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J Bacteriol. 1979 December; 140(3): 987-995
Copyright © 1979, American Society for Microbiology. All Rights Reserved.

Manganese, an Essential Trace Element for N₂ Fixation by Rhodospirillum rubrum and Rhodopseudomonas capsulata: Role in Nitrogenase Regulation

Department of Biology, University of South Carolina, Columbia, South Carolina 29208

ABSTRACT

Nitrogenase (N₂ase) from the photosynthetic bacterium Rhodospirillum rubrum can exist in two forms, an unregulated form (N₂ase A) and a regulatory form (N₂ase R), the latter being identified in vitro by its need for activation by a Mn²⁺-dependent N₂ase activating system. The physiological significance of this Mn²⁺-dependent N₂ase activating system was suggested here by observations that growth of R. rubrum and Rhodopseudomonas capsulata on N₂ gas (a condition that produces active N₂ase R) required Mn²⁺, but growth on ammonia or glutamate did not. Manganese could not be shown to be required for the biosynthesis of either nitrogenase or glutamine synthetase or for glutamine synthetase turnover, but it was required for the in vitro activation of N₂ases from N₂ and glutamate-grown R. rubrum and R. capsulata cells. Chromatium N₂ase, in contrast, was always fully active and did not require Mn²⁺ activation, suggesting that only the purple nonsulfur bacteria are capable of controlling their N₂ase activity by this new type of regulatory system. Although R. rubrum could not substitute Fe²⁺ for Mn²⁺ in the in vivo N₂ fixation process, Fe²⁺ and, to a lesser extent, Co²⁺ could substitute for Mn²⁺ in the in vitro activation of N₂ase. Electron paramagnetic resonance spectroscopy of buffer-washed R. rubrum chromatophores showed lines characteristic of Mn²⁺. Removal of the Mn²⁺-dependent N₂ase activating factor by a salt wash of the chromatophores removed 90% of the Mn²⁺, which suggested a specific coupling of this metal to the activating factor. The data presented here all indicate that Mn²⁺ plays an important physiological role in regulating the N₂ fixation process by these photosynthetic bacteria.