some cases (see
below), 30 ml of diluted starting culture was grown in a 250-ml bottle. At the
times indicated, the bottles were swirled gently, and 0.75 ml of culture was
removed to a 1-cm-path length cuvette for OD620 determination.
Construction and verification of S. pneumoniae mutants. Strains containing
antibiotic markers were constructed by transforming linear DNA amplicons
synthesized by overlapping fusion PCR into competent pneumococcal cells as
described previously (60). For antibiotic selections, TSAII BA plates were supplemented with 200 ␮g kanamycin per ml, 100 ␮g spectinomycin per ml, 150 ␮g
streptomycin per ml, 0.3 ␮g erythromycin per ml, or 2.5 ␮g chloramphenicol per
ml. Strains and plasmids used and primers synthesized for this study are listed in
Tables S1 and S2, respectively, of the supplemental material. All constructs were
confirmed by DNA sequencing of the amplicon region used for transformation.
PCRs, purification of amplicons for transformation and sequencing, and sequencing reactions were similar to those described previously (58). D39
⌬spd-sr17::Pc-ermAM, D39 ⌬spd-sr37::Pc-ermAM, and D39 ⌬ccnA::Pc-ermAM
deletion/insertion mutants (see Table S1 and Fig. S1 in the supplemental material) were constructed by replacing DNA sequences from bp ⫺5 to ⫹71, bp ⫺32
to ⫹39, and bp ⫹46 to ⫹83 in spd-sr17, spd-sr37, and ccnA, respectively, relative
to transcription start sites, with the Pc-ermAM cassette amplified from strain
IU1547 (51). A ⌬ciaR::P-ermB mutant was constructed by replacing bp 186 to 591 of
ciaR (intact ciaR is 741 bp) with the P-ermB cassette derived from pAM␤1 (7).
Spd-sr17, Spd-sr37, and CcnA were expressed ectopically from the chromosomal expression platform (CEP) site (20) driven from their native promoters by
replacing the Pmal (PM) region of CEP with fragments that extended from 100 bp
upstream to 20 bp downstream of spd-sr17, 85 bp upstream to 28 bp downstream
of spd-sr37, or 153 bp upstream to 27 bp downstream of ccnA. For the
CEP::Pmal(c)-spd-sr17, CEP::Pmal(c)-spd-sr37, and CEP::Pmal(c)-ccnA constructs,
the respective sRNA gene sequence extending 20, 28, or 27 bp downstream was
placed at the ⫹1 site of the Pmal promoter of CEP. These constructions deleted
the MalR operator site (53) and thereby created the constitutive Pmal(C) promoter (see Fig. 2D, below; see also Table S2 in the supplemental material). The
CEP::PspxB ccnA construct (see Table S2) contains 150 bp upstream of the spxB
transcription start site (58). CEP::PrRNA-spd-sr17 contains 104 bp of the rRNA
promoter region identified by comparison to the Bacillus subtilis rrnB and rrnO
P2 promoters (36). The construct was designed to retain a G at the ⫹1 position
to maximize promoter strength, and the expressed spd-sr17 contains an extra G
at its 5⬘ end. CEP::PHspac-spd-sr37 and CEP::PHspac(⫺1)-spd-sr37 contain 128 and
127 bp, respectively, of the PHyperspank promoter of pDR111 (4) driving spd-sr37
from the ⫹1 and the ⫺1 positions, respectively.
A control CEP::P-kan (kanamycin-resistant) construct lacking sRNA inserts
was constructed by transforming an amplicon synthesized from pCEP (20) as
template and primers VR49 and VR54 (see Tables S1 and S2 in the supplemental material). A CEP::P-aad9 construct was made by replacing kan in CEP::P-kan
with aad9 (spectinomycin resistant) (50). Microarray control analysis of strains
IU1690 (D39 parent) versus IU 2586 (D39 CEP::P-kan) grown in BHI broth to
mid-exponential phase showed no changes in the relative transcript amounts
from amiF and treR, the genes flanking the CEP region, or from any other genes
(data not shown).
Markerless comC3 and comC6 alleles (see Fig. S2 in the supplemental material) were generated using the kanr-rpsL⫹ (Janus cassette) allele replacement
method described in reference 67. D39 ⌬comC::[Pc-kanr-rpsL⫹] was constructed
by replacing DNA sequence from bp ⫺122 to ⫹122 relative to the comC coding
region with a [Pc-kanr-rpsL⫹] cassette. The rpsL1 allele in strain IU1781 was
repaired to rpsL⫹ by transformation with a fusion amplicon containing a rpsL⫹rpsG⫹-cat, which contains a chloramphenicol resistance marker (cat), its ribosomal binding site, and four upstream nucleotides (see Table S1 in the supplemental material) inserted immediately downstream of the rpsG stop codon. One
transformant (IU3373) of 79 patched was Cmr and Strs and contained the
expected rpsL⫹ sequence. Amplicon from IU3373 was used to transform IU3598
(D39 comC6 rpsL1). Nine of 75 transformants were Cmr and Strs. One transformant was stored as IU3631(D39 comC6 rpsL⫹-rpsG⫹-cat).
Determination of CFU per ml per OD620 unit. CFU per ml were determined
for strains grown to an OD620 of 0.075 to 0.09 by serially diluting cultures in
phosphate-buffered saline (PBS) and spreading aliquots of dilutions onto TSAII
BA plates. CFU per ml obtained on plates for strains IU2678 or IU2841 were
similar to the number of chains per ml counted manually using a Petroff-Hausser
chamber on a phase-contrast microscope (data not shown).
Microscopy and chain length determinations. After cultures reached an
OD620 of ⬇0.06 and ⬇0.2, 500 ␮l was removed and centrifuged at 16,000 ⫻ g for
2 min at room temperature. Pellets were suspended in 100 ␮l of BHI broth. Cells
were examined using a Nikon E-400 phase-contrast microscope, and images were
captured using a cooled digital SPOT camera (1). At least 25 to 30 chains from
each of two independent cultures of each strain were counted to determine
distributions of numbers of cells per chain.
Determination of natural transformation frequencies. Strains were diluted
into 30 ml of BHI broth in 250-ml bottles to a starting OD620 of ⬇0.001, and
separate parallel, unperturbed cultures were used for each time point. Starting
from the initial inoculation and at 1-h intervals thereafter, 1 ml of cell suspension
was removed and mixed with 40 ng of amplicon DNA carrying a novobiocin
resistance marker (see Table S1 in the supplemental material), which was obtained by PCR of CP1500 genomic DNA using primers XXV-F-036 and XXVR-040 (see Table S2 in the supplemental material). After incubation for 55 min
at 37°C in an atmosphere of 5% CO2, 800-␮l aliquots of the cell suspensions were
mixed with 3 ml of melted 42°C 0.8% nutrient broth plus 0.7% (wt/vol) Bacto
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discovered by serendipity in a search for genes directly regulated by the CiaRH two-component system (TCS) (22), which
plays roles in resistance to ␤-lactam antibiotics, responding to
stress, and competence (11, 16, 19, 56). Halfmann et al. defined
a consensus CiaR binding site upstream of several genes
known to be directly regulated by CiaRH, including lic (modification of teichoic acids), mal and man (sugar metabolism),
htrA (stress response protease), parB (chromosomal segregation), and ppmA (protease maturation) (22). Scanning the
pneumococcal genome revealed CiaR binding sites upstream
of five genes (ccnA to -E) that encode redundant sRNAs
(csRNA1 to -5 or CcnA to -E) containing segments of similar
sequences and predicted secondary structures. CiaR was shown
to positively regulate expression of the CcnA to -E sRNAs,
which were undetectable in a ⌬ciaR mutant (22). However, the
functions of CcnA to -E are not known, other than that CcnD
and CcnE affect stationary-phase autolysis (22).
To understand regulatory mechanisms and RNA metabolism more fully in S. pneumoniae, we performed an empirical
search for sRNAs in virulent serotype 2 strain D39 (39). We
based this search on a previous bioinformatic analysis by Livny
et al. (41), which predicted putative sRNAs in the genome of
pneumococcal serotype 4 strain TIGR4 (41). This method
(sRNAPredict2) predicted possible sRNAs based on appropriate spacing of putative promoters and terminators in intergenic
regions and conservation of primary sequences and secondary
structures among closely related species (42). We converted
predictions of sRNAs in the TIGR4 genome to the genome of
strain D39 and then experimentally tested for 40 of these
predicted sRNAs by Northern analysis. We report here the
identification of nine new pneumococcal sRNAs and the previously reported CcnA sRNA (22). We also report the operon
structures of three of these pneumococcal sRNA genes and
phenotypic characterizations of deletion mutants and ectopic
constructs overexpressing the three sRNAs.
265
266
TSUI ET AL.
