On the dependence of spontaneous mutation rates on the functional state of genes.

Spontaneous mutation of some genes was studied in haploid adenine and leucine auxotrophic yeast Saccharomyces. It was shown that a decrease in the amount of adenine (from 500 to 0 mg l-1) or leucine (from 300 to 0.3 mg l-1) in the medium, simultaneously with the transition from repression to derepression of the biosynthesis of these metabolites, resulted in a 15- to 150-fold increase in the reversion rate of genes ade 2 and leu2, respectively, for different strains. At the same time the mutation rate of suppressor genes varied relatively little (up to five-fold), and that of gene lys did not change at all. It was also demonstrated (on gene leu2) that the mutation rate is determined by the composition of the nutrient medium at the time of the S-phase of the cell cycle and it does not depend on the cultivation conditions during the presynthetic period. We discuss the hypothesis that derepressed genes mutate with a significantly higher rate than genes in the repressed state.

[Spontaneous mutation of leu2 in Saccharomyces cerevisiae]. / Spontannoe mutirovanie leu2 u Saccharomyces cerevisiae.

The data obtained indicate that spontaneous mutations in Saccharomyces cerevisiae are formed during DNA replication. With no DNA replication in the lag-period, in the stationary growth phase, spontaneous mutations are not formed in cell culture during the G1 phase of cell cycle. Experimental data show the absence of primary spontaneously occurring DNA lesion accumulation in the cell G1 phase. Spontaneous mutations of yeasts are formed in the S phase of cell cycle, apparently as DNA replication errors. It is established that the frequency of spontaneous reversions of the leu2 gene in Saccharomyces cerevisiae strain NA3-24 increases when the cells are cultivated on the culture medium with different concentrations of leucine.

[Regularities in formation of gamma-induced mutants of Saccharomyces cerevisiae]. / Zakonomernosti obrazovaniia gamma-indutsirovannykh mutantov Saccharomyces cerevisiae.

As shown by an example of the formation of reversions in the leu2 gene of Saccharomyces cerevisiae, the process of mutant formation after gamma-irradiation is associated with the post-irradiation replication phases. When no DNA replication takes place, the mutants are not formed. The periods of realizing premutation lesions into mutations depend on what cell cycle phase the cells were in when irradiated.

[Characteristics of detection of mutants on selective media]. / Zakonomernosti vyiavleniia mutantov na selektivnykh sredakh.

The regularities of formation and appearance of mutants after transfer or plating yeast cells on selective media are analysed. The knowledge of these patterns is necessary for quantitative evaluation of mutants' content in the culture. Mutants formed in a cell culture, prior to transfer onto selective medium, are revealed on this medium as colonies and form a wave in time that is being observed during several days. Due to residual growth of cells, new mutants are formed and revealed as the second wave. Following two waves, an increasing front may approach which is formed from mutants of unknown origin. For evaluation of the number of mutants in the culture, prior to transfer onto selective medium, it is necessary to count up only the "first wave" of mutant colonies' appearance.