Portal de Periódicos da CAPES

Two Different Signals Regulate Repression and Induction of Gene Expression by Glucose

2002; Elsevier BV; Volume: 277; Issue: 49 Linguagem: Inglês

10.1074/jbc.m208726200

1083-351X

Sabire Özcan,

Plant Gene Expression Analysis

In addition to being the universal carbon and energy source, glucose also regulates gene expression in many organisms. In the yeast Saccharomyces cerevisiae glucose regulates gene expression via two different pathways known as the glucose repression and glucose induction pathways. The signal for glucose induction of hexose transporter (HXT) genes is generated via two glucose-transporter like molecules, Snf3 and Rgt2. A strain lacking both sensors is unable to induce HXT gene expression and is defective in glucose uptake. The snf3 rgt2 double mutant is also defective in glucose repression of transcription, raising the possibility that Snf3 and Rgt2 are also involved in generating the glucose repression signal. In this report, I show that induction and repression of gene expression by glucose in yeast is regulated by two independent signals. While the signal for induction of HXT gene expression is generated by Snf3 and Rgt2 glucose receptors, the repression signal requires the uptake and metabolism of glucose. In addition, the glucose induction of theHXT genes is required for repression of gene expression by glucose. Therefore the glucose repression defect of the snf3 rgt2 strain is indirect and is due to the lack of glucose uptake in this double mutant. In addition to being the universal carbon and energy source, glucose also regulates gene expression in many organisms. In the yeast Saccharomyces cerevisiae glucose regulates gene expression via two different pathways known as the glucose repression and glucose induction pathways. The signal for glucose induction of hexose transporter (HXT) genes is generated via two glucose-transporter like molecules, Snf3 and Rgt2. A strain lacking both sensors is unable to induce HXT gene expression and is defective in glucose uptake. The snf3 rgt2 double mutant is also defective in glucose repression of transcription, raising the possibility that Snf3 and Rgt2 are also involved in generating the glucose repression signal. In this report, I show that induction and repression of gene expression by glucose in yeast is regulated by two independent signals. While the signal for induction of HXT gene expression is generated by Snf3 and Rgt2 glucose receptors, the repression signal requires the uptake and metabolism of glucose. In addition, the glucose induction of theHXT genes is required for repression of gene expression by glucose. Therefore the glucose repression defect of the snf3 rgt2 strain is indirect and is due to the lack of glucose uptake in this double mutant. In the yeast Saccharomyces cerevisiae, glucose has two major effects on gene transcription. First it represses expression of genes encoding enzymes for the metabolism of alternate sugars such as galactose and sucrose (1Gancedo J.M. Microbiol. Mol. Biol. Rev. 1998; 62: 334-361Google Scholar, 2Carlson M. Curr. Opin. Microbiol. 1999; 2: 202-207Google Scholar). Second it induces the transcription of genes encoding glycolytic enzymes and glucose transporters required for efficient glucose metabolism (3Johnston M. Trends Genet. 1999; 15: 29-33Google Scholar, 4Özcan S. Johnston M. Microbiol. Mol. Biol. Rev. 1999; 63: 554-569Google Scholar).Although the repressive effect of glucose on gene expression has been known for decades, the primary signal required for glucose repression remains unknown. Recent data indicate that glucose repression requires the uptake and metabolism of glucose (5Ye L. Kruckeberg A.L. Berden J.A. Dam K.v. J. Bacteriol. 1999; 181: 4673-4675Google Scholar, 22Lafuente M.J. Gancedo C. Jauniaux J.C. Gancedo J.M. Mol. Microbiol. 2000; 35: 161-172Google Scholar). Derepression of glucose-repressed genes (such as GAL1 andSUC2) in the absence of high concentrations of glucose requires the protein kinase Snf1 (6Zimmermann F.K. Kaufmann I. Rasenberger H. Haubetamann P. Mol. Gen. Genet. 