Crystal structure of a three-tetrad, parallel, K⁺-stabilized human telomeric G-quadruplex at 1.35 ? resolution

The crystal structure of the G-rich human telomeric DNA Tel22 has been determined at 1.35 angstrom resolution in space group P6. Tel22 forms a non-canonical DNA structure called the G-quadruplex. The space group and unit-cell parameters are comparable to those in the crystal structures with PDB codes 6ip3 (1.40 angstrom resolution) and 1kf1 (2.15 angstrom resolution). The G-quadruplexes are highly similar in all of the structures. However, this structure of Tel22 displays clear density for polyethylene glycol and two potassium ions, which are located outside the ion channel in the G-quadruplex and play an important role in stabilizing the crystal contacts. In addition, 111 water molecules were identified (compared with 79 and 68 in PDB entries 6ip3 and 1kf1, respectively) that participate in intricate and extensive networks providing high stability to the G-quadruplex.