Conversion of the lycopene monocyclase of Myxococcus xanthus into a bicyclase

Depending on the cyclized hydrocarbon backbone ends, carotenoids can be acyclic, monocyclic, or bicyclic. Lycopene cyclases are the enzymes responsible for catalyzing the formation of cyclic carotenoids from acyclic lycopene. Myxococcus xanthus is a bacterium that accumulates monocyclic carotenoids such as a glycoside ester of myxobacton. We show here that this bacterium possesses a cyclase belonging to the group of the heterodimeric cyclases CrtYc and CrtYd. These two individual proteins are encoded by crtYc and crtYd, which are located in the carotenogenic carA operon of the carB-carA gene cluster, and the presence of both is essential for the cyclization of lycopene. CrtYc and CrtYd from M. xanthus form a heterodimeric cyclase with beta-monocyclic activity, which converts lycopene into monocyclic gamma-carotene, but not into bicyclic beta-carotene like most beta-cyclases. This is an unusual case where two different proteins constitute a lycopene cyclase enzyme with monocyclic activity. We were able to convert this lycopene monocyclase into a lycopene bicyclase enzyme producing beta-carotene, by fusing both proteins with an extra transmembrane domain. The chimeric protein appears to allow a proper membranal disposition of both CrtYc and CrtYd, to perform two cyclization reactions, while a hybrid without the extra transmembrane helix performs only one cyclization.