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Proton NMR study on a histone-like protein, HU alpha, from Escherichia coli and its complex with oligo DNAs.

It was confirmed that the flexible arm region of HU alpha forms an antiparallel beta-sheet and that all of the residues of phenylalanines, together with some of leucines and/or valines, form a hydrophobic core within the dimer of HU alpha. HU alpha protein alone is thermally labile and melts at 38 degrees C, but it becomes remarkably stabilized and melts at 59 degrees C in the presence of DNA. Several resonances from both HU alpha and DNA perturbed by their complex formation, notably those of His C-2 and C-4 protons, downfield shifted C alpha protons in the antiparallel beta-sheet, as well as Arg C delta and Lys C epsilon protons. The results indicated that a beta-sheet region of HU alpha binds to DNA, and also showed that rapid equilibrium occurs on the NMR time scale between bound and unbound states of HU alpha. A few intermolecular nuclear Overhauser effects (NOEs) were also observed between the protein and H1' protons of DNA in the complex, suggesting that HU alpha binds primarily to the minor groove of DNA.