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The involvement of glutamate dehydrogenase and glutamine synthetase in ammonia assimilation by the rapidly growing ectomycorrhizal ascomycete, Cenococcum geophilum Fr.
Author(s) -
MARTIN F.,
STEWART G. R.,
GENETET I.,
MOUROT B.
Publication year - 1988
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/j.1469-8137.1988.tb00294.x
Subject(s) - glutamate synthase , glutamine synthetase , glutamate dehydrogenase , glutamine , biochemistry , transamination , nitrogen assimilation , amino acid , biology , amino acid synthesis , alanine , enzyme , glutamate receptor , lysine , receptor
summary In order to determine significance of the glutamate dehydrogenase pathway and a glutamine synthetase/glutamate synthase cycle in NH 4 + assimilation, we followed a number of different metabolikc parameters (nutrient uptake, free amino acid pools, NH 4 + ‐induced glutamine accumulation) and 15 N 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 NH 4 + . 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 15 N 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 NH 4 + in rapidly growing C. geophilum is brought about by concurrent activity of the GDH and GS pathways. The pathway of primary assimilation of NH 4 + 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.