Expression of Uptake Hydrogenase and Molybdenum Nitrogenase in Rhodobacter capsulatus Is Coregulated by the RegB-RegA Two-Component Regulatory System

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Abstract
Purple photosynthetic bacteria are capable of generating cellular energy from several sources, including photosynthesis, respiration, and H2 oxidation. Under nutrient-limiting conditions, cellular energy can be used to assimilate carbon and nitrogen. This study provides the first evidence of a molecular 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-component regulatory system in Rhodobacter capsulatus. Footprint analyses and in vivo transcription studies showed that RegA indirectly activates nitrogenase synthesis by binding to and activating the expression of nifA2, which encodes one of the two functional copies of the nif-specific transcriptional activator, NifA. Expression of nifA2 but notnifA1 is reduced in the reg mutants up to eightfold under derepressing conditions and is also reduced under repressing conditions. Thus, although NtrC is absolutely required fornifA2 expression, RegA acts as a coactivator ofnifA2. We also demonstrated that in regmutants, [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 controls such diverse processes as energy-generating photosynthesis and H2 oxidation, as well as the energy-demanding processes of N2 fixation and CO2 assimilation.

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