Improved Properties of Baker's Yeast Mutants Resistant to 2-Deoxy- d -Glucose

Abstract

We isolated spontaneous mutants from Saccharomyces cerevisiae (baker's yeast V1) that were resistant to 2-deoxy-D-glucose and had improved fermentative capacity on sweet doughs. Three mutants could grow at the same rate as the wild type in minimal SD medium (0.17% Difco yeast nitrogen base without amino acids and ammonium sulfate, 0.5% ammonium sulfate, 2% glucose) and had stable elevated levels of maltase and/or invertase under repression conditions but lower levels in maltose-supplemented media. Two of the mutants also had high levels of phosphatase active on 2-deoxy-D-glucose-6-phosphate. Dough fermentation (CO 2 liberation) by two of the mutants was faster and/or produced higher final volumes than that by the wild type, both under laboratory and industrial conditions, when the doughs were supplemented with glucose or sucrose. However, the three mutants were slower when fermenting plain doughs. Fermented sweet bakery products obtained with these mutants were of better quality than those produced by the wild type, with regard to their texture and their organoleptic properties.Saccharomyces cerevisiae may utilize a variety of carbon sources, but glucose and fructose are preferred. When one of these sugars is present, carbon catabolite repression occurs and the enzymes required for utilization of the alternative carbon sources are synthesized at low rates or not at all (12,13,14). Carbon catabolite repression alters transcription and is regulated mainly by the Mig1p protein (13, 14, 23), a transcriptional repressor of glucose-repressible genes involved in metabolic processes other than glucose fermentation (such as utilization of the alternative carbon sources sucrose, maltose, or galactose; gluconeogenesis; and respiratory metabolism [13,14]). Transcription of genes required for growth in nonfermentable carbon sources is activated by the Hap complex, which is repressed by Mig1p (5,14).S. cerevisiae baker's yeasts commonly are grown in molasses, which contains sucrose as the primary carbon source, and genotypes with the highest growth rate and productivity in molasses are favored (8,9,10,11). Further increases in invertase expression and redirection of the respiro-fermentative flux through the deregulation of Mig1p or Hap complex (5) would improve utilization of molasses and production of sweet doughs by these strains. Expression of the SUC genes, which code for the invertase required for catabolism of sucrose and raffinose, is repressed at high levels of glucose (12,13,14). Various regulatory regions have been identified in the SUC2 promoter. Mig1p binds to SUC2A and SUC2B (activation sequences) in the presence of glucose. Repression mediated by the upstream repression sequence for SUC2 (URS SUC2 ) occurs in the absence of glucose (14).In dough without addition of sugar, the principal fermentable sugar for yeast is maltose, liberated from the starch of the flour by amylases. The leavening ability of sponge dough is closely related to maltose fermentability (4,20). Maltose utilization requires a MAL locus and tr...

A putative sensor kinase, Hik31, is involved in the response of Synechocystis sp. strain PCC 6803 to the presence of glucose

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Genes involved in glucose repression and oxidative stress response in the fission yeast Schizosaccharomyces pombe

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