Fig. 1.

The sui gene cluster and the reaction catalyzed by SuiB. (A) The sui gene cluster is highly homologous to str. It encodes a 22-mer precursor peptide (SuiA), a tailoring radical SAM enzyme (SuiB), and a putative transporter/protease (SuiC). The sequence of SuiA is depicted with the 8-mer sequence of the mature product shown in bold. Cross-linked residues are shown in red. (B) SuiB catalyzes Lys–Trp cross-link formation in SuiA. The new bond installed is shown in red. (C) SuiB contains three [4Fe–4S] clusters and multiple functional domains. (D) The radical SAM domain (blue) forms a partial TIM barrel that is laterally closed by the auxiliary cluster-containing SPASM domain (green). The canonical catalytic [4Fe–4S] cluster-binding motif following β1 is shown in yellow. The bridging region and N-terminal domains are shown in light blue and red, respectively. (E) Placement of the three [4Fe–4S] clusters, shown in ball and stick representation (Fe, orange; S, yellow). Distances are calculated between the nearest atoms. (F) Topologies of the N-terminal domain, SPASM domain, and bridging region.