Maintenance and copy number control of ARS1 plasmids in Saccharomyces cerevisiae

Summary Following mating of a and isogenic haploids we observe that the frequency of plasmid bearing cells, during selective growth, increases three fold. By examining the mitotic stability, the frequency of plasmid bearing cells during the cell cycle and the copy number of ARS1 plasmids in isogenic haploid and diploid cells, we show that the apparent stability of circular ARS1 plasmids in a/ cells is largely due to a diminished copy number in these cells. This observation is fully comprehensible with the model for plasmid segregation as presented by Murray and Szostak (1983). In order to account for the differences in copy numbers, a and a/a isogenic strains were compared. Likewise a number of mating type nonspecific sterile mutants were compared with the parental Ste⁺ strain. It seems that a diminished copy number is established when the MATa1/MAT2 regulatory system (Klar et al. 1981) is switched on, since the effect is observed in Sir^– strains only.