Structure and phase diagram of Nucleosome Core Particles aggregated by Multivalent cations

¹ Nogales lab. Berkeley
² Université Paris-Sud, Bat. 510
³ IBBMC
⁴ Université Paris Sud, Bat. 510

^* To whom correspondence should be addressed. E-mail: mangenot{at}lps.u-psud.fr.

Submitted on March 7, 2007
Revised on April 9, 2007
Accepted on 20 July 2007

	Abstract

The degree of compaction of the eukaryotic chromatin in vivo and in vitro is highly sensitive to the ionic environment. We address the question of the effect of multivalent ions on the interactions and mutual organization of the chromatin structural units, the Nucleosome Core Particles (NCPs). Conditions of precipitation of NCPs in the presence of 10 mM Tris buffer and various amounts of either magnesium (Mg²⁺) or spermidine (Spd³⁺) are explored, compared, and discussed in relation to theoretical models. In addition, the structure of the aggregates is analyzed by complementary techniques: freeze-fracture EM, Cryo-EM and X-rays diffraction. NCPs tend to stack on top of one another to form columns that are not organized long-ranged in Mg²⁺-NCPs aggregates. In the presence of Spd³⁺, NCPs precipitate to form a dense isotropic phase, a disordered phase of columns, a 2D columnar hexagonal phase or a 3D crystal. The more ordered phases (2D or 3D hexagonal) are found close to the precipitation line, where the number of positive charges carried by cations is just above the number of available negative charges of the NCPs. They coexist with the dense isotropic phases. The formation of the hexagonal and columnar phases is prevented by an excess of polycations.

Key Words: Nucleosome Core particle, Phase diagram, Precipitation, mesophase, supramolecular organization