Primer extension assays. Primer extension using the primers in Table S2 of the
supplemental material was performed as described before (58) to map the 5⬘
ends of the spd-sr17, spd-sr37, and ccnA sRNA transcripts in strain D39 (IU1690)
grown exponentially in BHI broth (OD620, 0.2). The comC transcription start
point (see Fig. S2 in the supplemental material) was mapped for strains D39
(IU1690) and IU2678 (D39 ⌬ciaR::P-ermB CEP::pnative-ccnA). The 5⬘ U residue
determined by this method was 1 nt shorter than the 5⬘-G start reported elsewhere (21).
RNA preparation for microarray and qRT-PCR analyses. To determine relative amounts of comD transcript by quantitative reverse transcription-PCR
(qRT-PCR) at different growth points and for microarray analyses of exponential
cultures (OD620, 0.06 to 0.1), total RNA was prepared from cultures growing at
37°C in BHI broth in separate bottles as described above for the natural transformation assays. RNA from 8 to 12 ml of culture was prepared by a rapid lysis
procedure followed by purification using an RNAeasy minikit (Qiagen) as described previously, including on-column treatment with DNase I (Qiagen) (58,
59). This procedure excludes RNA species smaller than 200 nt. For qRT-PCR
and RT-PCR assays, 5 ␮g of total RNA was further digested with DNase using
a DNA-free kit (Ambion).
Microarray analysis. Synthesis, labeling, and hybridization to S. pneumoniae
microarrays (Ocimum Biosolutions) covering 2,018 open reading frames (ORFs)
of the R6/D39 genome, scanning, and analysis using the Cyber-T web interface
were performed as described previously (32, 39, 52, 58). Data were normalized
without background subtraction by the global Lowess method using BASE (BioArray Software Environment; http://iubase.cgb.indiana.edu), excluding empty
wells and Arabidopsis thaliana control spots. Intensity and expression ratio data
for all transcripts have been deposited in the GEO database (accession number
GSE14688).
qRT-PCR analysis. qRT-PCR analysis was performed as described before (58)
with the exception that cDNA was prepared with a qScript Flex cDNA synthesis
kit (Quanta Biosciences) according to the manufacturer’s protocol. PCR primers
(see Table S2 in the supplemental material) covering the center regions of
spd_0239 (F1 and R1 [see Fig. 2C, below]), spd_0240 (F2 and R2 [see Fig. 2C]),
or the 3⬘ region of the comD transcript were used for RNA quantitation. All
primer sets showed standard curves with R2 values of ⬎0.985, 90 to 110%
reaction efficiencies, and only one peak in dissociation curves. spd_0240 and
comD transcript amounts were normalized to 16S rRNA amounts by using
primers in Table S2 of the supplemental material as described previously (58).
The 16S-normalized comD transcript amounts were then expressed as ratios
relative to the amount of comD transcript in the D39 parent strain (IU1690) at
mid-exponential phase (OD620 of 0.1 to 0.2), which was set to 1.
RT-PCR detection of spd_0239-spd_0240-ccnA and trmU-spd-sr37-spd_0128gidA cotranscripts. For the detection of the spd_0239-spd_0240-ccnA cotranscript, cDNA synthesis was performed with a StrataScript first-strand synthesis
system (Stratgene) using 242 ng of total RNA from IU1690 and 700 nM ccnA
Northern probe (see Fig. S1C in the supplemental material) as the reverse
primer. PCRs were performed with 0.25 ␮l of the cDNA reaction mixture,
forward primers F1 or F2 (see Fig. 2C), ccnA Northern probe as the reverse
primer, and PCR enzyme GoTag (Promega). Amplicons of 1,096 bp and 456 bp
from F1 and F2 to the ccnA probe, respectively, were predicted and detected
(data not shown). Detection of trmU-spd-sr37-spd_0128-gidA cotranscript was
performed similarly with R3 as the reverse primer for the RT reaction and
primers F3 and R3 for the PCRs (see Fig. 2B). A 2,109-bp amplicon was predicted
and detected (data not shown). Control amplifications carried out by omitting
reverse transcriptase in the first step of the procedure did not show any product
indicative of DNA contamination in the RNA preparations (data not shown).
RESULTS
Detection of nine new sRNAs and the previously reported
CcnA sRNA in S. pneumoniae D39. Livny, Waldor, and coworkers predicted 63 putative sRNAs with lengths of 66 to 365 nt in
the intergenic regions of the TIGR4 strain (41). We numbered
these candidate sRNAs from 1 to 63 according to the order in
Table S10 of reference 41. BLAST searches were run for the 63
sRNAs against the genome of laboratory strain R6. Forty candidate sRNAs were located in similar regions of the TIGR4
and R6 genomes (coordinates can be found in Table S3 of the
supplemental material) and showed more than 90% sequence
identity between the two genomes. Twelve of the 40 sequences
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Agar containing 2.5 ␮g novobiocin per ml and poured onto TSAII BA plates
containing the same concentration of novobiocin. The transformation frequency
was determined as the ratio of Novr CFU to total CFU per unit volume of cell
suspension. The detection limits for this assay was ⬇10⫺5 and ⬇10⫺8 at early and
late time points in growth curves, respectively.
Salt and antibiotic stress tests and virulence studies of sRNA deletion mutants. Strains IU1690 (D39 wild type [WT]), IU2083 (D39 ⌬ccnA::Pc-ermAM),
IU2084 (D39 ⌬spd-sr17::Pc-ermAM), and IU2086 (D39 ⌬spd-sr37::Pc-ermAM)
were grown exponentially in 9 ml of BHI broth to an OD620 of 0.1. Three
milliliters of cultures was transferred to tubes containing the following chemicals
to give the final concentrations indicated: 0.1 or 0.2 M NaCl, 25 or 50 ng
mupirocin per ml, or 0.1 or 0.03 ␮g ampicillin per ml. Growth was further
monitored and compared to that of control cultures lacking additives. For further
antibiotic resistance testing, the same set of strains was grown exponentially in
BHI broth to an OD620 of 0.1 to 0.2. One milliliter of culture was added to
molten soft agar (see above) and poured onto TSAII BA plates. After the soft
agar solidified, Sensi-Disks (BD BBL) containing 10 IU penicillin, 10 ␮g ampicillin, 75 ␮g ticarcillin, 1 ␮g oxacillin, 30 ␮g vancomycin, or 10 ␮g amdinocillin
were placed on the surface of the plates. Plates were incubated for 24 h at 37°C
in an atmosphere of 5% CO2, and the diameters of inhibition zones were
determined. Virulence studies were carried out on this set of strains using a
murine pneumonia model of infection with intranasal inoculation (⬇2 ⫻ 108
CFU) of 4- to 5-week-old, male BALB/c mice as described elsewhere (58) All
procedures were approved in advance by the Institutional Animal Care and Use
Committee and were performed according to the guidelines of the National
Research Council.
RNA extraction for sRNA Northern analysis and primer extension assays.
Total RNA was prepared by a method that retained RNA species of all sizes.
Thirty-milliliter BHI broth cultures of the D39 parent contained in 50-ml conical
centrifuge tubes were grown from a starting OD620 of 0.005 to a final OD620 of
0.2 (exponential phase), for an additional 1 h after reaching an OD620 of 0.6
(early stationary phase), or from a starting OD620 of 0.002 to a final OD620 of 0.1,
after which synthetic competence stimulatory peptide 1 (CSP1) was added to a
final concentration of 100 ng per ml for 12 min. Ten milliliters of exponentialphase and CSP-treated cultures and 3 ml of stationary-phase cultures were
chilled in precooled flasks on ice, transferred to precooled 50-ml centrifuge
tubes, and centrifuged at 13,000 ⫻ g for 6 min at 4°C. Pellets were resuspended
in 0.5 ml of boiling solution A (1% [wt/vol] sodium dodecyl sulfate [SDS], 20 mM
sodium acetate, 8 mM EDTA, pH 5.5) and heated in a boiling water bath for 2
min. The 0.5 ml of lysate was extracted with acidified phenol (pH 4.3) at 65°C
with shaking, followed by extraction with phenol-chloroform-isoamyl alcohol and
chloroform at room temperature as described previously (74). Nucleic acids were
precipitated with ethanol, dried, treated with DNase (Promega RQ1 DNase; 12.5
units per 250 ␮g total RNA), and the phenol-chloroform extraction, ethanol
precipitation, and drying steps were repeated.
Northern analysis. Total RNA (5 ␮g per lane) and size markers (RNA Century-plus [Ambion] and a 49-nt DNA oligomer) were fractionated by polyacrylamide gel electrophoresis (PAGE) on Tris-buffered EDTA (TBE)–8% polyacrylamide–7 M urea gels (5) and transferred electrophoretically to BrightStarPlus positively charged nylon membranes (Ambion) using a semidry blot
apparatus (Bio-Rad). RNA was cross-linked to membranes with UV light using
the optical cross-linking setting on an XL-1000 UV Spectro linker (Spectronics
Corp.). Hybridizations were conducted according to the protocol accompanying
the Ultrahyb-oligo buffer (Ambion) at 37°C for 16 to 20 h with rotation. Blots
were washed twice with 2⫻ SSC (1⫻ is 0.15 M NaCl plus 0.015 M sodium citrate)
and 0.5% (wt/vol) SDS for 30 min each at 37°C with rotation.
