The DLP1 mutant of the yeast Saccharomyces cerevisiae with an increased copy number of the 2 mu plasmid shows a shortened lifespan

We isolated and characterized a recessive mutant, named dlp1, which shows t he Dlp phenotype (delayed loss of proliferation activity) during the autoph agic death of cdc28. The dlp1 mutant was found to consist of two subtypes o f cells based on colony morphology. One subtype with the Dlp phenotype, nam ed dlp1-1, became large, red, and nibbled during the incubation, suggesting that the cells on the surface of the colonies were dying. The other withou t the Dlp phenotype, named dlp1-s, retained small, white colonies even afte r a prolonged incubation and was found to be a petite mutant. The change fr om dlp1-1 to dlp1-s (petite) occurred much more frequently (about 15%) than that from the wild-type to petite mutant (less than 1%). The lifespan of b oth subtypes of cells was severely shortened. The copy number of the endoge nous 2 mu plasmid of dlp1-1 was 68-fold that of the original cdc28, and dec reased by half after the conversion to dlp1-s (petite). A 4.0-kbp fragment of the 2 mu plasmid containing REP2 decreased the copy number of the endoge nous 2 mu plasmid to 8-fold that of the original cdc28 cells and partially rescued the shortened lifespan, in addition to resulting in the complete co mplementation of the Dlp and nibbled-colony phenotypes. These results sugge st that DLP1 is a chromosomal gene that regulates the copy number of the 2 mu plasmid, and that the shortening of the lifespan and other effects of th e dlp1 mutation are likely caused by the increased copy number of the endog enous 2 mu plasmid. (C) 1999 Elsevier Science Ireland Ltd. All rights reser ved.