NifA- and CooA-Coordinated cowN Expression Sustains Nitrogen Fixation by Rhodobacter capsulatus in the Presence of Carbon Monoxide

Rhodobacter capsulatus fixes atmospheric dinitrogen via two nitrogenases, Mo- and Fe-nitrogenase, which operate under different conditions. Here, we describe the functions in nitrogen fixation and regulation of thercc00574 (cooA ) andrcc00575 (cowN ) genes, which are located upstream of the structural genes of Mo-nitrogenase,nifHDK . Disruption ofcooA orcowN specifically impaired Mo-nitrogenase-dependent growth at carbon monoxide (CO) concentrations still tolerated by the wild type. ThecooA gene was shown to belong to the Mo-nitrogenase regulon, which is exclusively expressed when ammonium is limiting. Its expression was activated by NifA1 and NifA2, the transcriptional activators ofnifHDK . AnfA, the transcriptional activator of Fe-nitrogenase genes, repressedcooA , thereby counteracting NifA activation. CooA activatedcowN expression in response to increasing CO concentrations. Base substitutions in the presumed CooA binding site located upstream of thecowN transcription start site abolishedcowN expression, indicating thatcowN regulation by CooA is direct. In conclusion, a transcription factor-based network controlscowN expression to protect Mo-nitrogenase (but not Fe-nitrogenase) under appropriate conditions.