Structural characterization of a new crystal form of the four-way Holliday junction formed by the DNA sequence d(CCGGTACCGG)2: sequence versus lattice?

DNA-strand exchange is a vital step in the recombination process, of which a key intermediate is the four-way DNA Holliday junction formed transiently in most living organisms. Here, the single-crystal structure at a resolution of 2.35 A of such a DNA junction formed by d(CCGGTACCGG)(2), which has crystallized in a more highly symmetrical packing mode to that previously observed for the same sequence, is presented. In this case, the structure is isomorphous to the mismatch sequence d(CCGGGACCGG)(2), which reveals the roles of both lattice and DNA sequence in determining the junction geometry. The helices cross at the larger angle of 43.0 degrees (the previously observed angle for this sequence was 41.4 degrees) as a right-handed X. No metal cations were observed; the crystals were grown in the presence of only group I counter-cations.