Figure 2.

The refined model superimposed on a 2Fo-Fc electron density map of the TAR RNA Ca1-binding site contoured at 1.3 σ. Ca1 (yellow sphere) stabilizes the bulge backbone structure through direct ligands with the phosphate oxygens of U23, C24, and G26. Three water molecules, depicted as cyan-colored spheres, complete the octahedral coordination. All Ca²⁺-phosphate oxygen distances are 2.3 Å. All Ca²⁺-water distances are 2.4 Å.