1977; 151: 95-103Google Scholar, 7Ciriacy M. Mol. Gen. Genet. 1977; 154: 213-220Google Scholar, 8Carlson M. Osmond B.C. Botstein D. Genetics. 1981; 98: 25-40Google Scholar). Based on work carried out with the mammalian homologue of Snf1, the AMP-activated kinase (AMPK), it has been suggested that changes in AMP:ATP ratio generate the signal for glucose repression (9Hardie D.G. Carling D. Carlson M. Annu. Rev. Biochem. 1998; 67: 821-855Google Scholar, 10Hardie D.G. Hawley S.A. BioEssays. 2001; 23: 1112-1119Google Scholar, 11Wilson W.A. Hawley S.A. Hardie D.G. Curr. Biol. 1996; 6: 1426-1434Google Scholar).The signal for glucose induction of hexose transporter (HXT) gene expression is generated by two membrane receptors, Snf3 and Rgt2 (12Özcan S. Dover J. Rosenwald A.G. Wölfl S. Johnston M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 12428-12432Google Scholar, 13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar). Both proteins have similarities to glucose transporters and function as sensors of extracellular glucose. In the snf3 rgt2 double mutant, induction of HXT gene expression by glucose is completely abolished. As a consequence of this defect thesnf3 rgt2 strain is defective for glucose uptake and displays impaired growth on glucose-containing media. Interestingly, the snf3 rgt2 double mutant is also defective for glucose repression of GAL1 and SUC2 genes, indicating the requirement of these sensors for the glucose repression pathway (13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar, 14Schmidt M.C. McCartney R.R. Zhang X. Tillman T.S. Solimeo H. Wölfl S. Almonte C. Watkins S.C. Mol. Cell. Biol. 1999; 19: 4561-4571Google Scholar).This report provides evidence that two different signals are responsible for repression and induction of transcription by glucose in yeast. It demonstrates that the generation of the signal for glucose repression of gene expression requires the uptake and the metabolism of glucose. Furthermore, it shows that the induction of HXTgene expression by glucose is essential for the generation of the glucose repression signal.DISCUSSIONIn the yeast S. cerevisiae, glucose has both positive and negative regulatory effects on gene expression. While expression of the genes encoding glucose transporters such as theHXT genes is 10- to 300-fold induced by glucose, transcription of genes required for metabolism of alternate carbon sources such as GAL1 and SUC2 genes is inhibited by 10- to 1000-fold (1Gancedo J.M. Microbiol. Mol. Biol. Rev. 1998; 62: 334-361Google Scholar, 2Carlson M. Curr. Opin. Microbiol. 1999; 2: 202-207Google Scholar). The signal for induction of HXTgene expression by glucose is generated by two glucose sensors, Snf3 and Rgt2, and does not require the transport and metabolism of glucose (4Özcan S. Johnston M. Microbiol. Mol. Biol. Rev. 1999; 63: 554-569Google Scholar, 12Özcan S. Dover J. Rosenwald A.G. Wölfl S. Johnston M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 12428-12432Google Scholar, 13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar). The nature of the primary signal mediating glucose repression of gene expression is unknown. Recent data indicate that the signal for glucose repression of gene expression is determined by the intracellular concentration of the glucose (5Ye L. Kruckeberg A.L. Berden J.A. Dam K.v. J. Bacteriol. 1999; 181: 4673-4675Google Scholar, 19Meijer M.C. Boonstra J. Verkleij A.J. Verrips C.T. J. Biol. Chem. 1998; 273: 24102-24107Google Scholar).It has been previously shown that the snf3 rgt2 double mutant, which is defective in induction of the HXT gene expression, is also defective in glucose repression of GAL1and SUC2 gene transcription (13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar, 14Schmidt M.C. McCartney R.R. Zhang X. Tillman T.S. Solimeo H. Wölfl S. Almonte C. Watkins S.C. Mol. Cell. Biol. 1999; 19: 4561-4571Google Scholar). This raised the possibility that Snf3 and Rgt2 may be also involved in generation of the glucose repression signal. Alternatively, it was