DNA-SEQUENCE DIVERGENCE AND FUNCTIONAL CONSERVATION AT THE STB LOCUS OF YEAST 2-MU-M CIRCLE VARIANTS

2-mu-m DNA isolated from industrial Saccharomyces cerevisiae yeasts exhibited extensive restriction fragment length polymorphisms. At least five 2-mu-m species were identified from eleven [cir+] strains. Southern hybridization mapped restriction fragment length polymorphisms at STB, a cis-acting locus essential for plasmid partitioning. Some 2-mu-m variants (e.g., 4110-2-mu-m and 4108-2-mu-m) had an altered number of 125-bp consensus repeats at STB. However, the corresponding region of 7754-2-mu-m has only approximately 70% nucleotide sequence homology with the 125-bp STB consensus repeat. YRp plasmids containing 7754-2-mu-m STB behave as YEp plasmids in laboratory yeasts, thereby indicating STB sequence divergence coupled to conservation of function.