Glutamine as a feedback inhibitor of nitrogenase in whole cells and cell-free extracts of the photosynthetic bacterium Ṟẖo̲ḏo̲p̲s̲e̲u̲ḏo̲m̲o̲ṉa̲s̲ S̲p̲ẖa̲e̲ṟo̲i̲ḏe̲s̲

This inhibition of nitrogenase was pursued at the cell-free level, where only the hydrogen production assay was used. Cell-free nitrogenase of R. Sphaeroides was found to be like that from other bacteria in that its activity was oxygen labile, cold labile, absolutely dependent on ATP, and unaffected by ammonia, glutamine or glutamate at concentrations up to 10 mM. The nitrogenase activity was unusual in that it pelleted at 250,000 x g in three hours and, unlike that of other photosynthetic bacteria, was unaffected by manganese. A macromolecular inhibitor of nitrogenase activity was found in cell-free extracts, and appeared to be a ubiquitous constituent of the bacterium, irrespective of growth conditions. The inhibitor was oxygen and heat labile. Its inhibition of nitrogenase was concentration-dependent, rather than requiring a fixed ratio of inhibitor to nitrogenase. The addition of this inhibitor to an active nitrogenase preparation permitted a previously unobserved potentiation of inhibition of nitrogenase by 10 mM glutamine, but not by ammonia, glutamate, methionine sulfoximine, or methionine sulfone at the same concentration. As the concentration of inhibitor was increased, the potentiation by glutamine became larger. Both the inhibition and the potentiation by glutamine seemed to occur by interaction with the MoFe protein of nitrogenase. Although the identity of the inhibitor remains unknown, it was found not to be either the FeS protein or the MoFe protein of nitrogenase, or glutamine synthetase. A model is proposed to explain the observed results. bThis data represents the first report of a macromolecular inhibitor of nitrogenase as well as the first observation of inhibition of nitrogenase by glutamine.