functions as a precursor of vitamin A in mammals (G. A. J. Pitt, in I. Osler, H. Gutmonn, and U. Solms, ed., Carotenoids-1971Carotenoids- , 1971). The enzymes and genes which mediate the biosynthesis of cyclic carotenoids such as 13-carotene are virtually unknown. We have elucidated for the first time the pathway for biosynthesis of these carotenoids at the level of enzyme-catalyzed reactions, using bacterial carotenoid biosynthesis genes. These genes were cloned from a phytopathogenic bacterium, Erwinia uredevora 20D3 (ATCC 19321)

, for their kind and helpful advices for the preparation of bacteriochlorophyll a and gener ous gift of the culture of Chromatium vinosum. * Amounts of bacteriochlorophyll a were calculated from light absorbancies at 770 nm on the bases of the values of millimolar extinction coefficients: 65.3 in acetone-methanol (7: 2)2) and 42.0 in methanol.9)

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