EFFECT OF CARBON AND NITROGEN-METABOLISM ON NITRATE REDUCTASE-ACTIVITY OF RHODOBACTER-CAPSULATUS E1F1

The phototrophic bacterium Rhodobacter capsulatus E1F1 possesses an assimilatory, inducible nitrate reductase that is regulated by carbon and nitrogen metabolism. Nitrate reductase activity was detected in cells cultured with amino acids and nitrate as simultaneous nitrogen source but it required an additional carbon source such as D,L-malate. A significant rise in nitrate reductase activity was observed in media with increasing nitrate concentrations up to 10 mM KNO3, although higher nitrate concentrations had an inhibitory effect. Growth yield, generation time, and nitrate reductase activity were also dependent on the concentration of D,L-malate in cells growing with 10 mM nitrate. In carbon-starved cells, nitrate reductase activity dropped even in the presence of nitrate. The intracellular concentration of keto acids such as oxaloacetate or 2-oxoglutarate fluctuated widely depending on the presence of nitrogen and carbon sources in the culture medium. The increase in the intracellular concentration of oxaloacetate or 2-oxoglutarate in R. capsulatus E1F1 correlated well with a rise in nitrate reductase activity. These results suggest that the intracellular carbon-nitrogen balance regulates nitrate uptake in R. capsulatus E1F1, thus affecting the expression of nitrate reductase.