Acidic polypeptides can assemble both histones and chromatin in vitro at physiological ionic strength.

We provide evidence that nucleosomes can assemble in vitro at physiological ionic strength (0.1-0.2 M NaCl/10 mM Tris . HCl, pH 8.0) in the absence of "assembly factors" and that poly(glutamic acid) greatly facilitates chromatin assembly under these conditions. We also show that in the presence of either poly(glutamic acid) or poly(aspartic acid), core histones assemble into octamers at physiological ionic strength. We suggest that it is a property of histones to assemble into octamers upon their interaction with macromolecules containing regions of high negative charge density, and we discuss several implications of this property.