In Physiological Salt Conditions the Core Proteins of the Nucleosomes in Large Chromatin Fragments Denature at 73 °C and the DNA Unstacks at 85 °C

Previous studies of chromatin melting had been done at low salt concentrations to maintain optically clear solutions permitting observation of hyperchromic effects that accompany the unstacking of bases in DNA. Scanning calorimetry does not require clear solutions and so allowed more nearly physiological levels of salt to be used in the present work. HeLa chromatin went through two major structural transitions of 73 and 85 degrees C in 150 mM NaCl, 1 mM MgCl2, 1 mM CaCl2. Proteinase K treatment eliminated the 73 degrees C transition without affecting the other one. DNase I digestion eliminated the 85 degrees C transition and shifted the other from 73 to 58 degrees C. Supported by previous reports that salt concentrations like these would largely eliminate the effect of histones on the Tm of DNA, the present observations indicate that the collapse of the core histone complex occurs at 73-76 degrees C (transition II) and the nucleosomal DNA unstacks at 85-87 degrees C (transition III). They also show that the thermal stability of the histone octamer is enhanced by the DNA folded around it in the nucleosome; although the histone core raises the Tm of the DNA in low salt, in physiological salt conditions it is the DNA that stabilizes the protein complex.