Abstract
Glucose plays an important regulatory role in the yeast Saccharomyces cerevisiae, which is mostly reflected at the transcriptional level by glucose repression. 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 in 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 independently 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 lower 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.
Highlights
In addition to its function as a nutrient, glucose plays an important regulatory role in the metabolism of the yeast Saccharomyces cerevisiae
Culturing S. cerevisiae in a nitrogen-limited continuous culture in the presence of various concentrations of glucose in the feed medium allows the possibility of modulating the glucose concentration in the medium and, the glucose flux without changing the growth rate
Glucose regulates carbon metabolism in S. cerevisiae by transcription inhibition of a number of genes involved in the utilization of carbon sources other than glucose
Summary
In addition to its function as a nutrient, glucose plays an important regulatory role in the metabolism of the yeast Saccharomyces cerevisiae. In agreement with these data Schaaff et al [17] showed that various intermediates of glycolysis do not contribute to the repression phenomenon These observations suggest that the following parameters may play a role in the generation of the initial signal for glucose repression: 1) an increase in glucose concentration, either extracellular or intracellular, or 2) an increase in glucose flux over the plasma membrane. Yeast cells were grown under continuous culture conditions in a chemostat, which generated a physiologically stable and well defined environment Using nitrogen limitation, both glucose flux and extracellular glucose concentration could be modulated by cultivating the cells at different but constant glucose concentrations at different but fixed growth rates. The glucose concentration is most likely to be involved in the activation of glucose repression, whereas the glucose flux is not important in this respect
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