4750–4754 Nucleic Acids Research, 2000, Vol. 28, No. 23
© 2000 Oxford University Press
Mitochondrial RNAs of myxomycetes terminate with
non-encoded 3′ poly(U) tails
Tamara L. Horton and Laura F. Landweber*
Departments of Ecology and Evolutionary Biology, Molecular Biology, Princeton University, Princeton, NJ 08544,
USA
Received July 26, 2000; Revised and Accepted October 10, 2000
ABSTRACT
We examined the 3′ ends of edited RNAs from the
myxomycetes Stemonitis flavogenita and Physarum
polycephalum using a modified anchor PCR
approach. Surprisingly, we found that poly(A) tails
are missing from the cytochrome c oxidase subunit 1
mRNA (coI ) from both species and the cytochrome c
oxidase subunit 3 mRNA (cox3 ) from P.polycephalum.
Instead, non-encoded poly(U) tails of varying length
were discovered at the 3′ ends of these transcripts.
These are the first described examples of 3′ poly(U)
tails on mature mRNAs in any system.
INTRODUCTION
RNA molecules are routinely processed by splicing, editing
and 3′ polyadenylation, resulting in transcripts that contain
information not encoded by the DNA genome. In eukaryotic
nuclear genes, 3′ end processing consists of mRNA cleavage
followed by the addition of a poly(A) tail by a group of factors,
including poly(A) polymerase and the C-terminal domain of
RNA polymerase II (1). Some prokaryotic, mitochondrial and
chloroplast mRNAs also have poly(A) tails. While polyadenylation of eukaryotic nuclear genes enhances stability and
translation initiation (2), in prokaryotes and chloroplasts polyadenylation provides a signal for rapid RNA degradation (3,4).
In mitochondrial systems, polyadenylation has a variety of
functions: creating stop codons on most human mitochondrial
transcripts (5), possibly signaling for translation initiation in
trypanosomes (6), and stimulating quick degradation of a plant
mitochondrial mRNA (7) and some trypanosome RNAs (8).
Adenosine is not the only nucleotide found in unencoded 3′
tails. Kinetoplastids, unicellular eukaryotes with extensive
uridine insertional/deletional editing in their mitochondria,
have 3′ polyuridine tails on guide RNAs (gRNAs), a population
of mitochondrial RNAs involved in editing (9). These nonencoded poly(U) tails may aid the editing complex in holding
together the broken halves of the purine-rich mRNA during the
cleavage stage of the editing process (10,11), or might denature
secondary structure in the regions of the mRNAs being edited
(12). The other class of non-messenger RNA encoded by the
kinetoplastid mitochondrial genome, the ribosomal RNAs,
also have non-encoded 3′ poly(U) tails (13). Poly(U) tails have
even been detected on the ends of many unedited and partially
edited pre-mRNAs in Trypanosoma brucei, and are thought to
be added by a rampant terminal uridyl transferase activity
operating on editing intermediates (14).
Physarum polycephalum is a myxomycete, or plasmodial
slime mold, that is amenable to cellular study in the laboratory.
The production of functional mitochondrial transcripts for
almost all of P.polycephalum’s messenger and structural RNAs
requires several types of RNA editing. Many single cytidine
insertions, a small number of uridine and mixed dinucleotide
insertions, and a few instances of cytidine to uridine base
conversions modify the RNA sequences. For instance, the
cytochrome c oxidase subunit 1 (coI) mRNA is edited by insertion
of 59 Cs, a single U and three mixed dinucleotides. Four C to
U conversions are also found in this transcript (15).
Here, we present our discovery of a new form of RNA
processing that alters myxomycete mitochondrial transcripts.
We show that P.polycephalum and Stemonitis flavogenita both
have non-encoded 3′ poly(U) tails added to edited mitochondrial mRNAs. The unusual tails on mRNAs that have also
undergone editing suggest a possible connection between these
types of RNA sequence change.
MATERIALS AND METHODS
Cultures
Freeze-dried cultures of S.flavogenita (24714) were obtained
from the American Type Culture Collection, and grown on
half-strength cornmeal agar plates into full size plasmodia.
Plates of P.polycephalum plasmodia were obtained from
Carolina Biologicals.
Isolation of nucleic acids
RNA and DNA were extracted from the slime mold plasmodia
by use of Trizol reagent from Life Technologies. RNA was
treated with DNase (Promega); DNA was treated with RNase A
(Sigma). Nucleic acids were then extracted with phenol/
chloroform, ethanol precipitated, and resuspended in 10 mM Tris,
pH 7.4, 0.1 mM EDTA.
