Two different signals regulate repression and induction of gene expression by glucose.

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 the HXT 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.