title: Ammonium assimilation in Rhizobium meliloti.
abstract: We have characterized a mutant of Rhizobium meliloti strain 2011 which cannot use ammonium as a nitrogen source. This mutant, RTm2620, was found to have significantly altered glutamate synthase activity. Both the mutant and the wild-type strains had glutamate dehydrogenase activity, which, although stimulated in the presence of glutamate and ammonium, was apparently insufficient to allow ammonium assimmilation. We conclude that the glutamine synthetase-glutamate synthase pathway may be the normal mode of ammonium assimilation by this strain in the free-living state. Independent revertants of Rm2620 were isolated and fell into two classes. Class I revertants regained partial glutamate synthase activity and had the same levels of glutamate dehydrogenase activity as Rm2620. Class II revertants retained the altered glutamate synthase activity but acquired a very high level of assimilatory glutamate dehydrogenase activity. Both classes were found to be altered in their symbiotic properties, although the original Rm2620 mutant was normal in this regard.
mesh_terms: D005969:Glutamate Dehydrogenase / Q000378:metabolism*; D005970:Glutamate Synthase / Q000378:metabolism*; D005974:Glutamate-Ammonia Ligase / Q000378:metabolism*; D009154:Mutation; D009586:Nitrogen Fixation; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D012231:Rhizobium / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D000637:Transaminases / Q000378:metabolism*
chemical_list: D000644:Quaternary Ammonium Compounds; D005970:Glutamate Synthase; D005969:Glutamate Dehydrogenase; D000637:Transaminases; D005974:Glutamate-Ammonia Ligase