Phytoene desaturase, CrtI, of the purple photosynthetic bacterium, Rubrivivax gelatinosus, produces both neurosporene and lycopene

Biosynthetic pathways for carotenoids in the purple photosynthetic bacteriu m, Rubrivivax gelatinosus, which synthesizes spirilloxanthin in addition to spheroidene and OH-spheroidene, were investigated by means of genetic mani pulation. A phytoene desaturase gene (crt]) found in the photosynthesis gen e cluster of this bacterium was expressed in an Escherichia coli strain tha t can produce phytoene. Both neurosporene and lycopene were synthesized in the recombinant, probably by three- and four-step desaturation reactions of CrtI. A mutant of Rvi. gelatinosus lacking the crtI gene produced only phy toene, indicating that this organism had no other phytoene desaturases. Whe n the crtI deletion mutant was complemented by the three-step phytoene desa turase of Rhodobacter capsulatus, spirilloxanthin and its precursors were n ot synthesized, although spheroidene and OH-spheroidene were accumulated. I t was concluded that neurosporene and lycopene are produced by a single phy toene desaturase in Rvi. gelatinosus resulting in the synthesis of spheroid ene and spirilloxanthin, and that there are no pathways for spirilloxanthin synthesis via spheroidene.