SUMMARY: The enzymes involved in the assimilation of ammonia by free-living cultures of spp. are glutamine synthetase (EC., glutamate synthase (L-glutamine: 2-oxoglutarate amino transferase) and glutamate dehydrogenase (EC Under conditions of ammonia or nitrate limitation in a chemostat the assimilation of ammonia by cultures of and proceeded via glutamine synthetase and glutamate synthase. Under glucose limitation and with an excess of inorganic nitrogen, ammonia was assimilated via glutamate dehydrogenase, neither glutamine synthetase nor glutamate synthase activities being detected in extracts. The coenzyme specificity of glutamate synthase varied according to species, being linked to NADP for the fast-growing and but to NAD for the slow-growing and

Glutamine synthetase, glutamate synthase and glutamate dehydrogenase activities were assayed in sonicated bacteroid preparations and in the nodule supernatants of and nodules. All bacteroid preparations, except those from and contained glutamate synthase but substantial activities were found only in and The glutamine synthetase activities of bacteroids were low, although high activities were found in all the nodule supernatants. Glutamate dehydrogenase activity was present in all bacteroid samples examined. There was no evidence for the operation of the glutamine synthetase/glutamate synthase system in ammonia assimilation in root nodules, suggesting that ammonia produced by nitrogen fixation in the bacteroid is assimilated by enzymes of the plant system.


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