RGT1P OF SACCHAROMYCES-CEREVISIAE, A KEY REGULATOR OF GLUCOSE-INDUCEDGENES, IS BOTH AN ACTIVATOR AND A REPRESSOR OF TRANSCRIPTION

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 f unction 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 gl ucose: (i) in the absence of glucose, it functions as a transcriptiona l repressor; (ii) high concentrations of glucose cause it to function as a transcriptional activator; and (iii) in cells growing on low leve ls of glucose, Rgt1p has a neutral role, neither repressing nor activa ting transcription, Glucose alters Rgt1p function through a pathway th at includes two glucose sensors, Snf3p and Rgt2p, and Grr1p, The gluco se transporter Snf3p, which appears to be a low-glucose sensor, is req uired for inhibition of Rgt1p repressor function by low levels of gluc ose, Rgt2p, a glucose transporter that functions as a high-glucose sen sor, is required for conversion of Rgt1p into an activator by high lev els of glucose, Grr1p, a component of the glucose signaling pathway, i s required both for inactivation of Rgt1p repressor function by low le vels of glucose and for conversion of Rgt1p into an activator at high levels of glucose, Thus, signals generated by two different glucose se nsors act through Grr1p to determine Rgt1p function.