The enzymes involved in ammonia assimilation by Rhizobium meliloti 4l and their role in the regulation of nitrogen metabolism were studied. Glutamine synthetase (GS) and glutamate synthase (GOGAT) were present at relatively high levels in cells grown in media containing either low or high concentrations of ammonia. NADP-linked glutamate dehydrogenase could not be detected. GOGAT and GS mutants were isolated and characterised. A mutant lacking GOGAT activity did not grow even on high concentrations of ammonia, it was a glutamate auxotroph and was effective in symbiotic nitrogen fixation. The GS and assimilatory nitrate reductase activities of this mutant were not repressible by ammonia but still repressible by casamino acids. A mutant with low GS activity required glutamine for optimal growth. It was ineffective and its nitrate reductase was not inducible. These findings indicate that ammonia is assimilated via the GS/GOGAT pathway in free-living R. meliloti and bacterial GOGAT is not important in symbiosis. Furthermore, GS is suggested to be a controlling element in the nitrogen metabolism of R. meliloti.
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