possible that the glucose repression defect of the snf3 rgt2 mutant was due to diminished glucose uptake and metabolism in this mutant. In this report I demonstrate that the glucose repression defect of the snf3 rgt2 mutant is due to the lack of HXT gene expression in this mutant (Fig. 7). Consistent with this idea, a strain deleted for six of the HXT genes (hxt null), unable to grow on glucose due to lack of glucose transport, is also defective in glucose repression GAL1 andSUC2 genes. Furthermore, restoration of the glucose transport defect in the hxt null mutant by overexpression of Hxt1 glucose transporter restores the glucose repression defect. Similarly, overexpression of HXT1 from the ADH1promoter in the snf3 rgt2 double mutant restores the glucose repression but not the glucose induction defect of this mutant.In summary, these data indicate that repression and induction of gene expression by glucose is regulated by two different primary signals. While the glucose repression signal requires the uptake and metabolism of glucose, the glucose induction signal is generated in a receptor-mediated process. Furthermore, the glucose induction pathway is connected to the glucose repression pathway via the expression of glucose transporter (HXT) genes. The snf3 rgt2mutant, unable to induce the expression of the HXT genes is also defective in glucose repression of GAL1 andSUC2 genes.While Snf3 and Rgt2 sense extracellular glucose and generate the signal for induction of HXT gene expression, the nature of the intracellular signal and how it is transmitted from the cytoplasm into the nucleus is unknown. Recent data indicate that two homologues proteins, Mth1 and Std1, may be involved in the transmission of the signal from the cytoplasm into the nucleus. 1Sabire Özcan, unpublished data. Generation of the primary signal for glucose repression requires the uptake and metabolism of glucose; however the protein(s) involved in sensing the glycolytic flux remain to be identified. The protein kinase Snf1 has been shown to be essential for derepression of glucose-repressed genes (6Zimmermann F.K. Kaufmann I. Rasenberger H. Haubetamann P. Mol. Gen. Genet. 1977; 151: 95-103Google Scholar, 7Ciriacy M. Mol. Gen. Genet. 1977; 154: 213-220Google Scholar, 8Carlson M. Osmond B.C. Botstein D. Genetics. 1981; 98: 25-40Google Scholar). In a snf1 mutant, expression of GAL1 andSUC2 genes is repressed even in the absence of glucose. The mammalian homologue of Snf1, the AMP-activated protein kinase (AMPK) has been shown to be regulated by the AMP:ATP ratio (9Hardie D.G. Carling D. Carlson M. Annu. Rev. Biochem. 1998; 67: 821-855Google Scholar, 10Hardie D.G. Hawley S.A. BioEssays. 2001; 23: 1112-1119Google Scholar). Therefore it has been proposed that changes in the AMP:ATP ratio provide the signal for glucose repression (9Hardie D.G. Carling D. Carlson M. Annu. Rev. Biochem. 1998; 67: 821-855Google Scholar). Although it is known that switching yeast cells from high to low glucose causes increases in AMP levels; however, this increase appears to have no effect on Snf1 kinase activity in vitro (11Wilson W.A. Hawley S.A. Hardie D.G. Curr. Biol. 1996; 6: 1426-1434Google Scholar).Transcription of genes coding for lipogenic and glycolytic enzymes in liver and/or adipose tissue is up-regulated by glucose. To generate the signal for this up-regulation, glucose needs to be metabolized. The signal metabolite required for induction of gene expression in the liver appears to be glucose-6 phosphate (20Foufelle F. Girard J. Ferre P. Curr. Opin. Clin. Nutr. Metab. Care. 1998; 1: 323-328Google Scholar, 21Vaulont S. Vasseur-Cognet M. Kahn A. J. Biol. Chem. 2000; 275: 31555-31558Google Scholar). In yeast, repression of transcription by glucose also requires the transport and metabolism of glucose. Although, mutations in Hxk2, the major glucose-phosphorylating enzyme in yeast, lead to defects in glucose repression, recent data indicate that glucose-6 phosphate is not the signal metabolite in yeast. The signal for glucose repression in yeast appears to be upstream of glucose-6-phosphate (5Ye L. Kruckeberg A.L. Berden J.A. Dam K.v. J. Bacteriol. 