SOLUTION STRUCTURE OF THE OXYTRICHA TELOMERIC REPEAT D[G(4)(T(4)G(4))(3)] G-TETRAPLEX

The solution structure of Oxytricha telomere sequence d[G(4)(T(4)G(4))(3)] in 0.1 M Na+ containing solution has been determined using a combined NMR-molecular dynamics approach including relaxation matrix refinement. This four G(4) repeat sequence folds intramolecularly into a right-handed G-tetraplex containing four stacked G-tetrads which are connected by two lateral T-4 loops and a central diagonal T-4 loop. The guanine glycosidic bonds adopt a syn-anti alternation along the full length of the d[G(4)(T(4)G(4))(3)] sequence while the orientation around adjacent G-tetrads switches between syn . syn . anti . anti and anti . anti . syn . syn alignments. Four distinct grooves are formed by the parallel (two of medium width) and anti-parallel (one wide and one narrow width) alignment of adjacent G-G-G-G segments in the G-tetraplex. The T-4 residues in the diagonal loop are well-defined while the T-4 residues in both lateral loops are under-defined and sample multiple conformations. The solution structure of the Na+-stabilized Oxytricha d[G(4)(T(4)G(4))(3)] G-tetraplex and an earlier solution structure reported from our laboratory on the Na+-stabilized human d[AG(3)(T(2)AG(3))(3)] G-tetraplex exhibit a common folding topology defined by the same syn/anti distribution of guanine residues along individual strands and around individual G-tetrads, as well as a common central diagonal loop which defines the strand directionalities. The well-resolved proton NMR spectra associated with the d[G(4)(T(4)G(4))(3)] G-tetraplex opens the opportunity for studies ranging from cation-dependent characterization of G-tetraplex conformation and hydration to ligand and protein recognition of the distinct grooves associated with this folding topology. (C) 1995 Academic Press Limited