Structural and functional analyses of photosynthetic regulatory genes regAand regB from Rhodovulum sulfidophilum, Roseobacter denitrificans, and Rhodobacter capsulatus

Genes coding for putative RegA, RegB, and SenC homologues were identified a nd characterized in the purple nonsulfur photosynthetic bacteria Rhodovulum sulfidophilum and Roseobacter denitrificans, species that demonstrate weak or no oxygen repression of photosystem synthesis. This additional sequence information was then used to perform a comparative analysis with previousl y sequenced RegA, RegB, and SenC homologues obtained from Rhodobacter capsu latus and Rhodobacter sphaeroides. These are photosynthetic bacteria that e xhibit a high level of oxygen repression of photosystem synthesis controlle d by the RegA-RegB two-component regulatory system. The response regulator, RegA, exhibits a remarkable 78.7 to 84.2% overall sequence identity, with total conservation within a putative helix-turn-helix DNA-binding motif, Th e RegB sensor kinase homologues also exhibit a high level of sequence conse rvation (55.9 to 61.5%) although these additional species give significantl y different responses to oxygen. A Rhodovulum sulfidophilum mutant lacking regA or regB was constructed. These mutants produced smaller amounts of pho topigments under aerobic and anaerobic conditions, indicating that the RegA -RegB regulon controls photosynthetic gene expression in this bacterium as it does as in Rhodobacter species. Rhodobacter capsulatus regA- or regB-def icient mutants recovered the synthesis of a photosynthetic apparatus that s till retained regulation by oxygen tension when complemented with reg genes from Rhodovulum sulfidophilum and Roseobacter denitrificans. These results suggest that differential expression of photosynthetic genes in response t o aerobic and anaerobic growth conditions is not the result of altered redo x sensing by the sensor kinase protein, RegB.