New insights into pradimicin biosynthesis revealed by two O-methyltransferases

Pradimicins are a group of antiviral and antifungal natural products from Actinomadura hibisca. Two putative O-methyltransferase genes, pdmF and pdmT, are present in the pradimicin biosynthetic gene cluster. However, there is only one methoxy group (11-OCH₃) in pradimicins. Through heterologous expression and in vitro reactions with various substrates, PdmF was characterized as the C-11 O-methyltransferase with a relatively broad substrate specificity. To probe the role of PdmT in pradimicin biosynthesis, the corresponding gene was disrupted through homologous recombination, leading to the production of pradimicinone II. This enzyme was then expressed in Escherichia coli with an N-terminal His₆ tag and purified by Ni-NTA chromatography. Reaction of pradimicinone II with PdmT generated 7-O-methylpradimicinone II, confirming that this enzyme is a C-7 O-methyltransferase. Characterization of PdmT suggests a novel pathway that leads to the “flip” of 7-OH to C-14 in pradimicin biosynthesis.