Figure 2.

(A) Proposed mechanism for carbon-centered methylation catalyzed by RlmN; the enzyme is first conventionally methylated on Cys355 by SAM; the reaction is then initiated by hydrogen abstraction from the thiomethyl group by Ado•. (B) Proposed mechanism for methylthiolation catalyzed by MiaB. Methylation by SAM of a persulfide ion (derived from the pentasulfide bridge) ligated to the unique iron of the auxiliary [Fe₄S₄] cluster precedes the radical SAM chemistry to generate Ado•, which abstracts hydrogen from the substrate. The substrate radical then attacks the methylated sulfur atom to afford methythiolated product. (C) Crystal structure of holo-TmRimO. (Right) ribbon diagram showing UPF0004 domain (red), radical SAM domain (blue) and TRAM domain (pink). (Left) A pentasulfide bridge links the two unique iron atoms in each of the Fe₄S₄ clusters, with iron and sulfur shown in orange and yellow.