A METHOD FOR DIRECT SELECTION OF MATING-COMPETENT CLONES FROM MATING-INCOMPETENT INDUSTRIAL STRAINS OF SACCHAROMYCES-CEREVISIAE

Industrial strains of Saccharomyces cerevisiae are generally incompete nt for mating with others and hard to sporulate. Physical and function al analyses of Kyokai no. 7, a typical sake yeast, revealed that it ha s functional a and alpha mating-type genes and the mating incompetency has the regular non-mater phenotype due to alpha2 and a1-alpha2 repre ssion. An effective selection method for isolation of mating-competent mutants of this strain was developed using plasmids bearing fused gen es, STE6p-PHO5 and MFalpha1p-PHO5, consisting of the promoter of STE6, an a-specific gene, and MFalpha1, an alpha-specific gene, respectivel y, connected to the coding region of PHO5 encoding repressible acid ph osphatase. Mating-competent mutants having the a or alpha mating type were selected by detection of repressible acid phosphatase activity by specific staining of colonies. This method was applied successfully t o two other sake yeasts. Hybrid yeast strains superior for sake making were constructed by crossing these mating-competent derivatives.