Functional analysis of spheroidene mono-oxygenase, CrtA, of the purple photosynthetic bacterium, Rubrivivax gelatinosus

Abstract

Many purple bacteria, such as Rhodospirillum and Rhodopseudomonas, synthesize spirilloxanthin and it¿s precursor, but further oxidation of these carotenoids is not observed. On the other hand, some species, such as Rhodobacter, synthesize spheroidene and OH-spheroidene as the major carotenoids, and these are oxidized to spheroidenone and OH-spheroidenone, respectively, under semi-aerobic conditions. This oxidation is known to be catalyzed by spheroidene mono-oxygenase (CrtA). Physiological significance of the oxidation, however, has never been studied. A purple photosynthetic bacterium, Rubrivivax (Rvi.) gelatinosus, can synthesize both carotenoids; spirilloxanthin and the major carotenoids, spheroidene and OH-spheroidene. The occurrence of both carotenoids was due to the unique characteristics of the phytoene desaturase, CrtI, of this bacterium (Harada et al. unpublished). We have shown that the mono-oxygenation of spirilloxanthin as well as spheroidene and OH-spheroidene occurs under the semi-aerobic conditions; spirilloxanthin changes to 2,2¿-diketospirilloxanthin via 2-ketospirilloxanthin (Takaichi and Shimada 1999 Plant Cell Physiol. 40: 613-617). To clarify the way of mono-oxygenation of carotenoids in Rvi. gelatinosus, we cloned crtA from the genome and constructed a crtA-deleted mutant. This crtA mutant did not synthesize keto-derivatives of spirilloxanthin nor spheroidenone at all. It is suggested that spirilloxanthin can be oxidized in the presence of CrtA and substitute the functions of?spheroidenone and OH-spheroidenone under semi-aerobic conditions in purple bacteria that have no spheroidene and OH-spheroidene.