The sequences of and distance between two cis-acting signals determine the efficiency of ribosomal frameshifting in human immunodeficiency virus type 1 and human T-cell leukemia virus type II in vivo.

We have analyzed in cell culture the sequence elements that control the level of ribosomal frameshifting in the human T-cell leukemia virus type II (HTLV-2) gag-pro junction. The slippery sequence of HTLV-2 is sufficient to dictate a basal level of frameshifting. This level is enhanced by its upstream sequence context and by the downstream stem-loop structure which is located at an optimal distance of 7 bases. Frameshifting in human immunodeficiency virus gag-pol is similar to that of HTLV-2 gag-pro. However, experiments using hybrid cassettes of HTLV-2 and human immunodeficiency virus type 1 frameshift elements show that while the slippery sequence of HTLV-2 is less efficient, the stem-loop structure is a more efficient enhancer.