1999; 181: 4673-4675Google Scholar, 19Meijer M.C. Boonstra J. Verkleij A.J. Verrips C.T. J. Biol. Chem. 1998; 273: 24102-24107Google Scholar). In the yeast Saccharomyces cerevisiae, glucose has two major effects on gene transcription. First it represses expression of genes encoding enzymes for the metabolism of alternate sugars such as galactose and sucrose (1Gancedo J.M. Microbiol. Mol. Biol. Rev. 1998; 62: 334-361Google Scholar, 2Carlson M. Curr. Opin. Microbiol. 1999; 2: 202-207Google Scholar). Second it induces the transcription of genes encoding glycolytic enzymes and glucose transporters required for efficient glucose metabolism (3Johnston M. Trends Genet. 1999; 15: 29-33Google Scholar, 4Özcan S. Johnston M. Microbiol. Mol. Biol. Rev. 1999; 63: 554-569Google Scholar). Although the repressive effect of glucose on gene expression has been known for decades, the primary signal required for glucose repression remains unknown. Recent data indicate that glucose repression requires the uptake and metabolism of glucose (5Ye L. Kruckeberg A.L. Berden J.A. Dam K.v. J. Bacteriol. 1999; 181: 4673-4675Google Scholar, 22Lafuente M.J. Gancedo C. Jauniaux J.C. Gancedo J.M. Mol. Microbiol. 2000; 35: 161-172Google Scholar). Derepression of glucose-repressed genes (such as GAL1 andSUC2) in the absence of high concentrations of glucose requires the protein kinase Snf1 (6Zimmermann F.K. Kaufmann I. Rasenberger H. Haubetamann P. Mol. Gen. Genet. 1977; 151: 95-103Google Scholar, 7Ciriacy M. Mol. Gen. Genet. 1977; 154: 213-220Google Scholar, 8Carlson M. Osmond B.C. Botstein D. Genetics. 1981; 98: 25-40Google Scholar). Based on work carried out with the mammalian homologue of Snf1, the AMP-activated kinase (AMPK), it has been suggested that changes in AMP:ATP ratio generate the signal for glucose repression (9Hardie D.G. Carling D. Carlson M. Annu. Rev. Biochem. 1998; 67: 821-855Google Scholar, 10Hardie D.G. Hawley S.A. BioEssays. 2001; 23: 1112-1119Google Scholar, 11Wilson W.A. Hawley S.A. Hardie D.G. Curr. Biol. 1996; 6: 1426-1434Google Scholar). The signal for glucose induction of hexose transporter (HXT) gene expression is generated by two membrane receptors, Snf3 and Rgt2 (12Özcan S. Dover J. Rosenwald A.G. Wölfl S. Johnston M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 12428-12432Google Scholar, 13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar). Both proteins have similarities to glucose transporters and function as sensors of extracellular glucose. In the snf3 rgt2 double mutant, induction of HXT gene expression by glucose is completely abolished. As a consequence of this defect thesnf3 rgt2 strain is defective for glucose uptake and displays impaired growth on glucose-containing media. Interestingly, the snf3 rgt2 double mutant is also defective for glucose repression of GAL1 and SUC2 genes, indicating the requirement of these sensors for the glucose repression pathway (13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar, 14Schmidt M.C. McCartney R.R. Zhang X. Tillman T.S. Solimeo H. Wölfl S. Almonte C. Watkins S.C. Mol. Cell. Biol. 1999; 19: 4561-4571Google Scholar). This report provides evidence that two different signals are responsible for repression and induction of transcription by glucose in yeast. It demonstrates that the generation of the signal for glucose repression of gene expression requires the uptake and the metabolism of glucose. Furthermore, it shows that the induction of HXTgene expression by glucose is essential for the generation of the glucose repression signal. DISCUSSIONIn the yeast S. cerevisiae, glucose has both positive and negative regulatory effects on gene expression. While expression of the genes encoding glucose transporters such as theHXT genes is 10- to 300-fold induced by glucose, transcription of genes required for metabolism of alternate carbon sources such as GAL1 and SUC2 genes is inhibited by 10- to 1000-fold (1Gancedo J.M. Microbiol. Mol. Biol. Rev. 1998; 62: 334-361Google Scholar, 2Carlson M. Curr. Opin. Microbiol. 