Extended Data Figure 2. Representative electron density and bound trivalent ions.

a, Unbiased, density-modified electron density from SAD phasing using data to 2.5 Å (grey mesh, 2σ), superimposed on the final model. The T-loop and part of the D-loop is shown. For simplicity, density and structure of water and ions is not shown. b, Final 2F_o-F_c electron density map after model building and refinement to 1.99 Å (2σ). c, Structure with the location of 12 Iridium (III) hexammine ions. Although many of these hexammine binding sites may also be Mg²⁺ binding sites important for stabilizing the fold, the trivalent hexammine was present at 8 mM and thus many weaker Mg²⁺ binding sites could have been occupied. For this reason, and because there is not a one-to-one correlation of Mg²⁺ binding sites and trivalent hexammine sites, we do not make conclusions about Mg²⁺ binding based on this structure.