Two pathways of electron transport to nitrogenase in Rhodospirillum rubrum: the major pathway is dependent on the fix gene products

In the photosynthetic bacterium Rhodospirillum rubrum, as in many other diazotrophs, electron transport to nitrogenase has not been characterized in great detail. In this study, we show that there are two pathways operating in R. rubrum. The products of the fix genes constitute the major pathway operating under heterotrophic conditions, whereas a pyruvate:ferredoxin oxidoreductase, encoded by the nifJ gene, may play a central role under anaerobic conditions in the dark. In both systems, ferredoxin N is the main direct electron donor to dinitrogenase reductase. Furthermore, we suggest from studying mutants lacking components in one or both systems under different conditions, that the Fix system operates most efficiently under conditions when a proton motive force is generated. A model for our current view of the electron transfer pathways in R. rubrum is presented.