FIGURE 4.

Crystal structure of the dimeric C-terminal domain of FbiB. A, overlay of F₄₂₀-bound (yellow) and FMN-bound (blue) structures. F₄₂₀ and FMN molecules are shown as ball-and-stick models with carbon atoms colored in yellow and cyan, respectively. Selected secondary structure elements are labeled. B, electron density (2F_o − F_o, omit map) surrounding F₄₂₀ and FMN molecules in their respective structures. C, multiple sequence alignment of FbiB (C-terminal domain), the putative FMN-dependent nitroreductase from C. difficile (Protein Data Bank entry 3GFA), and a nitroreductase from A. tumefaciens (Protein Data Bank entry 3K6H). Secondary structure elements are shown for the FbiB structure, and residues involved in F₄₂₀ and FMN binding are indicated by open circles and solid stars, respectively. Many residues involved in FMN binding appear conserved across the three structures, whereas those involved in F₄₂₀ binding are not conserved. D, overlay of enlarged view of FbiB (yellow) and Protein Data Bank entry 3GFA (white). Incorporation of an F₄₂₀ binding site (left) produces a large shift in the top of helix α4 and in the β2-α5 loop compared with the C. difficile homolog structure. Another large difference occurs at the bottom of the molecule (right), involving the β5-β6 loop, that opens up in FbiB to produce the binding site for the phosphate group of FMN.