A NOVEL TYPE OF LIFE-CYCLE DELAYED HOMOTHALLISM IN SACCHAROMYCES-CEREVISIAE WY2 SHOWED SLOW INTERCONVERSION OF MATING-TYPE

Saccharomyces cerevisiae wy2 segregated to 2 mater and 2 non-mater in relation to mating ability, The non-mater segregants behaved as the no rmal type of homothallic life cycle. On the other hand, the mater segr egants gradually formed spores during successive subcultures, indicati ng that slow interconversion of mating-type happened to occur during s ubcultures. We termed this novel type of life cycle ''delayed homothal lism''. The results of complementation tests with standard ho strains and introduction of a wild type HO gene showed that delayed homothalli sm was caused by a defective HO gene. The amino acid sequence deduced from the nucleotide sequence of the wy2 HO gene differed from the wild type HO gene in three amino acid residues. In the carboxy terminus of HO protein, there are three repeats of cysteine and histidine that ar e postulated to play a role in binding of HO protein to DNA. However, wy2 HO protein lacked one such repeat at residues Cys470-His475, where His was replaced by Leu.