Short‐Term Regulation of the Nitrogenase Activity in Rhodopseudomonas sphaeroides

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Abstract
The nitrogenase activity in whole cells of R. sphaeroides could be inhibited by lowering the electrical potential across the cytoplasmic membrane. The membrane potential was partly dissipated by lowering the light intensity or by the addition of a lipophilic cation, tetraphenylphosphonium. Under these circumstances, it was shown that the intracellular ATP/ADP ratio was not affected and that the inhibition of the whole cell nitrogenase activity was not due to an inactivation of the nitrogenase enzyme. Electron transport to nitrogenase in R. sphaeroides is apparently dependent on a high membrane potential. The nitrogenase enzyme in whole cells could be inactivated by lowering the membrane potential across the cytoplasmic membrane by incubating the cells in the dark or in the light in the presence of uncouplers. Nitrogenase could be reactivated in the light in the absence of uncouplers. Some possible mechanisms of action of NH4+ inhibition of whole cell nitrogenase activity could be excluded. Inhibition by NH4Cl of whole cell nitrogenase activity in R. sphaeroides could not be explained by a rapid inactivation of the nitrogenase enzyme or by an effect on the intracellular ATP/ADP ratio or the membrane potential. NH4+ inhibits whole cell nitrogenase activity not directly but probably after being assimilated by glutamine synthetase. The role of glutamine, glutamate and 2-oxoglutarate on the regulation of electron transport to nitrogenase will be discussed.

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