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 mol photons m ؊2 s ؊1 , photomixotrophic growth of cells lacking sll1213 was poor. When grown at 40 mol 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 ⌬slr0940K, a strain lacking -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.

Upon depletion of Sll0254 in Synechocystis sp. strain PCC 6803, cyclized carotenoids were replaced by linear, relatively hydrophilic carotenoids, and the amount of the two photosystems decreased greatly. Full segregants of the sll0254 deletion in Synechocystis were not obtained, implying that this gene is essential for survival, most likely to allow normal cell division. The N-terminal half of Sll0254 has limited similarity to the family of lycopene cyclases, has an additional dehydrogenase motif near the N terminus, and is followed by a Rieske 2Fe-2S center sequence signature. To test whether Sll0254 serves as a lycopene cyclase in Synechocystis, the corresponding gene was expressed in Escherichia coli strains that can produce lycopene or neurosporene. In the presence of Sll0254 these linear carotenoids were converted into cyclized, relatively hydrophilic pigments, with masses consistent with the introduction of two hydroxyl groups and with spectra indicative of only small changes in the number of conjugated double bonds. This suggests that Sll0254 catalyzes formation of oxygenated, cyclized carotenoids. We interpret the appearance of the hydroxyl groups in the carotenoids to be due to dioxygenase activity involving the Rieske 2Fe-2S center and the additional dehydrogenase domain. This dioxygenase activity is required in the myxoxanthophyll biosynthesis pathway, after or concomitant with cyclization on the other end of the molecule. We interpret Sll0254 to be a dual-function enzyme with both lycopene cyclase and dioxygenase activity and have named it CrtL diox .

To elucidate the biosynthetic pathways of carotenoids, especially myxol 2-glycosides, in cyanobacteria, Anabaena sp. strain PCC 7120 (also known as Nostoc sp. strain PCC 7120) and Synechocystis sp. strain PCC 6803 deletion mutants lacking selected proposed carotenoid biosynthesis enzymes and GDP-fucose synthase (WcaG), which is required for myxol 2-fucoside production, were analyzed. The carotenoids in these mutants were identified using high-performance liquid chromatography, field desorption mass spectrometry, and Cyanobacteria synthesize carotenoids, as do all phototrophic organisms. The carotenoids in cyanobacteria are not limited to just the common carotenoids, such as ␤-carotene, but also include some unique ketocarotenoids and carotenoid glycosides, which are not found in higher plants (6). It has become evident from several recent studies that these unique carotenoids are different in different cyanobacterial species, and cyanobacteria can be classified into several groups based on their unique carotenoids (39). Members of the first group, which includes the genera Anabaena and Nostoc, contain ␤-carotene, keto derivatives of ␤-carotene such as echinenone, and myxol glycosides, but little or no zeaxanthin. Members of the second group, including Synechocystis sp. strain PCC 6803 and Thermosynechococcus elongatus strain BP-1, contain these carotenoids as well as zeaxanthin. Members of the third group, which includes the genera Synechococcus and Prochlorococcus, contain ␤-carotene, zeaxanthin, and nostoxanthin but lack both ketocarotenoids and carotenoid glycosides. Prochlorococcus also contains ␣-carotene, and the lycopene cyclases in these genera have homology to those of plants.We recently identified the molecular structures of carotenoids in some Anabaena and Nostoc strains, and we proposed carotenogenesis pathways and genes (37,38). In these genera, the major carotenoids were ␤-carotene and echinenone, and the polar carotenoids were myxol and 4-ketomyxol glycosides.

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When grown at high light intensity, more than a quarter of the total carotenoids in the unicellular cyanobacterium Synechocystis consists of myxoxanthophyll, a polar carotenoid glycoside. The biosynthetic pathway of myxoxanthophyll is unknown but is presumed to involve a number of enzymes, including a C-3,4 desaturase required to add one double bond to generate 11 conjugated double bonds in the monocyclic myxoxanthophyll. A candidate for this desaturase is Slr1293, which was identified by genome similarity searching. To determine whether Slr1293 is a desaturase recognizing neurosporene and lycopene, slr1293 was expressed in Escherichia coli strains accumulating neurosporene or lycopene. Confirming such a desaturase function for Slr1293, these E. coli strains accumulated 3,4-didehydroneurosporene and 3,4-didehydrolycopene, respectively. Indeed, deletion of slr1293 in Synechocystis provides further evidence that Slr1293 is a desaturase recognizing neurosporene: In the slr1293 deletion mutant, neurosporene was found to accumulate and was further processed to produce neurosporene glycoside. Neurosporene hereby becomes a primary candidate to be the branch point molecule between carotene and myxoxanthophyll biosynthesis in this cyanobacterium. The slr1293 gene was concluded to encode a C-3,4 desaturase that is essential for myxoxanthophyll biosynthesis, and thus it was designated as crtD. Furthermore, as Slr1293 appears to recognize neurosporene and to catalyze the first committed step on the myxoxanthophyll biosynthesis pathway, Slr1293 plays a pivotal role in directing a portion of the precursor pool for carotenoid biosynthesis toward myxoxanthophyll biosynthesis in Synechocystis sp. strain PCC 6803.