YEAST CARBON CATABOLITE REPRESSION

Glucose and related sugars repress the transcription of genes encoding enzymes required for the utilization of alternative carbon sources; s ome of these genes air also repressed by other sugars such as galactos e, and the process is known as catabolite repression. The different su gars produce signals which modify the conformation of certain proteins that, in turn, directly or through a regulatory cascade affect the ex pression of the genes subject to catabolite repression. These genes ar e not all controlled by a single set of regulatory proteins, but there are different circuits of repression for different groups of genes. H owever, the protein kinase Snf1/Cat1 is shared by the various circuits and is therefore a central element in the regulatory process. Snf1 is not operative in the presence of glucose, and preliminary evidence su ggests that Snf1 is in a dephosphorylated stare under these conditions . However the enzymes that phosphorylate and dephosphorylate Snf1 have not been identified, and it is not known how the presence of glucose may affect their activity. What has been established is that Snf1 rema ins active in mutants lacking either the proteins Grr1/Cat80 or Hxk2 o r the Glc7 complex, which functions as a protein phosphatase. One of t he main roles of Snf1 is to relieve repression by the Mig1 complex, bu t it is also required for the operation of transcription factors such as Adr1 and possibly other factors that are still unidentified. Althou gh our knowledge of catabolite repression is still very incomplete, it is possible in certain cases to propose a partial model of the way in which the different elements involved in catabolite repression may be integrated.