The involvement of the RAD6 gene in starvation-induced reverse mutation in Saccharomyces cerevisiae.

The accumulation of Ade+ revertants during adenine starvation and Trp+ revertants during tryptophan starvation in haploid polyauxotrophic strains of Saccharomyces cerevisiae occurs in a time-dependent manner. Accumulation of revertants is enhanced in Rad6- strains, suggesting that starvation-induced reversion is influenced by some of the RAD6 gene functions. The higher frequency of adaptive reversions in Rad6- strains is somewhat influenced by, but does not totally depend on, the genetic background. Therefore, the RAD6 gene product is involved in maintaining a low level not only of spontaneous mutation but also of starvation-induced reversion. The starvation-induced Ade+ and Trp+ reversions both appear to be adaptive. The analysis of growth characteristics and the genotype of revertants shows a difference between early and late-appearing revertants. These results support the hypothesis that the adaptivity of starvation-induced reversion is based on the selective fixation of random mutations, and particularly on transcription-enhanced repair and/or mutagenesis processes.