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Functional significance of overlapping transcripts of crtEF, bchCA, and puf photosynthesis gene operons in Rhodobacter capsulatus

The Rhodobacter capsulatus crtEF, bchCA, and puf operons each encode products that function together to produce photosynthetic pigment-protein complexes. The crtEF operon encodes two enzymes of the carotenoid biosynthetic pathway, the bchCA operon encodes two enzymes of the bacteriochlorophyll biosynthetic pathway, and the puf operon contains six genes, four of which are known to code for proteins that bind pigment cofactors and position them in the intracytoplasmic membrane. These operons are adjacent on the chromosome and are transcribed in the same direction. Although each of these operons can be expressed from its own promoter, it is possible for transcription to begin at the crtEF promoter and continue through the bchCA and puf operons. We herein present the results of experiments which demonstrate that this overlapping transcriptional arrangement is important for establishment of normal levels of transcripts of the bchCA and puf operons and that read-through transcription from the bchCA operon contributes significantly to the expression of a pufB'::lac'Z gene fusion. Growth studies of crtF and bchC interposon mutants show that this read-through transcription is necessary for efficient transition from respiratory to photosynthetic growth modes.