An overlap between operons involved in carotenoid and bacteriochlorophyll biosynthesis inRhodobacter capsulatus

A new example of superoperonal gene arrangement has been documented in the Rhodobacter capsulatus photosynthetic gene cluster. The promoter for the operon initiated by the bchl gene is embedded within an upstream operon for carotenoid synthesis. The stop codon for the crtA gene, the only gene in the first operon, overlaps the start codon of the downstream bchl gene. As a consequence of this overlap, the promoter(s) for the bch operon must be located within the crtA structural gene. The bchI gene is shown here for the first time to be required for the conversion of protoporphyrin IX to subsequent intermediates in bacteriochlorophyll biosynthesis.