The involvement of glutamate dehydrogenase and glutamine synthetase in ammonia assimilation by the rapidly growing ectomycorrhizal ascomycete, Cenococcum geophilum Fr.

summary In order to determine significance of the glutamate dehydrogenase pathway and a glutamine synthetase/glutamate synthase cycle in NH4+ assimilation, we followed a number of different metabolikc parameters (nutrient uptake, free amino acid pools, NH4+‐induced glutamine accumulation) and 15N incorporation into amino acids of rapidly growing Cennoccum geophilum. Arginine was a major free amino acid in C. geophilum during its entire growth period. C. geophilum synthesized and accumulated very large amounts of glutamine at the beginning of the rapid phase of growth in low nitrogen medium, during the whole growth period in high nitrogen medium, and immediately after addition of NH4+. Therefore, the accumulation of a large amount of glutamine tool; place when the external ammonium concentration was high. The current data identify four pathways of N metabolism in rapidly growing C. geophilum: (1) glutamine synthesis, invoking transfer of N to both amino and amino moieties; (2) glutamate formation; (3) transamination with pyruvate to yield alanine; (4) transamination with oxaloacetate to yield asparate. The higher accumulation of glutamate and related amino acids (alanine and aspartate) in the presence of the glutamine synthetase inhibitor methionine sulphoximine indicates that glutamate, the precursor of glutamine, was formed by a pathway insensitive to methionine sulphoximine, the glutamate dehydrogenase pathway. Up to 40% of the assimilated 15N terminated in the amido‐N of glutamine. These data are consistent with a pivotal role for glutamine synthetase activity and indicates that the primary assimilation of NH4+ in rapidly growing C. geophilum is brought about by concurrent activity of the GDH and GS pathways. The pathway of primary assimilation of NH4+ by C. geophilunt in the rapid phase of growth therefore differs from those operating in the stationary phase of growth where N flax through GDH is higher than the flux through the GS pathway.