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 comb ined NMR-molecular dynamics approach including relaxation matrix refin ement. This four G(4) repeat sequence folds intramolecularly into a ri ght-handed G-tetraplex containing four stacked G-tetrads which are con nected by two lateral T-4 loops and a central diagonal T-4 loop. The g uanine glycosidic bonds adopt a syn-anti alternation along the full le ngth 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 na rrow 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 c onformations. The solution structure of the Na+-stabilized Oxytricha d [G(4)(T(4)G(4))(3)] G-tetraplex and an earlier solution structure repo rted 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 sam e syn/anti distribution of guanine residues along individual strands a nd around individual G-tetrads, as well as a common central diagonal l oop which defines the strand directionalities. The well-resolved proto n NMR spectra associated with the d[G(4)(T(4)G(4))(3)] G-tetraplex ope ns the opportunity for studies ranging from cation-dependent character ization of G-tetraplex conformation and hydration to ligand and protei n recognition of the distinct grooves associated with this folding top ology. (C) 1995 Academic Press Limited