Derepression of a baker's yeast strain for maltose utilization is associated with severe deregulation of HXT gene expression

Aims: We undertook to improve an industrial Saccharomyces cerevisiae strain by derepressing it for maltose utilization in the presence of high glucose concentrations. Methods and Results: A mutant was obtained from an industrial S. cerevisiae strain following random UV mutagenesis and selection on maltose/5-thioglucose medium. The mutant acquired the ability to utilize glucose simultaneously with maltose and possibly also sucrose and galactose. Aerobic sugar metabolism was still largely fermentative, but an enhanced respirative metabolism resulted in a 31% higher biomass yield on glucose. Kinetic characterization of glucose transport in the mutant revealed the predominance of the high-affinity component. Northern blot analysis showed that the mutant strain expresses only the HXT6/7 gene irrespective of the glucose concentration in the medium, indicating a severe deregulation in the induction/repression pathways modulating HXT gene expression. Interestingly, maltose-grown cells of the mutant display inverse diauxy in a glucose/maltose mixture, preferring maltose to glucose. Conclusion: In the mutant here reported, the glucose transport step seems to be uncoupled from downstream regulation, because it seems to be unable to sense abundant glucose, via both repression and induction pathways. Significance and Impact of the Study: We report here the isolation of a S. cerevisiae mutant with a novel derepressed phenotype, potentially interesting for the industrial fermentation of mixed sugar substrates.