Rgt1p of Saccharomyces cerevisiae, a Key Regulator of Glucose-Induced Genes, Is both an Activator and a Repressor of Transcription

Abstract

The RGT1 gene of Saccharomyces cerevisiae plays a central role in the glucose-induced expression of hexose transporter (HXT) genes. Genetic evidence suggests that it encodes a repressor of the HXT genes whose function is inhibited by glucose. Here, we report the isolation of RGT1 and demonstrate that it encodes a bifunctional transcription factor. Rgt1p displays three different transcriptional modes in response to glucose: (i) in the absence of glucose, it functions as a transcriptional repressor; (ii) high concentrations of glucose cause it to function as a transcriptional activator; and (iii) in cells growing on low levels of glucose, Rgt1p has a neutral role, neither repressing nor activating transcription. Glucose alters Rgt1p function through a pathway that includes two glucose sensors, Snf3p and Rgt2p, and Grr1p. The glucose transporter Snf3p, which appears to be a low-glucose sensor, is required for inhibition of Rgt1p repressor function by low levels of glucose. Rgt2p, a glucose transporter that functions as a high-glucose sensor, is required for conversion of Rgt1p into an activator by high levels of glucose. Grr1p, a component of the glucose signaling pathway, is required both for inactivation of Rgt1p repressor function by low levels of glucose and for conversion of Rgt1p into an activator at high levels of glucose. Thus, signals generated by two different glucose sensors act through Grr1p to determine Rgt1p function.Glucose is the preferred carbon and energy source for the yeast Saccharomyces cerevisiae, as well as for most mammalian cells, and has major and wide-ranging effects on gene expression. Glucose represses expression of many genes that are unnecessary for its metabolism and induces expression of many other genes required for its utilization. Among the genes whose expression is induced by glucose are several HXT genes encoding glucose transporters (5,30,31,47). Three HXT genes respond differently to different levels of glucose: HXT1 expression is induced only by high levels of glucose, whereas HXT2 and HXT4 are induced only by low concentrations of glucose.We have previously shown that glucose induction of HXT expression is due to a repression mechanism mediated by Rgt1p (31). In cells growing in the absence of glucose, Rgt1p represses HXT expression; addition of glucose causes Rgt1p function to be inhibited, leading to derepression of HXT expression. The different responses of HXT1 and HXT2 (and HXT4) to different levels of glucose are due to two additional regulatory mechanisms that act on these genes: HXT2 and HXT4 are also subject to glucose repression mediated by Mig1p, a repressor that is activated by high levels of glucose (32, 47), and HXT1 responds to an additional regulatory mechanism, whose components have not yet been identified, that requires high levels of glucose for function (31).Glucose-stimulated inhibition of Rgt1p requires Grr1p, a leucine-rich repeat-containing protein, and Snf3p, a glucose transporter thought to be involved in sensing glucose and generating an intracell...

Glucose-mediated Phosphorylation Converts the Transcription Factor Rgt1 from a Repressor to an Activator

“…We report that the requirement of Rgt1 for activation of HXT1 gene expression in response to high concentrations of glucose is mediated by an indirect mechanism, because Rgt1 is unable to bind to the HXT1 gene promoter in vivo when high levels of glucose are present. Previous data indicated that the transcription of Rgt1 is not regulated by glucose, suggesting the idea that the transcriptional activity of Rgt1 is regulated post-translationally (12). We demonstrate that Rgt1 is hyperphosphorylated in response to high concentrations of glucose and that the lack of Rgt1 phosphorylation abolishes its ability to activate transcription.…”

“…It was previously shown that Rgt1 is required for both repression and activation of HXT1 gene expression and binds to the HXT1 promoter in vitro by gel shift assays (12). We report that the requirement of Rgt1 for activation of HXT1 gene expression in response to high concentrations of glucose is mediated by an indirect mechanism, because Rgt1 is unable to bind to the HXT1 gene promoter in vivo when high levels of glucose are present.…”

“…Construction of Strains and Plasmids-The yeast S. cerevisiae strains used in this study are the wild type strain YM4127, rgt1 (YM4509), snf3 rgt2 double mutant (YM6107), and the grr1 mutant (YM4576), which have been described previously (5,7,12). Yeast cells were grown on YNB (0.67% yeast nitrogen base (Difco) plus 0.5% ammonium sulfate) medium lacking the appropriate amino acids.…”

“…The target of the glucose induction signal is the Cys 6 DNAbinding protein Rgt1, which belongs to the family of the Gal4 transcription factors (12). In the absence of glucose, Rgt1 represses HXT gene expression, whereas at high concentrations of glucose Rgt1 is required for maximal activation of HXT1 gene expression.…”

“…Repression of transcription by Rgt1 in the absence of glucose requires the general repressor complex, Ssn6 and Tup1. Activation of transcription by Rgt1 in response to high concentrations of glucose requires the glucose sensors Snf3 and Rgt2 and the ubiquitin ligase Grr1 (12).…”