[Fundamental and applied aspects of research on the virus-like plasmids of the yeast Saccharomyces].

Six types of the antagonistic activity (AA) in Saccharomyces were distinguished and characterized. The K1, K2, K3 and K6 activities were associated with the presence of the two kinds of cytoplasmic killer virus-like particles (VLP)--cytoplasmic double-stranded RNAs (dsRNAs) consisting of the main (L) and minor (M) species. All four antagonistic groups contained the L dsRNAs of the same size. The size of the M dsRNAs are dependent on the origin of the host strain. The cytoplasmic inheritance of the K4 activity was observed among the strains which contained no dsRNAs. The AA of this type was not connected with the integrity of mitochondria, also. The K5 activity was under chromosomal control. The optimum and the limits of pH for detection of the AA were dependent on the type of activity. The natural route of extracellular infection by the killer VLPs was the penetration of the VLP into the early protoplast-like spore sprouts. The variability of the K2 activity was studied through mutagenesis of the marked genetic stocks. Twenty six chromosomal genes were involved in the control of this activity. Most of these genetic determinants differed from the chromosomal determinants of the K1 activity in their phenotypes or localization. Four phenotypically different mutational defects of the M-2 plasmid affected the manifestation of the K2 character. Epistatic and cumulative interactions between mutant chromosomal genes controlling the reproduction of the K2 plasmids were observed. Having compared the peculiarities of genetic control of the mutant phenotypes of the K1 and the K2 killers, we acquired the ability to propose a functional model of regulation of the killer virus-like plasmids reproduction. This model defines the interactions between hypothetical gene products and killer dsRNAs or their intermediates. The perspectives of selection of dsRNAs producers were studied. The methods for construction and maintaining the productive strains from the diploids homozygous for a chromosomal mutation increasing the copy number of both K2 killer plasmids were developed. These methods assisted in obtaining the producers suitable for large-scale cultivation.