In vitro evidence that the untranslated leader of the HIV-1 genome is an RNA checkpoint that regulates multiple functions through conformational changes

Bimodal loop-loop interactions increase the affinity of RNA aptamers for HIV-1 RNA structures

Multiple loop-loop interactions between adjacent RNA hairpins regulate gene expression in different organisms. To demonstrate that such natural interactions could be mimicked for generating RNA ligands that are able to recognize simultaneously at least two structured RNA targets, a double kissing complex model was designed. The target consisted of two HIV-1 transactivating responsive (TAR) RNA variants, BRU and MAL, connected by a non-nucleotidic linker. The double ligand was generated by combining the corresponding hairpin aptamers, R06BRU and R06MAL, identified previously by in vitro selection [Ducongé, F., and Toulmé, J. J (1999) RNA 5, 1605-1614]. The resulting interaction was analyzed by thermal denaturation monitored by UV spectroscopy, electrophoretic mobility shift assays (EMSAs), and surface plasmon resonance (SPR) experiments. The bimodal complex was characterized by a binding equilibrium constant increased by at least 1 order of magnitude compared to that of the complexes between the individual parent hairpins. This resulted from a slower dissociation rate. We then made use of such a strategy for targeting two structured functional motifs of the folded 5' untranslated region (5'UTR) of HIV-1. Two bivalent RNA ligands were designed that targeted simultaneously the TAR and dimerization initiation site (DIS) hairpins or the TAR and poly(A) ones. The results show that these ligands also displayed enhanced affinity for their target compared to the individual molecules. The work reported here suggests that bimodal structured RNA ligands might provide a way of increasing the affinity of aptamers for folded RNA targets.

Graphical exploratory data analysis of RNA secondary structure dynamics predicted by the massively parallel genetic algorithm

Studies indicate that RNA may enter intermediate and multiple conformational states, which may impact gene expression and molecular function. It is known that the biologically functional states of RNA molecules may not correspond to their minimum energy conformations, that kinetic barriers may trap the molecule in a local minimum, that folding often occurs during transcription, and that cases exist in which a molecule will transition between one or more functional conformations. Thus, methods for simulating the folding pathway and dynamic behavior of an RNA molecule are important for the prediction of RNA structure and its associated functions. We have developed several data mining techniques guided by interactive visualization tools associated with our massively parallel genetic algorithm for RNA/DNA secondary structure prediction, MPGAfold, and StructureLab analysis workbench. Most of the methods and tools are also applicable to dynamic programming algorithm (DPA) folding data analysis. When applied to MPGAfold results these methodologies are used to determine the significant intermediate and final structures associated with co-transcriptional and full length RNA folding. Since the genetic algorithm is essentially stochastic, multiple runs are required to develop a consensus understanding of an RNA structure. The interactive visualizations facilitate interpretation of results from sequential or full length individual MPGAfold runs, final results of multiple folding runs, including multiple population sizes, and the results from multiple RNA sequences of one family. This paper describes several of these techniques and shows how they are used to help solve this highly combinatoric problem.

The T12I mutation within the SP1 region of Gag restricts packaging of spliced viral RNA into human immunodeficiency virus type 1 with mutated RNA packaging signals and mutated nucleocapsid sequence

Specific packaging of human immunodeficiency virus type 1 (HIV-1) RNA is attributable to the high affinity of nucleocapsid (NC) sequence of Gag for the cis-acting RNA packaging signals located within the 5' un-translated region (5' UTR). Interestingly, we have previously reported that the T12I mutation (named MP2) within SP1 of Gag prevented incorporation of spliced viral RNA into mutated viruses that lacked the stem-loop 1 (SL1) RNA element (also named dimerization initiation site, DIS), suggesting a role for the SP1 sequence in viral RNA packaging. In this study, we have further tested this activity of MP2 in the context of a variety of mutations that affect viral RNA incorporation. The results showed that MP2 was able to effectively restrict packaging of spliced viral RNA into viruses containing either NC mutations R10A and K11A or mutated 5' UTR sequence, such as DeltaGU3 that lacked the 112-GUCUGUUGUGUG-123 sequence of U5, D1 that was deleted of a 27 nt fragment immediately downstream of the primer binding site (PBS), Delta(306-325) that had the SL3 RNA element removed and MD2 that was missing the 328-GGAG-331 sequence. As a result, MP2 contributed increased infectivity to the related viruses. Therefore, the MP2 mutation demonstrates a distinct role in HIV-1 RNA packaging that is neither pertained to the specific viral RNA packaging signal nor to the NC sequence.

