G-DNA - A TWICE-FOLDED DNA-STRUCTURE ADOPTED BY SINGLE-STRANDED OLIGO(DG) AND ITS IMPLICATIONS FOR TELOMERES

Our dimethyl sulfate modification experiments suggest that (dG)(n) stretches within single-stranded DNA fragments, which represent the simplest model for telomeric sequences, adopt a complex intrastrand structure other than a simple hairpin. We present a molecular model for the DNA structure that conforms to dimethyl sulfate methylation data. The principal element of this G-DNA structure is a quadruple helix formed by pairwise antiparallel segments of the twice-folded (dG)(n) stretch. This quadruple core has two wide and two narrow grooves connected by three loop-shaped segments. The strong stacking interactions of the neighboring guanine tetrads and the large number of hydrogen bonds formed can be the primary reasons that such structures are favored over a common hairpin for long (dG)(n) stretches. Such compact structures may be formed from (dG)(n) stretches of telomeric sequences.