THE INVOLVEMENT OF THE RAD6 GENE IN STARVATION-INDUCED REVERSE MUTATION IN SACCHAROMYCES-CEREVISIAE

The accumulation of Ade(+) revertants during adenine starvation and Tr p(+) revertants during tryptophan starvation in haploid polyauxotrophi c strains of Saccharomyces cerevisiae occurs in a time-dependent manne r. Accumulation of revertants is enhanced in Rad6(-) strains, suggesti ng 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, t he 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 charact eristics and the genotype of revertants shows a difference between ear ly and late-appearing revertants. These results support the hypothesis that the adaptivity of starvation-induced reversion is based on the s elective fixation of random mutations, and particularly on transcripti on-enhanced repair and/or mutagenesis processes.