Interactions between arginine-rich histones and deoxyribonucleic acids. I. Thermal denaturation.

Physical properties of histone-DNA complexes very often depend upon the method of complex formation. In an attempt to make the studies of histone-DNA interactions more relevant to biological systems, results from thermal denaturation of native chromatin were used as references for determining how closely a given histone-DNA complex approaches its native state in chromatin. In the case of arginine-rich histones H3 (III or f3) and H4 (IV or f2a1), four methods were used for making complexes with calf thymus DNA: (A) NaCl gradient dialysis with urea; (B) NaCl gradient dialysis without urea; (C) direct mixing in 2.5 x 10(-4) EDTA, pH 8.0; and (D) direct mixing in 0.01 M sodium phosphate, pH 7.0. It was observed that a complex made by direct mixing in phosphate (method D) is closer to the native than is one made by direct mixing in EDTA (method C) than the one made by gradient dialysis with urea (method A) or without urea (method B). Regardless of the method used for complex formation, no substantial differences were observed between complexes with histone H3 dimer with disulfide bond(s) and a reduced H3 without disulfide bond, implying that perhaps a dimer with or without disulfide bond is a natural fundamental subunit in our experimental conditions. When the method of direct mixing in EDTA is used, the melting properties of the complexes vary only slightly with any one of the following H3 histones: from calf thymus, H3 without disulfide bond, H3 dimer, and H3 oligomer with disulfide bonds, also, from duck erythrocyte, H3 monomer and dimer. The complexes formed between DNA and a mixture of H3 and H4 by method D have melting properties similar to those of native chromatin. Since an equimolar mixture of histone H3 and H4 in 0.01 M phosphate, pH 7.0, was shown to form a tetramer (D'Anna, J.A., and Isenberg, I. (1974), Biochem. Biophys. Res. Commun. 61, 343), our results suggest that, a tetramer of H3 and H4, likely to be (H3)2(H4)2, formed from one H3 dimer and one H4 dimer, can bind DNA in a manner similar to that in native chromatin.