Glucose repression in Saccharomyces cerevisiae is related to the glucose concentration rather than the glucose flux

Glucose plays an important regulatory role in the yeast Saccharomyces cerevisiae, which is mostly reflected at the transcriptional level by glucose regression. The signal that initiates glucose repression is unknown, but data indicate that it is located at or above the level of glucose 6-phosphate, suggesting the involvement of either the intracellular or extracellular glucose concentration or the glucose flux in triggering glucose repression. We have investigated the role of the glucose flux and the extracellular glucose concentration in glucose repression by growing the cells in continuous culture under nitrogen limitation. By a step-wise increase ire the glucose feed concentration, the glucose flux and extracellular glucose concentrations were modulated in an accurate way. Furthermore, the glucose flux and glucose concentrations were modulated undependently of each other by increasing the dilution rate or by the use of fructose as a substrate. Using these approaches we demonstrate that glucose repression is related to the extracellular (or intracellular) glucose concentration rather than the glucose flux, At external glucose concentrations Bower than 14 mM, glucose repression of SUC2 gene transcription was not triggered,, whereas glucose repression of this gene was activated when the glucose concentration exceeded 18 mM. A comparable effect was observed for the glucose-repressible carbon source fructose.