Sioxanthin, a novel glycosylated carotenoid, reveals an unusual subclustered biosynthetic pathway

Abstract

Members of the marine actinomycete genus Salinispora constitutively produce a characteristic orange pigment during vegetative growth. Contrary to the understanding of widespread carotenoid biosynthesis pathways in bacteria, Salinispora carotenoid biosynthesis genes are not confined to a single cluster. Instead, bioinformatic and genetic investigations confirm that four regions of the Salinispora tropica CNB-440 genome, consisting of two gene clusters and two independent genes, contribute to the in vivo production of a single carotenoid. This compound, namely (2'S)-1'-(β-D-glucopyranosyloxy)-3',4'-didehydro-1',2'-dihydro-φ,ψ-caroten-2'-ol, is novel and has been given the trivial name 'sioxanthin'. Sioxanthin is a C40 -carotenoid, glycosylated on one end of the molecule and containing an aryl moiety on the opposite end. Glycosylation is unusual among actinomycete carotenoids, and sioxanthin joins a rare group of carotenoids with polar and non-polar head groups. 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. Additionally, this study's investigations of clustering of carotenoid biosynthetic genes in heterotrophic bacteria show that a non-clustered genome arrangement is more common than previously suggested, with nearly half of the investigated genomes showing a non-clustered architecture.

Figure 1

Proposed biosynthesis of sioxanthin. Early steps (black enzymes) are common to carotenoid biosynthetic pathways in general. Structural diversity derives from modification of lycopene, shown here for sioxanthin. Colors correspond to genome localization of the genes required for each step where black is terp2 genes, blue is terp1 genes, red are non-clustered genes.

Figure 2

Carotenoid biosynthetic genes in Salinispora are subclustered and dispersed throughout the genome. Genome map shows the relative locations of carotenoid biosynthetic genes including two terpene clusters (terp1 blue and terp2 black) and two non-clustered genes (strop_0241 and strop_2408 red). Numbers refer to the gene locus, gene code is the predicted carotenoid biosynthesis homolog. Table shows results of gene inactivation of carotenoid biosynthesis with the blue line highlighting the sioxanthin biosynthetic peak. Genes determined to be part of the sioxanthin biosynthetic pathway, based on altered retention time, are shown in bold. Photographs show visual phenotypes of wildtype and mutant carotenoid producers. Numbers in parenthesis refer to the structure.

Figure 3

Chemical structures of sioxanthin and related compounds.

Figure 4

Tree showing relative relationship of the Micromonosporaceae genera and other actinomycetes, highlighting the time of entry of sioxanthin genes in to the genomes. Three genes were present prior to the subdivision of the order actinomycetales. Three others entered later (blue arrow) followed by some gene loss (red arrow).

Figure 5

Chart of percentage of clustered (dark grey) and non-clustered (light grey) carotenoid biosynthetic pathways in selected bacterial genomes, shown by total genomes, individual phylogenetic groupings, and by environment in which the host organism was isolated. The total number of genomes in each category is in parenthesis.