ISOLATION AND CHARACTERIZATION OF NEW NUC LEAR SRM GENE-MUTATIONS THAT CHANGE THE MAINTENANCE OF BOTH NUCLEAR AND MITOCHONDRIAL GENETIC STRUCTURES IN THE YEAST SACCHAROMYCES

From grown cultures of UV-irradiated Saccharomyces cerevisiae cells wi th disomy at chromosome IV, clones with nuclear gene mutations were is olated, each of which was suggested to change both mitochondrial spont aneous rho- mutability and the mitotic stability of extra natural chro mosomes. Four such nonallelic mutations (srm8, srm12, srm15, and srm17 ) were isolated, and their phenotypic expression characterized. All fo ur mutations are associated with decreased spontaneous rho- mutability and virtually block sporulation in homozygous mutant diploids. Mutati on srm8 is temperature-sensitive and, most probably, involves an essen tial gene. Double mutants of genotypes srm8 cdc28-srm and srm8 srm12 a re nonviable. Mutation srm12 increases the rate of spontaneous loss of extra chromosome XIV by disomics by a factor of about 30. Mutation sr m15 induces a small (about twofold) but statistically significant decr ease of this rate. Mutations srm8 and srm17 drastically decelerate rep roduction of cells with disomy, which prevents quantitative estimation s of rates of loss of extra chromosomes.