Artificial RNA tailing, reverse transcription and PCR of cDNA
GTP tails were added to total RNA by yeast poly(A)
polymerase as described (16) in the presence of 0.5 mM GTP.
Reverse transcription was performed using SuperScript II
*To whom correspondence should be addressed. Tel: +1 609 258 1947; Fax: +1 609 258 1682; Email: [email protected]
Present address:
Tamara L. Horton, Laboratory of Molecular Parasitology, The Rockefeller University, 1230 York Avenue, New York, NY 10021, USA
Nucleic Acids Research, 2000, Vol. 28, No. 23 4751
reverse transcriptase from Life Technologies, and primer UXR′C12
on S.flavogenita RNA, and TXRC12 for P.polycephalum RNA
(primer sequences listed below). The nested PCR of S.flavogenita coI cDNA was performed for 20 cycles with primer
CUAUXR′ and primer coi551st, followed by 20 cycles with
CUAUXR′ and primer coi561st. Physarum polycephalum coI
cDNA was amplified in 37 cycles of PCR, with primers
3PPcoiF and TXR. Physarum polycephalum cox3 cDNA was
amplified in 40 cycles of PCR with primers 3PPcox3F and
TXR. Physarum polycephalum mitochondrial LSU cDNA was
amplified in a nested PCR of 20 cycles with primers 3PLSUF1
and TXR, followed by 25 cycles with primers 3PPLSUF and
TXR. Physarum polycephalum nuclear SSU cDNA was amplified
in 37 cycles of PCR with primers 3PPSSUF and TXR.
Amplification of DNA
Walking PCR of S.flavogenita DNA was performed as
described (17). The single strand amplification was 40 cycles
with primer coi551b. The second PCR was 22 cycles with
primer coi561st and UXR′C12. The third PCR was 25 cycles
with primer coi581st and CUAUXR′. A single clone was
obtained by this method, the plasmid isolated and sequenced.
Primer stem-ptR was designed at the 3′ most extreme of this
clone sequence, then used in a 40 cycle PCR with primer
coi551st. The PCR product was precipitated and directly
sequenced with primer coi551st and stem-ptR.
Figure 1. Poly(U) tails on S.flavogenita coI cDNA. The 3′ untranslated region
of S.flavogenita coI cDNA clones are aligned with directly sequenced DNA
PCR product (GenBank accession no. AF239222). The alignment begins with
nucleotide 1805 (T) in the GenBank record, and the inferred stop codon is underlined. Dotted regions within clone sequences indicate identity with the DNA
sequence. The regions with white letters inside a black box indicate poly(U) tails.
The gray shaded regions designate the primer used for reverse transcription. Slight
variation in the length of poly(G) regions of reverse transcription primers may be
due to minor imperfections in the primer pool, or PCR slippage through the
homopolymeric region. The final base of the primer is a non-C anchor base,
intended to direct annealing location of this primer to the most 5′ end of a
potentially long poly(G) run during reverse transcription. Non-primer-derived
nucleotides beyond the poly(U) tails (black letters on a white background
within the clone sequences) are probably artifacts of the tailing proceedure, as
explained in the Results.
Cloning, purification and sequencing
PCR products were cloned with the TOPO TA cloning kit from
Invitrogen. Plasmids were purified with the High Pure Plasmid
Isolation Kit from Boehringer-Mannheim/Roche. Both strands
of all plasmids were sequenced at the Princeton University
SynSeq facility.
Primer sequences
(Note that D designates a 1:1:1 mixture of A, G, and T.)
UXR′C12 (5′-CUACUACUACUACTCGAGAATTCCCCCCCCCCCCD-3′)
TXRC12 (5′-CATCATCATCATCTCGAGAATTCCCCCCCCCCCCD-3′)
CUAUXR ′ (5′-CUACUACUACUACTCGAGAATT-3′)
TXR
(5′-CATCATCATCATCTCGAGAATT-3′)
coi551st (5′-TTGTTAGCAAATGATTATCG-3′)
coi551b (5′-biotin-TTGTTAGCAAATGATTATCG-3′)
coi561st (5′-TACATTTCCTTTAACTGTTGC-3′)
3PPcoiF (5′-CGCCGTATTCCAGATTATCCTGATGC-3′)
3PPcox3F (5′-CATGCTCCTTTCTCTATTTCTGATGG-3′)
3PLSUF1 (5′-TCTGTCTAGTACGAAAGGACTGG-3 ′)
3PPLSUF (5′-TGAGCTGTTTGCGCACGCTCATTCGC-3′)
3PPSSUF (5′-GTAAAACGAGTGCTTGAACAAGGCGTCC-3′)
stem-ptR (5′-TAAGTAAATGCAGTAACATTTG-3′)
RESULTS
While investigating the distribution and types of RNA editing
in myxomycetes, we attempted to recover the 3′ end of the coI
mRNA of S.flavogenita by anchor PCR, a technique that relies
on the presence of a 3′ poly(A) tail (18). Though sequences
obtained by this method extended to near the 3′ end of the
predicted coding region, the sequences lacked a stop codon.
