| The RegB/RegA regulatory system has a central role in controlling cellular redox poise in Rhodobacter capsulatus. As such, the Reg regulon encodes proteins involved in numerous diverse energy-generating and energy-utilizing processes such as photosynthesis, carbon fixation, nitrogen fixation, hydrogen utilization, aerobic and anaerobic respiration, denitrification, electron transport and aerotaxis. RegB is a membrane bound sensor kinase with the ability to sense environmental oxygen tension and respond to diminishing oxygen levels by autophosphorylating a conserved histidine residue. The mechanism of redox control surprisingly involves a redox active cysteine in the cytosolic region of RegB, with only reduced RegB capable of autophosphorylating. Also, RegB can sense the redox state of the ubiquinone pool, which fluctuates in response to oxygen tension. Once phosphorylated, RegB can transfer this phosphoryl group to an aspartate residue on the cognate response regulator, RegA. Both phosphorylated and unphosphorylated forms of the cognate response regulator RegA are capable of activating or repressing a variety of genes in the regulon. Highly conserved homologues of RegB and RegA have been found in a wide number of photosynthetic and non-photosynthetic bacteria. Interestingly, the newly discovered redox active cysteine and the ubiquinone-binding motif are both 100% conserved among 20 RegB homologues from a broad distribution of eubacteria. Thus, genetic evidence is emerging which indicates that RegB/RegA plays a fundamental role in the transcriptional redox-regulation of numerous operons in many bacterial species. |
