A Circular Plasmid from the YeastTorulaspora delbrueckii

A new member of the 2-μm family of plasmids, named pTD1, was found in the yeastTorulaspora delbrueckii,a widespread yeast associated with food. Nucleotide sequences revealed the presence of a pair of inverted repeats and three open reading frames, one of which is a homologue of the FLP recombinase gene of 2-μm plasmid. An ARS region was identified, by replication inSaccharomyces cerevisiaeandT. delbrueckii,near one of the inverted repeats. By the use of pTD1 derivatives and auxotrophic mutant hosts, an efficient host–vector system was established forT. delbrueckii.So far, the 2-μm family of plasmids is restricted to four closely related genera (Q6 group):Saccharomyces, Zygosaccharomyces, Kluyveromyces,andTorulaspora.After a survey of 2500 strains belonging to about 500 species (80 genera) of yeast, no circular plasmids were found in other genera.