RT and PCR products were not full length, due to annealing of
our modified poly(T) primer to the A-rich sequences still within
the coding region. To circumvent the problems associated with
traditional anchor RT and PCR of a transcript of such a high A/T
content (70%), we used poly(A) polymerase to add an artificial
G tail to the RNA (16), then reverse-transcribed and amplified
from the introduced tail into the coding region.
Sequence analysis of six cloned cDNA fragments revealed
that the 3′ end of S.flavogenita coI mRNA has a conventional
termination codon (UAA), followed by a 24 nt untranslated region
and a homopolymeric tail of 20–31 nt (Fig. 1). Surprisingly, the
tail is not composed of poly(A), as expected, but consists
primarily of uridines. We recovered the DNA sequence in this
region by a walking PCR approach (17), and found that the
poly(U) tail is not encoded in the genomic sequence (Fig. 1).
Didymium nigripes coI cDNA clones also terminate with
similar poly(U) tails (data not shown), although we did not
determine the corresponding DNA sequence.
To expand our survey of the distribution of this type of
poly(U) tailing in a myxomycete with a greater number of
published mitochondrial gene sequences, we amplified the 3′
region of several P.polycephalum RNAs by the same modified
anchor PCR technique. We examined the termini of two edited
mitochondrial mRNAs, an edited mitochondrial structural
RNA, and a non-edited nuclear structural RNA. We found that
the non-encoded poly(U) tail is a common feature of the edited
mitochondrial mRNAs in both species.
Although both species share the presence of a 3′ poly(U) tail
on the coI transcripts, P.polycephalum’s coI tails are shorter
than those of S.flavogenita. Physarum polycephalum’s coI tails
are only 9–25 nt long, and vary in their start site on the RNA
over a 33 nt region (Fig. 2). In seven clones analyzed, only one
tail contained a single cytidine residue, as compared to two out
of six S.flavogenita clones, which contain one and three
cytidines apiece. Another P.polycephalum clone contained a
4752 Nucleic Acids Research, 2000, Vol. 28, No. 23
Figure 2. Poly(U) tails on P.polycephalum coI cDNA. The 3′ untranslated
region of P.polycephalum coI cDNA clones are aligned with DNA sequence
from Jonatha Gott (personal communication). Numerical notation is continuous
with GenBank accession no. L14779. The inferred stop codon is underlined.
Annotation as in Figure 1.
guanidine residue amidst the uridine run. When the P.polycephalum tail sequences are aligned with their corresponding
regions on the mitochondrial genomic DNA sequence
(J.M.Gott, personal communication), the Us are not encoded in
the genomic copy. The cDNA sequences agree with nuclease
protection experiments that imply that the end of the P.polycephalum coI mRNA is ~50 bases downstream of the stop codon
(L.M.Visomirski-Robic and J.M.Gott, personal communication).
RNA editing adds 32 Cs and a single UC dinucleotide to the
cytochrome c oxidase subunit 3 (cox3) mRNA in P.polycephalum (19). Five out of six clones of the 3′ end of cox3
terminate with poly(U) tails (Fig. 3). The tails range from 12 to
37 bases in length, with the starting point of the tails spanning
a 21 base region. The tails in these clones were composed
uniformly of U residues. We note that for all of the P.polycephalum cDNA transcripts, some clones contain a non-encoded
A/G-rich sequence just upstream of the 3′ primer. These A/G
stretches are probably caused by a few contaminating adenosines
in the poly(A) polymerase-catalyzed G tailing reaction, since
yeast poly(A) polymerase catalyzes the addition of adenosine
twice as efficiently as guanosine (16). If the artificial tail were
contaminated with a few As, then the 3′ anchor primer
(TXRC12) would anneal slightly downstream of the real
beginning of the artificial tail, resulting in the presence of these
A residues interspersed with poly(G) tails. The fact that these
A/G regions are found in common on all P.polycephalum
sequences, even those without poly(U) tails (see below),
supports this explanation, and confirms a real difference
between poly(U)-tailed and non-poly(U)-tailed transcripts.
