SPECIAL PEPTIDYL-TRANSFER-RNA MOLECULES CAN PROMOTE TRANSLATIONAL FRAMESHIFTING WITHOUT SLIPPAGE

Recently we described an unusual programmed +1 frameshift event in yea st retrotransposon Ty3. Frameshifting depends on the presence of pepti dyl-tRNA(CGC)(Ala) on the GCG codon in the ribosomal P site and on a t ranslational pause stimulated by the slowly decoded AGU codon. Framesh ifting occurs on the sequence GCG-AGU-U by out-of-frame binding of a v alyl-tRNA to GUU without slippage of peptidyl-tRNA(CGC)(Ala). This mec hanism challenges the conventional understanding that frameshift effic iency must correlate with the ability of mRNA-bound tRNA to slip betwe en cognate or near-cognate codons. Though frameshifting does not requi re slippery tRNAs, it does require special peptidyl-tRNAs. We show tha t overproducing second isoacceptor whose anticodon had been changed to CGC eliminated frameshifting; peptidyl-tRNA(CGC)(Ala) must have a spe cial capacity to induce +1 frameshifting in the adjacent ribosomal A s ite. 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. W e found no correlation between the ability to stimulate +1 frameshifti ng and the ability of the cognate tRNA to slip on the mRNA-several cod ons predicted to slip efficiently do not stimulate frameshifting, whil e several predicted not to slip do stimulate frameshifting. By inducin g a severe translational pause, we identified eight tRNAs capable of i nducing measurable +1 frameshifting, only four of which are predicted to slip on the mRNA. We conclude that in Saccharomyces cerevisiae, spe cial peptidyl-tRNAs can induce frameshifting dependent on some charact eristic(s) other than the ability to slip on the mRNA.