a, A diagram of the secondary structures found in the N-terminal domain (blue), central helical bundle domain (orange), and C-terminal cupin domain (green) of SznF. Feii ligands and proposed active-site residues are indicated as black dots. Disordered regions are shown as dashed lines. The cupin Feii-binding site is depicted as a circle. b, An Fe anomalous difference map (orange mesh, contoured at 5.0σ) is shown for the fully occupied mononuclear histidine-coordinated Feii site (orange sphere) in the cupin domain. Selected amino acids are shown in stick format. c, The central domain contains a partially occupied (around 50%) iron-binding site in selected crystals, with a smaller peak in the iron anomalous difference map (orange mesh, contoured at 3.0σ).