Special Peptidyl-tRNA Molecules Can Promote Translational Frameshifting without Slippage

Abstract

Recently we described an unusual programmed +1 frameshift event in yeast retrotransposon Ty3. Frameshifting depends on the presence of peptidyl-tRNA ^Ala_CGC on the GCG codon in the ribosomal P site and on a translational pause stimulated by the slowly decoded AGU codon. Frameshifting occurs on the sequence GCG-AGU-U by out-of-frame binding of a valyl-tRNA to GUU without slippage of peptidyl-tRNA^Ala_CGC. This mechanism challenges the conventional understanding that frameshift efficiency must correlate with the ability of mRNA-bound tRNA to slip between cognate or near-cognate codons. Though frameshifting does not require slippery tRNAs, it does require special peptidyl-tRNAs. We show that overproducing a second isoacceptor whose anticodon had been changed to CGC eliminated frameshifting; peptidyl-tRNA^Ala_CGC must have a special capacity to induce +1 frameshifting in the adjacent ribosomal A site. In order to identify other special peptidyl-tRNAs, we tested the ability of each of the other 63 codons to replace GCG in the P site. We found no correlation between the ability to stimulate +1 frameshifting and the ability of the cognate tRNA to slip on the mRNA—several codons predicted to slip efficiently do not stimulate frameshifting, while several predicted not to slip do stimulate frameshifting. By inducing a severe translational pause, we identified eight tRNAs capable of inducing measurable +1 frameshifting, only four of which are predicted to slip on the mRNA. We conclude that in Saccharomyces cerevisiae, special peptidyl-tRNAs can induce frameshifting dependent on some characteristic(s) other than the ability to slip on the mRNA.