In G4s, four G residues from different runs assemble to form a planar G-quartet that is stabilized by Hoogsteen hydrogen bonding.

Our results suggest that higher-order quadruplex assemblies may be a common feature within the genome, rather than simple single quadruplex structures.ĭNA and RNA sequences with four runs of typically three consecutive guanine residues separated by 1–7 nucleotides may fold into stable four-stranded structures called G-quadruplexes (G4s) under defined solution conditions ( 1, 2). We also report methodological advances in our integrated structural biology approach, which now includes quantitative CD for counting stacked G-tetrads, DNaseI cleavage for hairpin detection and SAXS model refinement. Each structure exhibits interfacial regions between stacked quadruplexes or novel loop geometries that are possible druggable targets. A longer c-Kit sequence forms a quadruplex-hairpin structure. One longer sequence from c-Myc forms a structure with three contiguous stacked quadruplexes. Three sequences form structures with two contiguous, stacked, G-quadruplex units. Each sequence studied has a unique structure. We report here five structures of long (34–70 nt) wild-type sequences selected from three cancer-related promoters: c-Myc, c-Kit and k-Ras. The formal resolution of our approach is 18 angstroms, but in some cases structural features of only a few nucleotides can be discerned. Our approach uses quantitative biophysical tools (analytical ultracentrifugation, small-angle X-ray scattering, and circular dichroism spectroscopy) combined with modeling and molecular dynamics simulations, to derive self-consistent structural models. We report on higher-order G-quadruplex structures adopted by long promoter sequences obtained by an iterative integrated structural biology approach.

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