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Glucose, the most abundant carbon and energy source, regulates the expression of genes required for its own efficient metabolism. In the yeast Saccharomyces cerevisiae, glucose induces the expression of the hexose transporter (HXT) genes by modulating the activity of the transcription factor Rgt1 that functions as a repressor when glucose is absent. However, in the presence of high concentrations of glucose, Rgt1 is converted from a repressor to an activator and is required for maximal induction of HXT1 gene expression. We report that Rgt1 binds to the HXT1 promoter only in the absence of glucose, suggesting that Rgt1 increases HXT1 gene expression at high levels of glucose by an indirect mechanism. It is likely that Rgt1 stimulates the expression of an activator of theHXT1 gene at high concentrations of glucose. In addition, we demonstrate that Rgt1 becomes hyperphosphorylated in response to high …