A Radically Different Mechanism for S-Adenosylmethionine–Dependent Methyltransferases

Methylation of small molecules and macromolecules is crucial in metabolism, cell signaling, and epigenetic programming and is most often achieved by S -adenosylmethionine (SAM)–dependent methyltransferases. Most employ an S N 2 mechanism to methylate nucleophilic sites on their substrates, but recently, radical SAM enzymes have been identified that methylate carbon atoms that are not inherently nucleophilic via the intermediacy of a 5′-deoxyadenosyl 5′-radical. We have determined the mechanisms of two such reactions targeting the sp 2 -hybridized carbons at positions 2 and 8 of adenosine 2503 in 23 S ribosomal RNA, catalyzed by RlmN and Cfr, respectively. In neither case is a methyl group transferred directly from SAM to the RNA; rather, both reactions proceed by a ping-pong mechanism involving intermediate methylation of a conserved cysteine residue.