Figure 3. Poly(U) tails on P.polycephalum cox3 cDNA. The 3′ untranslated
region of P.polycephalum cox3 cDNA clones are aligned with corresponding
DNA sequence (GenBank accession no. AF084526). The first nucleotide of
the alignment corresponds to nucleotide 3368 (T) in the GenBank record, and
the inferred stop codon is underlined. Annotation as in Figure 1.
We also amplified the terminal region of a structural RNA,
the mitochondrial large subunit rRNA (23S), which is edited
by 52 C insertions and five dinucleotide insertions (19).
Analysis of six clones corresponding to the 3′ end of the mitochondrial LSU transcript revealed only a few U residues: two
transcripts terminated with one U each, and one ended with
three Us (Fig. 4A). These ‘tails’ were added over a 7 bp region,
though one of the clones that lacked any apparent U tail ended
30 bases downstream of the end of the earliest-terminating
clone. Some clones had small poly(A) tails, but these could be
artifacts of the artificial tailing process, as described above.
To ascertain whether the tails were unique to mitochondrially
encoded transcripts, which had undergone RNA editing, we
also examined the 3′ end of the P.polycephalum 18S nuclearencoded small subunit (SSU) rRNA. No nuclear transcripts in
P.polycephalum are known to exhibit editing. Six clones of the
SSU transcript displayed no evidence of poly(U) tails on the SSU
rRNA (Fig. 4B). These sequences also serve as a negative control
for the poly(G) tailing and reverse transcription, as both
treatments were performed singly for the total P.polycephalum
RNA sample. Detection of different RNA transcripts both with
and without 3′ poly(U) tails by this method strengthens our
assertion that the poly(U) tails are naturally present on the
mitochondrial RNA transcripts.
DISCUSSION
Although poly(A) tails are present on nuclear mRNAs of
P.polycephalum (20), and common on mitochondrial mRNAs
in some organisms, they are not known to be universally
present, and have not been directly detected on mitochondrial
RNAs of myxomycetes. We have found that at least one
Nucleic Acids Research, 2000, Vol. 28, No. 23 4753
Figure 4. Physarum polycephalum mitochondrial large subunit ribosomal
RNA (mit LSU) cDNA and nuclear small subunit ribosomal RNA (SSU)
cDNA. (A) Physarum polycephalum mitochondrial large subunit ribosomal
RNA (mit LSU) cDNA clones are aligned with the DNA sequence (GenBank
accession no. AF080602). The first position of the alignment corresponds to
nucleotide 2783 (T) in the GenBank record. (B) Physarum polycephalum
nuclear small subunit ribosomal RNA (SSU) cDNA clones are aligned with
the DNA sequence (GenBank accession no. X13160). The alignment begins
with nucleotide 1950 (G) in the GenBank record. Annotation as in Figure 1.
(The extreme ends of the lower clones’ reverse transcription primer sequences
have been omitted for clarity.)
S.flavogenita mitochondrial mRNA and two P.polycephalum
mitochondrial mRNAs are not polyadenylated, but rather
polyuridylated. With the exception of the incompletely edited
pre-mRNAs of kinetoplastid mitochondria (14), these are the
first described poly(U) tails on mRNA transcripts in any
organism. Interestingly, both kinetoplastids and myxomycetes
share RNA editing of mitochondrial transcripts, albeit by very
dissimilar and presumably independently evolved mechanisms. However, in contrast to the poly(U)-tailed kinetoplastid
pre-mRNAs, which appear to be transient intermediates in the
editing process, the myxomycete poly(U)-tailed transcripts
appear to be mature mRNAs, as they show complete editing in
the 3′ region that was amplified and sequenced (21). Because
insertional editing in myxomycetes is believed to be
cotranscriptional, proceeding from the 5′ to the 3′ end of the
transcript (22,23), the mRNAs are probably completely translatable.
