Suppression of two missense alleles of the TRP5 locus of Saccharomyces cerevisiae.

A suppressor SUP101 of alleles trp5-67 and trp5-18 of the trp5 locus of Saccharomyces cerevisiae is described. The two suppressible mutations have been previously classified as missense. The suppression does not result from a physiological bypass of the tryptophan synthetase-catalyzed reaction, since the suppression is allele-specific. IU alleles trp5-70, tryp5-95, and trp5-102; IA alleles trp5-81, trp5-101, and trp5-103; and the ochre alleles trp5-33 and trp5-48 are not suppressed by SUP101. SUP101 does not suppress ochre alleles ade2-1, his5-2, arg4-17, lys1-1, amber alleles trp1-1, tyr7-1, or unclassified alleles at a number of other loci. These results indicate SUP101 is a missense suppressor. Growth on tryptophanless media is dependent upon gene dosage of both the suppressor and the suppressible alleles. Only the diploids homozygous both for the suppressor and suppressible alleles produce growth equivalent to growth of the haploids bearing a suppressible allele and the suppressor. Suppressor-bearing strains grow poorly even on tryptophan-supplemented media. In more than 100 asci analyzed partial growth inhibition on the complete medium always segregated with the suppressor.