Journal
of General Virology (1998), 79, 2877–2882. Printed in Great Britain
...................................................................................................................................................................................................................................................................................
Retroviral RNA dimer linkage
Jane Greatorex and Andrew Lever
Department of Medicine, University of Cambridge, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2QQ, UK
Introduction
All retroviral genomes apparently consist of two identical
positive-strand molecules of RNA, making them the only
known diploid viruses. The evidence for this is discussed
below, but the advantages conferred by carrying two genomes
are as yet unclear, although the diploid genome will permit
virus recombination and template switching during reverse
transcription. The conservation of the diploid genome throughout the Retroviridae argues that it has a key role in the virion
life-cycle. The diploid genome appears to be physically linked
(see below) and the site at which the linkage occurs is referred
to as the dimer linkage site (DLS). Because of its proximity, the
DLS has been proposed to contribute to the encapsidation
signal of retroviruses.
The Retroviridae are a diverse family, and their individual
life-cycles vary widely : thus, to assign a single generic role to
the dimer linkage may be too simplistic. This review will cover
the evidence for the dimeric genome, mapping of the various
DLS in different retroviruses, and recent attempts to assign a
role in vivo for the dimer linkage. As with many aspects of
retrovirology, most data have come from studies of human
immunodeficiency virus (HIV-1).
RNA dimers and dimer linkage – the evidence
The dimeric nature of the retroviral genomic RNA is
supported by sedimentation analysis (Stoltzfus & Snyder,
1975) and evidence from a number of electron microscopic
(EM) studies looking at viruses as diverse as HIV-1, endogenous murine ecotropic virus, endogenous murine xenotropic virus, avian reticuloendotheliosis virus, Moloney murine
leukaemia virus (MoMLV) and Rous sarcoma virus (RSV)
(Bender et al., 1978 ; Clever & Parslow, 1997 ; Murti et al.,
1981). It has been demonstrated in several viruses, including
MoMLV, HIV-1 and Harvey sarcoma virus, that when virion
RNA is isolated, run out on native polyacrylamide gels, and
probed by Northern blotting, two species of RNA can be
detected, consistent in size with a monomer and a dimer. The
dimer can be denatured by heat to become monomeric. The
EM studies show that there appears to be a particularly stable
contact point between the RNA molecules, close to the 5« end
Author for correspondence : Andrew Lever.
Fax ­44 1223 336846. e-mail amll1!medschl.cam.ac.uk
of the genome. Under less stringent conditions (Mangel et al.,
1974) multiple points of contact have been noted, suggesting
that the single linkage observed under more restrictive
conditions might be a primary association or dimerization
initiation site (DIS). Recent work from Ho$ glund et al. (1997),
looking at the dimer linkage of HIV-1 showed, interestingly,
that there did not appear to be any free 5« ends, indicating that
the two RNAs were interacting in addition at site(s) upstream
of the recognized DLS. A combination of length determination
and computer modelling suggested that more than one region
might be involved in dimer linkage in this virus.
The structure/nature of the linkage
Studies with in vitro transcribed RNA showed that
transcripts generated from the 5« end of a variety of retroviral
genomes exhibited electrophoretic profiles consistent with
their being dimeric (Darlix et al., 1992). This observation
provided a system within which the sequences and structures
involved in the linkage of the two RNA molecules could be
investigated. Since those initial studies, biochemical analysis,
mutagenesis and physical probing of the RNAs in vitro have
helped to elucidate critical sequences and predict secondary
structures. Recently, the tertiary structure of at least the initial
dimerization contact in HIV-1 was elucidated by NMR
(Mujeeb et al., 1998) (see Fig. 3).
Early theories as to the nature of the interaction ignored the
EM evidence that the molecules appeared to be aligned in a
parallel fashion (Coffin, 1982). Subsequently purine tracts
involved in non-canonical bonds, and apparently essential for
dimer formation in vitro, were identified in the 5« leader
sequence of HIV-1 (Marquet et al., 1994). However, when
identical tracts were deleted from the 5« leader sequence of
HIV-2, dimeric transcripts were still obtained (Berkhout et al.,
1993).
In a refinement of this work, two groups of investigators
(Awang & Sen, 1993 ; Sundquist & Heaphy, 1993) separately
suggested that since in vitro dimer formation in HIV-1 is cation
dependent, this might implicate groups of guanine tetrads coordinated by cations linking the RNA molecules together,
analogous to the structure of telomeres. In a number of
different studies, however, it has been shown that dimer
formation occurs in the absence of sequences containing these
guanine motifs (Marquet et al., 1994 ; Muriaux et al., 1995).
