S2-025 Functional analysis of spheroidene mono-oxygenase, CrtA, of the purple photosynthetic bacterium, Rubrivivax gelatinosus J Harada 1 , S Takaichi 2 , KVP. Nagashima 1 , K Matsuura 1 , K Shimada 1 1 Dept. Biol., Tokyo Metropolitan Univ., Minami-ohsawa 1-1, Hachiouji, Tokyo 192-0397, Japan. Fax: +81 426 77 2559, e-mail: j-harada@comp.metro-u.ac.jp 2 Biol. Lab., Nippon Medical School, Kawasaki, 211-0063, Japan. Keywords: Carotenoid, CrtA, Rubrivivax gelatinosus, Spheroidenone, Purple photosynthetic bacteria Introduction In purple photosynthetic bacteria, carotenoids are bound to the pigment-protein complexes, i.e., the reaction center complex and the light-harvesting 1 and 2 complexes. One of the two important functions of carotenoids in photosynthetic bacteria is the absorption of light energy with wavelengths ranging from 400 to 550 nm and the transfer of the absorbed energy to bacteriochlorophylls (Bchl). The other is photoprotection against the formation of singlet oxygen and other harmful free radicals. In carotenoid biosynthesis, many purple bacteria synthesize spirilloxanthin and its precursors by the normal spirilloxanthin pathway. On the other hand, some species, such as Rhodobacter species, synthesize spheroidene and OH-spheroidene under photosynthetic conditions and keto-carotenoids, spheroidenone and OH spheroidenone, under semi-aerobic conditions by the spheroidene pathway (see Fig. 3) (Takaichi 1999). It is known that this oxidation is catalyzed by spheroidene mono-oxygenase CrtA in Rhodobacter. Keto-carotenoids have not been found from bacteria which have the normal spirilloxanthin pathway. The purple photosynthetic bacterium, Rubrivivax (Rvi.) gelatinosus, has a unique carotenoid composition among purple bacteria (Takaichi and Shimada 1999). This bacterium synthesizes spirilloxanthin in addition to the major carotenoids, spheroidene and OH-spheroidene. The occurrence of these carotenoids is due to the unique characteristics of phytoene desaturase CrtI of this bacterium (Harada et al.