ABSTRACT

The 2-μm plasmid is a 6318-bp double-stranded circular DNA found in practically all strains of Saccharomyces cerevisiae. The 2-μm plasmid appears to behave as a selfish DNA26-28 and has developed a very elaborate system to maintain itself in yeast populations. The 2-μm circle plasmid exists in the cell at a high number of copies, is efficiently transmitted at cell division, and is able to amplify. When such a strain is transferred on galactose, the FLP protein is made and excises the integrated flp- 2-μm circle. This plasmid then inverts by recombination between its FLP recognition target sites, unless one of them has been inactivated by mutation. The 2-μm plasmid is present in the nuclei of most Saccharomyces strains at 50 to 100 copies per haploid genome. In order to follow the kinetics of amplification,62 a diploid strain bearing an integrated invertible plasmid and the GAL10-FLP construction was induced by galactose.