0001-5884 # 1998 SGM
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J. Greatorex and A. Lever
Fig. 1. Putative secondary structure of the DIS and packaging signal (Ψ) in HIV-1 (Subject to modification after Harrison et al.,
1998.).
This does not completely rule out a role for guanine tetrads
since they might still be involved in maturation or stabilization
of a dimer, but indicates that they are not pivotal in actual
dimer formation.
Watson–Crick interactions are now the focus of attention.
In the past 2–3 years, most work on the sequences involved in
retroviral RNA dimer linkage has centred around palindromic
regions found in the 5« leader sequences of MoMLV, avian
leukosis virus (ALV) and certain lentiviruses. The leader region,
including the encapsidation signal of HIV-1, is predicted to
consist of a series of stem–loop structures (Baudin et al., 1993 ;
Berkhout, 1996 ; Clever et al., 1996 ; Harrison & Lever, 1992)
(Fig. 1). One of these, containing a terminal palindrome
(designated SL1 by some workers), was identified as being
essential for dimer formation in vitro. This, and other
palindromic sequences, are sometimes termed dimer initiation
sites (DIS), indicating their importance in the very first stage of
the interaction between the two RNA strands. As indicated
below, however, the DIS structures so far postulated cannot
alone be responsible for the stable intermolecular linkage seen
in vitro, and the heat lability of RNAs containing only the DLS
differ considerably from more extended structures containing
the additional DLS elements. Those investigating the role of
these sequences in vitro have proposed various theories for the
nucleotide interaction. Watson–Crick bonds could form between the palindromic sequences, and the stem–loops subsequently interact in a so-called kissing-hairpin manner (Fig. 2 a).
In these in vitro experiments, the structure depends not only on
complementarity in bases between the strands, but it is also
suggested that flanking purine residues are important (Paillart
et al., 1997) (Fig. 2 b). This model has been supported by
chemical probing of the RNA secondary structure and
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computer predictions of putative tertiary structure (Paillart et
al., 1997 ; Skripkin et al., 1994) (Fig. 2 b). Comparable RNA–
RNA interactions based on the interaction between autocomplementary single-stranded loops have been described
elsewhere for the regulation of plasmid regulation (Persson et
al., 1990). Autocomplementarity between palindromes is
insufficient for dimerization as some constructs containing
inverted palindromes barely dimerize at all. However, since
similar palindromic sequences occur in the leader regions of
other lentiviruses and in ALV and MoMLV (Girard et al., 1995 ;
Khan & Giedroc, 1992), this mechanism was postulated as
being common to all retroviruses. However, the stability of in
vitro produced dimeric RNA to heat and denaturing agents
strongly argues against Watson–Crick pairing as being the
only interaction involved in the linkage (if the in vitro system
is representative of the in vivo linkage). Removal of no single
element or motif consistently abolishes dimer formation. The
recent tertiary structure already alluded to supports the model
of two interacting stem–loop structures (Mujeeb et al., 1998).
The loop structures proposed by this study, however, are
atypical and are dissimilar to those seen in kissing-hairpin
models. The loops are distorted by interstrand stacking of the
adenosines, and the authors of this paper suggest that this
might contribute to the instability of the structure, perhaps
bringing about the melting of the initial contact and leading to
the formation of a more stable structure. The actual NMR was
obtained by proton 2-D NMR, using D O. It remains to be
#
seen whether other studies back up these findings, and whether
or not a structure can be solved for the more stable dimer
complex.
In vivo, viral and cellular proteins may have a role to play
in the dimerization process. The nucleocapsid protein (NC) has
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Review : Retroviral RNA dimer linkage
(a)
(b)
Fig. 2. Proposed ‘ kissing-hairpin ’ DIS in HIV-1. (a) The extended interaction occurring between the two RNA molecules after
the initial contact between the two palindromic sequences. The sequence shown is from HIV-1NL4-3. (b) A proposed tertiary
structure showing the inter- and intra-molecular bonds. The important adenine residues are also indicated. Adapted from Paillart
et al. (1997).
Fig. 3. NMR structure of the kissing-loop dimer of HIV-1. The structure shown is the symmetrized global average of 34
restraint-minimized structures. The minor groove of the loop helix is oriented to the right. Adapted from Mujeeb et al. (1998).
potent annealing activity, as well as the ability to destabilize
nucleic acids (Khan & Giedroc, 1992). NC protein was shown
to strongly activate dimerization in vitro, presumably because
of these two properties (Feng et al., 1996). It has been
suggested that NC interacts with the kissing-hairpin itself
(Muriaux et al., 1996).
