6-Hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme induced in carrot cells, and catalyzes the condensation of 1 mol of acetyl-and 4 mol of malonyl-CoAs. An NADPH-dependent ketoreduction of the top carbonyl group takes place at the triketide intermediate stage to form the dihydroisocoumarin skeleton directly. The synthase is composed by two multifunctional subunits, and forms a homodimer structure with the two equivalent reaction centers. Organization of the reaction center of the synthase with the multicatalytic properties and factors regulating the activities of the reactions are discussed in relation to animal fatty acid synthase of which properties resembles to polyketide synthase.