Electron transport to nitrogenase in Rhodospirillum rubrum: Role of energization of the chromatophore membrane

Nitrogen fixation is dependent on a source of ATP and the generation of a reductant at low enough red-ox potential to transfer electrons to nitrogenase. In Rhodospirillum rubrum, grown photoheterotrophically, ATP is produced by photophosphorylation, a process studied in great detail, but the source of reductant for nitrogenase is as yet unidentified. In this report we have studied the effect on nitrogen fixation when the energization of the chromatophore membranes was changed, by decreasing the light intensity or by addition of uncouplers. When the light intensity was lowered a pronounced decrease in nitrogenase activity was observed although there was no decrease in the ATP/ADP ratio. The inhibition observed was not due to ADP-ribosylation, as the same effect was observed in a mutant devoid of the enzymes in the metabolic regulatory cascade operating in R. rubrum and some other diazotrophs. Even at low concentrations of the uncouplers used, a drastic decrease in the ATP/ADP ratio was observed. However, this decrease in the ATP/ADP ratio did not cause a decrease in nitrogenase activity. At higher concentrations of uncouplers, nitrogenase activity decreased but the ATP/ADP ratio remained essentially at a constant low level. These results support a model in which reduction of the electron donor(s) to nitrogenase in R. rubrum is coupled to the energization of the chromatophore membranes.