Highly conserved RNA pseudoknots at the gag-pol junction of HIV-1 suggest a novel mechanism of −1 ribosomal frameshifting

Four putative pseudoknots (PK1–PK4) at the gag-pol frameshift junction of HIV-1. (A) A schematic diagram for the four pseudoknot-forming sequences of PK1–PK4 in the reference strain HXB2 (GenBank accession No. K03455). The slippery sequence (ss, residues 2084–2091) UUUUUUA is highlighted in black. The numbers indicate the starting and ending residues of the pseudoknot-forming sequences. (B,C) Predicted secondary structures for the four putative pseudoknots PK1–PK4 in two representative strains. The slippery sequence is boxed.