SOLUTION STRUCTURE OF THE TETRAHYMENA TELOMERIC REPEAT D(T(2)G(4))(4)G-TETRAPLEX

Background: Telomeres in eukaryotic organisms are protein-DNA complexe s which are essential for the protection and replication of chromosoma l termini. The telomeric DNA of Tetrahymena consists of T(2)G(4) repea ts, and models have been previously proposed for the intramolecular fo lded structure of the d(T(2)G(4))4 sequence based on chemical footprin ting and cross-linking data. A high-resolution solution structure of t his sequence would allow comparison with the structures of related G-t etraplexes. Results: The solution structure of the Na+-stabilized d(T( 2)G(4))(4) sequence has been determined using a combined NMR-molecular dynamics approach. The sequence folds intramolecularly into a right-h anded G-tetraplex containing three stacked G-tetrads connected by link er segments consisting of a G-T-T-G lateral loop, a central T-T-G late ral loop and a T-T segment that spans the groove through a double chai n reversal. The latter T-T connectivity aligns adjacent G-G-G segments in parallel and introduces a new G-tetraplex folding topology with un precedented combinations of strand directionalities and groove widths, as well as guanine syn/anti distributions along individual strands an d around individual G-tetrads. Conclusions: The four repeat Tetrahymen a and human G-tetraplexes, which differ by a single guanine for adenin e substitution, exhibit strikingly different folding topologies. The o bserved structural polymorphism establishes that G-tetraplexes can ado pt topologies which project distinctly different groove dimensions, G- tetrad base edges and linker segments for recognition by, and interact ions with, other nucleic acids and proteins.