RegA, the regulator of the two-component system RegB/RegA of Brucella suis, is a controller of both oxidative respiration and denitrification required for chronic infection in mice.

Adaptation to oxygen deficiency is essential for virulence and persistence of Brucella inside the host. The flexibility of this bacterium with respect to oxygen depletion is remarkable, since Brucella suis can use an oxygen-dependent transcriptional regulator of the FnrN family, two high-oxygen-affinity terminal oxidases, and a complete denitrification pathway to resist various conditions of oxygen deficiency. Moreover, our previous results suggested that oxidative respiration and denitrification can be simultaneously used by B. suis under microaerobiosis. The requirement of a functional cytochrome bd ubiquinol oxidase for nitrite reductase expression evidenced the linkage of these two pathways, and the central role of the two-component system RegB/RegA in the coordinated control of both respiratory systems was demonstrated. We propose a scheme for global regulation of B. suis respiratory pathways by the transcriptional regulator RegA, which postulates a role for the cytochrome bd ubiquinol oxidase in redox signal transmission to the histidine sensor kinase RegB. More importantly, RegA was found to be essential for B. suis persistence in vivo within oxygen-limited target organs. It is conceivable that RegA acts as a controller of numerous systems involved in the establishment of the persistent state, characteristic of chronic infections by Brucella.