Polymorphism of 2-microns plasmids in industrial strains of Saccharomyces cerevisiae.

Restriction fragment length polymorphism (RFLP) analyses of industrial Saccharomyces yeast DNA have identified eight 2-microns plasmid variants that fall into two distinct types. Type-I plasmids are of unique form, whereas type-II plasmids exist in seven distinct RFLP forms. Only two different 2-microns variants were observed in 35 bakers' strains analysed. One variant was the unique type-I whereas the second variant represents an ancestral form of the type-II plasmid. Sixteen of nineteen wine yeasts carried a distinctive type-II plasmid with a homologous STB repeat whereas ale and lager yeasts had a wide range of type-II variants. Relative to nuclear and mtDNA, 2-microns polymorphism is less diverse and not diagnostic for a specific strain. This 2-microns DNA polymorphism is a convenient and useful addendum to nuclear and mtDNA RFLP analyses but cannot serve as the sole marker for strain identification. A tentative phylogeny of industrial S. cerevisiae yeasts is suggested with origins in bakers' yeast carrying the ancestral type-II form.