UC San Diego

Members of the marine actinomycete genus Salinispora constitutively produce an orange pigment during vegetative growth. Investigations into the genome sequences identified putative carotenoid biosynthetic genes in four regions of the genome. Gene inactivation experiments in S. tropica CNB-440 confirmed that these four regions, consisting of two gene clusters and two independent genes, contribute to the production of a single carotenoid that is responsible for Salinispora pigmentation. This distributed biosynthetic genome arrangement is unusual among literature about bacterial carotenogenesis and is counter to what is understood about the genome organization of secondary metabolites. Isolation and purification of carotenoids from S. tropica enabled the structural elucidation of the compound responsible for orange pigmentation. The compound is a novel carotenoid (2'S)-1'-([Beta]-D-glucopyranosyloxy)-3',4'-didehydro-1', 2'-dihydro-[Phi], [Psi]-caroten-2'-ol, which has been given the trivial name "sioxanthin". Sioxanthin is a C₄₀ carotenoid, glycosylated on one end of the molecule and containing an aryl functional group on the other end. Glycosylation is unusual among the actinomycetes and sioxanthin represents a poorly studied group of carotenoids which are polar on one end and non-polar on the other. The addition of a hydroxyl group on the 2'- carbon was not predicted by bioinformatics and has not yet been identified in Salinispora. Gene sequence homology predicts that the sioxanthin biosynthetic pathway is present in all of the Salinispora as well as other members of the family Micromonosporaceae including the genera Micromonopsora, Verrucosispora, and Actinoplanes. Additionally, investigations of clustering of carotenoid biosynthetic genes in heterotrophic bacteria showed that a non-clustered genome arrangement is more common than the literature suggests, with nearly half of the investigated genomes showing a non-clustered architecture. The sioxanthin evolutionary history was explored via character trees that predicted the ancestral traits for each region of the pathway. These indicate that genes responsible for cyclization, isomerization, and glycosylation were horizontally acquired more recently in Micromonosporaceae evolution than the rest of the pathway. Comparisons of gene and species trees confirm that each of these acquisitions were from separate bacterial groups. It seems that the novel and unique carotenoid, sioxanthin, is the result of integration of carotenoid biosynthetic genes from multiple sources