MUTATIONAL ANALYSIS OF THE BIPARTITE DIMER LINKAGE STRUCTURE OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 GENOMIC RNA

The genome of all retroviruses consists in two homologous RNA molecules associated near their 5' end in a region called the dimer linkage structure. Dimerization of genomic RNA is thought to be important for several functions of the retroviral cycle such as encapsidation, reverse transcription, and translation. In human immunodeficiency virus type 1 (HIV-1), a region downstream of the splice donor site was initially postulated to mediate dimerization. However, we recently showed that the dimerization initiation site is located upstream of the splice donor site and suggested that dimerization may initiate through a loop-loop interaction. Here, we show that single base mutations in the palindromic loop of the dimerization initiation site completely abolish dimerization, while introduction of compensatory mutations restores the process. Furthermore, two single nucleotide mutants that are unable to form homodimers efficiently codimerize, while the wild type RNA and the compensatory mutant, which both form homodimers, are unable to codimerize. These results unambiguously prove the interaction between the palindromic loops of each monomer. By contrast, none of the deletions that we introduced downstream of the splice donor site abolishes dimerization. However, deletions of two purine tracts located in the vicinity of the initiation codon of the gag gene significantly decrease the thermal stability of the HIV-1 RNA dimer.