Characterization of Nonsense Suppressor tRNAs from Saccharomyces cerevisiae: Identification of the Mutational Alterations That Give Rise to the Suppressor Function

Abstract

Nonsense suppressor mutations of Saccharomyces cerevisiae have been identified from their ability to suppress auxotrophic markers that are known to correspond to either UAA, UAG, or UGA nonsense mutations. Each of these mutations, with only a few exceptions, causes suppression at only one of the nonsense codons. UAA (ocher) and UAG (amber) suppressors have also been characterized from analyses on the iso-1-cytochrome c of strains bearing ocher or amber mutations in the structural gene for this protein, and it has been found that these suppressors each insert one of just three amino acids during translational suppression of UAA or UAG codons (Gilmore et al. 1971; Sherman et al. 1973; Liebman et al. 1975, 1976; Brandriss et al. 1976; Sherman et al. 1979). These amino acids are tyrosine, serine, and leucine. The amino acid insertion specificities of S. cerevisiae UGA suppressors (Hawthorne 1976) are as yet unknown.

Evidence that nonsense suppression in yeast is mediated, as in prokaryotes, by altered tRNA molecules has come from two different lines of experimentation. On the one hand, it has been demonstrated that total tRNA isolated from suppressor-carrying yeast strains can permit the insertion of amino acids in response to nonsense codons in in vitro protein-synthesizing systems (Capecchi et al. 1975; Gesteland et al. 1976). On the other hand, it has been shown that certain suppressor mutations determine anticodon base changes in tRNAs from nucleotide sequencing studies of suppressor tRNAs and genes that encode them (Piper et al. 1976; Goodman et al. 1977; Piper 1978).