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 un known, but data indicate that it is located at or above the level of g lucose 6-phosphate, suggesting the involvement of either the intracell ular or extracellular glucose concentration or the glucose flux in tri ggering glucose repression. We have investigated the role of the gluco se flux and the extracellular glucose concentration in glucose repress ion by growing the cells in continuous culture under nitrogen limitati on. By a step-wise increase ire the glucose feed concentration, the gl ucose flux and extracellular glucose concentrations were modulated in an accurate way. Furthermore, the glucose flux and glucose concentrati ons were modulated undependently of each other by increasing the dilut ion rate or by the use of fructose as a substrate. Using these approac hes we demonstrate that glucose repression is related to the extracell ular (or intracellular) glucose concentration rather than the glucose flux, At external glucose concentrations Bower than 14 mM, glucose rep ression 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-repre ssible carbon source fructose.