No abstract

The 5 leader of the human immunodeficiency virus type 1 (HIV-1) genomic RNA contains highly structured domains involved in key steps of the viral life cycle. These RNA domains inhibit cap-dependent protein synthesis. Here we report that the HIV-1 5 leader harbors an internal ribosome entry site (IRES) capable of driving protein synthesis during the G 2 /M cell cycle phase in which cap-dependent initiation is inhibited. The HIV-1 IRES was delineated with bicistronic mRNAs in in vitro and ex vivo assays. The HIV-1 leader IRES spans nucleotides 104 to 336 and partially overlaps the major determinants of genomic RNA packaging. These data strongly suggest that, as for HIV-1 transcription, IRES-mediated translation initiation could play an important role in virus replication during virus-induced G 2 /M cell cycle arrest.

No abstract

The virion cores of the replication competent type 1 human immunodeficiency virus (HIV‐1), a retrovirus, contain and RNA genome associated with nucleocapsid (NC) and reverse transcriptase (RT p66/p51) molecules. In vitro reconstructions of these complexes with purified components show that NC is required for efficient annealing of the primer tRNALys,3. In the absence of NC, HIV‐1 RT is unable to retrotranscribe the viral RNA template from the tRNA primer. We demonstrate that the HIV‐1 RT p66/p51 specifically binds to its cognate primer tRNALys,3 even in the presence of a 100‐fold molar excess of other tRNAs. Cross‐linking analysis of this interaction locates the contact site to a region within the heavily modified anti‐codon domain of tRNALys,3.

The nucleocapsid (NC) of human immunode-

No abstract

The retroviral gag nucleocapsid protein NCp7 (72 amino acids) of HIV‐1 (LAV strain), which contains two successive zinc fingers of the Cys‐X2‐Cys‐X4‐His‐X4‐Cys form linked by a stretch of basic residues, promotes viral RNA dimerization and encapsidation and activates annealing of the primer tRNA to the initiation site of reverse transcription. The structure of NCp7 and other shorter fragments was studied by 600 MHz 1H nuclear magnetic resonance (NMR) in aqueous solution to account for its various biological properties. Complete sequence specific 1H NMR assignments of the 13–51 residues of NCp7 encompassing the two zinc fingers was achieved by two‐dimensional NMR experiments and the three‐dimensional structure of (13–51)NCp7 was deduced from DIANA calculations, using nuclear Overhauser effects as constraints. The structure of the zinc complexed form of NCp7 is characterized by a kink at the Pro31 level in the basic Arg29‐Ala‐Pro‐Arg‐Lys‐Lys‐Gly35 RNA binding linker leading to a proximity of the Lys14‐Cys18 to the Gly35‐Cys39 sequences, which belong to the folded proximal and distal zinc fingers, respectively. Accordingly, the aromatic residues Phe16 and Trp37 were found to be spatially close. The Lys33 and Lys34 side‐chains involved in viral RNA dimerization were solvent exposed. The N‐ and C‐terminal sequences of NCp7 behave as flexible independent domains. The proposed structure of NCp7 might be used to rationally design new anti‐viral agents aimed at inhibiting its functions.

The retroviral genome consists of two identical RNA molecules joined close to their 5' ends by the dimer linkage structure. Recent findings indicated that retroviral RNA dimerization and encapsidation are probably related events during virion assembly. We studied the cation-induced dimerization of HIV-1 RNA and results indicate that all in vitro generated HIV-1 RNAs containing a 100 nucleotide domain downstream from the 5' splice site are able to dimerize. RNA dimerization depends on the concentration of RNA, mono- and multivalent cations, the size of the monovalent cation, temperature, and pH. Up to 75% of HIV-1 RNA is dimeric in the presence of spermidine. HIV-1 RNA dimer is fairly resistant to denaturing agents and unaffected by intercalating drugs. Antisense HIV-1 RNA does not dimerize but heterodimers can be formed between HIV-1 RNA and either MoMuLV or RSV RNA. Therefore retroviral RNA dimerization probably does not simply proceed through mechanisms involving Watson-Crick base-pairing. Neither adenine and cytosine protonation, nor quartets containing only guanines appear to determine the stability of the HIV-1 RNA dimer, while quartets involving both adenine(s) and guanine(s) could account for our results. A consensus sequence PuGGAPuA found in the putative dimerization-encapsidation region of all retroviral genomes examined may participate in the dimerization process.