Genome Mining and Biosynthesis Study of a Type B Linaridin Reveals a Highly Versatile α-N-Methyltransferase

Abstract

Linaridins are a small but growing family of natural products belonging to the ribosomally synthesized and post-translationally modified peptide (RiPP) superfamily. In this study, a genome mining approach led to the identification of a novel linaridin, mononaridin (MON), from Streptomyces monomycini. In-frame deletion genetic knockout studies showed that, in addition to many genes essential for MON biosynthesis, monM encodes an S-adenosyl methionine (SAM)-dependent α-N-methyltransferase that is responsible for installing two methyl groups in the MON N-terminus. Besides SAM, MonM also accepts ethyl-SAM and allyl-SAM, in which the methyl of SAM is replaced by an ethyl and an allyl, respectively. We showed that ethyl-SAM and allyl-SAM have distinct reactivities in MonM catalysis, and this observation was further investigated in detail by density functional theory (DFT) calculations. Remarkably, MonM acts efficiently on nisin, a prototypic lantibiotic that is structurally very different from the native substrate, and the ability of MonM to transfer an allyl group to the nisin N-terminus allowed production of a fluorescently labeled nisin, which can be further used in microscopic cell analysis. Our studies provide new insights into linaridin biosynthesis and demonstrate the potential of linaridin methyltransferases in bioengineering applications.