The cbb(3) terminal oxidase of Rhodobacter sphaeroides 2.4.1: Structural and functional implications for the regulation of spectral complex formation

We have previously shown that the flow of reductant through the cbb3 termin al cytochrome c oxidase of Rhodobacter sphaeroides is essential to the repr ession of photosynthesis (PS) gene expression in the presence of oxygen by inhibiting the functional activity of the Prr two-component activation syst em. To gain further insight into the role of the cbb(3) oxidase and the cog nate ccoNOQP operon in the oxygen regulation of PS gene expression, we cons tructed nonpolar, in-frame deletions within the ccoN and ccoQ genes. Wherea s mutations in ccoN, ccoQ, and ccoP resulted in PS gene expression in the p resence of oxygen, only the ccoQ mutation showed both the normal flow of re ductant through the cbb(3) oxidase and the absence of any alteration in the relative levels of spheroidene and spheroidenone, as is observed for those mutations in the cco operon that result in the loss of terminal oxidase ac tivity. Consistent with these findings is the observation that extra copies of the ccoNOQP operon in trans resulted in the decreased formation of both the B800-850 and B875 spectral complexes under anaerobic growth conditions . These results in conjunction with our earlier findings indicate that (1) the flow of reductant through the cbb(3) terminal oxidase is a prerequisite to the regulation of PS gene expression by the Prr two-component regulator y system, (2) the CcoQ protein is involved in conveying the signal derived from reductant flow through the cbb(3) terminal oxidase to the Prr regulato ry pathway, (3) there is reductant flow through this terminal oxidase under anaerobic conditions, and as a result, the activity of the Pn system is st ill subject to cbb3 regulation, and (4) the acceptor for reductant flow thr ough cbb3 under anaerobic conditions is in whole or in part involved in the conversion of spheroidene to spheroidenone.