Relation between repair mechanisms and induced mitotic recombination after UV irradiation, in the yeast Schizosaccharomyces pombe. Effects of caffeine

Summary Two different pathways A and 1 are known to control the repair of UV lesions in the yeast Schizosaccharomyces pombe. The relation between the UV-induced intergenic mitotic crossing over (MCO) and the repair of prelethal lesions controlled by these pathways were studied in the following strains: UVS1,₁/UVS1,₁, where pathway A acts; UVSA/UVSA where pathway 1 acts, UVS⁺/UVS⁺ (wild type) and UVS1A/UVS1A (double mutant). The analysis of the survival and MCO induction curves, and the comparison, as a function of the dose and as a function of survival, of the MCO induction curves corresponding to the different strains, show that the repair pathway 1 controls a mechanism involving recombination, and that the repair pathway A controls a mechanism which removes prerecombinational lesions. Studies were done with UVS1,₁/UVS1,₁ cells in different physiological conditions affecting the repair efficiency of prelethal lesions (irradiation during the logarithmic growth phase, liquid holding). In all cases the more efficient the repair of prelethal lesions is, the smaller is the recombination inducibility. This is expected if pathway A controls an excision repair mechanism.The effect of the repair inhibitor, caffeine, was studied. It inhibits only the repair of UV prelethal lesions controlled by pathway 1. The involvement of recombination in the repair of UV lesions in UVS⁺/UVS⁺ and UVSA/UVSA cells is also shown by the fact that the sensitization to the lethal effect of UV by caffeine in these strains is correlated with a decrease in UV MCO inducibility. Caffeine has no effect either on the UV survival, or on the MCO inducibility in UVS1,₁/UVS1,₁ cells. It is concluded that it inhibits the recombinational repair pathway and not the excision repair pathway.The MCO induction observed in UVS1/UVS1 and UVS1A/UVS1A cells could be due to the presence of a second recombinational pathway, not sensitive to caffeine. At least a fraction of the prerecombinational lesions would not be prelethal, and they are repairable by the excision repair mechanism.