The origin of spontaneous mutation in Saccharomyces cerevisiae.

Characterization of two antimutator loci in yeast shows that both are members of the same mutagenic repair system known to be responsible for almost all induced mutation (LAWRENCE and CHRISTENSEN 1976, 1979a,b; PRAKASH 1976). One of the these newly isolated antimutator mutations is an allele of rev3 (LEMONTT 1971b). Two other alleles of rev3 were tested and were also found to be antimutators. Double mutants carrying rev3 and mutator mutations of rad3, rad51 or rad18 are like rev3 single mutants with respect to spontaneous mutation rate, supporting the hypothesis (HASTINGS, QUAH and VON BORSTEL, 1976) that many mutators in yeast act by channelling spontaneous lesions from accurate to mutagenic repair. However, the enhanced mutation rate seen in a radiation-resistant mutator mutant mut1 is not dependent on REV3, but is dependent on another gene designated ANT1. An additive effect on the reduction in spontaneous mutation, seen in the ant1 rev3 double-mutant strain, leads to the conclusion that at least 90% of spontaneous mutations seen in the wild type are caused by mutagenic repair of spontaneous lesions.