Other retroviral DLS which have been mapped include
those of bovine leukaemia virus (BLV) and human T-cell
lymphotropic virus type I (HTLV-I). Although these two
viruses are related, the evidence to date does not point to a
similar mechanism for dimer linkage (Greatorex et al., 1996 ;
Katoh et al., 1991, 1993). In HTLV-I, sequences required for the
dimerization of in vitro transcribed RNAs have been mapped to
a region downstream of the splice donor (SD) and upstream of
the primer binding site (PBS). Potential secondary structures
identified in this region of the HTLV-1 genome include
palindromic sequences, and a conserved stem–loop. However,
disruptive mutagenesis of these putative structures reduced
but did not abolish dimer formation, suggesting that they were
not essential (Greatorex et al., 1996). In contrast to HTLV-I,
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J. Greatorex and A. Lever
BLV, like RSV, appears to have a discontinuous DLS (Katoh et
al., 1991, 1993 ; Kurg et al., 1995) although work is still required
to pinpoint the site of dimer association.
In RSV, the dimer linkage has been visualized by EM (Murti
et al., 1981). In vitro mapping studies indicate that it is located
between nucleotides 544 and 564 from the 5« end of the
genome (Bieth et al., 1990). There appear to be discontinuous
sequences within positions 208–270 and 400–460 which also
promote dimer formation.
To date, there is no known single type of nucleotide
interaction which can explain all the findings in vivo and in vitro
concerning the nature of the dimer linkage interaction. The
frequent occurrence of palindromic sequences in regions shown
to be associated with dimer linkage strongly supports their
involvement, but it must be remembered that there is also
much experimental evidence (described above) which indicates
that Watson–Crick bonding alone cannot be the sole linkage.
The role of the dimer linkage in the virus lifecycle
In all retroviruses, dimer linkage and the diploid genome
provide the opportunity for switching from a damaged to a
physically linked intact template, and this is probably advantageous. In highly variable viruses like HIV where variation
is linked to ‘ fitness ’, the ability to recombine is also an asset. In
MoMLV the DLS overlap with sequences encoding a hypervariable region, and it has been proposed (Mikkelsen et al.,
1996) that the DLS might, in this instance, be involved in
recombination, although this is difficult to reconcile with the
highly conserved genome of MoMLV.
Recent reports have further investigated whether or not the
kissing-hairpin sequences have a role in virion dimer formation,
and also if they have a biological significance (see below).
Summarized in Table 1 are some of the data from the six
studies which have attempted to examine the role of the dimer
linkage in the HIV-1 life-cycle. The conclusions vary, possibly
reflecting differences in assay methods. Two of the five studies
quantifying relative amounts of dimer demonstrated that this
was decreased in viruses mutated in the kissing-hairpin domain
(Haddrick et al., 1996 ; Laughrea et al., 1997). However, this
decrease was nowhere near as profound as that seen in vitro,
and three of the five studies showed no difference at all
(Berkhout & van Wamel, 1996 ; Clever & Parslow, 1997 ;
Sakuragi & Panganiban, 1997). In vitro, the melting temperatures of the transcripts in which the kissing-hairpin sequences
have been deleted differ from that of undeleted transcripts
(Laughrea & Jette, 1994 ; Muriaux et al., 1995). There is
conflicting evidence as to whether or not this is the case in vivo,
however, some authors seeing no difference between wildtype and mutated viruses (Berkhout, 1996 ; Clever & Parslow,
Table 1. Summary of data from studies investigating the role of the kissing-hairpin
sequences in some aspects of the HIV-1 life-cycle
Reference
Laughrea et al. (1997)
Paillart et al. (1996)
Berkhout & van Wamel
(1996)
Clever & Parslow (1997)
Haddrick et al. (1996)
Sakuragi & Panganiban
(1997)
Study
Observations
Stem–loop mutations
Saw difference in amount of dimer
Deletion of stem and palindrome Mutants packaged to a lesser extent than
wild-type
Transcription efficiency reduced
Splicing reduced
Infectivity reduced C 99 %
Deletion of KLD*³bulge
2–5¬ decrease in packaging
Mutations in palindrome
10–1000¬ decrease in infectivity
Disruption of palindrome
Dimers had similar thermal stabilities
Insertion of larger palindrome
Mutants showed 2¬ decrease in
packaging
Mutants showed 10¬ decrease in
infectivity
Mutations in stem and loop
Dimers had similar thermal stabilities
Mutants showed slight packaging defects
Mutants package more spliced RNA
Mutants show infectivity defect
Mutation in palindrome
Reduction in the amount of dimeric RNA
Delayed replication
Stem–loop mutations
Dimers had similar thermal stabilities
KLD mutations
* KLD, kissing loop domain.
