RIBOSOMAL FRAMESHIFTING IN THE YEAST RETROTRANSPOSON TY - TRANSFER-RNAS INDUCE SLIPPAGE ON A 7-NUCLEOTIDE MINIMAL SITE

Ribosomal frameshifting regulates expression of the TYB gene of yeast Ty retrotransposons. We previously demonstrated that a 14 nucleotide sequence conserved between two families of Ty elements was necessary and sufficient to support ribosomal frameshifting. This work demonstrates that only 7 of these 14 nucleotides are needed for normal levels of frameshifting. Any change to the sequence CUU-AGG-C drastically reduces frameshifting; this suggests that two specific tRNAs, tRNALeuUAG and tRNAArgCCU, are involved in the event. Our tRNA overproduction data suggest that a leucyl-tRNA, probably tRNALeuUAG, an unusual leucine isoacceptor that recognizes all six leucine codons, slips from CUU-Leu onto UUA-Leu (in the +1 reading frame) during a translational pause at the AGG-Arg codon induced by the low availability of tRNAArgCCU, encoded by a single-copy essential gene. Frameshifting is also directional and reading frame specific. Interestingly, frameshifting is inhibited when the "slip" CUU codon is located three codons downstream, but not four or more codons downstream, of the translational initiation codon. © 1990.