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 o f a reductant at low enough red-ox potential to transfer electrons to nitrogenase. In Rhodospirillum rubrum, grown photoheterotrophically, A TP is produced by photophosphorylation, a process studied in great det ail, but the source of reductant for nitrogenase is as yet unidentifie d. In this report we have studied the effect on nitrogen fixation when the energization of the chromatophore membranes was changed, by decre asing the light intensity or by addition of uncouplers. When the light intensity was lowered a pronounced decrease in nitrogenase activity w as observed although there was no decrease in the ATP/ADP ratio. The i nhibition observed was not due to ADP-ribosylation, as the same effect was observed in a mutant devoid of the enzymes in the metabolic regul atory cascade operating in R. rubrum and some other diazotrophs. Even at low concentrations of the uncouplers used, a drastic decrease in th e ATP/ADP ratio was observed. However, this decrease in the ATP/ADP ra tio did not cause a decrease in nitrogenase activity. At higher concen trations of uncouplers, nitrogenase activity decreased but the ATP/ADP ratio remained essentially at a constant low level. These results sup port a model in which reduction of the electron donor(s) to nitrogenas e in R. rubrum is coupled to the energization of the chromatophore mem branes.