Physical and functional structure of a yeast plasmid, pSB3, isolated from Zygosaccharomyces bisporus

The plasmid pSB3 of yeast Zygosacharomyces bisporus has been sequenced. It contains 6,615 base pairs, including a pair of inverted repeats (IR) consisting of 391 base pairs and 3 large open reading frames (ORF). One of the ORFs (A gene) participates in the recombination at the IRs and the other two (B and C genes) are necessary for the stable maintenance of this plasmid. The ARS sequence, which functions in a Saccharomyces cerevisiae host, was localized within 168 base pairs consisting of part of one of the IRs and a unique sequence contiguous to it. pSB3 can be maintained as stably in Z. rouxii as in the natural host Z.bisporus. In contrast, pSB3 is maintained fairly unstably in S.cerevisiae. The reason for this instability was found to be inefficient partitioning of pSB3 in S.cerevisiae. The molecular construction of pSB3 resembles that of 2-micron DNA, however, sequence homology at the DNA level was very poor.