Three Different Regulatory Mechanisms Enable Yeast Hexose Transporter (HXT) Genes To Be Induced by Different Levels of Glucose

Abstract

The HXT genes (HXT1 to HXT4) of the yeast Saccharomyces cerevisiae encode hexose transporters. We found that transcription of these genes is induced 10-to 300-fold by glucose. Analysis of glucose induction of HXT gene expression revealed three types of regulation: (i) induction by glucose independent of sugar concentration (HXT3); (ii) induction by low levels of glucose and repression at high glucose concentrations (HXT2 and HXT4); and (iii) induction only at high glucose concentrations (HXT1). The lack of expression of all four HXT genes in the absence of glucose is due to a repression mechanism that requires Rgt1p and Ssn6p. GRR1 seems to encode a positive regulator of HXT expression, since grr1 mutants are defective in glucose induction of all four HXT genes. Mutations in RGT1 suppress the defect in HXT expression caused by grr1 mutations, leading us to propose that glucose induces HXT expression by activating Grr1p, which inhibits the function of the Rgt1p repressor. HXT1 expression is also induced by high glucose levels through another regulatory mechanism: rgt1 mutants still require high levels of glucose for maximal induction of HXT1 expression. The lack of induction of HXT2 and HXT4 expression on high levels of glucose is due to glucose repression: these genes become induced at high glucose concentrations in glucose repression mutants (hxk2, reg1, ssn6, tup1, or mig1). Components of the glucose repression pathway (Hxk2p and Reg1p) are also required for generation of the high-level glucose induction signal for expression of the HXT1 gene. Thus, the glucose repression and glucose induction mechanisms share some of the same components and may share the same primary signal generated from glucose.

Signalling pathways leading to transcriptional regulation of genes involved in the activation of glycolysis in yeast

“…This result suggests that a signalling pathway independent of sugar phosphorylation mediates the initial stages of the transcriptional response of the FBP1 gene to high glucose concentrations. The involvement of different signalling pathways in the responses to low and high glucose concentrations has been proposed previously for the transcriptional regulation of hexose transporter (HXT ) genes (Ö zcan and Johnston, 1995).…”

“…Evidence has been found for the involvement of multiple signalling pathways in sensing glucose (Beullens et al, 1988;Ö zcan and Johnston, 1995;Griffioen et al, 1996;Pernambuco et al, 1996;Yin et al, 1996). Among these, the main glucose repression pathway and the Ras-adenylate cyclase pathway have been characterized in some detail (Thevelein, 1994, and references therein).…”

Carbohydrate utilization and the lager yeast transcriptome during brewery fermentation

“…The results of the current study are generally consistent with the known properties of the genes, in that carriers with high or moderate affinity for glucose (HXT2, HXT6, HXT7 ) are repressed when glucose levels are high and induced after glucose depletion. HXT1, which encodes a lowaffinity glucose transporter, is known to be induced under high glucose conditions, while HXT3 is only weakly regulated by glucose concentration (Ozcan and Johnston, 1995). The gene encoding Hxt5p (HXT5 ) is known to be induced in response to non-fermentable carbon sources and a drop in growth rate (Diderich et al, 2001;Verwaal et al, 2002), as well as being STRE-regulated (Verwaal et al, 2004), which would explain the increased transcription after the 30 h sampling time.…”

Nob1p is required for biogenesis of the 26S proteasome and degraded upon its maturation in Saccharomyces cerevisiae

“…Only those plasmids that reproducibly gave rise to a positive interaction with Nob1p bait protein were studied further. ␤-galactosidase activity was measured using permeabilized mid-log phase cells as an enzyme source (Ozcan and Johnston 1995).…”