Structural polymorphism of the HIV-1 leader region explored by computational methods

Experimental studies revealed that the elements of the human immunodeficiency virus type 1 (HIV-1) 5′-untranslated leader region (5′-UTR) can fold in vitro into two alternative conformations, branched (BMH) and ‘linearized’ (LDI) and switch between them to achieve different functionality. In this study we computationally explored in detail, with our massively parallel genetic algorithm (MPGAfold), the propensity of 13 HIV-1 5′-UTRs to fold into the BMH and the LDI conformation types. Besides the BMH conformations these results predict the existence of two functionally equivalent types of LDI conformations. One is similar to what has been shown in vitro to exist in HIV-1 LAI, the other is a novel conformation exemplified by HIV-1 MAL long-distance interactions. These novel MPGAfold results are further corroborated by a consensus probability matrix algorithm applied to a set of 155 HIV-1 sequences. We also have determined in detail the impact of various strain mutations, domain sizes and folds of elongating sequences simulating folding during transcription on HIV-1 RNA secondary structure folding dynamics.

How retroviruses select their genomes

As retroviruses assemble in infected cells, two copies of their full-length, unspliced RNA genomes are selected for packaging from a cellular milieu that contains a substantial excess of non-viral and spliced viral RNAs. Understanding the molecular details of genome packaging is important for the development of new antiviral strategies and to enhance the efficacy of retroviral vectors used in human gene therapy. Recent studies of viral RNA structure in vitro and in vivo and high-resolution studies of RNA fragments and protein-RNA complexes are helping to unravel the mechanism of genome packaging and providing the first glimpses of the initial stages of retrovirus assembly.

Kinetics and thermodynamics make different contributions to RNA folding in vitro and in yeast

RNAs somehow adopt specific functional structures despite the capacity to form alternative nonfunctional structures with similar stabilities. We analyzed RNA assembly during transcription in vitro and in yeast using hairpin ribozyme self-cleavage to assess partitioning between functional ribozyme structures and nonfunctional stem loops. Complementary insertions located upstream of the ribozyme inhibited ribozyme assembly more than downstream insertions during transcription in vitro, consistent with a sequential folding model in which the outcome is determined by the structure that forms first. In contrast, both upstream and downstream insertions strongly inhibited assembly of the same ribozyme variants when expressed as chimeric mRNAs in yeast, indicating that inhibitory stem loops can form even after the entire ribozyme sequence has been transcribed. Evidently, some feature unique to the intracellular environment modulates the influence of transcription polarity and enhances the contribution of thermodynamic stability to RNA folding in vivo.

Analysis of the contribution of reverse transcriptase and integrase proteins to retroviral RNA dimer conformation

All retroviruses contain two copies of genomic RNA that are linked noncovalently. The dimeric RNA of human immunodeficiency virus type 1 (HIV-1) undergoes rearrangement during virion maturation, whereby the dimeric RNA genome assumes a more stable conformation. Previously, we have shown that the packaging of the HIV-1 polymerase (Pol) proteins reverse transcriptase (RT) and integrase (IN) is essential for the generation of the mature RNA dimer conformation. Analysis of HIV-1 mutants that are defective in processing of Pol showed that these mutant virions contained altered dimeric RNA conformation, indicating that the mature RNA dimer conformation in HIV-1 requires the correct proteolytic processing of Pol. The HIV-1 Pol proteins are multimeric in their mature enzymatically active forms; RT forms a heterodimer, and IN appears to form a homotetramer. Using RT and IN multimerization defective mutants, we have found that dimeric RNA from these mutant virions has the same stability and conformation as wild-type RNA dimers, showing that the mature enzymatically active RT and IN proteins are dispensable for the generation of mature RNA dimer conformation. This also indicated that formation of the mature RNA dimer structure occurs prior to RT or IN maturation. We have also investigated the requirement of Pol for RNA dimerization in both Mason-Pfizer monkey virus (M-PMV) and Moloney murine leukemia virus (MoMuLV) and found that in contrast to HIV-1, Pol is dispensable for RNA dimer maturation in M-PMV and MoMuLV, demonstrating that the requirement of Pol in retroviral RNA dimer maturation is not conserved among all retroviruses.

