Type I polyketide synthase requiring a discrete acyltransferase for polyketide biosynthesis

Type I polyketide synthases (PKSs) are multifunctional enzymes that are organized into modules, each of which minimally contains a β-ketoacyl synthase, an acyltransferase (AT), and an acyl carrier protein. Here we report that the leinamycin (LNM) biosynthetic gene cluster from Streptomyces atroolivaceus S-140 consists of two PKS genes, Inml and InmJ, that encode six PKS modules, none of which contain the cognate AT domain. The only AT activity identified within the Inm gene cluster is a discrete AT protein encoded by InmG. Inactivation of InmG, Inml, or InmJ in vivo abolished LNM biosynthesis. Biochemical characterization of LnmG in vitro showed that it efficiently and specifically loaded malonyl CoA to all six PKS modules. These findings unveiled a previously unknown PKS architecture that is characterized by a discrete, iteratively acting AT protein that loads the extender units in trans to "AT-less" multifunctional type I PKS proteins for polyketide biosynthesis. This PKS structure provides opportunities for PKS engineering as exemplified by overexpressing InmG to improve LNM production.