Structural basis for tRNA methylthiolation by the radical SAM enzyme MiaB.

Numerous post-transcriptional modifications of transfer RNAs have vital roles in translation. The 2-methylthio-N-isopentenyladenosine (msiA) modification occurs at position 37 (A37) in transfer RNAs that contain adenine in position 36 of the anticodon, and serves to promote efficient A:U codon-anticodon base-pairing and to prevent unintended base pairing by near cognates, thus enhancing translational fidelity. The msiA modification is installed onto isopentenyladenosine (iA) by MiaB, a radical S-adenosylmethionine (SAM) methylthiotransferase. As a radical SAM protein, MiaB contains one [FeS] cluster used in the reductive cleavage of SAM to form a 5'-deoxyadenosyl 5'-radical, which is responsible for removing the C hydrogen of the substrate. MiaB also contains an auxiliary [FeS] cluster, which has been implicated in sulfur transfer to C of iA37. How this transfer takes place is largely unknown. Here we present several structures of MiaB from Bacteroides uniformis. These structures are consistent with a two-step mechanism, in which one molecule of SAM is first used to methylate a bridging µ-sulfido ion of the auxiliary cluster. In the second step, a second SAM molecule is cleaved to a 5'-deoxyadenosyl 5'-radical, which abstracts the C hydrogen of the substrate but only after C has undergone rehybridization from sp to sp. This work advances our understanding of how enzymes functionalize inert C-H bonds with sulfur.