Interactions among genes controlling sensitivity to radiation and alkylation in yeast

Summary In the simple eucaryote Saccharomyces cerevisiae there are at least three phenotypically distinct classes of mutants sensitive to inactivation by radiations and alkylating agents: class I mutants are sensitive to ultraviolet light and nitrogen mustard (HN2); class II mutants are sensitive to X-rays and methylmethane sulphonate (MMS); and class III mutants are sensitive to all four of these agents. We have constructed doubly mutant strains of types (I, I), (I, II), (I, III), and (II, III) and have measured their sensitivity to UV, X-rays, HN2 and MMS in order to characterize the interactions of the various mutant gene pairs. Class (I, III) double mutants proved to be supersensitive to UV and HN2 and class (II, III) double mutants proved to be supersensitive to X-rays and MMS. All other double mutants showed little or no enhancement of sensitivity over their most sensitive single mutant parents. Mutants of class I are known to be defective in excision repair and our results are consistent with the idea that there exist at least two additional pathways for dark repair in yeast, one capable of repairing X-ray and MMS damage to DNA, and another, possibly analogous to post-replication repair in bacteria, that competes with the other two for damaged regions in DNA.