Host-plasmid interactions in Saccharomyces cerevisiae: effect of host ploidy on plasmid stability and copy number.

The segregational stability of two chimaeric plasmids has been examined in an isogenic series of haploid, diploid and tetraploid strains of Saccharomyces cerevisiae, constructed by transformation-associated spheroplast fusion. For the highly unstable, ARS-based plasmid YRp7M, a significant increase in its segregational stability was observed with increasing ploidy, while the relatively stable, 2 microns-based plasmid pMA3a showed only a small increase in stability in strains of higher ploidy. The copy number of both pMA3a and the endogenous 2 microns plasmid increased in proportion with the host cell ploidy, while the copy number of TRp7M was increased in the higher ploidy strains but did not correlate with ploidy. These results suggest that the copy numbers of both the 2 microns plasmid and a plasmid derived from it are controlled by a nuclear gene and that, in addition, there are 2 microns sequences, other than those required for the FLP-mediated recombination system, that play a role in maintaining copy number.