A set of closely related methyltransferases for site-specific tailoring of anthraquinone pigments

Abstract

Modification of the polyketide anthraquinone AQ-256 in the entomopathogenic Photorhabdusluminescens involves several O-methylations, but the biosynthetic gene cluster antA-I lacks corresponding tailoring enzymes. We here describe the identification of five putative, highly homologous O-methyltransferases encoded in the genome of P.luminescens. Activity assays invitro and deletion experiments invivo revealed that three of them account for anthraquinone tailoring by producing three monomethylated and two dimethylated species of AQ-256. X-ray structures of all five enzymes indicate high structural and mechanistic similarity. As confirmed by structure-based mutagenesis, a conserved histidine at the active site likely functions as a general base for substrate deprotonation and subsequent methyl transfer in all enzymes. Eight complex structures with AQ-256 as well as mono- and dimethylated derivatives confirm the substrate specificity patterns found invitro and visualize how single amino acid differences in the active-site pockets impact substrate orientation and govern site-specific methylation.