Nucleotide sequence, organization, and nature of the protein products of the carotenoid biosynthesis gene cluster of Rhodobacter capsulatus

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Abstract
Carotenoid pigments are essential for the protection of both photosynthetic and non-photosynthetic tissues from photooxidative damage. Although carotenoid biosynthesis has been studied in many organisms from bacteria to higher plants, little is known about carotenoid biosynthetic enzymes, or the nature and regulation of the genes encoding them. We report here the first DNA sequence of carotenoid genes from any organism. We have determined the complete nucleotide sequence (11 039 bp) of a gene cluster encoding seven of the eight previously known carotenoid genes (crt A, B, C, D, E, F, I) and a new gene, designated crtK, from Rhodobacter capsulatus, a purple non-sulfur photosynthetic bacterium. The 5′ flanking regions of crtA, I, D and E contain a highly conserved palindromic sequence homologous to the consensus binding site for a variety of prokaryotic DNA-binding regulatory proteins. This putative regulatory palindrome is also found 5′ to the puc operon, encoding the light-harvesting II antenna polypeptides. Escherichia coli-like σ70 promoter sequences are located 5′ to crtI and crtD, suggesting for the first time that such promoters may exist in purple photosynthetic bacteria. The crt genes form a minimum of four distinct operons, crtA, crtIBK, crtDC and crtEF, based on inversions of transcriptional orientation within the gene cluster. Possible rho-independent transcription terminators are located 3′ to crtI, B, K, C and F. The 3′ end of crtA may overlap transcription initiation signals for a downstream gene required for bacteriochlorophyll biosynthesis. We have also observed two regions of exceptional amino acid homology between CrtI and CrtD, both of which are dehydrogenases.