DNA oligonucleotides used as probes for Northern blots are listed in Table S3
of the supplemental material. Two probes that target different regions were
designed for some of the predicted sRNA sequences: 12(1)-12(2); 17(1)-17(2);
37(1)-37(2); 52(1)-52(2). T4 polynucleotide kinase (New England Biolabs) was
used to end label 2 pmol of each synthetic DNA oligonucleotide with 1.67 pmol
of [␥-32P]ATP (6,000 Ci per mmol; Perkin-Elmer). Radiolabeled oligonucleotides were purified using Sephadex G-25 quick spin columns (Roche). Labeled
Northern blots were exposed to X-ray film to obtain an image and to a phosphor
screen (Amersham) for 10 to 30 min for quantitation. The phosphor screen was
scanned with a Typhoon 9200 variable mode imager (Amersham), and quantitation of bands was performed with ImageQuant software (Molecular Dynamics). sRNA amounts were normalized to 5S rRNA amounts by stripping blots (5)
and rehybridizing them with a radiolabled 5S rRNA probe (see Table S2 in the
supplemental material). To allow comparisons, normalized amounts of sRNA
from CSP-treated and stationary-phase samples were divided by the normalized
amounts of sRNA from exponential-phase samples, which were set to 1.
J. BACTERIOL.
VOL. 192, 2010
sRNAs OF STREPTOCOCCUS PNEUMONIAE D39
267
TABLE 1. sRNAs detected in S. pneumoniae D39 by Northern blotting
Predicted sRNAa
sRNA probe
name
7
b,c
10
12(1)b,d
12(2)d
14b
17(1)d,e
17(2)d
37(1)b,e
37(2)f
38
39g
48
52(1)h
h
52(2)
54j
56 (ccnA)
b,e,i
820,183–820,245
(812,653–812,715)
1,750,985–1,751,149
(1,743,453–1,743,617)
967,854–968,170
(960,323–960,639)
149,307–149,416
(149,307–149,416)
912,572–912,715
(905,012–905,175)
131,773–131,842
(131,738–131,829)
131,773–131,842
(131,738–131,829)
769,933–770,085
(762,402–762,554)
1,678,490–1,678,641
(1,671,110–1,670,959)
862,664–862,819
(855,134–855,289)
1,687,151–1,687,017
(1,679,621–1,679,486)
1,687,151–1,687,017
(1,679,621–1,679,486)
1,216,035–1,215,850
(1,208,504–1,208,319)
231,143–231,235
(231,164–231,228)
Flanking gene
5⬘
3⬘
Predicted
length (nt)
Location (nt) of
Northern blot band
in D39j
63
⬃400
ndk
165
⬃120
ppc
spd_0954 (spr_0975)
317
ugd
mutR
110
⬃200 (S2)
⬃200 (S2)
⬃200 (S2)
tRNAThr1
asd
164
trmU
spd_0128 (spr_0123)
92
⬃144 (S2)
⬃144 (S2)
⬃80
trmU
spd_0128 (spr_0123)
92
No signal at ⬃80
IS1167 transposase spd_0758
(spr_0767)
IS1167 truncation transposase
spd_1666 (spr_1701)
spd_0846 (spr_0860)
spd_0759 (spr_0768)
153
⬃80
spd_1665 (spr_1700)
152
⬃80
infC
156
⬃150
spd_1672 (spr_1708)
amiA
135
⬃60
spd_1672 (spr_1708)
amiA
135
⬃65
trzA
rplJ
186
⬃145 (S2)
spd_0240 (spr_0237)
ccnB
65
93 (C1)
spd_0803 (spr_0810)
spd_0804 (spr_0811)
spd_1756 (spr_1774)
a
As reported by by Livny et al. in 2006 (41). The equivalent gene in strain R6 is in parentheses.
Identical sequences in the R6, D39, and TIGR4 genomes.
Second copy of gene possibly expressing Spd-sr10 is located between kgdA (5⬘) and hysA (3⬘) at coordinates 288,782 to 288,669 (288,732 to 288,619).
d
A band of the same size was detected with two separate probes of different sequences.
e
Coordinates are based on mapped 5⬘ ends and predicted termination points and not on predictions in reference 41 (see also Fig. S1 in the supplemental material).
f
Probe 37(2) was found to correspond to a region upstream of the spd-sr37 sequence (see Fig. S1B in the supplemental material).
g
Spd-sr38 and Spd-sr39 are redundant and are transcribed opposite to the transposase genes at one end of IS1167 transposon elements. There are seven other
complete or truncated IS1167 elements in the D39 and R6 genomes that likely express Spd-sr38/39 sRNAs at the following D39 coordinates (with 3⬘-flanking genes):
30,017 to 30,123 (spd0029); 41,501 to 41,608 (comA); 980,059 to 980,189 (murZ); 999,548 to 999,669 (spd0987); 1,654,960 to 1,655,086 (spd1640); 1,697,548 to 1,697,669
(birA); 1,708,477 to 1,708,605 (spd1707).
h
Two nonoverlapping bands of slightly different sizes were detected. Two additional copies of putative genes for sRNAs that potentially hybridized with probe 52(1)
are located in D39 between murZ (5⬘) and IS1167 at 980,476 to 980,535 and between a transposase H gene (spd_2008) (5⬘) and spd_2009 (3⬘) at 1,985,576 to 1,985,477.
Two additional copies of putative genes for sRNAs that potentially hybridized with probe 52(2) are located in D39 between rplA (5⬘) and spd_0554 (ABC-NBD) at
coordinates 563,660 to 563,745 and between a transposase H gene (spd_2008) (5⬘) and spd_2009 (3⬘) at 1,985,576 to 1,985,477. The two genes located adjacent to the
transposase H gene (spd_2008) whose putative sRNA transcripts may hybridize with probes 52(1) and 52(2) are tandem, similar to the arrangement in the spd-sr52
region.
i
Probe 56 is specific for CcnA sRNA.
j
S2, the relative transcript amount decreased in stationary phase; C1, the relative transcript amount increased when treated with CSP.
b
c
were identical in the TIGR4, R6, and D39 genomes (Table 1;
see also Table S3 in the supplemental material), and all 40
predicted sRNAs were identical between related strains R6
and D39 (39).
Probes were designed for the 40 putative sRNAs (see Table
S3 in the supplemental material), and Northern analyses were
performed on total RNA prepared from strain D39 growing in
exponential phase, at early stationary phase, and after treatment with CSP for 12 min in BHI broth (see Materials and
Methods). Of the 40 sRNA candidates analyzed, 22 probes
showed no signal, and 6 probes showed multiple bands on the
Northern blots (data not shown). Distinct bands corresponding
to sRNA species of approximately 80 to 400 nt were found for
12 probes (Fig. 1A and B; Table 1). We designated these
sRNAs temporarily as Spd-sr (S. pneumoniae D39 sRNA) fol-
lowed by the numbers assigned above. The genomic coordinates in the R6 and D39 strains, flanking genes, predicted lengths,
and approximate sizes of these detected sRNAs are shown in
Table 1.
In the course of our study, Spd-sr56 was reported as the
ccnA-transcribed csRNA1 (22), and we refer to Spd-sr56
(csRNA1) as CcnA sRNA here. One candidate, sRNA (Spdsr7), was considerably longer (⬇400 nt) than its predicted size
of 63 nt (data not shown). This longer transcript likely corresponded to a contiguous transcript with the upstream gene,
spd_0803, which contains an open reading frame of 303 bp.
BLAST searches of the probes used for the Northern blots
showed that four candidate sRNAs, Spd-sr10, Spd-38, Spd-39,
and Spd-52, may be redundant and transcribed from multiple
sites in the D39 chromosome (Table 1). There is a second
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g
sRNA coordinates in D39
genome (5⬘–3⬘)
(coordinates in R6)
268
TSUI ET AL.
J. BACTERIOL.
putative gene with a nearly identical sequence to that of spdsr10 at a different chromosomal location. spd-sr38, spd-sr39,
and possibly seven other similar sRNAs are transcribed divergently from the transposase gene within one end of complete
and truncated IS1167 elements located around the chromosome. Another candidate, Spd-sr52, may arise from multiple
intergenic regions with similar sequences (⬎85% identity) (Table 1). Probes 52(1) and 52(2), which were designed to detect
a predicted 135-nt sRNA (Table 1; see also Table S3 in the
supplemental material), hybridized instead to two slightly different sized transcripts of ⬇60 and ⬇65 nt, respectively (Fig.
1A). Primer extension assays showed a 5⬘ end consistent with
the ⬇60-nt transcript that hybridized to probe 52(1) (data not
shown). The origin of the ⬇65-nt transcript detected by probe
52(2) was not studied further.
