Genetic and biochemical characterization of carotenoid biosynthesis mutants of Rhodobacter capsulatus.

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We have used genetic and biochemical techniques to study carotenoid biosynthesis (crt) mutants of Rhodobacter capsulatus, a purple non-sulfur photosynthetic bacterium. All nine identified crt genes are located within the 46-kilobase pair photosynthesis gene cluster, and eight of the crt genes form a subcluster. We have studied the operon structure of the crt gene cluster using transposon Tn5.7 mutants. The Tn5.7 insertion sites in 10 mutants have been mapped to high resolution (25-267 base pairs) by Southern hybridization. Two insertions each map within the coding regions of the crtA, crtC, crtE, and crtF genes, and one insertion lies within the crtI gene. The insertion in crtI is not polar on the downstream crtB gene, suggesting that crtI and crtB may form two separate operons. Another insertion located in the 5' noncoding region between the divergent crtA and crtI genes has no effect on wild-type pigmentation and apparently lies between the promoters for these operons. A Tn5.7 mutation in the 3' region of crtA yields a bacteriochlorophyll-minus phenotype, while a 5' insertion affects only carotenoid biosynthesis. Regulatory signals for transcription of a downstream operon required for bacteriochlorophyll biosynthesis may thus overlap the coding region of crtA. We also present the first evidence for the functions of the crtB, crtE, and crtJ gene products using a new in vitro assay for the incorporation of [14C]isopentenyl pyrophosphate into carotenoid precursors and phytoene in cell-free extracts. Extracts from a crtE mutant accumulate [14C]prephytoene pyrophosphate, while those from crtB and crtJ mutants accumulate [14C]geranylgeranyl pyrophosphate. We therefore propose that CrtE is the phytoene synthetase and that CrtB, and possibly CrtJ, are components of the prephytoene pyrophosphate synthetase.