Variations in the transcription of nitrogen-regulated genes and in the activities of nitrogen-regulated enzymes of the yeast were studied by changing the carbon and nitrogen fluxes. was grown in continuous culture at various dilution rates () under nitrogen limitation with NHCl as sole nitrogen source. With an increase in from 0.05 to 0.29 h, both the glucose and the ammonia flux increased sixfold. The activities of the two ammonia-incorporating enzymes, NADPH-dependent glutamate dehydrogenase (NADPH-GDH) and glutamine synthetase (GS), encoded by and , respectively, increased with increasing , while the activity of the glutamate-degrading enzyme, NAD-dependent glutamate dehydrogenase (NAD-GDH), decreased. Surprisingly, no changes were observed in the transcription of and ; however increased was accompanied by an increase in transcription. At the metabolite level, the increase in the glucose and nitrogen flux did not result in changes in the intracellular 2-oxoglutarate, glutamate or glutamine concentrations. It is shown that growth on ammonia alone is not sufficient to cause repression of and transcription and that the regulation of transcription and both NADPH-GDH and GS activity is not an on/off switch, but is gradually modulated in correlation with the ammonia concentration.


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