Fig. 5. Structural details of HUαα-DNA assembly reveal promiscuity in HUαα-DNA and HUαα-HUαα interfaces.

a–b Orthogonal views of HUαα-DNA structures at pH 4.5 and pH 5.5 combining several asymmetric units (see also Supplementary Figs. 4 and 5a–c). a At pH 4.5, two distinct HUαα-HUαα interaction interfaces are seen–the highly electrostatic E34-K37 interface and the HUαα-HUαα arm β-zipper (some DNA strands have been removed for clarity). Distances between two sets of parallel DNA that correspond to diffraction peaks observed in solution scattering are highlighted. Inset–previously reported crystal structure of HUαα with artificially distorted DNA (PDBID: 1P71) with HUαα-arms intercalating between base pairs in the DNA minor grooves. b At pH 5.5, HUαα–HUαα interaction interface between residues Q5 and D5 is highlighted (some DNA strands have been removed for clarity). Distances between two sets of parallel DNA that correspond to diffraction peaks observed in solution scattering are highlighted. c Non-specific DNA-binding mode of HUαα results in several non-superimposable HUαα-DNA interfaces relative to the phosphate lock. Superimposition of the distinct HUαα-DNA interfaces at pH 4.5 and pH 5.5 reveals the ball-socket joint around the DNA minor grove. d Superimposition of the HUαα–HUαα interface from the crystal structure of HUαα–DNA at the pH 4.5 with interface seen in the crystal structure at the pH 5.5 (left panel) or crystal structure of HUα^E34Kα^E34K-DNA (right panel). Unpaired HUαα dimers at pH 5.5 and HUα^E34Kα^E34K dimers shows the 3 Å and 5 Å shift along the DNA minor grove relatively to the coupled HUαα dimers at pH 4.5 (see also Supplementary Figure 5d and Supplementary Movie 2).