Ammonia Assimilation by Rhizobium Cultures and Bacteroids

1975 Journal of General Microbiology  
S U M M A R Y The enzymes involved in the assimilation of ammonia by free-living cultures of Rhizobium spp. are glutamine synthetase (EC. 6.3. I . 2), glutamate synthase (Lglutamine : a-oxoglutarate amino transferase) and glutamate dehydrogenase (EC I . 4 . I .4). Under conditions of ammonia or nitrate limitation in a chemostat the assimilation of ammonia by cultures of R. leguminosarum, R. trifolii and R. japonicum proceeded via glutamine synthetase and glutamate synthase. Under glucose
more » ... ion 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 R. leguminosarum, R. melitoti, R. phaseoli and R. trifolii but to NAD for the slowgrowing R. japonicum and R. lupini. Glutamine synthetase, glutamate synthase and glutamate dehydrogenase activities were assayed in sonicated bacteroid preparations and in the nodule supernatants of Glycine max, Vicia faba, Pisum sativum, Lupinus luteus, Medicago sativa, Phaseolus coccineus and P. vulgaris nodules. All bacteroid preparations, except those from M . sativa and P. coccineus, contained glutamate synthase but substantial activities were found only in Glycine max and Lupinus luteus. 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. I N T R O D U C T I O N The assimilation of ammonia by bacteria proceeds via either glutamate dehydrogenase or the glutamine synthetase/glutamate synthase system, depending upon the organism and the ammonia concentration of the environment (Tempest, Meers & Brown, 1973; Brown, Macdonald-Brown & Meers, 1974). Dainty (1972) reported the presence of glutamate synthase in a strain of Clostridium pasteurianum which lacked glutamate dehydrogenase, while Nagatani, Shimizu & Valentine (1971) demonstrated the presence of glutamate synthase in a range of nitrogen-fixing bacteria and in bacteroids of Rhizobium japonicum prepared from Glycine max (soya bean) root nodules. Nagatani et al. (1971) claimed that the glutamine synthetase/glutamate synthase system was operative in ammonia assimilation
doi:10.1099/00221287-86-1-39 pmid:234505 fatcat:zg5yvt3jbzcqjgazhveobyccni