Figure 7

Stereo representation of the second Mg²⁺ion binding site within the active center of p21(mod)•GppNHp. (A) The displayed omit electron density map was calculated with the program REFMAC5 [16] by the randomized omit map procedure after removal of both Mg²⁺ ions and the coordinating water molecules around the second Mg²⁺ ion (in blue, 2F_obs-F_calc map with 2.0 σ cutoff level and in green, F_obs-F_calc map with 2.5 σ cutoff level; the figure was produced with the program PyMOL [24]). (B) Shown are residues 17, 33-36 and 60-66 (green sticks), the nucleotide which is coordinated to the two Mg²⁺ ions Mg-1 and Mg-2 (yellow spheres), and surrounding water molecules (cyan spheres). Both (A) and (B) show identical orientations of the active site with different zoom factors.