Antimutators of Mitochondrial and Nuclear DNA in Saccharomyces Cerevisiae. Relationship with Gamma-ray Sensitivity

In Saccharomyces cerevisiae ten antimutator mutants have been isolated. The spontaneous occurrence of mitochondrial mutants resistant to erythromycin, oligomycin, and diuron is decreased 2-60-fold in these strains. The rate of forward and reverse spontaneous mutations of the nuclear genome is also reduced. The meiotic progenies arising from the crosses of seven mutants (LB1, LB2, LB4, LB5, LB6, LB7, LB10) with an isogenic parental strain exhibit 2 : 2 segregations and therefore are the result of mutations in a single nuclear gene. The six mutants LB1, LB2, LB4, LB6, LB7, LB10 are semidominant and determine six complementation groups. The mutant LB5 is dominant and therefore cannot be assigned to any complementation group. The mutants, LB1, LB4 and LB10 are gamma-ray sensitive and, by tetrad analysis, it has been shown that gamma-ray sensitivity and spontaneous antimutability are the result of a single nuclear gene mutation. The other three mutants LB3, LB8, and LB9 exhibit complex tetrad segregations, typical of cytoplasmic inheritance and do not complement each other. However, although the mutations are semidominant, it has not been possible to detect any antimutator cytoductant among some 500 cytoductants carrying the karl-1 nucleus. These results suggest that either several nuclear genes are involved in the expression of the antimutator phenotype or that the antimutator gene is located on nonchromosomal elements of the nucleus. The present study leads to the conclusion that a large number of nuclear genes are able to control simultaneously the spontaneous mutation rate of nuclear and mitochondrial genes. Since out of the ten antimutator mutants, three are also deficient in the repair of gamma-ray damage, it is also concluded that spontaneous and gamma-ray-induced lesions of DNA can be repaired by the same error-free process.