The assimilation of ammonia by a wide variety of organisms is the primary route for the introduction of nitrogen into the biosphere. The assimilatory enzymes glutamine synthetase and glutamate synthase catalyze reactions that convert α-ketoglutarate and ammonia to glutamate, which is then used in a wide variety of biosynthetic reactions. These enzymes also play a major role in the reassimilation of ammonia derived from photorespiration in C3 plants. Recent biochemical, molecular and genetic studies are leading to a better understanding of the factors that determine the activity and function of glutamate synthase.
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Acknowledgements We wish to thank many colleaguesf or providing reprints and communicating recent results. The work was supported by National ScienceF oundationG rant No. IBN-9206890.T his paper is a joint contribution from the Plant SciencesR esearchU nit. United StatesD ept of Agriculture, Agricultural ResearchS ervice, and the Minnesota Agriculture Experiment Station (Paper No. 97 1130039.S cientific Journal Series).
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