Carotenoids, Their Diversity And Carotenogenesis In Cyanobacteria

Cyanobacteria grow by photosynthesis, and essentially contain chlorophyll and various carotenoids whose main functions are light-harvesting and photoprotection. In this chapter, we have summarized carotenoids, characteristics of carotenogenesis enzymes and genes, and carotenogenesis pathways in some cyanobacteria, whose carotenoids and genome DNA sequences have both been determined. Cyanobacteria contain various carotenoids: beta-carotene, its hydroxyl or keto derivatives, and carotenoid glycosides. Both ketocarotenoids, such as echinenone and 4-ketomyxol, and the carotenoid glycosides, such as myxol glycosides and oscillol diglycoside, are unique carotenoids in phototrophic organisms. Some cyanobacteria contain both unique carotenoids, while others do not contain such carotenoids. From these findings, certain carotenogenesis pathways can be proposed. The different compositions of carotenoids might be due to the presence or absence of certain gene(s), or to different enzyme characteristics. For instance, two distinct beta-carotene hydroxylases, CrtR and CrtG, are bifunctional enzymes whose substrates are both beta-carotene and deoxymyxol, and substrate specificities of CrtR vary across species. Two distinct beta-carotene ketolases, CrtO and CrtW, are found only in the first group and properly used in two pathways, beta-carotene and myxol, depending on the species. At present, the number of functionally confirmed genes is limited, and only a few species are examined. Therefore, further studies of carotenoids, characteristics of carotenogenesis enzymes and genes, and carotenogenesis pathways are needed.