Thiostrepton tryptophan methyltransferase expands the chemistry of radical SAM enzymes

TsrM, a member of the radical SAM enzyme family, is shown to catalyze tryptophan methylation en route to thiostrepton A with the help of a methylcobalamin cofactor and without generating the canonical 5-deoxyadenosyl radical. Methylation is among the most widespread chemical modifications encountered in biomolecules and has a pivotal role in many major biological processes. In the biosynthetic pathway of the antibiotic thiostrepton A, we identified what is to our knowledge the first tryptophan methyltransferase. We show that it uses unprecedented chemistry to methylate inactivated sp 2 -hybridized carbon atoms, despite being predicted to be a radical SAM enzyme.