The absence of clearly defined poly(U) tails on mitochondrial large subunit rRNA may mean that the tails perform
translation-related functions for mRNAs analogous to the roles
of poly(A) tails in other organisms. The tails may be actively
targeted exclusively to mRNAs. In eukaryotic nuclear systems,
where poly(A) tails are only found on mRNAs, their absence
on rRNAs is explained by transcription of messenger and
structural RNA by different polymerases, where pol II is
actively involved in the tail addition (1). In mitochondrial
systems, a single polymerase produces all transcripts. It is
unclear how addition of poly(A) tails is restricted to mRNAs in
mitochondria; in fact, a few mitochondrial RNAs with 3′ polyadenylation have been detected in Plasmodium falciparum,
mosquitoes and mammals (24). It is possible that some fundamental difference in sequence or secondary structure of the
P.polycephalum mitochondrial large subunit rRNA does not
promote its termination by poly(U), or that another process
specifically removes tails after addition.
The poly(U) tail sequences in this study vary somewhat in
length and overall composition; a few tails include cytidine
and guanine residues. A previous study of edited trypanosome
cDNAs concluded that though PCR-induced mutation
occurred at a low frequency overall, the PCR error within long
T homonucleotide stretches was much higher than the other
regions of the template. In fact, in nearly 3 kb of analyzed
sequence, 11 of 14 total PCR-induced mutations occurred in
homo(T) regions, with the majority of these in the longest T
stretch. Most mutations consisted of a single T insertion or
deletion per poly(T) run, probably due to polymerase slippage,
and there was also one T to C transition (25). However, even if
most of the variation we observed within the mononucleotide
runs of myxomycete poly(U) tails is real, similar or greater
mixed base composition has been noted in some poly(A)
sequences in organelles. The poly(A) rich 3′ tail sequences of
spinach chloroplast genes contain ~25% guanosines and a
combined 5% uridines and cytosines (3). Trypanosome
poly(A) tails also contain occasional U insertions, which may
be added either by the RNA polymerase or by the terminal
uridyl transferase (TUTase) activity present in these organisms
(26).
This study demonstrates that RNA processing of mitochondrial
mRNA transcripts in myxomycetes includes not only RNA
editing, but also 3′ polyuridylation. All of the mRNA transcripts
in which we found 3′ poly(U) tails were also processed by
editing (21). To discern whether these two unique forms of
RNA processing are related, one might examine the 3′ end of a
mitochondrial mRNA uninvolved in RNA editing. However,
all currently known P.polycephalum mitochondrial mRNAs
require editing (19), and the editing process is extremely
efficient (27). Thus, while mysteries abound in the organelles of
myxomycetes, further conclusions await progress in molecular
techniques, such as isolation of non-tailing or non-editing
mutants and the facile transformation of mitochondria.
ACKNOWLEDGEMENTS
We gratefully acknowledge many helpful discussions with
Dennis Miller and Jonatha Gott, and thank Jonatha for
generously supplying us with clones of P.polycephalum coI
and unpublished downstream DNA sequence. We also than
Catherine Lozupone for providing technical assistance. T.L.H.
was supported in part by a National Science and Engineering
Graduate Fellowship. This work was supported in part by
4754 Nucleic Acids Research, 2000, Vol. 28, No. 23
National Institute of General Medical Sciences Grant
GM59708 to L.F.L.
REFERENCES
1. Hirose,T., Kusumegi,T. and Sugiura,M. (1998) Translation of tobacco
chloroplast rps14 mRNA depends on a Shine-Dalgarno-like sequence in
the 5′-untranslated region but not on internal RNA editing in the coding
region. FEBS Lett., 430, 257–260.
2. Manley,J.L. and Proudfoot,N.J. (1994) RNA 3′ ends: formation and
function–meeting review. Genes Dev., 8, 259–264.
3. Lisitsky,I., Klaff,P. and Schuster,G. (1996) Addition of destabilizing poly
(A)-rich sequences to endonuclease cleavage sites during the degradation
of chloroplast mRNA. Proc. Natl Acad. Sci. USA, 93, 13398–13403.
4. Sarkar,N. (1997) Polyadenylation of mRNA in prokaryotes. Annu. Rev.
Biochem., 66, 173–197.
5. Ojala,D., Montoya,J. and Attardi,G. (1981) tRNA punctuation model of
RNA processing in human mitochondria. Nature, 290, 470–474.
6. Militello,K.T. and Read,L.K. (1999) Coordination of kRNA editing and
polyadenylation in Trypanosoma brucei mitochondria: complete editing is not
required for long poly(A) tract addition. Nucleic Acids Res., 27, 1377–1385.
7. Gagliardi,D. and Leaver,C.J. (1999) Polyadenylation accelerates the
degradation of the mitochondrial mRNA associated with cytoplasmic
male sterility in sunflower. EMBO J., 18, 3757–3766.
