(A) Structure of BioB, FeS clusters, and bound AdoMet and DTB. BioB exists as a homodimer in solution (11) and we find two possible dimeric relationships between BioB monomers in the crystal. The dimer shown here buries 17.6% of the monomer surface area (13,249 Å2) and is likely to be physiologically relevant. An anomalous Fourier electron density map, calculated with data collected at the Fe absorption peak wavelength (1.73827 Å) and phases from the polypeptide portion of the model, is contoured at 3σ in black mesh. These electron density peaks represent the positions of the four FeS clusters in this dimeric structure. There are no other features of a similar size in the electron density map. The FeS clusters are shown as ball-and-stick representations, with brown Fe atoms and yellow S atoms. In addition, we find one AdoMet (red) and one DTB (green) per subunit. Figures 1A and 2 were prepared with PyMOL (42). (B) Topology diagram of the BioB TIM barrel showing the location of important residues with respect to the β strands (arrows, numbered 1 to 8). The numbers to the left of each β strand correspond to the N-terminal residue of that secondary structure element. Ligands to the Fe4S4 cluster are in black, ligands to the Fe2S2 cluster are in red, and residues that contact the Fe2S2 cluster ligand Arg260 are in pink. AdoMet contacts (blue) include Ala100, Trp102, and Arg173, which form hydrogen bonds to the amino acid moiety; Asp155 and Asn153, which form hydrogen bonds to the ribose hydroxyl groups (Fig. 2A); Tyr59 and Ile192, which stack against the adenine ring (Fig. 2A); and Val225, which forms backbone hydrogen bonds to the adenine ring. Residues in position to form hydrogen bonds to DTB (brown) include Asn151, Asn153, and Asn222, which contact the DTB ureido ring (Fig. 2C), and Thr292 and Thr293, which contact the carboxylate tail. Asterisks denote main-chain interactions.