Structural elements of the tRNA TPsiC loop critical for nucleocytoplasmic transport are important for human immunodeficiency virus type 1 primer selection

Human immunodeficiency virus type 1 (HIV-1) selects a host cell tRNA as the primer for the initiation of reverse transcription. In a previous study, transport of the intact tRNA from the nucleus to the cytoplasm during tRNA biogenesis was shown to be a requirement for the selection of the tRNA primer by HIV-1. To further examine the importance of tRNA structure for transport and the selection of the primer, yeast tRNA(Phe) mutants were designed such that the native tRNA structure would be disrupted to various extents. The capacity of the mutant tRNA(Phe) to complement a defective HIV-1 provirus that relies on the expression of yeast tRNA(Phe) for infectivity was determined. We found a direct relationship between intact tRNA conformation and the capacity to be selected by HIV-1 for use in reverse transcription. tRNA(Phe) mutants that retained the capacity for nucleocytoplasmic transport, indicative of overall intact conformation, complemented the defective provirus. The mutant tRNAs were not aminoacylated, and the levels of complementation were lower than that for wild-type tRNA(Phe), which did undergo transport and aminoacylation. Taken together, these results demonstrate that HIV-1 primer selection is most dependent on a tRNA structure necessary for nucleocytoplasmic transport, consistent with primer selection occurring in the cytoplasm at or near the site of protein synthesis.

Inhibition of HIV-1 replication by RNA targeted against the LTR region

OBJECTIVE

The use of small RNA molecules able to effect gene inactivation has emerged as a powerful method of gene therapy. These small inhibitory RNAs are widely used for silencing malignant cellular and viral genes. We have assayed a series of inhibitory RNAs named catalytic antisense RNAs, consisting of a catalytic domain, hairpin or hammerhead ribozyme, and an antisense domain. The aim of the present study was to evaluate the effect of these inhibitory RNAs on HIV-1 replication.

METHODS

A series of expression vectors has been constructed for the intracellular synthesis of inhibitory RNAs, differing in the promoter that drives their synthesis. These inhibitory RNAs were designed to act at two possible cleavage sites in the long terminal repeat (LTR) region and the TAR domain was chosen as a target for the antisense domain. We have evaluated the effects of different inhibitory RNAs in HIV replication via changes in p24 antigen levels. Mobility shift assays have been used to check the binding capacity of inhibitory RNAs.

RESULTS

Catalytic antisense RNA designed to target the LTR region of HIV-1 inhibited viral replication in an eukaryotic cell environment by more than 90%. The conventional hairpin and hammerhead ribozymes, however, failed to inhibit viral replication.

CONCLUSIONS

The data provide preliminary evidence of a new class of inhibitory RNAs that can be used to block HIV replication. The results clearly show the importance of the ex vivo antisense effect in the inhibition achieved. A good correlation was found between the in vitro binding efficiency of the inhibitor RNA to the HIV-1 LTR and the inhibition of viral replication.

