Expression of uptake hydrogenase and molybdenum nitrogenase in Rhodobactercapsulatus is coregulated by the RegB-RegA two-component regulatory system

Purple photosynthetic bacteria are capable of generating cellular energy fr om several sources, including photosynthesis, respiration, and H-2 oxidatio n. Under nutrient-limiting conditions, cellular energy can be used to assim ilate carbon and nitrogen. This study provides the first evidence of a mole cular link for the coregulation of nitrogenase and hydrogenase biosynthesis in an anoxygenic photosynthetic bacterium. We demonstrated that molybdenum nitrogenase biosynthesis is under the control of the RegB-RegA two-compone nt regulatory system in Rhodobacter capsulatus. Footprint analyses and in v ivo transcription studies showed that RegA indirectly activates nitrogenase synthesis by binding to and activating the expression of nifA2, which enco des one of the two functional copies of the nif-specific transcriptional ac tivator, NifA. Expression of nifA2 but not nifA1 is reduced in the reg muta nts up to eightfold under derepressing conditions and is also reduced under repressing conditions. Thus, although NtrC is absolutely required for nifA 2 expression, RegA acts as a coactivator of nifA2. We also demonstrated tha t in reg mutants, [NiFe]hydrogenase synthesis and activity are increased up to sixfold, RegA binds to the promoter of the hydrogenase gene operon and therefore directly represses its expression. Thus, the RegB-RegA system con trols such diverse processes as energy-generating photosynthesis and H-2 ox idation, as well as the energy-demanding processes of N-2 fixation and CO2 assimilation.