Manganese, an essential trace element for N2 fixation by Rhodospirillum rubrum and Rhodopseudomonas capsulata: role in nitrogenase regulation

Abstract

Nitrogenase (N(2)ase) from the photosynthetic bacterium Rhodospirillum rubrum can exist in two forms, an unregulated form (N(2)ase A) and a regulatory form (N(2)ase R), the latter being identified in vitro by its need for activation by a Mn(2+)-dependent N(2)ase activating system. The physiological significance of this Mn(2+)-dependent N(2)ase activating system was suggested here by observations that growth of R. rubrum and Rhodopseudomonas capsulata on N(2) gas (a condition that produces active N(2)ase R) required Mn(2+), 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 N(2)ases from N(2) and glutamate-grown R. rubrum and R. capsulata cells. Chromatium N(2)ase, in contrast, was always fully active and did not require Mn(2+) activation, suggesting that only the purple nonsulfur bacteria are capable of controlling their N(2)ase activity by this new type of regulatory system. Although R. rubrum could not substitute Fe(2+) for Mn(2+) in the in vivo N(2) fixation process, Fe(2+) and, to a lesser extent, Co(2+) could substitute for Mn(2+) in the in vitro activation of N(2)ase. Electron paramagnetic resonance spectroscopy of buffer-washed R. rubrum chromatophores showed lines characteristic of Mn(2+). Removal of the Mn(2+)-dependent N(2)ase activating factor by a salt wash of the chromatophores removed 90% of the Mn(2+), which suggested a specific coupling of this metal to the activating factor. The data presented here all indicate that Mn(2+) plays an important physiological role in regulating the N(2) fixation process by these photosynthetic bacteria.