Yeast mutants affecting the spontaneous mutation frequency on both the mitochondrial and nuclear genomes

We have previously described two complementation groups of mutants affecting the spontaneous mutation frequency on the mitochondrial genome (Johnston, 1979). In a further search for such mutants the majority isolated fell into one of these groups, 3 into group I and 14 into group II. There are now a total of 12 alleles in the first group and 19 in the second complementation group, suggesting that these are the major classes of mutants affecting spontaneous mutation frequency in the mitochondrion. However, this search also identified a third complementation group, consisting of 2 mutants, which, like the existing groups, is recessive and is coded on the nuclear genome. In contrast to complementation groups I and II, these new mutants have no effect on spontaneous petite mutagenesis and they also increase the spontaneous mutation frequency on the nuclear genome. This nuclear mutation activity may be novel, as it complemented the existing nuclear mutators mut1-mut10. None of the three complementation groups has any detectable phenotype, other than the mutator activity.