The hexokinase 2 protein regulates the expression of the GLK1, HXK1 and HXK2 genes of Saccharomyces cerevisiae

The key glycolytic HXK2 gene, coding for the enzyme hexokinase 2 (Hxk2p), is expressed when cells of the yeast Saccharomyces cerevisiae are grown on a fermentable medium using glucose, fructose or mannose as a carbon source. After shifting the cells to a non-fermentable carbon source, the HXK2 gene is repressed and the HXK1 and GLK1 genes are rapidly de-repressed, producing the enzymes hexokinase 1 (Hxk1p) and glucokinase (Glk1p) respectively. Because the in vivo functions of the Hxk1p and Glk1p enzymes have remained a mystery so far, we have investigated this glucose-induced regulatory process. Here we demonstrate the involvement of Hsk2p in the glucose-induced repression of the HXK1 and GLK1 genes and the glucose-induced expression of the HSK2 gene. We have also demonstrated the involvement of Hxk1p as a negative factor in the expression of the GLK1 and HXK2 genes. Further experimental evidence, using mutant cells expressing a truncated version of Hxk2p unable to enter the nucleus, shows that nuclear localization of Hsk2p is necessary for glucose-induced repression signalling of the HSK1 and GLK1 genes and for glucose-induced expression of the HXK2 gene. Gel mobility-shift analysis shows that Hxk2p-mediated regulation is exerted through ERA (ethanol repression autoregulation)-like regulatory sequences present in the HXK1 and GLK1 promoters and in two downstream repressing sequences of the HXK2 gene. These findings reveal a novel mechanism of gene regulation whereby the product of a glycolytic gene, normally resident in the cytosol, interacts directly with nuclear proteins to regulate the transcription of the HXK1 and GLK1 gems and to autoregulate its own transcription.