title: The bi-loop, a new general four-stranded DNA motif.
abstract: The crystal structure of the cyclic octanucleotide d<pATTCATTC> contains two independent molecules that form a novel quadruplex by means of intermolecular Watson-Crick A.T pairs and base stacking. A virtually identical quadruplex composed of G.C pairs was found by earlier x-ray analysis of the linear heptamer d(GCATGCT), when the DNA was looped in the crystal. The close correspondence between these two structures of markedly dissimilar oligonucleotides suggests that they are both examples of a previously unrecognized motif. Their nucleotide sequences have little in common except for two separated 5'-purine-pyrimidine dinucleotides forming the quadruplex, and by implication these so-called "bi-loops" could occur widely in natural DNA. Such structures provide a mechanism for noncovalent linking of polynucleotides in vivo. Their capacity to associate by base stacking, demonstrated in the crystal structure of d(GCATGCT), creates a compact molecular framework made up of four DNA chains within which strand exchange could take place.
authors.affiliation: Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, United Kingdom.
authors.forename: S A
authors.initials: SA
authors.lastname: Salisbury
authors.forename: S E
authors.initials: SE
authors.lastname: Wilson
authors.forename: H R
authors.initials: HR
authors.lastname: Powell
authors.forename: O
authors.initials: O
authors.lastname: Kennard
authors.forename: P
authors.initials: P
authors.lastname: Lubini
authors.forename: G M
authors.initials: GM
authors.lastname: Sheldrick
authors.forename: N
authors.initials: N
authors.lastname: Escaja
authors.forename: E
authors.initials: E
authors.lastname: Alazzouzi
authors.forename: A
authors.initials: A
authors.lastname: Grandas
authors.forename: E
authors.initials: E
authors.lastname: Pedroso
mesh_terms: D001482:Base Composition; D001483:Base Sequence; D003460:Crystallization; D018360:Crystallography, X-Ray; D003596:Cytosine; D004247:DNA / Q000737:chemistry*; D006147:Guanine; D006860:Hydrogen Bonding; D008957:Models, Genetic; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D009838:Oligodeoxyribonucleotides / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D009838:Oligodeoxyribonucleotides; D006147:Guanine; D003596:Cytosine; D004247:DNA