Résumé
In order to determine whether the changes in the activities and mRNA levels of enzymes involved in intermediary carbon metabolism previously observed in glucose‐limited continuous cultures (Sierkstra et al., 1992a) were glucose specific, we have analysed their regulation in a galactose‐limited continuous culture of Saccharomyces cerevisiae. The Vmax of the galactose uptake system was shown to be dilution rate (D) dependent, comparable with the high‐affinity glucose uptake. The maximum uptake was observed at D 0·2 h−1 (0·25 mmol min−1 per g) and the minimum uptake (0·1 mmol min−1 per g) at D 0·05 h−1 and 0·3 h−1. The aerobic fermentation of galactose occurred at D 0·275–0·3 h−1 which is identical to the results obtained in glucose‐limited continuous cultures of this strain. Because galactose is not a repressing carbon source, this demonstrates that the Crabtree effect is not mediated by, or in any way related to glucose repression. Moreover, invertase and hexokinase I mRNA levels (both subject to glucose repression at the transcriptional level) were present when the yeast produced ethanol in galactose‐ and glucose‐limited continuous cultures. In glucose‐limited continuous cultures a decrease in alcohol dehydrogenase (I and II) mRNA levels and activity and phosphoglucomutase activity was observed with increasing dilution rates. In addition, at D 0·3 h−1, when the yeast produced ethanol, glucose‐6‐phosphate dehydrogenase and pyruvate decarboxylase were induced and a decrease in respiration was observed. The fact that the same changes were seen in a galactose‐limited culture and that invertase and hexokinase I mRNA levels were present demonstrates that glucose is not specifically involved in the regulation of these enzymes. The changes observed are therefore caused by the growth rate of the organism, the glycolytic flux (e.g. alcohol dehydrogenase) or the overflow metabolism when the yeast produces ethanol (e.g. pyruvate decarboxylase).