Crystallization and characterization of the thallium form of the Oxytricha nova G-quadruplex

The crystal structure of the Tl+ form of the G-quadruplex formed from the Oxytricha nova telomere sequence, d(G(4)T(4)G(4)), has been solved to 1.55 angstrom. This G-quadruplex contains five Tl+ ions, three of which are interspersed between adjacent G-quartet planes and one in each of the two thymine loops. The structure displays a high degree of similarity to the K+ crystal structure [Haider et al. (2002), J. Mol. Biol., 320, 189-200], including the number and location of the monovalent cation binding sites. The highly isomorphic nature of the two structures, which contain such a large number of monovalent binding sites (relative to nucleic acid content), verifies the ability of Tl+ to mimic K+ in nucleic acids. Information from this report confirms and extends the assignment of Tl-205 resonances from a previous report [Gill et al. (2005), J. Am. Chem. Soc., 127, 16 723-16 732] where Tl-205 NMR was used to study monovalent cation binding to this G-quadruplex. The assignment of these resonances provides evidence for the occurrence of conformational dynamics in the thymine loop region that is in slow exchange on the Tl-205 timescale.