Figure 2. Structural relationship between NFACT-N and FMN-DG domains. (A) Cartoon renderings of solved crystal structures (top) for the NFACT-N (left) and FMN-DG domains (right) accompanied by corresponding topology diagrams (bottom). Residues conserved across the domains are rendered as ball-and-stick in the cartoons; active site/enzymatic residues are colored in red, residues with a likely direct or indirect role in substrate recognition are colored in blue, and the conserved asparagine/histidine found in the HhH domains is colored in yellow. The PDB identifier is provided to the right of both cartoons. The labeling scheme provided below each diagram reflects the spatial/evolutionary conservation of each element as evident from the solved crystal structures and as referred to in the text. (B) Stepwise scenario for the emergence of the FMN-DG domain from the ancestral NFACT-N domain. Domains are depicted as arrays of secondary structure elements to show the wiring between elements.