Induction of forward mutations in mutationally defective yeast

Summary The 3 rev loci that reduce ultraviolet light (UV)-induced reversion in S. cerevisiae had a similar effect on forward mutation to auxotrophy induced by a single 400 erg/mm² UV dose: rev1-1, rev2-1 and rev3-1 reduced average frequencies of auxotrophs to 4%, 64% and 4% that in wild type and reduced frequencies of mutants at ade1 or ade2 to 19%, 88% and 2% wild type, respectively. The rev2-1 strain exhibited high frequencies of spontaneous mutation. It is suggested that rev1-1 and rev3-1 block steps in a general UV mutation mechanism controlling forward and reverse mutation throughout the genome. The small effect of rev2-1, compared to the effect of rev1-1 or rev3-1, is consistent with previously obtained data on UV reversion and could be due to a specificity for induced mutation involving only certain types of UV damage or, on the other hand, it may be related to mutator activity. Although rev caused varying degrees of sensitivity to ethylmethanesulfonate (EMS), there was little or no significant effect on mutation induced by a single 30 min. dose of 3% EMS. Auxotroph frequencies were 79%, 109% and 94% wild type, whild frequencies at ade1 or ade2 were 82%, 56% and 51% wild type in the respective strains. It is suggested that steps blocked by rev, although they may participate in repair of lethal EMS damage, do not themselves generate EMS-induced mutations.