The interaction of core histones with DNA: equilibrium binding studies

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Abstract
The binding of core histone proteins to DNA, neasured as a function of [NaCl] is a reversible process. Dissociation and reassociation occurs in two stages. Between 0.7 and 1.2 M NaCl H2a and H2b bind non-cooperatively as an equimolar complex with ΔGo=1.6 Kcals/mole at 4°C and 1.0 M NaCl. Between 1.2 and 2.0 M NaC1 H3 and H4 bind cooperatively as an equimolar complex with ΔGo=7.4 Kcals/mole at 4°C and 1.0 M NaCl. The proper binding of H2a and H2b requires the presence of bound H3 and H4. Nuclease digestion of H3-H4 DNA produces a tetramer of H3-H4 bound to fragments of DNA 145, 125 and 104 base pairs long. Thus an H3-H4 tetramer can protect fragments of DNA as long as those found in complete core particles and must therefore span the nucleosome core particle.