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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.