Interaction of the Escherichia coli HU protein with DNA. Evidence for formation of nucleosome-like structures with altered DNA helical pitch.

Nuclease digestion studies of DNA bound to the histone-like protein HU show that cuts in each strand of the DNA double helix are made with a periodicity of 8.5 base-pairs. By contrast, similar digestions of DNA in eukaryotic nucleosomes show a repeat of 10.4 base-pairs. This and other results (including circular dichroism studies) are consistent with the proposal that the pitch of the DNA double helix in the HU complex is reduced from a repeat length of 10.5 to 8.5 base-pairs per helical turn. Simultaneously, the DNA in the HU-DNA complex containing two dimers of HU per 60 base-pairs has its linking number decreased by 1.0 turn per 290 base-pairs. From these changes it is calculated that HU imposes a DNA writhe of 1.0 per three to four monomers of HU. The results suggest a model in which DNA is coiled in left-handed toroidal supercoils on the HU complex, having a stoichiometry resembling that of the half-nucleosome of eukaryotic chromatin. An important distinction is that HU complexes can restrain the same number of DNA superhelical turns as eukaryotic nucleosomes, yet the DNA retains more negative torsional tension, just as is observed in prokaryotic chromosomes in vivo. Another distinction is that HU-DNA complexes are less stable, having a dissociation half-life of 0.6 min in 50 mM-NaCl. This last property may explain prior difficulties in detecting prokaryotic nucleosome-like structures.