Construction of a minimal HIV-1 variant that selectively replicates in leukemic derived T-cell lines: towards a new virotherapy approach

T-cell acute lymphoblastic leukemia is a high-risk type of blood-cell cancer. We analyzed the possibility of developing virotherapy for T-cell acute lymphoblastic leukemia. Virotherapy is based on the exclusive replication of a virus in leukemic cells, leading to the selective removal of these malignant cells. We constructed a minimized derivative of HIV-1, a complex lentivirus encoding multiple accessory functions that are essential for virus replication in untransformed cells, but dispensable in leukemic T cells. This mini-HIV virus has five deletions (vif, vpR, vpU, nef, and U3) and replicated in the SupT1 cell line, but did not replicate in normal peripheral blood mononuclear cells. The stripped down mini-HIV variant was also able to efficiently remove leukemic cells from a mixed culture with untransformed control cells. In contrast to wild-type HIV-1, we did not observe bystander killing in mixed culture experiments with the mini-HIV variant. Furthermore, viral escape was not detected in long-term cultures. The mini-HIV variant that uses CD4 and CXCR4 for cell entry could potentially be used against CXCR4-expressing malignancies such as T-lymphoblastic leukemia/lymphoma, natural killer leukemia, and some myeloid leukemias.

A riboswitch regulates RNA dimerization and packaging in human immunodeficiency virus type 1 virions

The genome of retroviruses, including human immunodeficiency virus type 1 (HIV-1), consists of two identical RNA strands that are packaged as noncovalently linked dimers. The core packaging and dimerization signals are located in the downstream part of the untranslated leader of HIV-1 RNA-the Psi and the dimerization initiation site (DIS) hairpins. The HIV-1 leader can adopt two alternative conformations that differ in the presentation of the DIS hairpin and consequently in their ability to dimerize in vitro. The branched multiple-hairpin (BMH) structure folds the poly(A) and DIS hairpins, but these domains are base paired in a long distance interaction (LDI) in the most stable LDI conformation. This LDI-BMH riboswitch regulates RNA dimerization in vitro. It was recently shown that the Psi hairpin structure is also presented differently in the LDI and BMH structures. Several detailed in vivo studies have indicated that sequences throughout the leader RNA contribute to RNA packaging, but how these diverse mutations affect the packaging mechanism is not known. We reasoned that these effects may be due to a change in the LDI-BMH equilibrium, and we therefore reanalyzed the structural effects of a large set of leader RNA mutations that were presented in three previous studies (J. L. Clever, D. Mirandar, Jr., and T. G. Parslow, J. Virol. 76:12381-12387, 2002; C. Helga-Maria, M. L. Hammarskjold, and D. Rekosh, J. Virol. 73:4127-4135, 1999; R. S. Russell, J. Hu, V. Beriault, A. J. Mouland, M. Laughrea, L. Kleiman, M. A. Wainberg, and C. Liang, J. Virol. 77:84-96, 2003). This analysis revealed a strict correlation between the status of the LDI-BMH equilibrium and RNA packaging. Furthermore, a correlation is apparent between RNA dimerization and RNA packaging, and these processes may be coordinated by the same LDI-BMH riboswitch mechanism.

First snapshots of the HIV-1 RNA structure in infected cells and in virions

With the increasing interest of RNAs in regulating a range of cell biological processes, very little is known about the structure of RNAs in tissue culture cells. We focused on the 5'-untranslated region of the human immunodeficiency virus type 1 RNA genome, a highly conserved RNA region, which contains structural domains that regulate key steps in the viral replication cycle. Up until now, structural information only came from in vitro studies. Here, we developed chemical modification assays to test nucleotide accessibility directly in infected cells and viral particles, thus circumventing possible biases and artifacts linked to in vitro assays. The secondary structure of the 5'-untranslated region in infected cells points to the existence of the various stem-loop motifs associated to distinct functions, proposed from in vitro probing, mutagenesis, and phylogeny. However, compared with in vitro data, subtle differences were observed in the dimerization initiation site hairpin, and none of the proposed long range interactions were observed between the functional domains. Moreover, no global RNA rearrangement was observed; structural differences between infected cells and viral particles were limited to the primer binding site, which became protected against chemical modification upon tRNA(3) (Lys) annealing in virions and to the main packaging signal. In addition, our data suggested that the genomic RNA could already dimerize in the cytoplasm of infected cells. Taken together, our results provided the first analysis of the dynamic of RNA structure of the human immunodeficiency virus type 1 RNA genome during virus assembly ex vivo.

