EVIDENCE FOR THE ROLE OF REDOX CARRIERS IN PHOTOSYNTHESIS GENE-EXPRESSION AND CAROTENOID BIOSYNTHESIS IN RHODOBACTER-SPHAEROIDES-2.4.1

Previous work from this laboratory revealed that alterations in the st ructure of the ccoNOQP operon of Rhodobacter sphaeroides 2.4.1 could l ead to induction of the photosynthetic apparatus under aerobic growth conditions. Immediately downstream of the ccoNOQP operon is the rdxB g ene, the first gene of the rdxBHIS cluster. The rdxB gene product is p redicted to encode a membrane protein which can bind two [4Fe-4S] clus ters. The ccoP gene product is a diheme cytochrome which is a componen t of the cbb(3)-type cytochrome oxidase. Under aerobic growth conditio ns, strains possessing ccoP and rdxB mutations both singly and in comb ination produced light-harvesting complexes, suggesting that normal fu nctioning of these proteins is required to maintain repression of phot osynthesis gene expression in the presence of oxygen. Analysis of the expression of puc::lacZ fusions under aerobic conditions revealed an a pproximately 12-fold increase in puc operon expression in the RDXB1 an d CCOP1 mutant strains compared with that for wild-type 2.4.1. Similar ly, puf::lacZ activity was observed to be elevated fourfold above wild -type levels. Further indication of the importance of the RdxB and Cco P proteins was derived from studies of mutant and wild-type cells grow n under anoxygenic photosynthetic and nitrogen-fixing conditions. Thes e mutant strains were observed to accumulate spheroidenone to approxim ately 50% or more of the total carotenoid. In wild-type cultures, sphe roidenone normally accumulates to approximately 10 to 20% of the total carotenoid under the same growth conditions. This effect was most pro nounced when both the rdrB and the ccoP mutations were present togethe r in cells cultured under nitrogen-fixing photosynthetic growth condit ions in which spheroidenone represented approximately 90% of the total carotenoid. We propose that mutations in the rdxB or ccoP gene may le ad to changes in a membrane-generated redox signal or the accumulation of a critical redox intermediate in the mutant strains which results in increased photosynthesis gene expression under aerobic conditions b y alteration of the activity of a transcriptional regulator(s) of phot osynthesis gene expression. Mutations in these genes also appear to po sttranscriptionally influence the terminal step of carotenoid biogenes is. Potential regulators interacting with an aberrant redox signal in the mutants and the possible nature of such a redox signal are discuss ed.