Wild-type Aspergillus nidulans grew equally well on NH,Cl, K N 0 3 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.

~~~l 5N 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, 5N 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-ammoniagrown 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.

In a variety of C3 and C4 photosynthetic tissues it is widely recognized that ammonia assimilation proceeds via 9,28,29 To avoid infestation by fungus, the plants were watered twice (0.75 L per tray) with orthocid 50 (3 g captan/L) and also sprayed with the fungicide about 8 weeks prior to use.CO2 and H20 Gas Exchange Measurements. An open gas exchange system (2) was modified to simultaneously measure 3 samples. The CO2 concentration of the measuring gas was adjusted by mixing CO2 free air with CO2 using mass flow controllers (FC 2600, Tylan GmbH, D-8057 Eching). The gas stream was divided. The reference gas was dried and passed through the reference side of the CO2 gas analyzer (22 L/h). Internodes were enclosed in a tube shaped plexiglass cuvette (length 300 mm, internal diameter 32 mm). Four tubes were embedded in a closed water bath (about 10 mm water layer over the tubes) to control cuvette temperature. Each tube was flushed with 60 L/h of humidifed air (100% RH at 12.5C). The gas flow rates in the individual cuvettes were measured by flowmeters (Platon Flowbits GmbH, D-6903 Neckargemund and Rota D-7012 Oelflingen). Constant pressure conditions enabling constant gas fluxes through the cuvettes were obtained by bubbling excess gas into water (water column about 50 mm) using magnetic three way valves. The gas stream of one cuvette was led every 10 min to the humidity sensor (Vaisala, Driesen und Kern, D-2000 Hamburg-Tangstedt) and dried over CaCI2 and Mg(Cl04)2 before measuring the CO2 concentration with an IR gas analyzer (Unor 2, Maihak, D-2000 Hamburg). The temperature of the plant material was measured using two cooper constantan wires attached to the lower surfaces of two stem sections.The rates of CO2 uptake and evaporation were calculated as described previously (2).Experimental.