Nucleomeric organization of chromatin.

In the nuclei fixed in situ, as well as in nuclei in low-ionic-strength solutions containing magnesium ions, chromatin is represented by globular nucleomeric fibrils 20–25 nm in diameter. Staphylococcal or endogenous nucleases cleave chromatin fibrils to nucleomers and multinucleomers. On removal of firmly bound magnesium, the nucleomers unfold into chains of four, six or eight nucleosomes. Mild staphylococcal nuclease digestion of nuclear chromatin releases mononucleomers, dinucleomers and trinucleomers that sediment in the sucrose density gradient in the presence of EDTA as 37-S, 47-S and 55-S particles, respectively. The mononucleomers in the sucrose density gradient with MgCl2 sediment as 45-S particles. The determination of the length of staphylococcal-nuclease-digested DNAs contained in the chromatin fragments showed that a nucleomer is composed of 8, and a dimer and trimer of 14–16 and 21–24 nucleosomes, respectively. When deprived of Mg2+ ions, the monomers lose their compactness (45 S) and become loose particles (37 S). This transition is completely reversible if nucleomers contain histone HI. Removal of this histone or dialysis of the nucleomer against EDTA at low ionic strength results in the complete unfolding of the nucleomer into a chain of nucleosomes. A structural model of a nucleomer fibril is suggested where the helicity of the nucleosome chain in a nucleomer (two turns of four nucleosomes each) is periodically discontinued. Such an organization of chromatin apparently provides additional hindrances for site-specific recognition of DNA in chromatin but permits local changes (within a single nucleomer) in chromatin when a hindrance is abolished.