Effets de mutations vers la sensibilité au rayonnement ultraviolet chez la levure

Abstract

Resume The effects of mutations towards ultraviolet sensitivity in yeast UV-sensitive mutants were induced by the action of UV light and were isolated by the replica plating method. Highly homozygous stocks of yeast Saccharomyces cerevisiae descending from race XII were utilized as material for the present investigation. Two mutants, designated uvs 1 and uvs 2 , carrying nonallelic mutations of radiosensitivity, were studied more carefully. UV sensitivity appeared to be of a recessive monogenic character; heterozygous diploids were UV-resistant and segregated 1:1 resistant and sensitive clones in the random ascospore samples. The UV sensitivity of the strains bearing mutants uvs 1 and uvs 2 differs. The haploid uvs 1 mutant and the haploid uvs 2 mutant have sensitivities 25 and 1.8 times higher, respectively, than the wild-type haploid. The uvs 1 mutants exhibit an exponential survival curve, while the uvs 2 mutants and the wild-type haploid have sigmoidal shaped curves. The radioresistance of the isogenic strains, carrying the mutations at the uvs 1 locus, increases with polyploidy. We were able to study the effect of uvs mutations on the frequency of induced genic mutations due to the fact that the resistant mutants could easily be detected in the media with a high serine concentration. The results indicate that the uvs 1 and uvs 2 mutations significantly increase the sensitivity of haploids to mutagenic action of UV light. The uvs 1 and uvs 2 mutations also result in an increase in sensitivity to mutagenic action of UV light in regard to cytoplasmic determinants. The diploids homozygous for uvs 1 and uvs 2 mutations proved to be highly sensitive to the action of UV light causing recombination when the frequency of induced mitotic recombination in resistant and sensitive diploids heterozygous for ad 1 and ad 2 genes was recorded. The frequency of spontaneous mutations independent of adenine was recorded in haploids of genotype ad 2 , ad 2 uvs 1 and ad 2 uvs 2 . The mutations towards UV sensitivity cause a sharp increase in spontaneous mutability, the rise being especially significant due to uvs 2 mutations. The specific features of uvs 2 mutants of Saccharomyces are similar to those of UV-sensitive mutants of E. coli ; therefore, one might suggest that the uvs mutants in yeast also have a less efficient or blocked system for repair of UV damage in the DNA. The fact that the UV-sensitive mutants are sensitive not only to the lethal action of UV light but also to the mutagenic and recombinogenic ones suggests that similar molecular damages of the DNA underlie all these effects. The induction of cytoplasmic mutations in UV-sensitive mutants suggests that the repair system under consideration is localized not only in the nucleus but also in the cytoplasm of the cell. The sharp increase in spontaneous mutability in UV-sensitive mutants indicates that the main function of the repair system is to maintain the stability of genetic material under natural conditions.