Human immunodeficiency virus type 1 escapes from RNA interference-mediated inhibition

Short-term assays have suggested that RNA interference (RNAi) may be a powerful new method for intracellular immunization against human immunodeficiency virus type 1 (HIV-1) infection. However, RNAi has not yet been shown to protect cells against HIV-1 in long-term virus replication assays. We stably introduced vectors expressing small interfering RNAs (siRNAs) directed against the HIV-1 genome into human T cells by retroviral transduction. We report here that an siRNA directed against the viral Nef gene (siRNA-Nef) confers resistance to HIV-1 replication. This block in replication is not absolute, and HIV-1 escape variants that were no longer inhibited by siRNA-Nef appeared after several weeks of culture. These RNAi-resistant viruses contained nucleotide substitutions or deletions in the Nef gene that modified or deleted the siRNA-Nef target sequence. These results demonstrate that efficient inhibition of HIV-1 replication through RNAi is possible in stably transduced cells. Therefore, RNAi could become a realistic gene therapy approach with which to overcome the devastating effect of HIV-1 on the immune system. However, as is known for antiviral drug therapy against HIV-1, antiviral approaches involving RNAi should be used in a combined fashion to prevent the emergence of resistant viruses.

RNA branching and debranching in the yeast retrovirus-like element Ty1

Ty elements of Saccharomyces cerevisiae are long terminal repeat (LTR) retroelements related to retroviruses. Normal levels of Ty1 transposition require Dbr1p, a cellular enzyme that cleaves 2'-5' RNA bonds. We show that Ty1 RNAs lacking identifiable 5' ends accumulate in virus-like particles (VLPs) in dbr1 mutants. Debranching this RNA in vitro with Dbr1p creates an uncapped version of the normal Ty1 RNA 5' end. We show that the 5' nucleotide (nt) of Ty1 RNA forms a 2'-5' bond with a nt near the 3' end of the same RNA, creating a lariat. The properties of the lariat suggest it forms by a novel mechanism and that branching and debranching may play roles in Ty1 reverse transcription at the minus-strand transfer step.

Probing alternative foldings of the HIV-1 leader RNA by antisense oligonucleotide scanning arrays

Scanning arrays of antisense DNA oligonucleotides provide a novel and systematic means to study structural features within an RNA molecule. We used this approach to probe the structure of the untranslated leader of the human immunodeficiency virus type 1 (HIV-1) RNA genome. This 335 nt RNA encodes multiple important replication signals and adopts two mutually exclusive conformations. The poly(A) and the dimer initiation signal (DIS) sequences of the leader RNA are base-paired in the long-distance interaction (LDI) conformation, but both domains form distinct hairpins in the branched multiple hairpins (BMH) conformation. An RNA switch mechanism has been proposed to regulate the activity of the DIS dimerization signal that is masked in one, yet exposed in the other conformation. The two RNA conformations demonstrate discrete differences in the array-based hybridization patterns. LDI shows increased hybridization in the poly(A) region and decreased hybridization in the DIS region when compared with BMH. These results provide additional evidence for the structure models of the two alternative leader RNA conformations. We also found a correlation between the efficiency of oligonucleotide hybridization and the accessibility of the RNA structure as determined by chemical and enzymatic probing in previous studies. The array approach therefore provides a very sensitive method to detect structural differences in related transcripts.

Rev binds specifically to a purine loop in the SL1 region of the HIV-1 leader RNA

The leader RNA sequence of human immunodeficiency virus type 1 (HIV-1) consists of a complex series of stem loop structures that are critical for viral replication. Three-dimensional structural analysis by NMR of one of these structures, the SL1 stem loop of the packaging signal region, revealed a highly conserved purine rich loop with a structure nearly identical to the Rev-binding loop of the Rev response element. Using band-shift assays, surface plasmon resonance, and further NMR analysis, we demonstrate that this loop binds Rev. HIV-1 appears to have a second Rev-binding site close to the major splice donor site that may have an additional role in the viral life cycle.