8. Militello,K.T. and Read,L.K. (2000) UTP-dependent and -independent
pathways of mRNA turnover in Trypanosoma brucei mitochondria.
Mol. Cell. Biol., 20, 2308–2316.
9. Blum,B. and Simpson,L. (1990) Guide RNAs in kinetoplastid
mitochondria have a nonencoded 3′ oligo(U) tail involved in recognition
of the preedited region. Cell, 62, 391–397.
10. Kapushoc,S.T. and Simpson,L. (1999) In vitro uridine insertion RNA
editing mediated by cis-acting guide RNAs. RNA, 5, 656–669.
11. Burgess,M.L., Heidmann,S. and Stuart,K. (1999) Kinetoplastid RNA
editing does not require the terminal 3′ hydroxyl of guide RNA, but
modifications to the guide RNA terminus can inhibit in vitro U insertion.
RNA, 5, 883–892.
12. Leung,S.S. and Koslowsky,D.J. (1999) Mapping contacts between gRNA and
mRNA in trypanosome RNA editing. Nucleic Acids Res., 27, 778–787.
13. Adler,B.K., Harris,M.E., Bertrand,K.I. and Hajduk,S.L. (1991)
Modification of Trypanosoma brucei mitochondrial rRNA by
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
posttranscriptional 3′ polyuridine tail formation. Mol. Cell. Biol., 11,
5878–5884.
Decker,C.J. and Sollner-Webb,B. (1990) RNA editing involves
indiscriminate U changes throughout precisely defined editing domains.
Cell, 61, 1001–1011.
Gott,J.M., Visomirski,L.M. and Hunter,J.L. (1993) Substitutional and
insertional RNA editing of the cytochrome c oxidase subunit 1 mRNA of
Physarum polycephalum. J. Biol. Chem., 268, 25483–25486.
Martin,G. and Keller,W. (1998) Tailing and 3′-end labeling of RNA with
yeast poly(A) polymerase and various nucleotides. RNA, 4, 226–230.
Katz,L.A., Curtis,E.A., Pfunder,M. and Landweber,L.F. (2000)
Characterization of novel sequences from distantly related taxa by
walking PCR. Mol. Phylogenet. Evol., 14, 318–321.
Dorit,R.L., Ohara,O. and Gilbert,W. (1993) One-sided anchored
polymerase chain reaction for amplification and sequencing of
complementary DNA. Methods Enzymol., 218, 36–47.
Miller,D., Mahendran,R., Spottswood,M., Costandy,H., Wang,S.,
Ling,M.L. and Yang,N. (1993) Insertional editing in mitochondria of
Physarum. Semin. Cell Biol., 4, 261–266.
Martel,R., Tessier,A., Pollotta,D. and Lemieux,G. (1988) Selective gene
expression during sporulation of Physarum polycephalum. J. Bacteriol.,
170, 4784–4790.
Horton,T.L. and Landweber,L.F. (2000) Evolution of four types of RNA
editing in myxomycetes. RNA, 6, 1339–1346.
Visomirski-Robic,L.M. and Gott,J.M. (1997) Insertional editing in isolated
Physarum mitochondria is linked to RNA synthesis. RNA, 3, 821–837.
Visomirski-Robic,L.M. and Gott,J.M. (1997) Insertional editing of
nascent mitochondrial RNAs in Physarum. Proc. Natl Acad. Sci. USA, 94,
4324–4329.
Gillespie,D.E., Salazar,N.A., Rehkopf,D.H. and Feagin,J.E. (1999)
The fragmented mitochondrial ribosomal RNAs of Plasmodium
falciparum have short A tails. Nucleic Acids Res., 27, 2416–2422.
Landweber,L.F., Fiks,A.G. and Gilbert,W. (1993) The boundaries of
partially edited transcripts are not conserved in kinetoplastids:
implications for the guide RNA model of editing. Proc. Natl Acad. Sci.
USA, 90, 9242–9246.
Van der Spek,H., Speijer,D., Arts,G.J., Van den Burg,J., Van Steeg,H.,
Sloof,P. and Benne,R. (1990) RNA editing in transcripts of the
mitochondrial genes of the insect trypanosome Crithidia fasciculata.
EMBO J., 9, 257–262.
Rundquist,B.A. and Gott,J.M. (1995) RNA editing of the coI mRNA
throughout the life cycle of Physarum polycephalum. Mol. Gen. Genet.,
247, 306–311.