Conformation of nucleosome core particles and chromatin in high salt concentration

The conformation of nucleosome core particles and chromatin under different ionic strength conditions has been studied by electron microscopic, hydrodynamic, and spectroscopic techniques. In the range of ionic strength used (6--600 mM), all four core histones were bound to the DNA. The sedimentation coefficient of the core particle decreases from 11.3 in 6 mM NaCl to 9.4 in 600 mM NaCl, and an alteration of the circular dichroic spectrum was observed when the ionic strength was increased. Direct evidence for the alteration of the chromatin structure in high salt was obtained by electron microscopy where a very extended conformation of the nucleosome was observed. The protein cross-linking agent dimethylsuberimidate was used to study the histone--histone proximities in the core particles; our experiments reveal that the same histones are in contact in the extended particles and in the compact native particles.