1999; 2: 202-207Google Scholar). The signal for induction of HXTgene expression by glucose is generated by two glucose sensors, Snf3 and Rgt2, and does not require the transport and metabolism of glucose (4Özcan S. Johnston M. Microbiol. Mol. Biol. Rev. 1999; 63: 554-569Google Scholar, 12Özcan S. Dover J. Rosenwald A.G. Wölfl S. Johnston M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 12428-12432Google Scholar, 13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar). The nature of the primary signal mediating glucose repression of gene expression is unknown. Recent data indicate that the signal for glucose repression of gene expression is determined by the intracellular concentration of the glucose (5Ye L. Kruckeberg A.L. Berden J.A. Dam K.v. J. Bacteriol. 1999; 181: 4673-4675Google Scholar, 19Meijer M.C. Boonstra J. Verkleij A.J. Verrips C.T. J. Biol. Chem. 1998; 273: 24102-24107Google Scholar).It has been previously shown that the snf3 rgt2 double mutant, which is defective in induction of the HXT gene expression, is also defective in glucose repression of GAL1and SUC2 gene transcription (13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar, 14Schmidt M.C. McCartney R.R. Zhang X. Tillman T.S. Solimeo H. Wölfl S. Almonte C. Watkins S.C. Mol. Cell. Biol. 1999; 19: 4561-4571Google Scholar). This raised the possibility that Snf3 and Rgt2 may be also involved in generation of the glucose repression signal. Alternatively, it was possible that the glucose repression defect of the snf3 rgt2 mutant was due to diminished glucose uptake and metabolism in this mutant. In this report I demonstrate that the glucose repression defect of the snf3 rgt2 mutant is due to the lack of HXT gene expression in this mutant (Fig. 7). Consistent with this idea, a strain deleted for six of the HXT genes (hxt null), unable to grow on glucose due to lack of glucose transport, is also defective in glucose repression GAL1 andSUC2 genes. Furthermore, restoration of the glucose transport defect in the hxt null mutant by overexpression of Hxt1 glucose transporter restores the glucose repression defect. Similarly, overexpression of HXT1 from the ADH1promoter in the snf3 rgt2 double mutant restores the glucose repression but not the glucose induction defect of this mutant.In summary, these data indicate that repression and induction of gene expression by glucose is regulated by two different primary signals. While the glucose repression signal requires the uptake and metabolism of glucose, the glucose induction signal is generated in a receptor-mediated process. Furthermore, the glucose induction pathway is connected to the glucose repression pathway via the expression of glucose transporter (HXT) genes. The snf3 rgt2mutant, unable to induce the expression of the HXT genes is also defective in glucose repression of GAL1 andSUC2 genes.While Snf3 and Rgt2 sense extracellular glucose and generate the signal for induction of HXT gene expression, the nature of the intracellular signal and how it is transmitted from the cytoplasm into the nucleus is unknown. Recent data indicate that two homologues proteins, Mth1 and Std1, may be involved in the transmission of the signal from the cytoplasm into the nucleus. 1Sabire Özcan, unpublished data. Generation of the primary signal for glucose repression requires the uptake and metabolism of glucose; however the protein(s) involved in sensing the glycolytic flux remain to be identified. The protein kinase Snf1 has been shown to be essential for derepression of glucose-repressed genes (6Zimmermann F.K. Kaufmann I. Rasenberger H. Haubetamann P. Mol. Gen. Genet. 1977; 151: 95-103Google Scholar, 7Ciriacy M. Mol. Gen. Genet. 1977; 154: 213-220Google Scholar, 8Carlson M. Osmond B.C. Botstein D. Genetics. 1981; 98: 25-40Google Scholar). In a snf1 mutant, expression of GAL1 andSUC2 genes is repressed even in the absence of glucose. The mammalian homologue of Snf1, the AMP-activated protein kinase (AMPK) has been shown to be regulated by the AMP:ATP ratio (9Hardie D.G. Carling D. Carlson M. Annu. Rev. Biochem. 