Influence of Ions, Hydration, and the Transcriptional Inhibitor P4N on the Conformations of the Sp1 Binding Site

Three crystal structures containing the entire Sp1 consensus sequence d(GGGGCGGGG) with two or three additional base-pairs on either the 5′ or 3′ ends and overhangs have been determined. Despite the different lengths of DNA in the pseudo-dodecamers and pseudo-tridecamer, all three structures form A-DNA duplexes that share a common set of crystal contacts, including a T*(G·C) base triplet and a 5′-overhang that flips out and away from the helical axes to form a Hoogsteen base-pair with the 3′-overhang of a symmetry mate. The global conformations of the three structures differ, however, in the widths of their respective major grooves, the lengths of the molecules, and the extent of crystal packing. The structures were determined from crystals grown in an unusual precipitant for A-DNA, polyethylene glycol (PEG) 400, in combination with polyamines or ions; cobalt hexamine for the pseudo-tridecamer, and spermidine for the pseudo-dodecamers. As the Sp1 binding site is a target for antiviral and anticancer drugs, pseudo-dodecamer crystals were soaked with one such antiviral and anticancer compound, P4N. Although P4N was not visualized unambiguously in the electron density maps, the effect of the drug is evident from significant differences in the lattice constants, crystal packing, and overall conformation of the structure.