Physiological state of the glucose-limited chemostat corresponds to early diauxic shift.
Matthew J. Brauer, Alok J. Saldanha, David Botstein
Inst. for Integrative Genomics, Princeton University, Washington Rd., Princeton, NJ, 08544, USA
We studied the physiological response to limitation by glucose in batch and steady-state (chemostat) cultures of Saccharomyces cerevisiae, by following global patterns of gene expression. Unlike phosphate or sulfate limited cultures, standard glucose-limited batch cultures of yeast go through two sequential exponential growth phases beginning with a first, largely fermentative phase, followed by an essentially completely aerobic use of residual glucose and evolved ethanol (the 'diauxic shift'). We found that the patterns of gene expression indicate close correspondence between the state of the cells in the chemostat and the state of cells just before they undergo a transition from fermentation to respiration. This result was found at growth regimes in the chemostat corresponding to dilution rates differing by a factor of 5; in both cases the residual glucose similarly low. In neither case did the chemostat show purely aerobic growth, nor was there a full stress response. We conclude that despite the theoretical possibility of a complete switch to aerobic metabolism of glucose under conditions of very great glucose scarcity, the mechanisms controlling glucose metabolism appear to regulate growth as if fermentation of the glucose is the only option. As we found with other limitations, yeast growing in steady state in glucose-imited chemostats are 'poor', not 'starving', and the switch to aerobic metabolism in batch cultures must be triggered by even greater scarcity of carbon.
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