The Role of Hexose Transport on Glycolytic Flux and Glucose Induced Responses in Saccharomyces cerevisiae
In the yeast Saccharomyces cerevisiae, glucose is primarily fermented rather than respired. Only in cultivation modes such as chemostats or fed-batches, where the external glucose is kept low, a fully respiratory catabolism is observed. We have constructed a series of yeast strains starting from a mutant (the hxt Null strain) that is unable to take up glucose. In these strains glucose uptake is governed by chimeras between the two hexose transporters Hxt1 and Hxt7. These strains display a broad range of glucose uptake capacities. They have been used to show that glucose uptake has a high degree of control on the glycolytic flux only when the uptake rate is significantly reduced. One of the strains, with a low uptake capacity, presented a completely Crabtree negative phenotype. This shows that a switch between fermentative and respiratory metabolism can be achieved by changing only a single metabolic step. The transferability of the phenotype has been confirmed by integration of the chimeric construct into a hxt1-hxt7 haploid wine strain.
Strains with different glycolytic rates have also allowed quantitative studies on glucose-induced responses and dissection of different signaling mechanisms. A strong correlation between glycolytic rate and expression of glycolytic genes was observed. mRNA profiles of several groups of glucose-regulated genes suggest that sensing is primarily intracellular. Contrary to what is presently believed, SUC2 expression did not always correlate to high phosphorylation levels of Snf1 and Mig1, key regulators of glucose-controlled gene expression. These results suggest regulation either through additional pathway/s or independent regulation of the Snf1/Mig1 targeted kinases and phosphatases.