DIMERIZATION OF THE GENOMIC RNA OF HIV-1 AND MOMULV - MECHANISM AND STRUCTURAL AND FUNCTIONAL IMPLICATIONS

Dimerization is a key step in the retroviral cycle and is responsible for the high genetic recombination. It has also been proposed to positively regulate encapsidation and negatively, translation. In HIV1: (1) RNA fragments containing at least 100 nt downstream from the 5' splice site can dimerize spontaneously; (2) dimerization is dependant on pH, temperature, RNA, mono- and multivalent cation concentrations, and the size of monovalent cations; (3) antisense RNA does not dimerize; (4) heterodimers can be formed between RNA from HIV1 and RNA from either MoMuLV or RSV. A consensus minimal polypurine RGGARA sequence has been identified in about 40 retroviruses and is present 4 times in HIV1 and twice in MoMuLV. The high thermal stability, cation size dependance and the low dissociation process led us to propose a mechanism involving purine quartets. In MoMuLV, region 215-312 is proposed to be required for dimerization. The rate of spontaneous dimerization depends on temperature and RNA and cation concentrations. Thermal denaturation gives a Tm close to that found for the genomic RNA isolated from the virions. Thermodynamic parameters derived from denaturation curves of dimers also suggest that dimerization involves short sequences. The conformation of RNA fragments containing the 5' untranslated region and the 5' Gag-coding region has been probed by the use of a large variety of chemical structure probes. Secondary structural models have been proposed for RNA from HIV1 and MoMuLV. In both systems, RNA can be folded in structural domains containing the different functional sites. Dimerization induces structural rearrangements. In HIV1, the TAR and PBS are stable hairpin structures, while the dimer linkage structure and the Gag translational initiation domains undergo structural rearrangements. These structural rearrangements are discussed with respect to the proposed mechanism of dimerization and their functional implications. In MoMuLV, structural rearrangements are also evidenced in the Psi and translational initiation domains. The PBS is involved in a stable structure within the dimer, whereas in the monomer form this region can adopt alternative conformations.