THE CONCENTRATION OF AMMONIA REGULATES NITROGEN-METABOLISM IN SACCHAROMYCES-CEREVISIAE

Saccharomyces cerevisiae was grown in a continuous culture at a single dilution rate with input ammonia concentrations whose effects ranged from nitrogen limitation to nitrogen excess and glucose limitation. Th e rate of ammonia assimilation (in millimoles per gram of cells per ho ur) was approximately constant. Increased extracellular ammonia concen trations are correlated with increased intracellular glutamate and glu tamine concentrations, increases in levels of NAD-dependent glutamate dehydrogenase activity and its mRNA (gene GDH2), and decreases in leve ls of NADPH-dependent glutamate dehydrogenase activity and its mRNA (g ene GDH1), as well as decreases in the levels of mRNA for the amino ac id permease-encoding genes GAP1 and PUT4. The governing factor of nitr ogen metabolism might be the concentration of ammonia rather than its flux.