Yeast UAA suppressors effective in psi+ strains: leucine-inserting suppressors.

We have previously reported the isolation and characterization of UAA suppressors from a haploid strain of yeast Saccharomyces cerevisiae containing the ψ⁺ non-Mendelian determinant which increases the efficiency of action of certain suppressors (Ono et al., 1979). Most of the suppressors caused the insertion of either tyrosine or serine. In contrast, the pattern of suppression of nutritional markers suggested that the rare suppressor, SUP26, inserted in an amino acid other than tyrosine or serine. In this investigation we report the characterization of additional suppressors, similar to SUP26, that were isolated on a medium lacking uracil and containing canavanine; this medium is expected to exclude serine-inserting suppressors because they do not suppress the ura4-1 marker, and to exclude tyrosine-inserting suppressors because they suppress the can1-100 marker. The total of 155 revertants similar to the SUP26 suppressor were analyzed genetically and these could be assigned to one or another of the six distinct loci SUP26, SUP27, SUP28, SUP29, SUP32 and SUP33. The SUP26, SUP27 and SUP29 loci mapped on chromosomes XII, IV and X, respectively. The detailed map position of the SUP29 suppressor suggests that it may be allelic to the SUP30 suppressor reported by Hawthorne &; Mortimer (1968). These six suppressors had the same pattern of suppression of UAA nutritional markers and all of them had a similar low efficiency of action on the iso-1-cytochrome c mutation cyc1-72. The efficiency of each of these suppressors was increased by a chromosomal allo-suppressor, sal. Each of the six suppressors caused the insertion of leucine in iso-1-cytochrome c at the UAA site of the cyc1-72 mutation. It is suggested that the gene products of these suppressors are redundant forms of the same leucine transfer RNA.