FIG. 1.

Schematic of the p4 region. The Gag and Gag-Pol polyproteins as well as the relevant amino acids are shown. The amino acids encoded fully or in part by the frameshift are underlined. The PR active site residues are in boldface. The PR cleavage sites in Gag and Gag-Pol are indicated by arrows. The p4 peptide is hatched. The large black dot represents the site of +1 translational frameshifting. The stop codon present in Gag is represented by an asterisk. Block substitution mutations s4 to s12 are shown as bars underneath the amino acids replaced by the peptide sequence AAGSAA. The transpositional competence of elements bearing these mutations is indicated.