Deletion of the HXK2 gene in Saccharomyces cerevisiae enables mixed sugar fermentation of glucose and galactose in oxygen-limited conditions

Galactose is one of the major sugar components of red seaweeds along with glucose. In Saccharomyces cerevisiae, synthesis of galactose-metabolizing enzymes (the Leloir proteins) is under tight and complex regulation in the presence of glucose by a mechanism called catabolite repression. As a result, when both sugars are concurrently present in the medium under oxygen-limited conditions, the yeast cannot utilize galactose even after glucose consumption. Research efforts on mixed sugar fermentation of glucose/galactose, therefore, were made in the presence of oxygen, thereby resulting in sequential substrate consumption, and low ethanol yield and productivity. In this study, mixed sugar fermentation of glucose and galactose in oxygen-limited conditions was achieved by deleting the HXK2 gene, a moonlighting protein acting as hexokinase or repressor involved in catabolite repression of S. cerevisiae. Remarkably, the S. cerevisiae D452-2δhxk2 strain utilized galactose after glucose depletion without a diauxic lag period, whereas the parental strain could not use galactose at all under oxygen-limited conditions. The δhxk2 strain fermented galactose at the consumption rate of 3.02 ± 0.10 g/L/h to produce ethanol with a yield of 0.44 ± 0.01 g ethanol/g galactose. Complementation of the δhxk2 strain with the plasmid-harbored HXK2 gene under the control of various promoters clearly showed that the galactose consumption rates were inversely related to the mRNA levels of the HXK2 gene. The transcription of the GAL genes was dramatically elevated by deleting the HXK2 gene, whereas the expressions of hexokinase genes were not significantly affected. We concluded that the HXK2-deleted strain is able to efficiently utilize both glucose and galactose in the oxygen-limited conditions by alleviating catabolite-repression.