Mutants of yeast defective in mutation induced by ultraviolet light.

NDUCED mutations are thought to arise as a result of enzymatic processes uti‘lizing DNA damage as a substrate (BRIDGES 1969). Although the molecular mechanism is not known, evidence reviewed by WITKIN (1969) suggests that in E. coli mutations are produced during postreplication repair of lethal damage induced by ultraviolet light (UV) and controlled by r e d and ezrf (or lexf) genes. Strains carrying rec or ezr, although normal in excision repair, exhibit reduced or no UV mutability compared to wild type. In addition, such strains are UV sensitive, X-ray sensitive, and recombination deficient in varying degrees ( WITKIN 1969). In previous studies of UV-induced mutation in fungi, UV-sensitive strains have been selected on the assumption that UV mutagenesis might be related to dark repair of lethal damage. In some of these UV-sensitive strains, the frequencies of UV mutation are reduced compared to wild type at equal UV doses (CHANG, LENNOX and TUVESON 1968; NASIM 1968; WOHLRAB and TUVESON 1969). whereas in others these frequencies are enhanced (AVERBECK et al. 1970; RESNICK 1969; ZAKHAROV, KOZINA and FEDOROVA, 1970). To identify new genes controlling UV-induced mutation, it is desirable to select strains directly for defective mutation induction, thereby avoiding the prior condition that all such mutants be UV-sensitive. Mutants of Saccharomyces cereuisiae were selected ( LEMONTT and MORTIMER 1970) for reduced ability to undergo UV-induced locus reversion of the ochresuppressible (GILMORE 1967; HAWTHORNE 1969) argl-17 allele. This paper describes the isolation and some characteristics of these “reversionless” mutants, The results are discussed in relation to current ideas about induced mutagenesis.