Carotenoids and carotenogenesis in cyanobacteria: unique ketocarotenoids and carotenoid glycosides
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Cyanobacteria grow by photosynthesis, and necessarily contain chlorophyll and carotenoids, whose main functions are light harvesting and photoprotection. In this review, we discuss the carotenoids, carotenogenesis pathways, and characteristics of carotenogenesis enzymes and genes in some cyanobacteria, whose carotenogenesis enzymes have been functionally confirmed. In these cyanobacteria, various carotenoids have been identified, including the unique ketocarotenoids, echinenone and 4-ketomyxol; and the carotenoid glycosides, myxol glycosides and oscillol diglycosides. From these findings, certain carotenogenesis pathways can be proposed. The different compositions of carotenoids among these species might be due to the presence or absence of certain gene(s), or to different enzyme characteristics. For instance, two distinct β-carotene ketolases, CrtO and CrtW, are properly used in two pathways depending on the species. One β-carotene hydroxylase, CrtR, has been identified, and its substrate specificities vary across species. At present, functionally confirmed genes have been found in only a few species, and further studies are needed.