Maintenance of the 2 microns circle plasmid in populations of Saccharomyces cerevisiae

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Abstract
The 2 .mu.m circle plasmid is maintained at high frequencies in populations of yeast cells. To find out how the plasmid is maintained, 3 forces were measured: the selective advantage or disadvantage conferred by 2 .mu.m circles, the rate of generation of [Cir0] cells and the rate of illegitimate transfer of 2 .mu.m circles from cell to cell. Under the conditions used, 2 .mu.m circles confer a selective disadvantage of about 1%, [Cir0] cells are generated at the rate of 7.6 .times. 10-5/[Cir+] cell per generation and illegitimate transfer of 2 .mu.m circles occurs at a rate < 10-7/recipient cell per generation. The most likely explanation of 2 .mu.m circle maintenance is that the plasmid is sexually transmitted at such a rate that it spreads through populations despite selection against it.