Structural and Functional Analyses of Photosynthetic Regulatory Genes regA and regB from Rhodovulum sulfidophilum , Roseobacter denitrificans , and Rhodobacter capsulatus

Abstract

Genes coding for putative RegA, RegB, and SenC homologues were identified and 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 previously sequenced RegA, RegB, and SenC homologues obtained from Rhodobacter capsulatus and Rhodobacter sphaeroides. These are photosynthetic bacteria that exhibit a high level of oxygen repression of photosystem synthesis controlled 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. The RegB sensor kinase homologues also exhibit a high level of sequence conservation (55.9 to 61.5%) although these additional species give significantly different responses to oxygen. A Rhodovulum sulfidophilum mutant lacking regA or regB was constructed. These mutants produced smaller amounts of photopigments 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-deficient mutants recovered the synthesis of a photosynthetic apparatus that still 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 to aerobic and anaerobic growth conditions is not the result of altered redox sensing by the sensor kinase protein, RegB.

Regulators of nonsulfur purple phototrophic bacteria and the interactive control of CO2assimilation, nitrogen fixation, hydrogen metabolism and energy generation

“…hvrA and spb encode light-responding transcriptional regulators of photosynthesis genes [28][29][30]. The reg/prr genes have also been isolated from the nonsulfur purple bacteria Rhodovulum sulfidophilum and Roseobacter denitrificans [12] and show the same pattern of gene organization with the exception that no ORFs similar to either hvrA or spb are found downstream of regA in these organisms. The transcription patterns of both the R. capsulatus and R. sphaeroides genes appear to be somewhat similar.…”

“…The RegB sequences that have been determined so far in NSP bacteria show a high degree of sequence conservation with sequence identities of approximately 56-62% [12]. The majority of sequence conservation occurs in the carboxy terminal portion beyond approximately amino acids 166 and 182 in R. capsulatus and R. sphaeroides, respectively [12].…”

“…The RegB sequences that have been determined so far in NSP bacteria show a high degree of sequence conservation with sequence identities of approximately 56-62% [12]. The majority of sequence conservation occurs in the carboxy terminal portion beyond approximately amino acids 166 and 182 in R. capsulatus and R. sphaeroides, respectively [12]. This region contains a conserved H box that spans a histidine, at amino acids 195 and 211 in R. capsulatus and R. sphaeroides RegB/PrrB, respectively, that is the site of autophosphorylation [13].…”

Sinorhizobium medicae genes whose regulation involves the ActS and/or ActR signal transduction proteins

“…RegBA act as a typical two-component system, with the membrane-spanning RegB histidine kinase phosphorylating RegA and allowing the activated regulator to modulate transcription by interacting with specific DNA targets [3][4][5]. RegBA homologs have been discovered in other closely related anoxygenic phototrophs [6][7][8][9], plant-associated and soil bacteria [10][11][12][13][14][15] and more recently in pathogens like Brucella melitensis [16] and Mycobacterim tuberculosis [17].…”

“…RegBA homologs regulate a diverse range of metabolic processes including photosystem formation [9,[18][19][20], CO 2 assimilation [21], nitrogen assimilation and hydrogen uptake [22,23], respiration and electron transport [24,25] and formaldehyde assimilation [26]. In the root nodule bacteria, the RegSR system from Bradyrhizobium japonicum is responsible for microaerobic induction of the nitrogen fixation regulator NifA, and regulation of CO 2 assimilation [11,27].…”