...nthesis or microbial biodegradation of aromatic compounds. We established UbiX acts as a novel flavin prenyltransferase, linking a dimethylallyl moiety to the flavin N5 and C6 atoms. Formation of the holo-UbiD enzyme involves oxidative maturation of the new cofactor, creating the novel azomethine ylide moiety. The dipolarophile substrate binds directly above the azomethine ylide group, and our dat ...