HIV type 1 C and C' subclusters based on long terminal repeat sequences in the Ethiopian type 1 subtype C epidemic

While the Ethiopian HIV-1 epidemic is dominated by subtype C, two distinguishable cocirculating C genotypes have been identified based on sequences of the C2V3 envelope region. In this study we sequenced and analyzed the long terminal repeat (LTR) sequence from 22 Ethiopian HIV-1-positive individuals. The two phylogenetically distinguishable genotypes C (n = 13) and C' (n = 4) are separated by significant bootstrap values. Nucleotide differences between the two groups were identified in the NF-AT, TCF-1alpha, and SP1 transcription factor binding sites, whereas the NF-kappaB and NRE-core sequences were identical between the two groups. Five isolates that could not be classified C or C' were found to be recombinants within the LTR sequence upon boots can analysis. Comparison of all the LTR sequences with their corresponding C2V3 envelope sequence revealed four intersubtype C/C' recombinant isolates. Thus, the prevalence of C/C' recombinant viruses is well over 40%. Interestingly, the C2V3 envelope sequences of all recombinant viruses belonged to the genotype C', whereas every LTR sequence belonged to the genotype C. This result indicates that recombination between the two genotypes is unidirectional, possibly as the result of evolutionary pressure on the respective biological functions of the LTR promoter and the envelope protein.

On the importance of the primer activation signal for initiation of tRNA(lys3)-primed reverse transcription of the HIV-1 RNA genome

Initiation of reverse transcription is a complex and regulated process in all retroviruses. Several base pairing interactions have been proposed to occur between the HIV-1 RNA genome and the specific tRNA(lys3) primer. The tRNA primer can form up to 21 bp with the primer binding site (PBS), and an additional 8 bp interaction may form between the primer activation signal (PAS) in the HIV-1 RNA and sequences within the T(Psi)C arm of the tRNA. The latter interaction is further analyzed in this in vitro study with mutant RNA transcripts that were designed to preclude the PAS interaction. These mutant transcripts are able to efficiently bind the tRNA primer, but they exhibit a profound defect at initiating reverse transcription. This defect is specific for the tRNA primer because it is not observed for PBS-bound DNA oligonucleotide primers. These results reinforce the model of regulated reverse transcription in which the PAS-mediated interaction is critical for efficient initiation.

A structural linkage between the dimerization and encapsidation signals in HIV-2 leader RNA

The 5' untranslated leader region of retroviral RNAs contains noncoding information that is essential for viral replication, including signals for transcriptional transactivation, splicing, primer binding for reverse transcription, dimerization of the genomic RNA, and encapsidation of the viral RNA into virions. These RNA motifs have considerable structural and functional overlap. In this study, we investigate the conformational dynamics associated with the use and silencing of a sequence in HIV-2 RNA that is involved in genomic RNA dimerization called stem-loop 1 (SL1) and its relationship with a flanking sequence that is known to be important for encapsidation of viral RNAs. We demonstrate that a long-distance intramolecular interaction between nucleotides located upstream of the primer-binding site domain and nucleotides encompassing the Gag translation start codon functionally silences SL1 as a dimerization element. This silencing can be relieved by mutation or by hybridization of an oligonucleotide that disrupts the long-distance interaction. Furthermore, we identify a palindrome within the packaging/encapsidation signal Psi (just 5' of SL1) that can either serve as an efficient dimerization signal itself, or can mediate SL1 silencing through base pairing with SL1. These results provide a tangible link between the functions of genomic RNA dimerization and encapsidation, which are known to be related, but whose physical relationship has been unclear. A model is proposed that accounts for observations of dimerization, packaging, and translation of viral RNAs during different phases of the viral replication cycle.

