Characterization of three amidinotransferases involved in the biosynthesis of ketomemicins.

We recently reported a novel class of amide bond forming enzymes (peptide ligases) involved in the biosynthesis of pheganomycins, resorcinomycins and ketomemicins. This class of enzymes exclusively utilizes Nα-amidino amino acids as the N-terminal substrate. In this Letter, we characterized three new amidinotransferases involved in the biosynthesis of ketomemicins and showed that l-arginine was the amidino-acceptor of amidinotransferases in both the Micromonospora sp. and Streptomyces mobaraensis clusters, while the Salinispora tropica enzyme recognized l-valine. Unexpectedly, the S. tropica enzyme accepted several different amino acids as amidino acceptors in addition to l-valine. Accordingly, we re-investigated the specific metabolites governed by the gene cluster of S. tropica and identified several minor congeners of ketomemicin C with different N-terminal amidino-amino acids. These results indicate that the amidinotransferase of S. tropica is promiscuous and could be useful to generate new ketomemicin-type natural products.