Peptide-derived natural products are a class of metabolites that afford the producing organism a selective advantage over other organisms in their biological niche. While the polypeptide antibiotics produced by the nonribosomal polypeptide synthetases (NRPS) are the most widely recognized, the ribosomally synthesized and post-translationally modified peptides (RiPPs) are an emerging group of natural products with diverse structures and biological functions. Both the NRPS derived peptides and the RiPPs undergo extensive post-translational modifications to produce structural diversity. Here we report the first characterization of the six cysteines in forty-five (SCIFF) [Haft, D. H. and Basu M. K. (2011) J. Bacteriol. 193, 2745-2755] peptide maturase Tte1186, which is a member of the radical S-adenosyl-l-methionine (SAM) superfamily. Tte1186 catalyzes the formation of a thioether cross-link in the peptide Tte1186a encoded by an orf located upstream of the maturase, under reducing conditions in the presence of SAM. Tte1186 contains three [4Fe-4S] clusters that are indispensable for thioether cross-link formation; however, only one cluster catalyzes the reductive cleavage of SAM. Mechanistic imperatives for the reaction catalyzed by the thioether forming radical SAM maturases will be discussed.