Angew. Chem. Int. Ed. Engl. doi:10.1002/anie.201404804

The pseurotins are a family of secondary metabolites from Aspergillus that have an unusual spiro-ring structure. Tsunematsu et al. have used several different approaches, including targeted gene deletion and in vitro enzymological assays, to probe the physiological functions of four enzymes in the pseurotin biosynthetic gene cluster from Aspergillus fumigatus. Bioinformatic analysis suggested that PsoF contains two domains, one of which is homologous to methyltransferases (MTs) and the other of which is homologous to FAD-containing monooxygenases (FMOs). Follow-up experiments indicated that the MT-like domain catalyzes the attachment of a methyl group to the C3 position of the pseurotin scaffold and that the FMO-like domain forms a 10,11-epoxide that is nonenzymatically converted to the 10,11-diol found in pseurotin A. The substrate specificity of the two domains of PsoF indicates that these two catalytic transformations occur at nonconsecutive steps in the biosynthetic pathway: the C3 position is methylated by the MT-like domain several steps before the 10,11-epoxide is formed by the FMO-like domain. Additional experiments suggested that PsoC methylates the hydroxyl group at the C8 position, that PsoD forms the ketone group at the C17 position and that PsoE isomerizes the C12-C13 olefin from trans to cis. More work is needed to reveal the exact mechanisms by which these enzymes carry out these transformations.