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Review : Retroviral RNA dimer linkage
1997), others seeing less dimeric RNA (Haddrick et al., 1996 ;
Laughrea et al., 1997) (see Table 1). Likewise, replication was
also impaired when the viral genome was mutated in the
kissing-loop domain. However, if assays are continued for a
period of time longer than that reported in these studies (! 10
days), it is apparent that replication of the mutants is only
delayed and can reach the same levels as wild-type (Harrison et
al., 1998).
As other authors have observed, the proximity of the DLS
to the packaging signal in HIV-1 makes it tempting to
speculate that dimerization and encapsidation are linked ; it is
thus difficult to separate the effect of a mutation on one
function from its effect on the other. Similarly, independently
of this, dimerization may affect other processes linked to
infectivity. The authors of the various studies in Table 1 used
different assays to assess viral infectivity, but 4}6 studies
showed a marked decrease in infectivity (ranging from
10–100 %) (Berkhout & van Wamel, 1996 ; Clever & Parslow,
1997 ; Laughrea et al., 1997 ; Paillart et al., 1996). Pertinent
observations on the link between encapsidation and dimerization have been made in other viruses : RSV is one of the few
retroviruses in which virion RNA has been directly analysed
by electrophoresis on native polyacrylamide gels (Lear et al.,
1995), and this study appeared to show that initially monomers
are packaged, maturing over time to dimers. Thus, in this virus,
encapsidation does not depend on dimerization or a DLS.
Fu & Rein (1993) showed in MoMLV that, in newly
released virions, the RNA was dimeric but dissociated to
monomers at a lower temperature than in mature virions.
Using protease negative and NC mutants, they suggested that
maturation of dimeric RNA requires the cleavage of the Gag
precursor or the presence of an intact cysteine array in the
released NC protein.
However, both RSV and MoMLV are simple retroviruses
and it is hazardous to extrapolate findings in either of these to
complex retroviruses such as HIV where the encapsidation
process may be quite distinct.
The way forward
The dimer linkage is a consistent feature of retroviruses as
evidenced by biochemical analysis, electron microscopy, and
examination of virion RNA. Its conservation, proximity and
potential role in RNA encapsidation also single it out as one of
a number of possible targets for gene and antiviral therapy.
Much effort has gone into in vitro studies teasing out the
possible nature of the actual interaction but a complete
understanding of the DLS awaits solution of all the tertiary and
quaternary structures. Until that time, molecular modelling by
mutagenesis provides only a tantalizing but incomplete
glimpse with which to speculate about the structure. This
information hopefully will supplement biological studies of
virus replication and infectivity and between them further our
understanding of the role of RNA dimer linkage in the virus
life-cycle.
We thank Ray Hicks for his help with the figures in this Review, and
Teresa Barnes for secretarial assistance. We would also like to thank Paul
Digard for critical reading of the manuscript. We are grateful to Anwar
Muyeeb and Tris Parslow for Fig. 3.
References
Awang, G. & Sen, D. (1993). Mode of dimerisation of HIV-1 genomic
RNA. Biochemistry 32, 11453–11457.
Baudin, F., Marquet, R., Isel, C., Darlix, J.-L., Ehresmann, B. &
Ehresmann, C. (1993). Functional sites in the 5« region of human
immunodeficiency virus type 1 RNA form defined structural domains.
Journal of Molecular Biology 229, 382–397.
Bender, W., Chein, Y.-H., Chattopadhyay, S., Vogt, P. K., Gardner,
M. B. & Davidson, N. (1978). High-molecular-weight RNAs of AKR,
NZB, and wild mouse viruses and avian reticuloendotheliosis virus all
have similar dimer structures. Journal of Virology 25, 888–896.
Berkhout, B. (1996). Structure and function of the human immunodeficiency virus leader RNA. Progress in Nucleic Acid Research and
Molecular Biology 54, 1–34.
Berkhout, B. & van Wamel, J. L. B. (1996). Role of the DIS hairpin in
replication of human immunodeficiency virus type 1. Journal of Virology
70, 6723–6732.