Characterization of the spd-sr17, spd-sr37, and ccnA transcription units. Five species of sRNAs showed differential expression in response to growth phase or CSP addition in the
parent D39 strain (Fig. 1; Table 1). The relative amounts of
Spd-sr12, Spd-sr14, Spd-sr17, and Spd-sr54 decreased in stationary phase and that of CcnA (Spd-sr56) increased upon
CSP addition. We initially focused on studying the Spd-sr17,
Spd-sr37, and CcnA sRNAs, because they produced single,
strong bands (Fig. 1B), and Spd-sr17 and CcnA showed strong
differential regulation (5-fold [P ⬍ 0.01] and 3.6-fold [P ⬍
Downloaded from jb.asm.org at WAYNE STATE UNIVERSITY on April 13, 2010
FIG. 1. Northern blot analyses of S. pneumoniae sRNAs. A. Detection of eight sRNAs (Spd-sr10, Spd-sr12, Spd-sr14, Spd-sr38, Spd-sr39,
Spd-sr48, Spd-sr52, and Spd-sr54). Total RNA was extracted from the D39 parent strain grown in BHI to exponential phase (OD620, 0.2) (E) and
treated with CSP for 12 min (C) or to early stationary phase (S). Northern blotting was performed as described in Materials and Methods. Size
standards are indicated by lines at the left of the blots. Two different probes (1) and (2) (see Table S3 in the supplemental material) were used
for Spd-sr12 and Spd-sr52. B. Quantitation of relative transcript amounts of Spd-sr17, Spd-sr37, and CcnA grown under the three conditions in
panel A. Blots were quantitated, exposed to X-ray film, stripped, and reprobed with 5S rRNA-specific probe as described in Materials and
Methods. Amounts of sRNAs were normalized to 5S rRNA from the same blot and are expressed relative to the normalized amounts in
exponentially (E) grown cultures, which were set to 1. The data are from three biological replicates, where standard errors of the means are
indicated and asterisks correspond to P ⬍ 0.01. C. Northern blots of sRNAs from deletion strains. Total RNA was prepared from the D39 parent
strain and strains IU2084 (⌬spd-sr17; ⌬17), IU2086 (⌬spd-sr37; ⌬37), and IU2082 (⌬ccnA) (Fig. 2; see also Fig. S1 and Table S1 in the supplemental
material). RNA was prepared from cultures grown in BHI broth to exponential phase (OD620 ⫽ 0.2). Blots were intentionally overexposed to show
that there were no bands corresponding to the sRNAs in the deletion/insertion mutants and that higher-molecular-weight bands present in the D39
strain were absent in the ⌬spd-sr37 and ⌬ccnA mutants.
VOL. 192, 2010
269
FIG. 2. Genetic loci expressing pneumococcal sRNAs (drawn to scale
in each panel). Transcription start sites (indicated by a boldface P) were
determined by primer extension assays (see Materials and Methods) (see
Fig. S1 in the suppplemental material). Likely terminators predicted
by TransTermHP (http://transterm.cbcb.umd.edu/cgi-bin/transterm
/predictions.pl.) (scores ⬎ 85) are indicated by lollipops. The predicted
terminators following spd-sr17, spd-sr37, and ccnA are consistent with the
lengths of the sRNAs detected in Northern blot assays (see Fig. 1, 3, and
5). Extents of deletion/Pc-ermAM insertion mutations in sRNA genes are
indicated by lines (see Materials and Methods and Table S1 in supplemental material). Exact end points of deletions are indicated in Fig. S1.
For convenience, the corresponding gene tag designations are given for
both strains D39 (spd) and R6 (spr). A. spd-sr17 locus with flanking genes
tRNAThr1, ccl (citrulline cluster-linked gene), and asd (aspartate ␤-semialdehyde dehydrogenase). B. spd-sr37 locus with flanking genes trmU
(tRNA [5-methylaminomethyl-2-thiouridylate]-methyltransferase),
spd_0128/spr_0123 (conserved hypothetical protein in the MutT/Nudix
family), and gidA (tRNA uridine 5-carboxymethylaminomethyl modification enzyme). Bent arrows indicate oligonucleotide primers used in RTPCR (F3 and R3) analyses (see Materials and Methods). The relatively
weak terminator (score ⫽ 55) after spd-sr37 (circled A) may act as an
attenuator (see text). C. ccnA locus with flanking genes spd_0239/
spr_0236-spd_0240/spr_0237 (conserved hypothetical proteins in the
GNAT family of acetyltransferases), ccnB (redundant sRNA to CcnA
[see Fig. S2 in the supplemental material]), and ruvB (Holiday junction
migration helicase). The 701-nt region between ccnB and ruvB is not
annotated and lacks ORFs over 50 amino acids. The 5⬘ and 3⬘ ends of
CcnA and CcnB were mapped previously (22). Bent arrows indicate
oligonucleotide primers used in Northern (probe 56), qRT-PCR (F1/R1
and F2/R2), and RT-PCR (F1, F2, and probe 56) analyses. D. CEP site
containing a selectable kan marker (20) used to express sRNAs ectopically. As an example, the construct expressing CcnA from the constitutive
Pmal(C) or CiaR-regulated Pnative promoter is shown (see Results). The
transcriptionally silent CEP site is flanked by amiF (ABC transporter ATP
binding proteins), IS1167 (truncated degenerate transposase gene), and
treR (trehalose operon transcriptional repressor). Predicted transcriptional terminators for amiF, ccnA, and treR are indicated. See Materials
and Methods for additional details of ectopic constructs.
S1B in the supplemental material) but not probe 37(2), which
corresponded to a region upstream of the spd-sr37 5⬘ end (data
not shown) (see Fig. S1B). Upon longer exposures, bands corresponding to transcripts of ⬇750 nt and ⬎1,000 nt were de-
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0.005], respectively) (Fig. 1C) under the limited number of
conditions tested here.
spd-sr17 is a single gene operon. spd-sr17 is located in the
intergenic region between the gene for tRNAThr1 and asd,
which encodes aspartate ␤-semialdehyde dehydrogenase from
the threonine-lysine biosynthetic pathway (Fig. 2A). Spd-sr17
was predicted to be 164 nt in length (Fig. 2A). Primer extension experiments mapped the 5⬘ end of spd-sr17 downstream
from an extended ⫺10 region (TCTAGTAATAT) (see Fig.
S1A in the supplemental material), characteristic of pneumococcal promoters (61). Therefore, the length of Spd-sr17 is 144
nt, assuming transcription termination after the last T of the
predicted strong terminator (see Fig. S1A). Pneumococcus
lacks Rho factor homologues, and this estimation of the 3⬘ end
is likely to be accurate to within a couple of nucleotides. The
end of this transcription terminator was 94 bp upstream of the
start codon of asd, and Spd-sr17 does not contain an apparent
antiterminator RNA segment, nor does it encode a threoninelysine-rich peptide, making it unlikely that Spd-sr17 acts as a
leader RNA for asd under the conditions tested so far.
A band consistent with a 144-nt Spd-sr17 species was detected in Northern blots using probes 17(1) (Fig. 1B andC; see
also Fig. S1A in the supplemental material) and 17(2) (data
not shown) (see Fig. S1A), and this band was missing in a D39
⌬spd-sr17 mutant deleted for part of spd-sr17 complementary
to the two probes (Fig. 1C). Upon longer exposure, faint bands
were visible above 750 nt on blots; however, these bands were
still present for the ⌬spd-sr17 mutant (Fig. 1C), indicating that
they were not specific to the spd-sr17 genomic region. To
confirm that spd-sr17 is an independent transcription unit,
Northern blot assays were performed with RNA prepared
from a D39 ⌬spd-sr17 CEP::Pnative-spd-sr17 construct, which
contains a copy of the spd-sr17 region (from ⫺100 to ⫹164)
inserted into the ectopic CEP site (Fig. 2D; see also Table S1
in the supplemental material) (20). This strain produced the
same 144-nt band as the IU1690 parent strain, consistent with
inclusion of the spd-sr17 promoter and terminator in the ectopic copy (Fig. 3A). Likewise, ectopic expression of Spd-sr17
from the constitutive Pmal(c) promoter or from the PrRNA promoter in the parent strain caused overexpression of the same
band (Fig. 3A). Microarray analyses of ⌬spd-sr17 mutants did
not show changes in the relative transcript amounts of the
flanking genes (data not shown), consistent with spd-sr17 as a
single-gene operon. Spd-sr17 likely folds into a compact, stable
structure that has stem-loop structures at the 5⬘ and 3⬘ ends
and a moderate-sized, unpaired internal loop (Fig. 4A).
The spd-sr37 region can be independently transcribed as a
sRNA but is also cotranscribed with upstream and downstream genes. spd-sr37 is in the intergenic region between trmU
encoding tRNA (5-methylaminomethyl-2-thiouridylate)-methyltransferase and spd_0128 encoding a conserved hypothetical
protein in the MutT/Nudix family. Spd-sr37 was predicted to
be 92 nt in length (41). Primer extension assays mapped a
transcript 5⬘ end downstream from an extended ⫺10 promoter
box (see Fig. S1B in the supplemental material); therefore, the
length of the spd-sr37 transcript is 80 nt, assuming transcription
termination at the last T of the second of two relatively weak
predicted terminators downstream of spd-sr37 (see Fig. S1B).
