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

Ribosomal frameshifting allows the synthesis of a protein from different reading frames. Generally, it involves a dissociation of the P-site tRNA from its codon and movement to the +1 codon setting the new frame for incoming tRNAs. This movement is stabilized by tRNA repairing with the +1 codon. However, in several occurrences in the yeastSaccharomyces cerevisiae, P-site tRNA repairing with the +1 codon is impossible. One hypothesis suggests that frameshifting involves P-site tRNA movement, with repairing not being essential. In the alternative model the A-site tRNA acceptance at the +1 codon occurs in the absence of P-site tRNA movement. Here we performed a comparative analysis of all known +1 frameshifting sites inS. cerevisiae. In addition, we discovered a novel gene requiring +1 frameshifting for its expression. We also identified a conserved RNA structure located upstream of theABP140frameshifting site. This structure substantially increases frameshifting efficiency. Its location suggests that it is formed in mRNA exiting the ribosome, creating a pulling effect favouring positioning of the +1 codon in the P-site. We show that the stimulation is limited to frameshifting sites where P-site tRNA repairing is possible, supporting the existence of two distinct mechanisms of +1 ribosomal frameshifting.