Reconstitution of the Rhodobacter sphaeroides cbb3-PrrBA signal transduction pathway in vitro

The PrrBA two-component system in Rhodobacter sphaeroides 2.4.1, which is composed of the PrrB histidine kinase and the PrrA response regulator, controls the expression of all of the photosynthesis genes, either directly or indirectly, in response to changes in oxygen tension. In vivo under aerobic conditions it is the cbb(3) cytochrome c oxidase which generates an inhibitory signal preventing the accumulation of activated PrrA. Using purified cbb(3) cytochrome c oxidase, PrrB, and PrrA, we demonstrate in vitro that the cbb(3) oxidase inhibits PrrB activity by apparently increasing the intrinsic PrrB phosphatase activity, which dephosphorylates phosphorylated PrrA without alteration of the PrrB kinase activity. The transmembrane domain of PrrB is required for the enhancement of PrrB phosphatase activity by the cbb(3) oxidase. Full-length PrrB has a significantly greater ability to phosphorylate PrrA than does truncated PrrB lacking the transmembrane domain. This is at least in part due to the lower autophosphorylation rate of the truncated PrrB relative to the full-length PrrB. This finding provides evidence that the sensing domain (transmembrane domain) of PrrB plays an important role not only in optimally sensing the state of the cbb(3) oxidase but also in maintaining the correct conformation of PrrB, providing optimal autokinase activity.