Comprehensive analysis of yeast +1 ribosomal frameshifting unveils a novel stimulator supporting two distinct frameshifting mechanisms

Ribosomal frameshifting is an important, albeit rare, mRNA decoding mechanism that generally allows the synthesis of a single protein from two different reading frames. +1 frameshifting is commonly presumed to involve re-pairing of the P-site tRNA with the +1 codon. However, in several occurrences in the yeast Saccharomyces cerevisiae, P-site tRNA re-pairing with the +1 codon is impossible. In one model, +1 frameshifting occurs according to a common mechanism involving P-site tRNA movement without re-pairing with the +1 codon. The alternative is a distinct mechanism allowing A-site tRNA acceptance at the +1 codon in the absence of P-site tRNA movement. Here, we experimentally compared all known +1 ribosomal frameshifting sites in S. cerevisiae, including a novel case discovered during this study in LLP1. We identified a conserved RNA secondary structure upstream of the ABP140 frameshifting site that increases frameshifting efficiency. The location of the structure suggests that it creates an mRNA-pulling effect favouring +1 codon in the P-site. Placing the stimulator upstream of various known frameshifting sites revealed that its stimulatory action is selective to those frameshifting sites where P-site tRNA re-pairing is possible, reinforcing the idea of two distinct mechanisms of +1 ribosomal frameshifting.