On the role of the P-site in leftward ribosome frameshifting at a hungry codon.

Previous work characterized ribosomal frameshifting within the sequence C UUC AAG provoked by lysyl-tRNA limitation. The ribosome frameshift is one base to the left of the AAG lysine codon, as shown by dotted overlining above. We now show that the frequency of this leftward ribosome frameshift is strongly influenced by the identity of the bases two, three and four positions to the left of the actual frameshift site. The nature of these influences coincides exactly with the possibilities of base-pairing between the sequence and the anticodon of the P-site peptidyl-tRNA when shifted one base to the left just upstream of the frameshift site. We conclude that a peptidyl shift in the P-site is intimately involved in leftward frameshifting in the adjacent A site when it codes for an aminoacyl-tRNA in short supply.