title: Self-association of a DNA loop creates a quadruplex: crystal structure of d(GCATGCT) at 1.8 A resolution.
abstract: BACKGROUND
  The flexibility of DNA enables it to adopt three interconvertible types of duplex termed the A-, B- and Z-forms. It can also produce hairpin loops, triplex structures and guanine-rich quadruplex structures. Conformational flexibility assists in the tight packaging of DNA, for example in chromosomes. This is important given the large quantity of genetic information that must be packaged efficiently. Moreover, the ability of DNA to specifically self-associate or interact with complementary sequences is fundamental to many biological processes. Structural studies provide information about DNA conformation and DNA-DNA interactions and suggest features that might be relevant to how the molecule performs its biological role.
  
  
  RESULTS
  We have characterized the structure of a synthetic heptanucleotide that folds into a novel loop structure. The loop is stabilized by association with a cation, by intra-strand hydrogen bonds between guanine and cytosine that are distinct from the normal Watson-Crick hydrogen bonds, and by van der Waals interactions. Two loops associate through the formation of four G.C pairs that exhibit pronounced base-stacking interactions. The formation of a symmetric A.A base pair further stabilizes loop dimerization. Stacking of the A.A pair on a symmetry-related A.A pairing assists the formation of a four-stranded assembly. A T.T pairing is also observed between symmetry-related loops.
  
  
  CONCLUSIONS
  This analysis provides a rare example of an experimentally determined non-duplex DNA structure. It provides conformational detail relevant to the tight packaging or folding of a DNA strand and illustrates how a cation might modulate phosphate-phosphate repulsion in a tightly packed structure. The observation of base quartets involving G.C base pairs suggests a further structure to be considered in DNA-DNA interactions. The structure also provides detailed geometries for A.A and T.T base pairs.
authors.affiliation: Department of Chemistry, University of Manchester, UK.
authors.forename: G A
authors.initials: GA
authors.lastname: Leonard
authors.forename: S
authors.initials: S
authors.lastname: Zhang
authors.forename: M R
authors.initials: MR
authors.lastname: Peterson
authors.forename: S J
authors.initials: SJ
authors.lastname: Harrop
authors.forename: J R
authors.initials: JR
authors.lastname: Helliwell
authors.forename: W B
authors.initials: WB
authors.lastname: Cruse
authors.forename: B L
authors.initials: BL
authors.lastname: d'Estaintot
authors.forename: O
authors.initials: O
authors.lastname: Kennard
authors.forename: T
authors.initials: T
authors.lastname: Brown
authors.forename: W N
authors.initials: WN
authors.lastname: Hunter
mesh_terms: D001482:Base Composition; D001483:Base Sequence; D003460:Crystallization; D004247:DNA / Q000648:ultrastructure*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation
chemical_list: D004247:DNA