Topology variation and loop structural homology in crystal and simulated structures of a bimolecular DNA quadruplex

The topology of DNA quadruplexes depends on the nature and number of the nucleotides linking G-quartet motifs. To assess the effects of a three-nucleotide TTT linker, the crystal structure of the DNA sequence d(G(4)T(3)G(4)) has been determined at 1.5 A resolution, together with that of the brominated analogue d(G(4)(Br)UTTG(4)) at 2.4 A resolution. Both sequences form bimolecular intermolecular G-quadruplexes with lateral loops. d(G(4)(Br)UTTG(4)) crystallized in the monoclinic space group P2(1) with three quadruplex molecules in the asymmetric unit, two associating together as a head-to-head stacked dimer, and the third as a single head-to-tail dimer. The head-to-head dimers have two lateral loops on the same G-quadruplex face and form an eight-G-quartet stack, with a linear array of seven K(+) ions between the quartets. d(G(4)T(3)G(4)) crystallized in the orthorhombic space group C222 and has a structure very similar to the head-to-tail dimer in the P2(1) unit cell. The sequence studied here is able to form several different folds; however, all four quadruplexes in the two structures have lateral loops, in contrast to the diagonal loops reported for the analogous quadruplex with T(4) loops. A total of seven independent T(3) loops were observed in the two structures. These can be classified into two discrete conformational classes, suggesting that these represent preferred loop conformations that are independent of crystal-packing forces.