Chromatin conformation and salt-induced compaction: three-dimensional structural information from cryoelectron microscopy

Cryoelectron microscopy has been used to examine the three-dimensional (3-D) conformation of small oligonucleosomes from chicken erythrocyte nuclei after vitrification in solutions of differing ionic strength. From tilt pairs of micrographs, the 3-D location and orientation of the nucleosomal disks, and the paths of segments of exposed linker can be obtained. In "low-salt " conditions (5 mM NaCI, 1 mM EDTA, pH 7.5), the average trinucleosome assumes the shape of an equilateral triangle, with nucleosomes at the vertices, and a length of exposed linker DNA between consecutive nucleosomes equivalent to ~46 bp. The two linker DNA segments converge at the central nucleosome. Removal of histones H1 and H5 results in a much more variable trinucleosome morphology, and the two