RNA signals for translation frameshift: influence of stem size and slippery sequence.

The pol gene of HIV-1 is expressed as a fusion protein with the upstream gag gene product after -1 ribosomal frameshifting. To get insights into the sequence requirements for the slippery heptanucleotide, generally present at the frameshift junction, and the downstream stem-loop structure, we prepared, starting from the gag-pol boundary sequence of HIV-1, serial deletion and base substitution derivatives for both of these signals, and inserted them into the amino-terminal proximal region of the luciferase gene, but downstream of the initiation codon, in out-of-frame fashion. The results of in vitro translation studies indicate that the stem-loop is not essential even if its size influences the efficiency of ribosomal frameshifting, and that the optimum repetition number of a single nucleotide within the slippery sequence exists for efficient ribosomal frameshifting.