How living organisms create carbon-sulfur bonds during the biosynthesis of critical sulfur-containing compounds is still poorly understood. The methylthiotransferases MiaB and RimO catalyze sulfur insertion into tRNAs and ribosomal protein S12, respectively. Both belong to a subgroup of radical-S-adenosylmethionine (radical-SAM) enzymes that bear two [4Fe-4S] clusters. One cluster binds S-adenosylmethionine and generates an Ado• radical via a well-established mechanism. However, the precise role of the second cluster is unclear. For some sulfur-inserting radical-SAM enzymes, this cluster has been proposed to act as a sacrificial source of sulfur for the reaction. In this paper, we report parallel enzymological, spectroscopic and crystallographic investigations of RimO and MiaB, which provide what is to our knowledge the first evidence that these enzymes are true catalysts and support a new sulfation mechanism involving activation of an exogenous sulfur cosubstrate at an exchangeable coordination site on the second cluster, which remains intact during the reaction.