Exposure to low dose of gamma radiation enhances the excision repair in Saccharomyces cerevisiae

The effect of low doses of ionizing and nonionizing radiation on the radiat ion response of yeast Saccharomyces cerevisiae toward ionizing and nonioniz ing radiation was studied. The wild-type strain D273-10B on exposure to 54 Gy gamma radiation (resulting in about 10% cell killing) showed enhanced re sistance to subsequent exposure to UV radiation. This induced UV resistance increased with the incubation time between the initial gamma radiation str ess and the UV irradiation. Exposure to low doses of UV light on the other hand showed no change in gamma or UV radiation response of this strain. The strains carrying a mutation at rad52 behaved in a way similar to the wild type, but with slightly reduced induced response. In contrast to this, the rad3 mutants, defective in excision repair, showed no induced UV resistance . Removal of UV-induced pyrimidine dimers in wild-type yeast DNA after UV i rradiation was examined by analyzing the sites recognized by UV endonucleas e from Micrococcus luteus, The samples that were exposed to low doses of ga mma radiation before UV irradiation were able to repair the pyrimidine dime rs more efficiently than the samples in which low gamma irradiation was omi tted. The nature of enhanced repair was studied by scoring the frequency of induced gene conversion and reverse mutation at trp and ilv loci respectiv ely in strain D7, which showed similar enhanced UV resistance induced by lo w-dose gamma irradiation. The induced repair was found to be essentially er ror-free. These results suggest that irradiation of strain D273-10B with lo w doses of gamma radiation enhances its capability for excision repair of U V-induced pyrimidine dimers.