A carotenoid biosynthesis gene cluster involved in canthaxanthin production was isolated from the photosynthetic Bradyrhizobium sp. strain ORS278. This cluster includes five genes identified as crtE, crtY, crtI, crtB, and crtW that are organized in at least two operons. The functional assignment of each open reading frame was confirmed by complementation studies.Bradyrhizobium strains isolated from Aeschynomene stem nodules are photosynthetic (4; see reference 6 for a review), which is a rare trait in Rhizobium bacteria. These strains exhibit a photoheterotrophic and strictly aerobic photosynthesis (6; A. Vermeglio, personal communication). In culture, most of these stem isolates show the same pink coloration, while a few strains produce orange pigmentation (12, 16). Pigment analyses showed that bacteriochlorophyll and spirilloxanthin, two pigments of the light harvesting system, are common to all of these photosynthetic Bradyrhizobium strains, whereas orange strains produce an additional bicyclic carotenoid, canthaxanthin (4,4Ј-diketo--carotene) (12). This was the first report on the presence of this carotenoid in photosynthetic bacteria. Bradyrhizobium sp. strain ORS278 produces the highest quantity of canthaxanthin of all tested photosynthetic bacteria; canthaxanthin represents 85% of its total carotenoid content (12).Contrary to anaerobic purple phototrophic bacteria, aerobic phototrophic bacteria synthesize an unusually diverse variety of carotenoids, including photosynthetic carotenoids such as spirilloxanthin or spheroidenone, and often a large amount of bicyclic carotenoid (-carotene and hydroxyl derivatives) (24; see reference 26 for a review). These carotenoids were shown to not be bound to the photosynthetic apparatus of these aerobic bacteria (17,27) and their function is still unclear-
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