Differential scanning calorimetry of nuclei reveals the loss of major structural features in chromatin by brief nuclease treatment.

Abstract

Differential scanning calorimetry revealed that chromatin melts in 4 distinct transitions in intact HeLa nuclei at 60.degree. C, 76.degree. C and 88.degree. C. Calorimetry of whole cells was characterized by the same 4 transitions along with another at 65.degree. C, which was probably RNA. Isolated chromatin melted in only 2 transitions at 72.degree. C and 85.degree. C. Very brief digestion of HeLa nuclei with either micrococcal nuclease or DNase I resulted in the conversion of a structure that melted at 105.degree. C to one that melted at 88.degree. C. Further digestion with micrococcal nuclease to the level of the mononucleosome did not result in any further substantial changes in either enthalpy or melting temperatures. Further DNase I digestion that resulted in cleavage within the nucleosome produced a pronounced shift in melting temperatures to 2 broad transitions at 62.degree. C and 78.degree. C.