Expression and functional analysis of a gene cluster involved in the synthesis of decaprenoxanthin reveals the mechanisms for C50 carotenoid formation

Corynebacterium glutamicum accumulates the C-50 carotenoid decaprenoxanthin. Rescued DNA from transposon color mutants of this Gram-positive bacterium was used to clone the carotenoid biosynthetic gene cluster By sequence comparison and functional, complementation, the genes involved in the synthesis of carotenoids with 50 carbon atoms were identified. The genes crtE, encoding a geranylgeranyl pyrophosphate synthase, crtB, encoding a phytoene synthase, and crtI, encoding a phytoene desaturase, are responsible for the formation of lycopene. The products of three novel genes, crtYe and crtYf, with sequence similarities to heterodimeric lycopene cyclase crtYe and crtYd, together with crtEb which exhibits a prenyl transferase motif, were involved in the conversion of C-40 acyclic lycopene to cyclic C-50 carotenoids. Using functional complementation in Escherichia coli, it could be shown that the elongation of lycopene to the acyclic C-50 carotenoid flavuxanthin by the addition of Cg isoprenoid units at positions C-2 and C-2' is catalyzed by the crtEb gene product. Subsequently, the gene products of crtYe and crtYf in a concerted action convert the acyclic flavuxanthin into the cyclic C-50 carotene, decaprenoxanthin, forming two epsilon -ionone groups. The mechanisms, involving two individual steps for the formation of cyclic C-50 carotenoids from lycopene, are proposed on the basis of these results.