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J. Bacteriol., 03 1997, 1951-1961, Vol 179, No. 6
JP O'Gara and S Kaplan
Previous work from this laboratory revealed that alterations in the
structure of the ccoNOQP operon of Rhodobacter sphaeroides 2.4.1 could lead
to induction of the photosynthetic apparatus under aerobic growth
conditions. Immediately downstream of the ccoNOQP operon is the rdxB gene,
the first gene of the rdxBHIS cluster. The rdxB gene product is predicted
to encode a membrane protein which can bind two [4Fe-4S] clusters. The ccoP
gene product is a diheme cytochrome which is a component of the cbb3-type
cytochrome oxidase. Under aerobic growth conditions, strains possessing
ccoP and rdxB mutations both singly and in combination produced
light-harvesting complexes, suggesting that normal functioning of these
proteins is required to maintain repression of photosynthesis gene
expression in the presence of oxygen. Analysis of the expression of
puc::lacZ fusions under aerobic conditions revealed an approximately
12-fold increase in puc operon expression in the RDXB1 and CCOP1 mutant
strains compared with that for wild-type 2.4.1. Similarly, puf::lacZ
activity was observed to be elevated fourfold above wild-type levels.
Further indication of the importance of the RdxB and CcoP proteins was
derived from studies of mutant and wild-type cells grown under anoxygenic
photosynthetic and nitrogen- fixing conditions. These mutant strains were
observed to accumulate spheroidenone to approximately 50% or more of the
total carotenoid. In wild-type cultures, spheroidenone normally accumulates
to approximately 10 to 20% of the total carotenoid under the same growth
conditions. This effect was most pronounced when both the rdxB and the ccoP
mutations were present together in cells cultured under nitrogen-fixing
photosynthetic growth conditions in which spheroidenone represented
approximately 90% of the total carotenoid. We propose that mutations in the
rdxB or ccoP gene may lead 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 by alteration of the activity of a transcriptional regulator(s)
of photosynthesis gene expression. Mutations in these genes also appear to
posttranscriptionally influence the terminal step of carotenoid biogenesis.
Potential regulators interacting with an aberrant redox signal in the
mutants and the possible nature of such a redox signal are discussed.
Copyright © 1997, American Society for Microbiology
Evidence for the role of redox carriers in photosynthesis gene expression and carotenoid biosynthesis in Rhodobacter sphaeroides 2.4.1
Department of Microbiology and Molecular Genetics, Medical School, University of Texas Health Science Center, Houston 77030, USA.
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