[Effect of ploidy and the status of the MAT locus in Saccharomyces cerevisiae yeast cells on the mitotic stability of episomal plasmids and the level of expression of the HBsAg gene of hepatitis B virus].

The frequency of the YEp13, pEF91, YEp13 + HBs, pNMVG3954 plasmids' elimination in a series of isogenic strains n. 2n and 3n was studied. Plasmid stability and the level of expression of the gene of hepatitis B surface antigen (HBsAg) increased in polyploid transformants as a result of increase in plasmid copy number. Heterozygotes MATa/MAT differed from the homozygotes in higher stability of YEp13, pEF91, and YEp13 + HBs plasmids having the same quantity of the HBsAg antigen. The appearance of negative properties--destabilization of episomal plasmids or decrease in synthesis of heterologous protein have been discovered in certain diploid cell. These results point to limitations in constructing a polyploid producer on the basis of the similar type of genome.