A band consistent with a length of 80 nt was detected in
Northern blots using probe 37(1) (Fig. 1B and C; see also Fig.
sRNAs OF STREPTOCOCCUS PNEUMONIAE D39
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TSUI ET AL.
J. BACTERIOL.
tected on Northern blots (Fig. 1B and C). The three bands (80,
⬇750, and ⬎1,000 nt) were absent in a D39 ⌬spd-sr37 mutant
(Fig. 1C), indicating that these transcripts contained sequences
that paired with probe 37(1). Cotranscription of spd-sr37 with
upstream trmU and downstream spd_0128 and gidA was confirmed by RT-PCR experiments (data not shown) (see Materials and Methods).
Independent transcription of Spd-sr37 was confirmed in constructs containing the Pnative-spd-sr37 gene expressed from the
CEP site in the D39 ⌬spd-sr37 mutant (Fig. 2D and 3B).
Consistent with this interpretation, Spd-sr37 was overexpressed
when driven from the constitutive Pmal(c) and PHspac(⫺1) promoters in the CEP site in the D39 parent strain (Fig. 3B). The
apparently identical size of Spd-sr37 expressed from the native
and ectopic sites argues against processing of a larger transcript as the origin of the Spd-sr37 sRNA. It seems likely that
the terminator after spd-sr37 (Fig. 2B) serves as an attenuator
that terminates the sRNA transcript but also allows transcription readthrough. Spd-Sr37 likely folds into a stable, highly
paired structure lacking unpaired regions (Fig. 4B).
Microarray analyses of the D39 ⌬spd-sr37 mutant showed
large (⬇12-fold) increases in the relative transcript amounts
from both upstream trmU and downstream spd_0128 compared to the D39 parent (data not shown). In addition, the
relative transcript amount from the next gene downstream,
gidA (tRNA uridine 5-carboxymethylaminomethyl modifica-
tion enzyme) increased about 5-fold in the D39 ⌬spd-sr37
mutant. Expression of Spd-sr37 at about the wild-type level
from the CEP::Pnative-spd-sr37 construct (Fig. 3B) did not reduce the relative transcript amounts of trmU, spd_0128, and
gidA in the D39 ⌬spd-sr37 mutant (data not shown). This result
indicates that the ⌬spd-sr37 mutation was exerting a cis-acting
effect on expression of the cotranscribed flanking genes, rather
than a trans-acting effect caused by lack of expression of the
sRNA.
The ccnA region is the 3ⴕ-untranslated region of a spd_0239spd_0240 cotranscript as well as an sRNA gene. Of the five
similar ccnA to -E genes dependent on CiaR activation (22),
ccnA and ccnB are located in tandem downstream from
spd_0239 and spd_0240, which encode conserved hypothetical
proteins similar to GNAT family acetyltransferases (Fig. 2C).
No predicted terminator sequences are located between
spd_0239 and spd_0240 or between spd_0240 and ccnA. Probe
56, which corresponded to the nt 50 to 84 region of CcnA that
was dissimilar to the other four CcnB to -E sRNAs (see Fig.
S1C in the supplemental material), hybridized to the distinct,
strong 93-nt CcnA band and to a weak ⬎1,000-nt band (Fig.
1C). Both bands were absent in blots of a D39 ⌬ccnA mutant,
suggesting possible cotranscription of the ccnA region with
adjacent genes, and probe 56 did not cross-hybridize with
CcnB to -E (nt 92 to 151) from the D39 ⌬ccnA mutant (Fig.
1C). Cotranscription of spd_0239-spd_0240 and the ccnA re-
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FIG. 3. (A and B) Northern blot analyses of Spd-sr17 (A) and Spd-sr37 (B). Total RNA was prepared from cultures grown exponentially
(OD620 ⫽ 0.2) and hybridized with probe 17(1) (A) or 37(1) (B) (see Materials and Methods and also Table S3 in the supplemental material).
sRNA amounts were normalized to 5S rRNA amounts and are expressed as relative amounts compared to that in the D39 parent strain (IU1690).
Strain numbers and genotypes (see Table S1) are listed below each lane. A. 1, IU1690 (D39 parent); 2, IU2647 (IU1690 CEP::Pmal(c)-spd-sr17);
3, IU3473 (IU1690 CEP::PrRNA-spd-sr17); 4, IU2084 (IU1690 ⌬spd-sr17::Pc-ermAM); 5, IU2811 (IU1690 ⌬spd-sr17 CEP::Pnative-spd-sr17). B. Lanes
1 to 4 and 5 to 7 were obtained from two separate Northern blot assays and independent RNA preparations. Lanes 2 to 4 and lanes 6 and 7 were
normalized to lane 1 and lane 5, respectively. 1, IU1690 (D39 parent); 2, IU3471 (IU1690 CEP::Pmal(c)-spd-sr37); 3, IU2086 (IU1690
⌬spd-sr37::Pc-ermAM); 4, IU3493 (IU1690 ⌬spd-sr37::Pc-ermAM CEP::Pnative-spd-sr37); 5, IU1690 (D39 parent); 6, IU3653 (IU1690
CEP::PHspac-spd-sr37); 7, IU3657 (IU1690 CEP:: PHspac(⫺1)-spd-sr37).
VOL. 192, 2010
sRNAs OF STREPTOCOCCUS PNEUMONIAE D39
271
gion was confirmed by RT-PCR experiments (see Materials
and Methods) in which forward primer F1 or F2 and reverse
primer (probe) 56 (Fig. 2C) yielded a PCR product consistent
with a size of 1,096 or 456 bp, respectively, but did not yield
bands in controls lacking reverse transcriptase (data not shown).
Thus, the ccnA region acts as a 3⬘-untranslated region of a
spd_0239-spd_0240 cotranscript.
We confirmed that the 93-nt CcnA sRNA was still independently transcribed (20) by ectopic expression from the CEP site
(Fig. 2D). Expression of CcnA from its native promoter or
from the constitutive Pmal(c) restored the 93-nt band in the D39
⌬ccnA mutant to about the level found in the D39 parent strain
(Fig. 5B). Likewise, expression of the ectopic constructs in the
D39 parent led to 2.6- to 4.0-fold-greater expression of CcnA
(Fig. 3A), with the highest overexpression from the constitutive
Pmal(c) promoter. Together, these results confirm the previous
conclusion of Halfmann et al. (22) that CcnA is transcribed
independently from its own promoter to a transcription terminator. As reported before, CcnA likely folds into a structure
with 5⬘ and 3⬘ stem-loop structures flanking a lengthy unpaired
region (Fig. 4C).
Microarray analyses of the D39 ⌬ccnA mutant compared to
the D39 parent showed that the relative transcript amounts
from the upstream spd_0239 and spd_0240 genes increased 4and 10-fold, respectively (data not shown). qRT-PCR analyses
confirmed overexpression of these two transcripts (data not
shown). These results are consistent with the Northern blot
and RT-PCR data mentioned above and further confirm that
the ccnA region is cotranscribed with the two upstream genes.
Ectopic expression of CcnA at a nearly wild-type level from the
Pnative or Pmal(c) promoters in a ⌬ccnA mutant failed to reduce
the relative amounts of the spd_0239 and spd_0240 transcripts
as determined by qRT-PCR and microarray assays (data not
shown). Thus, the increase in relative amounts of the spd_0239
and spd_0240 transcripts in the ⌬ccnA mutant was a cis-acting
effect of the construct, rather than a trans-acting effect of the
sRNA. Similar to the spd-sr37 gene described above (Fig. 2B;
see also Fig. S1B in the supplemental material), the region
transcribed independently into the CcnA sRNA is also cotranscribed with adjacent genes, and certain insertions in the ccnA
region perturb the expression of these other genes, possibly by
stabilizing the cotranscript.
Lack of overt phenotypes and changes in transcription profiles of sRNA deletion and overexpression strains. There were
no overt phenotypic differences between the D39 parent strain
and sRNA deletion strains IU2083 (D39 ⌬ccnA::Pc-ermAM),
IU2084 (D39 ⌬spd-sr17::Pc-ermAM), and IU2086 (D39 ⌬spdsr37::Pc-ermAM) in the following tests (see Materials and
Methods): growth in BHI broth or CDM at 30°C or 37°C;
growth in BHI broth adjusted to a pH between 6.8 and 7.4 at
37°C; growth on TSAII BA plates at 30°, 37°, or 40°C; cell
morphology or chaining of the cells grown exponentially in
BHI broth (OD620, ⬇0.2) at 37°C; sensitivity to 0.1 or 0.2 M
NaCl, 25 or 50 ng per ml mupirocin, or 0.1 or 0.03 ␮g per ml
ampicillin in BHI broth culture; sensitivity on TSAII BA plates
at 37°C to Sensi-Disks containing penicillin, ampicillin, ticarcillin, oxacillin, vancomycin, or amdinocillin. No statistically
significant difference in mean survival time was detected between the D39 parent and the three sRNA deletion mutants in
a BALB/c mouse model of pneumonia (see Materials and
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FIG. 4. Predicted secondary structures of Spd-sr17 (A), Spd-sr37 (B), and CcnA (C), predicted by Mfold (http://mfold.bioinfo.rpi.edu/cgi-bin
/rna-form1.cgi) (80). Arrows point to 5⬘ ends, and ⌬G units are in kcal/mol.
