Differential scanning calorimetry of chromatin at different levels of condensation

The thermal denaturation of calf thymus total chromatin and of fractions en riched in heterochromatin or euchromatin, has been investigated by differen tial scanning calorimetry and compared to that of calf thymus DNA and DNA-h istone complexes. In our experimental conditions, chromatin melts in three thermal transitions: the main one, assigned to separation of the DNA double helix, occurs at 83 degreesC, while the other two occur at 63 degreesC and 74 degreesC. The data show that: (a) the transition enthalpy for denaturat ion of DNA in the total chromatin and in DNA-histone complexes is nearly th e same as that of DNA in solution; (b) the transition at 63 degreesC is pre sent in the thermogram of the heterocromatin enriched fraction, while it is completely absent in that of the euchromatin enriched one. The results sug gest that this transition can be attributed to the higher order structures of heterochromatin.