The dimer initiation sequence stem-loop of human immunodeficiency virus type 1 is dispensable for viral replication in peripheral blood mononuclear cells

Human immunodeficiency virus type 1 (HIV-1) contains two copies of genomic RNA that are noncovalently linked via a palindrome sequence within the dimer initiation site (DIS) stem-loop. In contrast to the current paradigm that the DIS stem or stem-loop is critical for HIV-1 infectivity, which arose from studies using T-cell lines, we demonstrate here that HIV-1 mutants with deletions in the DIS stem-loop are replication competent in peripheral blood mononuclear cells (PBMCs). The DIS mutants contained either the wild-type (5'GCGCGC3') or an arbitrary (5'ACGCGT3') palindrome sequence in place of the 39-nucleotide DIS stem-loop (NL(CGCGCG) and NL(ACGCGT)). These DIS mutants were replication defective in SupT1 cells, concurring with the current model in which DIS mutants are replication defective in T-cell lines. All of the HIV-1 DIS mutants were replication competent in PBMCs over a 40-day infection period and had retained their respective DIS mutations at 40 days postinfection. Although the stability of the virion RNA dimer was not affected by our DIS mutations, the RNA dimers exhibited a diffuse migration profile when compared to the wild type. No defect in protein processing of the Gag and GagProPol precursor proteins was found in the DIS mutants. Our data provide direct evidence that the DIS stem-loop is dispensable for viral replication in PBMCs and that the requirement of the DIS stem-loop in HIV-1 replication is cell type dependent.

Multiple secondary structure rearrangements during HIV-1 RNA dimerization

HIV-1 RNA dimerization is a complex process that involves a series of RNA refolding events. The monomeric RNA can adopt two alternative conformations that largely determine the efficiency of dimerization. The dimeric RNA also exists in two different conformations, an initial kissing-loop complex and a stable dimer with extended intermolecular base pairing. We describe an ordered RNA folding pathway that incorporates this multitude of HIV-1 RNA conformers. Analysis of mutant transcripts designed to block distinct steps of the refolding cascade supports this model. The folding properties of the wild-type RNA and the defects caused by the mutations can be fully understood in terms of the free energy changes associated with secondary structure rearrangements.

A 5'-3' long-range interaction in Ty1 RNA controls its reverse transcription and retrotransposition

LTR-retrotransposons are abundant components of all eukaryotic genomes and appear to be key players in their evolution. They share with retroviruses a reverse transcription step during their replication cycle. To better understand the replication of retrotransposons as well as their similarities to and differences from retroviruses, we set up an in vitro model system to examine minus-strand cDNA synthesis of the yeast Ty1 LTR-retrotransposon. Results show that the 5' and 3' ends of Ty1 genomic RNA interact through 14 nucleotide 5'-3' complementary sequences (CYC sequences). This 5'-3' base pairing results in an efficient initiation of reverse transcription in vitro. Transposition of a marked Ty1 element and Ty1 cDNA synthesis in yeast rely on the ability of the CYC sequences to base pair. This 5'-3' interaction is also supported by phylogenic analysis of all full-length Ty1 and Ty2 elements present in the Saccharomyces cerevisiae genome. These novel findings lead us to propose that circularization of the Ty1 genomic RNA controls initiation of reverse transcription and may limit reverse transcription of defective retroelements.

Regulated HIV-2 RNA dimerization by means of alternative RNA conformations

The dimer initiation site (DIS) hairpin of the HIV-2 untranslated leader RNA mediates in vitro dimerization through 'loop-loop kissing' of a loop-exposed palindrome sequence. Premature RNA dimerization must be prevented during the retroviral life cycle. A regulatory mechanism has been proposed for the HIV-1 leader RNA that can adopt an alternative conformation in which the DIS motif is effectively masked by long-distance base pairing with upstream leader sequences. We now report that HIV-2 RNA dimerization is also regulated. Sequestering of the DIS motif by base pairing interactions with downstream leader sequences mediates a switch to a dimerization-impaired conformation. The existence of two alternative conformations of the HIV-2 leader RNA is supported by UV melting experiments. Furthermore, the equilibrium between the two conformations can be shifted by annealing of antisense oligonucleotides or by deletion of certain leader regions. These measures have a profound impact on the dimerization properties of the transcript, demonstrating a mutual exclusivity between the alternative conformation and dimerization, similar to what has been described for the HIV-1 leader. The overall resemblance in regulation of HIV-1 and HIV-2 RNA dimerization suggests that a similar mechanism may be operating in other lentiviruses and perhaps all retroviridae.