NITROGEN-REGULATED TRANSCRIPTION AND ENZYME-ACTIVITIES IN CONTINUOUS CULTURES OF SACCHAROMYCES-CEREVISIAE

Variations in the transcription of nitrogen-regulated genes and in the activities of nitrogen-regulated enzymes of the yeast Saccharomyces c erevisiae were studied by changing the carbon and nitrogen fluxes. S. cerevisiae was grown in continuous culture at various dilution rates ( D) under nitrogen limitation with NH4Cl as sole nitrogen source. With an increase in D from 0.05 to 0.29 h(-1), both the glucose and the amm onia flux increased sixfold. The activities of the two ammonia-incorpo rating enzymes, NADPH-dependent glutamate dehydrogenase (NADPH-CDH) an d glutamine synthetase (GS), encoded by GDH1 and GLN1, respectively, i ncreased with increasing D, while the activity of the glutamate-degrad ing enzyme, NAD-dependent glutamate dehydrogenase (NAD-GDH), decreased . Surprisingly, no changes were observed in the transcription of GDH1 and GLN1; however increased D was accompanied by an increase in GAP1 t ranscription. At the metabolite level, the increase in the glucose and nitrogen flux did not result in changes in the intracellular 2-oxoglu tarate, glutamate or glutamine concentrations. It is shown that growth on ammonia alone is not sufficient to cause repression of GAP1 and GL N1 transcription and that the regulation of GAP1 transcription and bot h NADPH-GDH and GS activity is not an on/off switch, but is gradually modulated in correlation with the ammonia concentration.