Ammonia assimilation by Aspergillus nidulans: [15N]ammonia study.

15N kinetic labelling studies were done on liquid cultures of wild-type Aspergillus nidulans. The labelling pattern of major amino acids under 'steady state' conditions suggests that glutamate and glutamine-amide are the early products of ammonia assimilation in A. nidulans. In the presence of phosphinothricin, an inhibitor or glutamine synthetase, 15N labelling of glutamate, alanine and aspartate was maintained whereas the labelling of glutamine was low. This pattern of labelling is consistent with ammonia assimilation into glutamate via the glutamate dehydrogenase pathway. In the presence of azaserine, an inhibitor of glutamate synthase, glutamate was initially more highly labelled than any other amino acid, whereas its concentration declined. Isotope also accumulated in glutamine. Observations with these two inhibitors suggest that ammonia assimilation can occur concurrently via the glutamine synthetase/glutamate synthase and the glutamate dehydrogenase pathways in low-ammonia-grown A. nidulans. From a simple model it was estimated that about half of the glutamate was synthesized via the glutamate dehydrogenase pathway; the other half was formed from glutamine via the glutamate synthase pathway. The transfer coefficients of nine other amino acids were also determined.

The involvement of glutamine synthetase/glutamate synthase in ammonia assimilation by Aspergillus nidulans.

Wild-type Aspergillus nidulans grew equally well on NH4Cl, KNO3 or glutamine as the only nitrogen source. NADP+-dependent glutamate dehydrogenase (EC 1.4.1.4) and glutamine synthetase (GS; EC 6.3.1.2) activities varied with the type and concentration of nitrogen source supplied. Glutamate synthase (GOGAT) activity (EC 1.4.7.1) was detected but it was almost unaffected by the type and concentration of nitrogen source supplied. Ion exchange chromatography showed that the GOGAT activity was due to a distinct enzyme. Azaserine, an inhibitor of the GOGAT reaction, reduced the glutamate pool by 60%, indicating that GOGAT is involved in ammonia assimilation by metabolizing the glutamine formed by GS.