Sa. Salisbury et al., THE BI-LOOP, A NEW GENERAL 4-STRANDED DNA MOTIF, Proceedings of the National Academy of Sciences of the United Statesof America, 94(11), 1997, pp. 5515-5518
The crystal structure of the cyclic octanucleotide d[pATTCATTC] contai
ns two independent molecules that form a novel quadruplex by means of
intermolecular Watson-Crick A.T pairs and base stacking, A virtually i
dentical quadruplex composed of G.C pairs was found by earlier x-ray a
nalysis 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 examp
les of a previously unrecognized motif. Their nucleotide sequences hav
e little in common except for two separated 5'-purine-pyrimidine dinuc
leotides 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 st
ructure of d(GCATGCT), creates a compact molecular framework made up o
f four DNA chains within which strand exchange could take place.