1998; 67: 821-855Google Scholar, 10Hardie D.G. Hawley S.A. BioEssays. 2001; 23: 1112-1119Google Scholar). Therefore it has been proposed that changes in the AMP:ATP ratio provide the signal for glucose repression (9Hardie D.G. Carling D. Carlson M. Annu. Rev. Biochem. 1998; 67: 821-855Google Scholar). Although it is known that switching yeast cells from high to low glucose causes increases in AMP levels; however, this increase appears to have no effect on Snf1 kinase activity in vitro (11Wilson W.A. Hawley S.A. Hardie D.G. Curr. Biol. 1996; 6: 1426-1434Google Scholar).Transcription of genes coding for lipogenic and glycolytic enzymes in liver and/or adipose tissue is up-regulated by glucose. To generate the signal for this up-regulation, glucose needs to be metabolized. The signal metabolite required for induction of gene expression in the liver appears to be glucose-6 phosphate (20Foufelle F. Girard J. Ferre P. Curr. Opin. Clin. Nutr. Metab. Care. 1998; 1: 323-328Google Scholar, 21Vaulont S. Vasseur-Cognet M. Kahn A. J. Biol. Chem. 2000; 275: 31555-31558Google Scholar). In yeast, repression of transcription by glucose also requires the transport and metabolism of glucose. Although, mutations in Hxk2, the major glucose-phosphorylating enzyme in yeast, lead to defects in glucose repression, recent data indicate that glucose-6 phosphate is not the signal metabolite in yeast. The signal for glucose repression in yeast appears to be upstream of glucose-6-phosphate (5Ye L. Kruckeberg A.L. Berden J.A. Dam K.v. J. Bacteriol. 1999; 181: 4673-4675Google Scholar, 19Meijer M.C. Boonstra J. Verkleij A.J. Verrips C.T. J. Biol. Chem. 1998; 273: 24102-24107Google Scholar). In the yeast S. cerevisiae, glucose has both positive and negative regulatory effects on gene expression. While expression of the genes encoding glucose transporters such as theHXT genes is 10- to 300-fold induced by glucose, transcription of genes required for metabolism of alternate carbon sources such as GAL1 and SUC2 genes is inhibited by 10- to 1000-fold (1Gancedo J.M. Microbiol. Mol. Biol. Rev. 1998; 62: 334-361Google Scholar, 2Carlson M. Curr. Opin. Microbiol. 1999; 2: 202-207Google Scholar). The signal for induction of HXTgene expression by glucose is generated by two glucose sensors, Snf3 and Rgt2, and does not require the transport and metabolism of glucose (4Özcan S. Johnston M. Microbiol. Mol. Biol. Rev. 1999; 63: 554-569Google Scholar, 12Özcan S. Dover J. Rosenwald A.G. Wölfl S. Johnston M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 12428-12432Google Scholar, 13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar). The nature of the primary signal mediating glucose repression of gene expression is unknown. Recent data indicate that the signal for glucose repression of gene expression is determined by the intracellular concentration of the glucose (5Ye L. Kruckeberg A.L. Berden J.A. Dam K.v. J. Bacteriol. 1999; 181: 4673-4675Google Scholar, 19Meijer M.C. Boonstra J. Verkleij A.J. Verrips C.T. J. Biol. Chem. 1998; 273: 24102-24107Google Scholar). It has been previously shown that the snf3 rgt2 double mutant, which is defective in induction of the HXT gene expression, is also defective in glucose repression of GAL1and SUC2 gene transcription (13Özcan S. Dover J. Johnston M. EMBO J. 1998; 17: 2566-2573Google Scholar, 14Schmidt M.C. McCartney R.R. Zhang X. Tillman T.S. Solimeo H. Wölfl S. Almonte C. Watkins S.C. Mol. Cell. Biol. 1999; 19: 4561-4571Google Scholar). This raised the possibility that Snf3 and Rgt2 may be also involved in generation of the glucose repression signal. Alternatively, it was possible that the glucose repression defect of the snf3 rgt2 mutant was due to diminished glucose uptake and metabolism in this mutant. In this report I demonstrate that the glucose repression defect of the snf3 rgt2 mutant is due to the lack of HXT gene expression in this mutant (Fig. 7). Consistent with this idea, a strain deleted for six of the HXT genes (hxt null), unable to grow on glucose