Histonelike proteins in prokaryotes and histone octamers in eukaryotes carry large positive charges, which are responsible of strong electrostatic interactions with DNA. As a result, DNA wraps around proteins and genetic information is condensed. We describe a generalized model of these electrostatic interactions mediated by salt that explains the wrapping of DNA around the nucleosome octamer, around remodeling factors in eukaryotes and around histonelike proteins in prokaryotes. It comes out that small changes in protein dimension and charge produce large effects in the supramolecular DNA-protein architecture.

The interaction free energy of like-charged polyelectrolytes in solution is calculated in the framework of the extended counterion condensation theory, recently given by Schurr and Fujimoto, Biophys. Chem. 2002, 101-102, 425-445. For sufficiently high linear charge density, the electrostatic free energy of two parallel identical rigid polyelectrolytes as a function of the distance between them shows a minimum at distances in the range of nanometers, increasing with the Debye screening length. This effect is due to the increasing of the counterion condensed charge and condensation volume as the two polyelectyrolytes approach.

Is linker DNA bent in the 30-nm chromatin fiber at physiological conditions? We show here that electrostatic interactions between linker DNA and histone tails including salt condensation and release may bend linker DNA, thus affecting the higher order organization of chromatin.

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