Yeast Carbon Catabolite Repression

Abstract

SUMMARY Glucose and related sugars repress the transcription of genes encoding enzymes required for the utilization of alternative carbon sources; some of these genes are also repressed by other sugars such as galactose, and the process is known as catabolite repression. The different sugars produce signals which modify the conformation of certain proteins that, in turn, directly or through a regulatory cascade affect the expression of the genes subject to catabolite repression. These genes are not all controlled by a single set of regulatory proteins, but there are different circuits of repression for different groups of genes. However, 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 suggests that Snf1 is in a dephosphorylated state 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 remains active in mutants lacking either the proteins Grr1/Cat80 or Hxk2 or the Glc7 complex, which functions as a protein phosphatase. One of the main roles of Snf1 is to relieve repression by the Mig1 complex, but it is also required for the operation of transcription factors such as Adr1 and possibly other factors that are still unidentified. Although 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.

Gal83 mediates the interaction of the Snf1 kinase complex with the transcription activator Sip4

“…The Snf1/AMPK protein kinase family has been conserved through evolution, and members have been identified in organisms ranging from plants to mammals (for a review see Hardie et al, 1998). In the yeast Saccharomyces cerevisiae, the Snf1 (Cat1, Ccr1) protein kinase has a major role in the glucose starvation signalling pathway (for reviews see Gancedo, 1998;Carlson, 1999;Johnston, 1999). The Snf1 kinase is activated when glucose is limiting Jiang and Carlson, 1996;Wilson et al, 1996) and is required for the transcription of glucose-repressed genes involved in alternate carbon source utilization, respiration and gluconeogenesis, as well as for sporulation, glycogen accumulation, thermotolerance, peroxisome biogenesis and meiosis (Celenza and Carlson, 1986;Schuller and Entian, 1987;Thompson-Jaeger et al, 1991;Simon et al, 1992;Honigberg and Lee, 1998).…”

Catalases protect cellular proteins from oxidative modification in

“…In our experiments, cell growth in glucose did not cause oxidative stress, but when cultivated in ethanol yeast demonstrated typical oxidative stress features that were mirrored by the strong activation of antioxidant enzymes. It should be noted that antioxidant enzyme expression is suppressed by glucose (Ruis and Ko¨ller, 1997;Gancedo, 1992). We have found that catalase in S. cerevisiae does not play an important role in protection of some proteins against inactivation by ROS under fermentation growth conditions.…”

RNA assemblages orchestrate complex cellular processes

“…The exact mechanism behind the coordination in terms of localization and/or translatability has been unclear, but a recent study suggests that the participation of RNP granules is pivotal for the physiology of the regulon [60]. Upon glucose starvation, Puf3p becomes heavily phosphorylated, mainly within its Nterminal low-complexity region, and this results in translational activation of bound mRNAs, thereby promoting mitochondrial biogenesis and the capacity for oxidative catabolism of carbon sources [61]. From a mechanistic point of view, it should be noted that phosphorylated Puf3p cosediments with its target mRNAs in polysomes, suggesting that the translational activation is due to interference with protein-protein, rather than RNA-protein, interactions.…”