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Nitrogenase (N2 ase) from the photosynthetic bacteriumRhodospirillum rubrum can exist in two forms, an unregulated form (N2 ase A) and a regulatory form (N2 ase R), the latter being identified in vitro by its need for activation by a Mn2+ -dependent N2 ase activating system. The physiological significance of this Mn2+ -dependent N2 ase activating system was suggested here by observations that growth ofR. rubrum andRhodopseudomonas capsulata on N2 gas (a condition that produces active N2 ase 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 N2 ases from N2 and glutamate-grownR. rubrum andR. capsulata cells.Chromatium N2 ase, in contrast, was always fully active and did not require Mn2+ activation, suggesting that only the purple nonsulfur bacteria are capable of controlling their N2 ase activity by this new type of regulatory system. AlthoughR. 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 N2 ase. Electron paramagnetic resonance spectroscopy of buffer-washedR. rubrum chromatophores showed lines characteristic of Mn2+ . Removal of the Mn2+ -dependent N2 ase 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.

journal of bacteriology

Manganese, an Essential Trace Element for N 2 Fixation by Rhodospirillum rubrum and Rhodopseudomonas capsulata : Role in Nitrogenase Regulation