due to lack of glucose transport, is also defective in glucose repression GAL1 andSUC2 genes. Furthermore, restoration of the glucose transport defect in the hxt null mutant by overexpression of Hxt1 glucose transporter restores the glucose repression defect. Similarly, overexpression of HXT1 from the ADH1promoter in the snf3 rgt2 double mutant restores the glucose repression but not the glucose induction defect of this mutant. In summary, these data indicate that repression and induction of gene expression by glucose is regulated by two different primary signals. While the glucose repression signal requires the uptake and metabolism of glucose, the glucose induction signal is generated in a receptor-mediated process. Furthermore, the glucose induction pathway is connected to the glucose repression pathway via the expression of glucose transporter (HXT) genes. The snf3 rgt2mutant, unable to induce the expression of the HXT genes is also defective in glucose repression of GAL1 andSUC2 genes. While Snf3 and Rgt2 sense extracellular glucose and generate the signal for induction of HXT gene expression, the nature of the intracellular signal and how it is transmitted from the cytoplasm into the nucleus is unknown. Recent data indicate that two homologues proteins, Mth1 and Std1, may be involved in the transmission of the signal from the cytoplasm into the nucleus. 1Sabire Özcan, unpublished data. Generation of the primary signal for glucose repression requires the uptake and metabolism of glucose; however the protein(s) involved in sensing the glycolytic flux remain to be identified. The protein kinase Snf1 has been shown to be essential for derepression of glucose-repressed genes (6Zimmermann F.K. Kaufmann I. Rasenberger H. Haubetamann P. Mol. Gen. Genet. 1977; 151: 95-103Google Scholar, 7Ciriacy M. Mol. Gen. Genet. 1977; 154: 213-220Google Scholar, 8Carlson M. Osmond B.C. Botstein D. Genetics. 1981; 98: 25-40Google Scholar). In a snf1 mutant, expression of GAL1 andSUC2 genes is repressed even in the absence of glucose. The mammalian homologue of Snf1, the AMP-activated protein kinase (AMPK) has been shown to be regulated by the AMP:ATP ratio (9Hardie D.G. Carling D. Carlson M. Annu. Rev. Biochem. 1998; 67: 821-855Google Scholar, 10Hardie D.G. Hawley S.A. BioEssays. 2001; 23: 1112-1119Google Scholar). Therefore it has been proposed that changes in the AMP:ATP ratio provide the signal for glucose repression (9Hardie D.G. Carling D. Carlson M. Annu. Rev. Biochem. 1998; 67: 821-855Google Scholar). Although it is known that switching yeast cells from high to low glucose causes increases in AMP levels; however, this increase appears to have no effect on Snf1 kinase activity in vitro (11Wilson W.A. Hawley S.A. Hardie D.G. Curr. Biol. 1996; 6: 1426-1434Google Scholar). Transcription of genes coding for lipogenic and glycolytic enzymes in liver and/or adipose tissue is up-regulated by glucose. To generate the signal for this up-regulation, glucose needs to be metabolized. The signal metabolite required for induction of gene expression in the liver appears to be glucose-6 phosphate (20Foufelle F. Girard J. Ferre P. Curr. Opin. Clin. Nutr. Metab. Care. 1998; 1: 323-328Google Scholar, 21Vaulont S. Vasseur-Cognet M. Kahn A. J. Biol. Chem. 2000; 275: 31555-31558Google Scholar). In yeast, repression of transcription by glucose also requires the transport and metabolism of glucose. Although, mutations in Hxk2, the major glucose-phosphorylating enzyme in yeast, lead to defects in glucose repression, recent data indicate that glucose-6 phosphate is not the signal metabolite in yeast. The signal for glucose repression in yeast appears to be upstream of glucose-6-phosphate (5Ye L. Kruckeberg A.L. Berden J.A. Dam K.v. J. Bacteriol. 1999; 181: 4673-4675Google Scholar, 19Meijer M.C. Boonstra J. Verkleij A.J. Verrips C.T. J. Biol. Chem. 1998; 273: 24102-24107Google Scholar). I thank E. Boles for providing thehxt null mutant, Keiko Kurioka for excellent technical assistance, and the reviewer of this manuscript for the useful suggestions.