Discovery of an Unnatural DNA Modification Derived from a Natural Secondary Metabolite

Abstract

Despite widespread interest for understanding how modified bases have evolved their contemporary functions, limited experimental evidence exists for measuring how close an organism is to accidentally creating a new, modified base within the framework of its existing genome. Here, we describe the biochemical and structural basis for how a single-point mutation in E. coli's naturally occurring cytosine methyltransferase can surprisingly endow a neomorphic ability to create the unnatural DNA base, 5-carboxymethylcytosine (5cxmC), invivo. Mass spectrometry, bacterial genetics, and structure-guided biochemistry reveal this base to be exclusively derived from the natural but sparse secondary metabolite carboxy-S-adenosyl-L-methionine (CxSAM). Our discovery of a new, unnatural DNA modification reveals insights into the substrate selectivity of DNA methyltransferase enzymes, offers a promising new biotechnological tool for the characterization of the mammalian epigenome, and provides an unexpected model for how neomorphic bases could arise in nature from repurposed host metabolites.