Glutamine as a feedback inhibitor of the Rhodopseudomonas sphaeroides nitrogenase system

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Abstract
In whole cells of R. sphaeroides, N2 fixation, as measured by H2 production and acetylene reduction, was totally inhibited by micromolar concentrations of NH3. This inhibition could not be duplicated by glutamate or glutamine alone. The inhibition by NH3 was abolished by methionine sulfoximine, a glutamine synthetase inhibitor. Inhibition by glutamine was complete in the presence of methionine sulfone, a preferential inhibitor of glutamate synthase, presumably by permitting a rise in the glutamine pool. The level of the glutamine pool controlled the activity of nitrogenase. None of these effects could be duplicated with cell-free nitrogenase, indicating there is probably a mediator which responds to the glutamine pool and inhibits nitrogenase, rather than glutamine itself being a direct inhibitor.