Berkhout, B., Oude Essink, B. B. & Schoneveld, I. (1993). In vitro
dimerisation of HIV-2 leader RNA in the absence of PuGGAPuA motifs.
FASEB Journal 7, 181–187.
Bieth, E., Gabus, C. & Darlix, J.-L. (1990). A study of the dimer
formation of Rous sarcoma virus RNA and of its effect on viral protein
synthesis in vitro. Nucleic Acids Research 18, 119–127.
Clever, J. L. & Parslow, T. G. (1997). Mutant human immunodeficiency
virus type 1 genomes with defects in RNA dimerization or encapsidation.
Journal of Virology 71, 3407–3414.
Clever, J. L., Wong, M. L. & Parslow, T. G. (1996). Requirements for
kissing-loop-mediated dimerization of human immunodeficiency virus
RNA. Journal of Virology 70, 5902–5908.
Coffin, J. (1982). Structure of the retroviral genome. In RNA Tumor
Viruses, pp. 261–368. Edited by R. Weiss, N. Teich, H. Varmus & J.
Coffin. Cold Spring Harbor, NY : Cold Spring Harbor Laboratory.
Darlix, J.-L., Gabus, C. & Allan, B. (1992). Analytical study of avian
reticuloendotheliosis virus dimeric RNA, generated in vivo and in vitro.
Journal of Virology 66, 7245–7252.
Feng, Y.-X., Copeland, T. D., Henderson, L. E., Gorelick, R. J., Bosche,
W. J., Levin, J. G. & Rein, A. (1996). HIV-1 nucleocapsid protein induces
‘‘ maturation ’’ of dimeric retroviral RNA in vitro. Proceedings of the
National Academy of Sciences, USA 93, 7577–7581.
Fu, W. & Rein, A. (1993). Maturation of dimeric viral RNA of Moloney
murine leukaemia virus. Journal of Virology 67, 5443–5449.
Girard, P.-M., Bonnet-Mathoniere, B., Muriaux, D. & Paoletti, J.
(1995). A short autocomplementary sequence in the 5« leader region is
responsible for dimerization of MoMuLV genomic RNA. Biochemistry
34, 9785–9794.
Greatorex, J. S., Laisse, V., Dokhelar, M.-C. & Lever, A. M. L. (1996).
Sequences involved in the dimerisation of human T-cell leukaemia virus
type-1 RNA. Nucleic Acids Research 24, 2919–2923.
Haddrick, M., Lear, A. L., Cann, A. J. & Heaphy, S. (1996). Evidence
that a kissing loop structure facilitates genomic RNA dimerisation in
HIV-1. Journal of Molecular Biology 259, 58–68.
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Sat, 17 Jun 2017 17:19:11
CIIB
J. Greatorex and A. Lever
Harrison, G. P. & Lever, A. M. L. (1992). The human immunodeficiency
virus type 1 packaging signal and major splice donor region have a
conserved stable secondary structure. Journal of Virology 66, 4144–4153.
Harrison, G. P., Miele, G., Hunter, E. & Lever, A. M. L. (1998).
Functional analysis of the core human immunodeficiency virus type 1
packaging signal in a permissive cell line. Journal of Virology 72,
5886–5896.
Ho$ glund, S., Ohagen, A., Goncalves, J., Panganiban, A. T. & Gabuzda,
D. (1997). Ultrastructure of HIV-1 genomic RNA. Virology 233,
271–279.
Mujeeb, A., Clever, J. L., Billeci, T. M., James, T. L. & Parslow, T. G.
(1998). Structure of the dimer initiation complex of HIV-1 genomic
RNA. Nature Structural Biology 5, 432–436.
Muriaux, D., Girard, P.-M., Bonnet-Mathoniere, B. & Paoletti, J.
RNA at low ionic strength. Journal of
(1995). Dimerization of HIV-1
LAI
Biological Chemistry 270, 8209–8216.
Muriaux, D., de Roquigny, H., Roques, B.-R. & Paoletti, J. (1996).
Katoh, I., Kyushiki, H., Sakamoto, Y., Ikawa, Y. & Yoshinaka, Y. (1991).
Bovine leukemia virus matrix-associated protein MA (p15) : further
processing and formation of a specific complex with the dimer of the 5«terminal genomic RNA fragment. Journal of Virology 65, 6845–6855.
Katoh, I., Yasunaga, T. & Yoshinaka, Y. (1993). Bovine leukaemia virus
RNA sequences involved in dimerization and specific Gag protein
binding : close relation to the packaging sites of avian, murine, and human
retroviruses. Journal of Virology 67, 1830–1839.
