SOLUTION STRUCTURE OF A PARALLEL-STRANDED TETRAPLEX FORMED BY D(TG(4)T) IN THE PRESENCE OF SODIUM-IONS BY NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

DNA sequences containing repetitive oligo-guanine sequences are observ ed in telomeric DNA and in recombination hotspots. In vitro, these seq uences fold into at least two classes of unusual conformations involvi ng the formation of guanine tetrads (G-quartets). Through the use of N MR, we have established that in solution in the presence of sodium ion s, the oligodeoxynucleotide dTG(4)T forms a parallel-stranded tetraple x, with hydrogen-bonded guanine tetrads. Here we present the detailed evidence for this configuration in 100 mM sodium ions at neutral pH. W e show that the parallel-stranded tetraplex structure is stabilised re lative to the single-strands when Na ions are replaced by K ions, with out any evidence of significant change in those chemical shifts associ ated with the tetraplex form. We also present a model for the Na+ solu tion structure of the tetraplex, by back-calculation from NOE volumes. We obtain excellent agreement with the data for structures with chara cteristics similar to B-DNA for the internal tetrads, G3 and G4. The o uter tetrads, particularly the 5'-terminal G2, display some unstacking . We see no evidence for formation of thymine tetrads analogous to the uridine tetrads observed in RNA, but rather we find the terminal thym ine conformations are not well determined by our data. me observe an u ninterrupted cylindrical channel through the centre of the complex.