THE INFLUENCE OF 5'-CODON CONTEXT ON TRANSLATION TERMINATION IN SACCHAROMYCES-CEREVISIAE

Translation termination in vivo was studied in the yeast Saccharomyces cerevisiae using a translation-assay system. Codon changes that were made at position -2-relative to the stop codon, gave a 3.5-fold effect on termination in a release-factor-defective (sup45) mutant strain, i n line with the effect observed in a wild-type strain. The influence o f the -2 codon could be correlated to the charge of the corresponding amino acid residue in the nascent peptide; an acidic residue favoring efficient termination. Thus, the C-terminal end of the nascent peptide influences translation termination both in the bacterium Escherichia coil and to a lesser extent in the yeast S. cerevisiae. However, the s ensitivity to the charge of the penultimate amino acid is reversed whe n the E. coli and S. cerevisiae are compared. Changing -1 (P-site) cod ons in yeast gave a 10-fold difference in effect on the efficiency of termination. This effect could not be related to any property of the e ncoded last amino acid in the nascent peptide. Iso-codons read by the same tRNA (AAA/G, GAA/G) gave similar readthrough values. Codons for g lutamine (CAA/G), glutamic acid (GAA/G) and isoleucine (AUA/C) that ar e read by different isoaccepting tRNAs are associated with an approxim ately twofold difference in each case in termination efficiency. This suggests that the P-site tRNAis able to influence termination at UGAC in yeast.