Carotenoids represent one of the most widely distributed and structurally diverse classes of natural pigments, with important functions in photosynthesis, nutrition, and protection against photooxidative damage. In the eubacterial community, yellow, orange, and red carotenoids are produced by anoxygenic photosynthetic bacteria, cyanobacteria, and certain species of nonphotosynthetic bacteria. Many eukaryotes, including all algae and plants, as well as some fungi, also synthesize these pigments. In noncarotenogenic organisms, such as mammals, birds, amphibians, fish, crustaceans, and insects, dietary carotenoids and their metabolites also serve important biological roles. Within the last decade, major advances have been made in the elucidation of the molecular genetics, the biochemistry, and the regulation of eubacterial carotenoid biosynthesis. These developments have important implications for eukaryotes, and they make increasingly attractive the genetic manipulation of carotenoid content for biotechnological purposes.

GENETICS OF EUBACTERIAL CAROTENOID BIOSYNTHESIS: A Colorful Tale

Carotenoid biosynthesis in extremophilic Deinococcus–Thermus bacteria

Myxoxanthophyll is a carotenoid glycoside in cyanobacteria that is of unknown biological significance. The sugar moiety of myxoxanthophyll in Synechocystis sp. strain PCC 6803 was identified as dimethyl fucose. The open reading frame sll1213 encoding a fucose synthetase orthologue was deleted to probe the role of fucose and to determine the biological significance of myxoxanthophyll in Synechocystis sp. strain PCC 6803. Upon deletion of sll1213, a pleiotropic phenotype was obtained: when propagated at 0.5 μmol photons m−2 s−1, photomixotrophic growth of cells lacking sll1213 was poor. When grown at 40 μmol photons m−2 s−1, growth was comparable to that of the wild type, but cells showed a severe reduction in or loss of the glycocalyx (S-layer). As a consequence, cells aggregated in liquid as well as on plates. At both light intensities, new carotenoid glycosides accumulated, but myxoxanthophyll was absent. New carotenoid glycosides may be a consequence of less-specific glycosylation reactions that gained prominence upon the disappearance of the native sugar moiety (fucose) of myxoxanthophyll. In the mutant, the N-storage compound cyanophycin accumulated, and the organization of thylakoid membranes was altered. Altered cell wall structure and thylakoid membrane organization and increased cyanophycin accumulation were also observed for Δslr0940K, a strain lacking ζ-carotene desaturase and thereby all carotenoids but retaining fucose. Therefore, lack of myxoxanthophyll and not simply of fucose results in most of the phenotypic effects described here. It is concluded that myxoxanthophyll contributes significantly to the vigor of cyanobacteria, as it stabilizes thylakoid membranes and is critical for S-layer formation.

Myxoxanthophyll is required for normal cell wall structure and thylakoid organization in the cyanobacterium Synechocystis sp. strain PCC 6803.

Carotenes have attracted much attention in recent years for their biological function in processes such as photosynthesis. The characterization of carotenes is difficult, however, because they consist of only carbon and hydrogen atoms, without oxygen. In the present study, we systematically examined the chemical structures of more than 30 carotenes, including most of the carotenes found in phototrophic organisms, and observed their elution order using a Novapak C18 HPLC column with simple isocratic elution. The elution order of the carotenes was C30 ,C 40,C45 then C50 .T he C 40 carotenes with fewer conjugated double bonds (N) had longer retention times. With respect to the end groups, the carotenes eluted in the following order: ,9, then end groups. Furthermore, absorption spectra in the HPLC eluent used were recorded with a photodiode-array detector. A greater N value was associated with a longer absorption maximum wavelength. Since the conjugated end groups ( and) influenced the absorption spectra and the non-conjugated end groups (9 and) did not, the number of conjugated end groups (zero, one and two) was clearly distinguishable. Therefore, the chemical structures of carotenes can be easily determined by a combination of the HPLC retention times and the absorption spectra.

Characterization of carotenes in a combination of a C18 HPLC column with isocratic elution and absorption spectra with a photodiode-array detector

The complete carotenoid composition of the thermophilic green sulfur bacterium Chlorobium tepidum strain TNO was determined by spectroscopic methods. Major carotenoids were four kinds of carotenes: γ-carotene, chlorobactene, and their 1′,2′-dihydro derivatives (1′,2′-dihydro-γ-carotene and 1′,2′-dihydrochlorobactene). In lesser amounts, hydroxyl γ-carotene, hydroxyl chlorobactene, and their glucoside fatty acid esters were found. The only esterified fatty acid present was laurate, and OH-chlorobactene glucoside laurate is a novel carotenoid. In other strains of C. tepidum, the same carotenoids were found, but the composition varied from strain to strain. The overall pigment composition in cells of strain TNO was 4 mol carotenoids and 40 mol bacteriochlorophyll c per mol bacteriochlorophyll a. The effects of nicotine on carotenoid biosynthesis in C. tepidum differed from those in the thermophilic green nonsulfur bacterium Chloroflexus aurantiacus.

New carotenoids from the thermophilic green sulfur bacterium Chlorobium tepidum : 1',2'-dihydro-γ-carotene, 1',2'-dihydrochlorobactene, and OH-chlorobactene glucoside ester, and the carotenoid composition of different strains

The precise chemical structures of two carotenoid glucoside esters, keto-myxocoxanthin glucoside ester isolated from Myxococcus sp. MY-18 and dehydrorhodopin glucoside ester from cells cultured in a nicotinecontaining medium, were determined as 1′-[(6-O-acyl-β-D-glucopyranosyl)oxy]-3′,4′-didehydro-l′,2′-dihydro-β,ψ-caroten-4-one and 1-[(6-O-acyl-β-D-glucopyranosyl)oxy]-3,4-didehydro-1,2-dihydro-ψ,ψ-carotene, respectively. They had β-D-glucoside, whose C-6 hydroxyl group was esterified. The major fatty acid was 11-methyldodecanoate with minor components of n-dodecanoate, n-tetradecanoate, and 13-methyltetradecanoate. This composition is different from that of cellular lipids, and the unusual accumulation of phytoene (more than 70% of total carotenoids) is characteristic of this strain.The possible biosynthetic pathway for the carotenoid glucoside ester starts with lycopene being cyclized to γ-carotene, from which myxocoxanthin is produced by hydration and desaturation. β-D-Glucose was attached to the carot...

Carotenoids of the Fruiting Gliding Myxobacterium, Myxococcus sp. MY-18, Isolated from Lake Sediment: Accumulation of Phytoene and Keto-myxocoxanthin Glucoside Ester

Hideyuki Yazawa

Papers

Carotenoids of the Fruiting Gliding Myxobacterium, Myxococcus sp. MY-18, Isolated from Lake Sediment: Accumulation of Phytoene and Keto-myxocoxanthin Glucoside Ester