Evaluation of carotenoid function by making mutants of a purple photosynthetic bacterium that accumulate specific carotenoid species

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Carotenoids are widespread natural pigments, occurring in animals, plants and microorganisms. Nevertheless, correlation between the chemical structure of various carotenoid species and the protective activity in vivo has rarely been investigated. In this study, photoprotective activities of several carotenoid species in vivo were comparatively investigated by using mutants of a purple photosynthetic bacterium, Rubrivivax (Rvi.) gelatinosus:Before constructing mutant strains, carotenoid biosynthesis pathway of Rvi. gelatinosus, which had been unclear, was determined. Carotenoid biosynthesis pathways of this bacterium were shown to be composed of two pathways due to characteristic specificity of the phytoene desaturase, CrtI. In the presence of oxygen, two types of ketocarotenoids were also synthesized as final products by a carotenoid mono-oxygenase, CrtA. Based on the elucidated biosynthesis pathways, mutant strains of Rvi. gelatinosus accumulating a particular carotenoid species were constructed.The protective activity of each carotenoid species against photooxidative damage and singlet oxygen was evaluated by the viability of each mutant of Rvi. gelatinosus under aerobic high-light condition or in the presence of singlet oxygen. Among the carotenoid species, ketocarotenoids, spheroidenone and diketospirilloxanthin, were shown to have remarkable activity of scavenging singlet oxygen. Spirilloxanthin and lycopene that have long conjugated double bonds showed less protectivity against singlet oxygen than ketocarotenoids, although they showed enough protectivity against photooxidative damage.