[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.

[Study of the stability of hybrid plasmids replicating in Saccharomyces cerevisiae due to DNA fragments from polyoma virus]. / Izuchenie stabil'nosti gibridnykh plazmid, replitsiruiushchikhsia v Saccharomyces cerevisiae za schet fragmentov DNK virusa poliomy.

Hybrid plasmid pSP97 carrying the entire genome of polyoma virus (PY), inserted into bacterial vector psV3, transforms yeast cells with the frequency 1 x 10(-2). Plasmid pSP97 is capable of autonomous replication in S. cerevisiae, while its structure remains unaltered, the stability of hybrid plasmid in transformants is 44%--100%. Plasmid pSP155 consisting of Ori-containing DNA segment from polyoma, pBR322 and yeast gene arg4, transforms yeast cells with the frequency 5 x 10(-3), the stability of plasmid in transformants is 23%--29%. Two types of plasmids were isolated from transformants: one was identical to SP155, while the another differed structurally and phenotypically from SP155. Plasmids pSP113 and pSP114, in addition to pBR322 and yeast gene arg4, contain a viral DNA segment that encodes genes from small and middle T-antigens. These plasmids transform yeast cells with low frequency (2 x 10(-4), 3 x 10(-5)), the stability of plasmids in yeast transformants is 100%. However, hybrid plasmids identical to pSP113 were isolated from transformants. Structural rearrangements have been observed in pSP114, which carries the arg4 gene in reversed orientation compared to pSP113.