Derepression of galactose metabolism in melibiase producing bakers' and distillers' yeast

Beet molasses is widely used as a growth substrate for bakers' and distille rs' yeast in the production of biomass and ethanol. Most commercial yeasts do not fully utilise the carbohydrates in molasses since they are incapable of hydrolysing the disaccharide melibiose to glucose and galactose. Also, expression of genes encoding enzymes for the utilisation of carbon sources that are alternatives to glucose is tightly regulated, sometimes limiting r ates of yeast growth and/or ethanol production. The GAL genes are regulated by specific induction by galactose and repression during growth on glucose . In an industrial distillers' yeast, two genes interacting synergistically in glucose repression of galactose utilisation, MIG1 and GAL80, have been disrupted with MEL1, encoding melibiase. The physiology of the wild-type st rain and the recombinant strains was investigated on mixtures of glucose an d galactose and on molasses. The recombinant strain started to ferment gala ctose when 9.7 g l(-1) glucose was still present during a batch fermentatio n, whereas the wild-type strain did not consume any galactose in the presen ce of glucose. The ethanol yield in the recombinant strain was 0.50 g ethan ol g sugar(-1) in an ethanol fermentation on molasses, compared with 0.48 g ethanol g sugar(-1) for the wild-type strain. The increased ethanol yield was due to utilisation of melibiose in the molasses. (C) 1999 Elsevier Scie nce B.V. All rights reserved.