272
TSUI ET AL.
J. BACTERIOL.
Methods). Likewise, with the exception of the genes flanking
spd-sr37 and ccnA mentioned above, only small changes
around the 2.0-fold cutoff were observed for relative transcript
amounts in microarray profiles of the D39 ⌬ccnA, D39 ⌬spdsr17, and D39 ⌬spd-sr37 mutants grown exponentially in BHI
broth at 37°C (data not shown). Finally, overexpression of
Spd-17 (⬇7⫻), Spd-37 (⬇5⫻), or CcnA (⬇4⫻) in D39 merodiploids containing ectopic constructs driven by the PrRNA,
PHspac(⫺1), or Pmal(c) promoters, respectively (Fig. 3 and 5) did
not cause strong changes in growth or microarray profiles in
bacteria grown exponentially in BHI broth at 37°C (data not
shown).
Ectopic expression of CcnA reverses some phenotypes of a
⌬ciaR mutant. Lack of phenotypes caused by deletion or overexpression of CcnA is consistent with redundancy with the
other four highly similar CcnB-E sRNAs (22). In addition, the
above results show that mutations in ccnA can lead to complicated cis-acting effects. To circumvent these problems and
study the functions of CcnA, we tried to take advantage of the
finding that expression of CcnA to -E is strictly dependent on
the CiaR response regulator (22). Therefore, a ⌬ciaR mutation
eliminates CcnA to -E redundancy, and ectopic expression of
CcnA from the CEP site avoids disrupting the ccnA region
(Fig. 2D). No CcnA transcript was detected in the D39 ⌬ciaR
CEP::Pnative-ccnA strain, indicating that the ectopic Pnative promoter was still fully dependent on CiaR for expression (Fig.
5C). In strains expressing CcnA constitutively from different
promoters in the CEP site, the amount of CcnA detected was
uniformly lower (10% to 60%) in the ⌬ciaR mutant than in the
⌬ccnA mutant (Fig. 5B and C). We do not know why the CcnA
amount is reduced in the ⌬ciaR mutant compared to the ciaR⫹
strain (Fig. 5B and C), but the expression level of CcnA from
the Pmal(c) promoter in CEP was still sufficient to reverse some
phenotypes compared to the nonexpressing ⌬ciaR CEP::
Pnative-ccnA strain.
CcnA expression partially restores growth defects of a ⌬ciaR
mutant. We observed that growth of the D39 ⌬ciaR mutant
was substantially defective and extremely sensitive to aeration
under these culture conditions. Growth of the D39 ⌬ciaR mutant in static BHI broth at 37°C in an atmosphere of 5% CO2
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FIG. 5. Northern blot analyses of CcnA expression in ciaR⫹ and ⌬ciaR strains. Total RNA samples used in the three panels were prepared from
cultures grown exponentially (OD620 ⫽ 0.2) in 25 ml of BHI broth in 50-ml conical tubes or in 60 ml of BHI in 250-ml bottles to the cell densities
indicated. sRNA amounts were normalized to 5S rRNA amounts and are expressed as relative amounts compared to that in the D39 parent strain
(IU1690). Strain numbers and genotypes (see Table S1 in the supplemental material) are listed for each lane below. A. 1, IU1690 (D39 parent);
2, IU2937 (IU1690 CEP::P-kan) control strain; 3, IU2496 (IU1690 CEP::Pnative-ccnA); 4, IU2942 (IU1690 CEP::Pmal(c)-ccnA); 5, IU2929 (IU1690
CEP::PspxB-ccnA). B. Lanes 1 to 3 and lanes 4 and 5 were obtained from two separate blots and independent RNA preparations. For lanes 4 and
5, the relative expression of CcnA was compared with D39 samples run on the same blot, but not shown in the Fig. Lanes: 1, IU1690 (D39 parent);
2, IU2083 (IU1690 ⌬ccnA::Pc-ermAM); 3 and 4, IU2527 (IU1690 ⌬ccnA::Pc-ermAM CEP:: Pmal(c)-ccnA); 5, IU2602 (IU1690 ⌬ccnA::Pc-ermAM
CEP::Pnative-ccnA). C. Lanes: 1, IU1690 (D39 parent); 2, IU2678 (IU1690 ⌬ciaR CEP::Pnative-ccnA); 3 and 4, IU2841 (IU1690 ⌬ciaR
CEP::Pmal(c)-ccnA); 5 and 6, IU2936 (IU1690 ⌬ciaR CEP::PspxB-ccnA). See text for additional details.
VOL. 192, 2010
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depended on the volume of medium relative to the size of the
vessel and aeration of the cultures during growth (Fig. 6A). In
tubes, growth of the D39 ⌬ciaR mutant was moderately biphasic and usually reached a lower growth yield than that of the
D39 parent strain (Fig. 6A), as reported previously (29). In
tubes without CO2, growth of the ⌬ciaR mutant was delayed
for hours (data not shown). In bottle cultures, the extent of
growth of the ⌬ciaR mutant was affected by how often cultures
were perturbed to take density readings and was again reduced
compared to that of the D39 parent strain (Fig. 6B). We did
not distinguish in these experiments whether these growth
differences were caused by slight differences in oxygen content,
pH, or both.
Because of this extreme sensitivity to growth conditions,
experiments involving the ⌬ciaR mutant were somewhat variable and were repeated numerous times (⬎16) with five independent constructs. Greater than 80% of ⌬ciaR CEP::
Pnative-ccnA cultures showed reduced growth, similar to that of
the ⌬ciaR mutant (Fig. 6). In contrast, constitutive expression
of CcnA from the Pmal(c) promoter in the CEP site improved
growth compared to the ⌬ciaR and ⌬ciaR CEP::Pnative-ccnA
mutants in greater than 85% of the cultures.
CcnA reduces the length of cell chains formed by the ⌬ciaR
mutant at low culture densities. We noticed that the CFU per
ml per OD620 unit was consistently reduced in the D39 ⌬ciaR
mutant compared to the D39 parent at low cell densities
(OD620, ⬇0.08) in early exponential phase (Fig. 7, bars 1 and
2). ⌬ciaR strains containing CEP::Pc-kan (control) or CEP::
Pnative-ccnA showed a lowered CFU per ml per OD620 unit,
similar to that of the ⌬ciaR mutant (Fig. 7, bars 3 and 4). In
contrast, ectopic expression of CcnA from Pmal(c) restored the
CFU per ml per OD620 unit to that of the D39 parent strain
(Fig. 7, bar 5).
This phenotype suggested that the chaining properties were
different between the strains expressing or not expressing
CcnA, because each chain of cells will form 1 CFU on plates.
Microscopic examination of low-density cultures (OD620, 0.05
to 0.06) confirmed that considerably longer chains of cells were
present in the ⌬ciaR, ⌬ciaR CEP::Pc-kan (control), and ⌬ciaR
CEP::Pnative-ccnA mutants compared to the chaining distribution of the ciaR⫹ parent (Fig. 8A and B and data not shown).
Constitutive ectopic expression of CcnA restored the chain
length distribution of the ⌬ciaR mutant to that of the ciaR⫹
FIG. 7. CFU per ml per OD620 of the ciaR⫹ parent and ⌬ciaR
strains that constitutively express [Pmal(c)-ccnA] or do not express
CcnA. Strains tested were as follows: bar 1, IU1690 (D39 parent); bar
2, IU2327 (IU1690 ⌬ciaR); bar 3, IU2750 (IU1690 ⌬ciaR CEP::P-kan
control); bar 4, IU2678 (IU1690 ⌬ciaR CEP::Pnative-ccnA); bar 5,
IU2841 (IU1690 ⌬ciaR CEP::Pmal(c)-ccnA). Cultures were grown in
glass tubes to an OD620 of 0.07 to 0.09, and CFU were determined as
described in Materials and Methods. Asterisks indicate statistically
significant differences in ratios relative to the ciaR⫹ parent strain (P ⬍
0.005) from three biological replicates.
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FIG. 6. Growth curves of ciaR⫹ and ⌬ciaR strains with or without ectopic expression of CcnA in BHI broth in glass tubes (A) or bottles (B) at
37°C in an atmosphere of 5% CO2 as described in Materials and Methods. Linear and semilog plots are shown to indicate growth yields and rates,
respectively. Strains had the following genotypes (see also Table S1 in the supplemental material): IU1690 (D39 parent); IU2841 (IU1690 ⌬ciaR
CEP::Pmal(c)-ccnA); IU2327 (IU1690 ⌬ciaR); IU2678 (IU1690 ⌬ciaR CEP::Pnative-ccnA).
