Comparative mutational analysis of cis-acting RNA signals for translational frameshifting in HIV-1 and HTLV-2

Human immunodeficiency virus type I (HIV-1) and human T cell leukemia virus type II (HTLV-2) use a similar mechanism for -1 translational frameshiftin g to overcome the termination codon in viral RNA at the end of the gag gene . Previous studies have identified two important RNA signals for frameshift ing, the slippery sequence and a downstream stem-loop structure. However, t here have been somewhat conflicting reports concerning the individual contr ibutions of these sequences. In this study we have performed a comprehensiv e mutational analysis of the cis-acting RNA sequences involved in HIV-I gag -pol and HTLV-2 gag-pro frameshifting. Using an in vitro translation system we determined frameshifting efficiencies for shuffled HIV-1/HTLV-2 RNA ele ments in a background of HIV-1 or HTLV-2 sequences. We show that the abilit y of the slippery sequence and stem-loop to promote ribosomal frameshifting is influenced by the flanking upstream sequence and the nucleotides in the spacer element. A wide range of frameshift efficiency rates was observed f or both viruses when shuffling single sequence elements. The results for HI V-1/HTLV-2 chimeric constructs represent strong evidence supporting the not ion that the viral wild-type sequences are not designed for maximal framesh ifting activity but are optimized to a level suited to efficient viral repl ication.