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The role of the cbb3 oxidase and the cognate NQQP operon in the oxygen regulation of photosynthesis gene expression was examined via nonpolar, in-frame deletions within the ccoN and ccoQ genes of Rhodobacter spaeroides. The results revealed the involvement of CcoQ in the transduction of the O2-sensing signal. The presence of either the redox state or the accumulation of a certain redox intermediate of the electron transport chain was also found necessary for the regulation of spectral complex formation.

On the basis of genetic and biochemical studies performed in vivo, the cbb3 cytochrome c oxidase possessing a high affinity for O2 was proposed to be the redox sensor that controls the equilibrium between the PrrB kinase/phosphatase activities in response to changes in O2 availability.

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 cbb3 cytochrome c oxidase which generates an inhibitory signal preventing the accumulation of activated PrrA. Using purified cbb3 cytochrome c oxidase, PnB, and PrrA, we demonstrate in vitro that the cbb3 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 cbb3 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 cbb3 oxidase but also in maintaining the correct conformation of PrrB, providing optimal autokinase activity. [ABSTRACT FROM AUTHOR]