Khan, R. & Giedroc, D. P. (1992). Recombinant human immunodeficiency virus type 1 nucleocapsid (NCp7) protein unwinds tRNA.
Journal of Biological Chemistry 267, 6689–6695.
Kurg, A., Sommer, G. & Metspal, A. (1995). An RNA stem–loop
structure involved in the packaging of bovine leukemia virus genomic
RNA in vivo. Virology 211, 434–442.
Laughrea, M. & Jette, L. (1994). A 19-nucleotide sequence upstream of
the 5« major splice donor is part of the dimerization domain of the human
immunodeficiency virus 1 genomic RNA. Biochemistry 33, 13464–13474.
Laughrea, M., Jette, L., Johnson, M., Kleiman, L., Liang, C. & Wainberg,
M. A. (1997). Mutations in the kissing-loop hairpin of human immuno-
deficiency virus type 1 reduce viral infectivity as well as genomic RNA
packaging and dimerization. Journal of Virology 71, 3397–3406.
Lear, A. L., Haddrick, M. & Heaphy, S. (1995). A study of the
dimerization of the Rous sarcoma virus RNA in vitro and in vivo. Virology
212, 47–57.
Mangel, W. F., Delius, H. & Duesburg, P. H. (1974). Structure and
molecular weight of the 60–70S RNA and the 30–40S RNA of the Rous
sarcoma virus. Proceedings of the National Academy of Sciences, USA 71,
4541–4545.
Marquet, R., Paillart, J.-C., Skripkin, E., Ehresmann, C. & Ehresmann,
B. (1994). Dimerization of human immunodeficiency virus type 1 RNA
involves sequences located upstream of the splice donor site. Nucleic
Acids Research 22, 145–151.
Mikkelsen, J. G., Lund, A. H., Kristensen, K. D., Duch, M., Sorensen,
M. S., Jorgensen, P. & Pedersen, F. S. (1996). A preferred region for
CIIC
recombinational patch repair in the 5« untranslated region of primer
binding site-impaired murine leukemia virus vectors. Journal of Virology
70, 1439–1447.
NCp7 activates HIV-1Lai RNA dimerization by converting a transient
loop–loop complex into a stable dimer. Journal of Biological Chemistry
271, 33686–33692.
Murti, K. G., Bondurant, M. & Tereba, A. (1981). Secondary structural
features in the 70S RNAs of Moloney murine leukemia and Rous sarcoma
viruses as observed by electron microscopy. Journal of Virology 37,
411–419.
Paillart, J.-C., Berthoux, L., Ottman, M., Darlix, J.-L., Marquet, R.,
Ehresmann, B. & Ehresmann, C. (1996). A dual role of the putative
RNA dimerization initiation site of human immunodeficiency virus type
1 in genomic RNA packaging and proviral synthesis. Journal of Virology
70, 8348–8354.
Paillart, J.-C., Westhof, E., Ehresmann, C., Ehresmann, B. & Marquet,
R. (1997). Non-canonical interactions in a kissing loop complex : the
dimerization initiation site of HIV-1 genomic RNA. Journal of Molecular
Biology 270, 36–49.
Persson, C., Wagner, E. G. H. & Nordstrom, N. (1990). Control of
replication of plasmid R1 : structures and sequences of the antisense RNA,
CopA, required for its binding to the target RNA, CopT. Journal of
European Molecular Biology 9, 3767–3775.
Sakuragi, J.-I. & Panganiban, A. T. (1997). Human immunodeficiency
virus type 1 RNA outside the primary encapsidation and dimer linkage
region affects RNA dimer stability. Journal of Virology 71, 3250–3254.
Skripkin, E., Paillart, J.-C., Marquet, R., Ehresmann, B. & Ehresmann,
C. (1994). Identification of the primary site of the human immuno-
deficiency virus type 1 RNA dimerization in vitro. Proceedings of the
National Academy of Sciences, USA 91, 4945–4949.
Stoltzfus, C. M. & Snyder, P. N. (1975). Structure of B77 sarcoma virus
RNA : stabilization of RNA after packaging. Journal of Virology 16,
1161–1170.
Sundquist, W. I. & Heaphy, S. (1993). Evidence for interstrand
quadruplex formation in the dimerization of human immunodeficiency
virus type 1 genome RNA. Proceedings of the National Academy of Sciences,
USA 90, 3393–3397.
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
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