Temperature and osmotic stress dependence of the thermodynamics for binding linker histone H1, Its carboxyl domain (H1-C) or globular domain (H1-G) to B-DNA.

Linker histones (H1) are the basic proteins in higher eukaryotes that are responsible for the final condensation of chromatin. In contrast to the nucleosome core histone proteins, the role of H1 in compacting DNA is not clearly understood. In this study ITC was used to measure the binding constant, enthalpy change, and binding site size for the interactions of H1, or its C-terminal (H1-C) and globular (H1-G) domains to highly polymerized calf-thymus DNA at temperatures from 288 K to 308 K. Heat capacity changes, ΔC, for these same H1 binding interactions were estimated from the temperature dependence of the enthalpy changes. The enthalpy changes for binding H1, H1-C, or H1-G to CT-DNA are all endothermic at 298 K, becoming more exothermic as the temperature is increased. The ΔH for binding H1-G to CT-DNA is exothermic at temperatures above approximately 300 K. Osmotic stress experiments indicate that the binding of H1 is accompanied by the release of approximately 35 water molecules. We estimate from our naked DNA titration results that the binding of the H1 to the nucleosome places the H1 protein in close contact with approximately 41 DNA bp. The breakdown is that the H1 carboxyl terminus interacts with 28 bp of linker DNA on one side of the nucleosome, the H1 globular domain binds directly to 7 bp of core DNA, and shields another 6 linker DNA bases, 3 bp on either side of the nucleosome where the linker DNA exits the nucleosome core.