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J. BACTERIOL.
parent (Fig. 8C). Direct counting of chains per unit volume
with a counting chamber (see Materials and Methods) confirmed that at the same low culture density (OD620, ⬇0.06), the
⌬ciaR CEP::Pnative-ccnA mutant produced fewer, but longer,
chains whereas the D39 parent and ⌬ciaR CEP::Pmal(c)-ccnA
construct produced shorter but more chains per volume of
culture (data not shown). We confirmed these growth and
chaining phenotypes with an independently constructed set of
strains (IU3067 and IU3036) (data not shown; see Table S1
in the supplemental material). As a control, we replaced the
ectopic CEP::Pmal(c)-ccnA locus in strain IU2841 (Fig. 8C) with
a CEP::P-aad9 marker to give strain IU3100 (see Table S1 in
the supplemental material). Strain IU3100 formed long chains
of cells, similar to those of the ⌬ciaR CEP::Pnative-ccnA mutant
(Fig. 8B and data not shown). Finally, this apparent difference
in chaining lessened as cultures continued to grow. At an
OD620 of ⬇0.2, all of the strains produced short or moderate-
length chains of cells, similar to the D39 parent, but lysis was
microscopically visible in cultures of the ⌬ciaR CEP::
Pnative-ccnA mutant that was not present for the D39 parent
and ⌬ciaR CEP::Pmal(c)-ccnA strains (data not shown). This
chain shortening and lysis seemed to coincide with the biphasic
shift in growth rate of the ⌬ciaR and ⌬ciaR CEP::Pnative-ccnA
mutants (Fig. 6A).
CcnA expression reverses the increase in relative transcript
amounts of the competence regulon. Previous studies of comCDE
operon expression and competence development in ⌬ciaR mutants showed dependencies on growth conditions, such as medium composition, degree of oxygenation, and pH (11, 12, 35,
44). We performed qRT-PCR analyses of relative comD transcript amounts and measured natural transformation efficiencies at different times during growth in bottles that were sampled only once (Fig. 9) (see Materials and Methods). The
growth curves for these conditions (Fig. 9A) were similar to
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FIG. 8. Chain formation of ciaR⫹ parent strain D39 (A) and ⌬ciaR mutants that do not (B) or do (C) express CcnA from the ectopic CEP site.
Strains IU1690 (D39 ciaR⫹ parent), IU2678 (IU1690 ⌬ciaR CEP::Pnative-ccnA), and IU2841 (IU1690 ⌬ciaR CEP::Pmal(c)-ccnA) were grown in BHI
in glass tubes to an OD620 of 0.05 and prepared for phase-contrast microscopy as described in Materials and Methods. Micrographs of typical cells
are shown. Distributions of chain lengths were based on 25 to 30 chains from at least two independent cultures of each strain.
VOL. 192, 2010
275
those of cultures in tubes (Fig. 6A) and different from those of
cultures in bottles that were sampled every hour (Fig. 6B).
Relative comD transcript levels normalized to 16S rRNA
were determined at different stages of growth 2.5 to 6.5 h after
inoculation as described in Materials and Methods (Fig. 9A
and B). Under these culture conditions, relative comD transcript amounts were substantially higher (⬎100-fold) in the
⌬ciaR CEP::Pnative-ccnA mutant compared to the D39 parent
or the ⌬ciaR CEP::Pmal(c)-ccnA strain in ⬎88% of the cultures
tested (Fig. 9B). Thus, ectopic CcnA expression normally reduced the amount of the comD transcript, and by inference the
comCDE cotranscript, strongly at all stages of growth. These
results were confirmed in a set of independently constructed
strains with the same genotypes as those shown in Fig. 9B
(strains IU2677 and IU2752, three isolates of IU3036, and
IU3067) (see also Table S1 in the supplemental material). As
a control, we determined the relative comD transcript amount
in strain IU3100, in which the ectopic CEP::Pmal(c)-ccnA locus
of strain IU2841 was replaced by a CEP::P-aad9 marker (see
Table S1). The relative comD transcript amount in IU3100
increased to that of the ⌬ciaR CEP::Pnative-ccnA strain (Fig. 9B
and data not shown). Therefore, even if a suppressor had
arisen in strain IU2841, its ability to lower comD transcription
would still be dependent on ectopic expression of CcnA. Finally, microarray experiments showed that the rest of the competence regulon was induced in the ⌬ciaR CEP::Pnative-ccnA
strain compared to the ⌬ciaR CEP::Pmal(c)-ccnA strain (see
Table S4 in the supplemental material).
We determined whether the natural transformation frequency followed the relative transcript amounts of comD and
the competence regulon in these strains. Under these culture
conditions (BHI at 37°C in a 5% CO2 atmosphere), the D39
parent was not naturally competent (asterisks) within the limits
of detection of this assay (Fig. 9C). In contrast, the ⌬ciaR
CEP::Pnative-ccnA mutant became measurably competent, consistent with the increased competence regulon transcript
amounts detected in the qRT-PCR and microarray experiments (Fig. 9B; see also Table S4 in the supplemental material). The natural transformation frequency of the ⌬ciaR
CEP::Pnative-ccnA mutant followed a reproducible pattern with
time in culture, with a high peak (5 ⫻ 10⫺3 to 10⫺4) about 2.5 h
after inoculation, followed by a drop to about 10⫺7 in the next
hour and then a rise to 10⫺6 for the next 2 h. Ectopic expression of CcnA in the ⌬ciaR CEP::Pmal(c)-ccnA strain abolished
or greatly reduced this natural competence (Fig. 9C), again
consistent with the results from transcript analyses. Similar
results were obtained in three biological replicates of this experiment (data not shown).
Finally, to test whether the putative pairing between CcnA
to -E and comC mRNA plays a role in negatively regulating
competence regulon transcription, we introduced point mutations into the comC leader region that should reduce pairing to
CcnA to -E (see Fig. S2 in the supplemental material). Unfortunately, the rpsL1 (streptomycin-resistant) mutation required
by the allele exchange method (67) to make the base change
mutations in comC altered the phenotypes of the D39 ⌬ciaR
mutant (data not shown). Therefore, we confined these experiments to the ciaR⫹ background, and we repaired the rpsL1
mutation in one of the mutants. Mutants IU3167 (D39 comC3
[5 base changes] rpsL1) and IU3631 (D39 comC6 [14 base
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FIG. 9. Relative comD transcript amount and natural transformation frequencies of ciaR⫹ parent strain D39 and ⌬ciaR mutants that do
not or do express CcnA from the ectopic CEP site. A. Growth curves
of strains IU1690 (D39 parent), IU2678 (D39 ⌬ciaR::P-ermB CEP::
Pnative-ccnA), and IU2841 [D39 ⌬ciaR::P-ermB CEP::Pmal(c)-ccnA].
Cultures were grown in separate bottles for each time point as described in Materials and Methods to minimize differences in handling
and aeration (see text). B. Representative graph of relative comD
transcript amounts determined by qRT-PCR at different time points of
growth. qRT-PCR was performed as described in Materials and Methods, and comD transcript amounts were normalized to amounts of 16S
rRNA. The relative comD transcript amount in the D39 parent strain
in midexponential growth (3.5 h) was set to 1 to allow comparisons.
Similar results were obtained for three biological replicates and other
independent constructs of these strains (see text). C. Representative
graph showing natural transformation of strain IU2678, but not
IU1690 and IU2841, at different time points of growth. Natural transformation frequencies were determined as described in Materials and
Methods. Transformants were obtained only for strain IU2678, where
numbers of transformants obtained in 800-␮l transformation reaction
mixtures are indicated above the arrows. No transformants (points
marked by asterisks indicate detection limits) were obtained at other
growth points of IU2678 and all growth points of IU1690 and IU2841.
Similar results were obtained in two other independent replicates of
this experiment, in which ectopic constitutive expression of CcnA in
the ⌬ciaR mutant abolished or greatly reduced natural competence.
sRNAs OF STREPTOCOCCUS PNEUMONIAE D39
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changes] rpsL⫹ linked to cat) (see Fig. S2) did not affect relative growth or comD transcript amount compared to the D39
ciaR⫹ strains during exponential growth in BHI broth (data
not shown). The implications of this negative result and possible mechanisms for the reversal of ⌬ciaR mutant phenotypes
by CcnA expression are considered in the Discussion section.
DISCUSSION
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A relatively limited number of sRNAs have been identified
so far in most Gram-positive bacteria, and considerably less is
known about the functions of sRNAs in Gram-positive than in
Gram-negative species (3, 6, 43, 54, 57, 62, 71, 73). In addition,
the functions of RNA chaperones, such as homologues of Hfq,
have been elusive in many of these Gram-positive species (2, 3,
15, 26), with the exception of Listeria monocytogenes, where
Hfq binds to sRNAs and plays roles in mRNA stability (6, 43).
