Mutagenesis by cytostatic alkylating agents in yeast strains of differing repair capacities.

Reversion of two nulcear ochre nonsense alleles and cell inactivation induced by mono-, bi-, and tri-functional alkylating agents and by UV has been investigated in stationary-phase haploid cells of yeast strains with differing capacities for DNA repair. The ability to survive alkylation damage is correlated with UV repair capacity, a UV-resistant and UV-mutable strain (RAD REV) being least and a UV-sensitive and UV-nonmutable strain (radi rev3) most sensitive. Mutagenicity of alkylating agents is highest in the former and is abolished in the latter strain. Deficiency in excision repair (rad1 rad2) or in the RAD18 function does not lead to enhanced mutability. Mutagenesis by the various agents is characterized by a common pattern of induction of locus-specific revertants and suppressor mutants. Induction kinetics are mostly linear, but UV-induced reversion in the RAD REV strain follows higher-than-linear (probably "quadratic") kinetics. The alkylating agent cyclophosphamide, usually considered inactive without metabolic conversion, reduces colony-forming ability and induces revertants in a manner similar but not identical to the other chemicals tested. These findings are taken to support the concept of mutagenesis by misrepair after alkylation, which albeit sharing common features with the mechanism of UV-induced reversion, can be distinguished therefrom.