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Biophys J, November 1999, p. 2725-2735, Vol. 77, No. 5

Chromatin Condensation Is Confined to the Loop and Involves an All-or-None Structural Change

Cecilia Balbi,* Paola Sanna,* Paola Barboro,* Ingles Alberti,* Marta Barbesino,*# and Eligio Patrone#

 *Istituto Nazionale per la Ricerca sul Cancro, 16132 Genoa, and  #Istituto di Studi Chimico-Fisici di Macromolecole Sintetiche e Naturali, 16146 Genoa, Italy

Using differential scanning calorimetry in combination with pulsed field gel electrophoresis, we relate here the changes in the thermal profile of rat liver nuclei induced by very mild digestion of chromatin by endogenous nuclease with the chain length distribution of the DNA fragments. The enthalpy of the endotherm at 106°C, which reflects the denaturation of the heterochromatic domains, decreases dramatically after the induction of a very small number of double-strand breaks per chromosome; the thermal transition disappears when the loops have undergone on average one DNA chain scission event. Quantitative analysis of the experimental data shows that the loop behaves like a topologically isolated domain. Also discussed is the process of heterochromatin formation, which occurs according to an all-or-none mechanism. In the presence of spermine, a strong condensation agent, only the loops that have undergone one break are able to refold, in confirmation of the extremely cooperative nature of the transition. Furthermore, our results suggest a relationship between the states that give rise to the endotherms at 90°C and 106°C and the morphologies referred to as class II and class III in a previous physicochemical study of the folding of chromatin fragments (Widom, 1986. J. Mol. Biol. 190:411-424) and support the view that the overall process of condensation follows a sequential (two-step) pathway.

Biophys J, November 1999, p. 2725-2735, Vol. 77, No. 5
© 1999 by the Biophysical Society   0006-3495/99/11/2725/11  $2.00




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Copyright © 1999 by the Biophysical Society.