A REDOX-RESPONSIVE PATHWAY FOR AEROBIC REGULATION OF PHOTOSYNTHESIS GENE-EXPRESSION IN RHODOBACTER-SPHAEROIDES-2.4.1

To further understand the proposed signal transduction pathway involvi ng the presumed redox proteins RdxBH and cbb(3) cytochrome oxidase in Rhodobacter sphaeroides 2.4.1, a series of mutants lacking components of both the Prr two component activation system and the cbb(3)-type cy tochrome oxidase or RdxBH mere constructed. We report that under highl y aerobic conditions, aberrant photosynthesis gene expression and spec tral complex formation typical of cbb(3)- or RdxBH-deficient mutants w ere no longer observed when either prrA (encoding the response regulat or of the Prr system) or prrB (encoding the presumed sensor kinase) wa s also deleted. These double-mutant strains are phenotypically identic al to single-mutant PrrA and PrrB strains, suggesting that the signal( s) originating from the cbb(3) terminal oxidase affects downstream pac and puf operon expression by acting exclusively through the Prr syste m. When the same double-mutant strains were examined under anaerobic d ark dimethyl sulfoxide growth conditions, photosynthesis gene expressi on was obligatorily linked to the two-component activation system. How ever, photosynthesis gene expression under the same growth conditions was significantly higher in the cbb(3) mutant strain when compared to that in the wild type, Similarly, under anaerobic photosynthetic condi tions the high levels of the oxidized carotenoid, spheroidenone, which accumulate in cbb(3)-deficient mutants were nearly restored to normal in a PrrB(-) CcoP(-) double mutant. This observation, together with p reviously published results, suggests that the regulation of the CrtA- catalyzed reaction possesses both transcriptional and posttranscriptio nal regulatory effecters. We propose that the cbb(3) cytochrome oxidas e, which by definition can interact with external oxygen, serves to co ntrol the activity of the Prr two-component activation system under bo th aerobic and anaerobic conditions. Although independent from the cbb (3) oxidase, the RdxBH proteins are also required for normal functioni ng of the Prr two-component activation system and are therefore believ ed to lie between the cbb(3) oxidase in this oxygen-sensing, redox sig naling pathway and the Prr activation system.