We report here that S. pneumoniae expresses at least nine
sRNAs besides the five related CcnA to -E sRNAs that were
reported previously under the control of the CiaR response
regulator (22). Besides CcnA to -E, three of these sRNAs
(Spd-sr10, Spd-sr38/39, and Spd-sr52) may be transcribed from
redundant genes, some of which are within transposon IS1167
elements (Table 1). Our initial attempts to identify pneumococcal sRNAs by cDNA cloning were not successful and cataloged numerous rRNA and tRNA degradation products (data
not shown). Subsequently, we turned to bioinformatic predictions and found that 28% (11 of 40) of the sRNAs predicted by
one of these methods (41) could be detected by Northern
blotting (Fig. 1A and B and Table 1; see also Table S3 in the
supplemental material). Analogous approaches have shown
similar success rates recently in identifying sRNAs in other
Gram-positive bacteria (43, 55, 62, 71). CcnB to -E were not
picked up in the bioinformatic analysis (41) or included among
the 40 candidate sRNAs tested in this study. The inclusion of
CcnA, but not CcnB to -E, in this set probably occurred,
because a short segment of 34 nt in ccnA matched a corresponding segment in Streptococcus agalactiae, which was used
as a BLAST partner but lacked intact ccnA to -E genes. Thus,
pneumococcus expresses at least 14 sRNAs, despite an absence
of homologues to Hfq and other known RNA chaperones (see
the introduction). Hfq homologues are also absent in other
Streptococcus species in which sRNAs have been detected,
such as Streptococcus pyogenes (55, 66).
Of the 11 pneumococcal sRNAs analyzed here, 5 showed
differential expression: relative amounts of Spd-sr12, Spd-sr14,
Spd-sr17, and Spd-sr54 decreased as cells entered stationary
phase, and the CcnA (Spd-sr56) amount increased in response
to CSP (Fig. 1). The mechanisms and physiological function
for these changes, if any, remain to be determined. Of the
three sRNAs validated as independently expressed by 5⬘-end
determinations and ectopic expression (Fig. 2, 3, and 5), Spdsr17 was transcribed as a monocistronic operon, whereas Spdsr37 and CcnA were transcribed as independent sRNAs and
the regions specifying these sRNAs were also cotranscribed as
parts of operons with adjacent genes (Fig. 2). Consequently,
deletion/insertion mutations in the spd-sr37 or ccnA region
increased the relative transcript amounts of the upstream and
downstream tRNA modification genes or upstream genes encoding putative GNAT family acetyltransferases, respectively
(Fig. 2). Ectopic expression of Spd-sr37 and CcnA failed to
reduce relative transcript amounts of these adjacent genes,
indicating that the mutations were cis-acting, rather than transacting, through absence of sRNA expression (see Results).
These results indicate that disruption of some pneumococcal
sRNA genes could lead to phenotypes caused by changes in
transcription of adjacent genes. Whether the genetic linkage of
these sRNA genes indicates some functional relationship to
the cotranscribed genes remains to be determined. On the
other hand, the observation that Spd-sr37 or CcnA had apparently identical sizes whether expressed chromosomally or ectopically (Fig. 3B and 5B) argues against these sRNAs arising
solely through processing of larger transcripts.
The genes encoding Spd-sr17, Spd-sr37, and CcnA are
present only in Streptococcus species. BLAST searches against
the nr/nt database (E values of ⬍10⫺4) showed that single
copies of homologues of spd-sr17 and spd-sr37 are present in
the genomes of S. pneumoniae, S. sanquis, S. gordonii, and S.
suis. Homologues of spd-sr17, but not spd-sr37, are present in
S. agalactiae, S. thermophilus, S. mutans, and S. equi. In species
other than S. agalactiae and S. suis, spd-sr17 is flanked by the
asd gene (Fig. 2A). Homologues of spd-sr37, but not spd-sr17,
are also present in S. pyogenes, where spd-sr37 is flanked by
trmU and mutT(Nudix)-like genes similar to those in S. pneumoniae (Fig. 2B). The conservation of spd-sr17 and spd-sr37 in
different Streptococcus species is in agreement with the BLAST
partners listed by Livny et al. (41). In contrast, BLAST
searches revealed that multiple genes encoding homologues of
CcnA to -E are present only in Streptococcus species of the S.
mitis group (S. pneumoniae, S. mitis, S. oralis, S. gondonii, and
S. sanguis), which also encode homologues of the comCDE
competence operon (24).
In E. coli, sRNA pairing sometimes regulates the stability of
target mRNAs directly and often modulates translation initiation, which indirectly can lead to changes in the stability of
mRNA targets (17, 18, 78); hence, changes in relative transcript amounts in deletion mutants lacking sRNAs or constructs overexpressing sRNAs can lead to target identification
(77). In S. pneumoniae D39, we did not detect overt phenotypes or changes in microarray analyses of deletion mutants or
strains overexpressing Spd-sr17, Spd-sr37, or CcnA (Fig. 3 and
5). These negative results suggest that we may have failed to
test the right growth or stress conditions, and only a single
animal model of infection has been evaluated so far for the
deletion mutants (see Results). It is also possible that these
pneumococcal sRNAs may function largely at the translational
level or by directly binding to proteins other than RNA chaperones. Little is known about RNA metabolism in S. pneumoniae, despite its importance as a human pathogen, and a
link between translation efficiency and mRNA stability has not
been established.
sRNAs structures can be classified into two groups. sRNAs
that bind target mRNAs often have 5⬘ and 3⬘ stem-loop structures flanking central unpaired regions, whereas sRNAs that
bind to proteins other than RNA chaperones often fold into
highly paired, extended hairpin structures (reviewed in references 3 and 78). Based on its predicted structure, it seems
likely that CcnA may bind to an mRNA target (Fig. 4C) (22).
TargetRNA analysis (72) of CcnA suggests strong possible
pairing to leader regions of several mRNAs, including that of
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277
tested so far (22). These results also suggest that ComC
mRNA may not be a binding target of CcnA to -E. However,
additional experiments are needed to determine whether these
comC leader mutants are affected at the translational level or
under other conditions that affect competence induction. It
also remains to be determined whether negative regulators of
competence that are in the CiaR regulon, notably the HtrA
protease (63), may override effects of CcnA to -E in ciaR⫹
strains. But the general lack of effects of deletion or overexpression of the three pneumococcal sRNAs studied so far on
transcription patterns raises the issue that these sRNAs may
operate primarily at the translational level, in response to
specific stress conditions that have not yet been tested, or by
modulating the activities of specific target proteins.
ACKNOWLEDGMENTS
We thank Kyle Wayne for performing the animal experiments, Krystyna Kazmierczak for the construction of strain IU3373, Donald Morrison (University of Illinois, Chicago) for providing strains and synthetic CSP, and Jean-Pierre Claverys (CNRS-Université Paul Sabatier,
Toulouse, France) for the pCEP plasmid. We thank the Center for
Genomics and Bioinformatics at Indiana University Bloomington for
access to microarray analysis equipment and programs.
This project was supported by grant numbers GM075068 (to A.L.F.
and M.E.W.) and AI060744 (to M.E.W.) from the National Institute of
General Medical Sciences and the National Institute of Allergy and
Infectious Diseases, respectively.
The contents of this report are solely the responsibility of the authors and do not necessarily represent the official views of the National
Institutes of Health.
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the CcnA to -E sRNAs prompted us to determine the effects of
ectopic CcnA expression in a D39 ⌬ciaR mutant. We confirmed here some phenotypes previously reported for D39
⌬ciaR mutants, including biphasic growth and lower yield (29)
and competence induction (35). In addition, we observed additional D39 ⌬ciaR phenotypes, such as chaining (Fig. 8). Our
results confirmed the previous finding (22) that CcnA expression is absolutely dependent on the CiaR response regulator,
and we observed that the level of constitutively expressed
CcnA was reduced in a ⌬ciaR mutant compared to a ⌬ccnA
mutant (Fig. 5B and C). This decrease in relative CcnA
amount was not due to CiaR regulation, since the Pmal(c) promoter lacks the second, upstream CiaR-dependent promoter
P1malM (20, 22). Despite this reduction, constitutive expression
of CcnA consistently reversed several phenotypes of the D39
⌬ciaR mutant, including reduced growth and a tendency to lyse
(Fig. 6), reduced CFU per ml per OD620 unit (Fig. 7), increased chaining at low OD620 values (Fig. 8), and increased
competence gene expression and natural competence induction (Fig. 9). Preliminary experiments confirmed that constitutive expression of CcnE also restored growth and reduced
chaining of a D39 ⌬ciaR mutant (data not shown). Together,
these results suggest that some of the complex phenotypes of
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the CcnA to -E sRNAs.
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comD transcript amount (see Results) is consistent with the
previous conclusion that CcnA to -E do not strongly affect
competence induction in ciaR⫹ strains under the conditions
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