GLUCOSE REPRESSION IN THE YEAST SACCHAROMYCES-CEREVISIAE

Understanding the mechanism of glucose repression in yeast has proved to be a difficult and challenging problem. A multitude of genes in different pathways are repressed by glucose at the level of transcription. The SUC2 gene, which encodes invertase, is an excellent reporter gene for glucose repression, since its expression is controlled exclusively by this pathway. Genetic analysis has identified numerous regulatory mutations which can either prevent derepression of SUC2 or render its expression insensitive to glucose repression. These mutations allow us to sketch the outlines of a pathway for general glucose repression, which has several key elements: hexokinase PII, encoded by HXK2, which seems to play a role in the sensing of glucose levels; the protein kinase encoded by SNF1, whose activity is required for derepression of many glucose-repressible genes; and the M1G1 repressor protein, which binds to the upstream regions of SUC2 and other glucose-repressible genes. Repression by M1G1 requires the activity of the CYC8 and TUP1 proteins. Glucose repression of other sets of genes seems to be controlled by the general glucose repression pathway acting in concert with other mechanisms. In the cases of the GAL genes and possibly CYC1, regulation is mediated by a cascade in which the general pathway represses expression of a positive transcriptional activator.