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

Sabire Özcan

doi:10.1074/jbc.m208726200

Abstract

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.

Highlights

In the yeast Saccharomyces cerevisiae, glucose has two major effects on gene transcription
The hxt Null Mutant, Defective in Glucose Uptake, Is Defective in Glucose Repression of Gene Expression—The snf3 rgt2 double mutant is defective in glucose induction of hexose transporter (HXT) gene expression and in glucose repression of GAL1 and SUC2 genes
Since the snf3 rgt2 double mutant grows very slowly on glucose, it is likely that the observed defect in glucose repression in this mutant is due to reduced glucose uptake and metabolism

Summary

EXPERIMENTAL PROCEDURES

Strains and Media—The yeast S. cerevisiae strains used in this study including the hxt null strain (RE700A) deleted for six of the HXT genes, have been described previously [12, 15]. The amplified PCR fragment was co-transformed with the pSF11 and pSO432 plasmids (cut within the URA3 gene with StuI) into a wild type strain. Transformants that are G418-resistant and unable to grow on media lacking uracil were selected and used to rescue the corresponding gap-repaired plasmids. To assays the induction of HXT1::lacZ and HXT2::lacZ gene expression, cells were pregrown on YNB containing 5% glycerol plus 0.5% galactose lacking uracil and transferred to YNB medium containing 2% glucose, 5% glycerol, plus 0.5% galactose or 5% glycerol, plus 0.2% glucose and incubated overnight before ␤-galactosidase activity was assayed. The expression of the GAL1::lacZ and SUC2::lacZ constructs was assayed by pregrowing cells on YNB with 2% glucose lacking uracil. The day cells containing GAL1::lacZ were transferred to YNB media containing either 2% galactose or 2% glucose plus 2% galactose.

RESULTS

DISCUSSION

Sabire Özcan