Identification of new genes involved in disaccharide fermentation in yeast.

Abstract

Maltose non-fermenting mutants were obtained from strains carrying a MAL4 allele which permits constitutive synthesis of maltase. Cells carrying this allele are able to utilize sucrose in the absence of the “classical” sucrose genes. All maltose non-fermenting mutants were also sucrose non-fermenters. Eight mutants had become maltase negative; 19 mutants could still form maltase constitutively. In crosses with segregational maltose and sucrose non-fermenting strains, enzyme negative mutants gave diploids unable to ferment maltose and sucrose. Enzyme positive, non-fermenting mutants gave diploids which readily fermented maltose and sucrose. This latter type of mutants was designated dsf (disaccharide fermentation) mutants. The diploids derived from crossing non-fermenting mutants with segregational non-fermenters were subjected to tetrad analysis. Enzyme negative non-fermenters gave only non-fermenting progeny. The dsf mutants segregated both fermenting and non-fermenting progeny, some of which showed the dsf phenotype. This indicated that none of the dsf mutants had a defect in a gene closely linked to MAL4. Crosses between dsf mutants and strains carrying the maltose genes MAL2 and MAL3 showed that the mutations affected maltose fermentation in general. Sucrose fermentation in the presence of the “classical” sucrose gene SUC3 was not affected, nor were fermentation of glucose, fructose and galactose. The uptake of radioactivity from uniformly labeled maltose appeared to be blocked in mutants of at least four of the dsf genes. Only one non-leaky and a leaky mutant showed a significant uptake. These results suggest that there is an extremely complex transport system for maltose and sucrose or that the utilization of these disaccharides requires a complex series of metabolic reactions.