HYDRODYNAMIC PROPERTIES OF NUCLEOSOMAL FIBERS AND CHROMATIN HIGHER-ORDER STRUCTURE

We studied the comparative hydrodynamic properties of chromatins diffe ring in transcriptional activity and in linker DNA length. Chromatins were prepared from the nuclei of pigeon brain cortical neurones, rat t hymus and sea urchin sperm. The sizes of linker DNA are 20, 50 and 100 base pairs (b.p.), respectively. Oligonucleosomes of different chain length were isolated from micrococcal nuclease-digested nuclei and stu died by the method of sedimentation velocity. Rheological behavior of nucleosome oligomers depends on the number of nucleosomes in the chain and on the ionic strength in the range between 5 and 85 mM was analyz ed on the basis of a cylinder model using different hydrodynamic theor ies. It is shown that hydrodynamic properties of oligonucleosomes from all types of investigated chromatins at low ionic strength can be wel l described by the model of a three-dimensional zig-zag chain with a d iameter of about 20 nm, the DNA packing ratio growing with the increas e of linker DNA length. Increasing ionic strength induces condensation of the zig-zag-shaped chain of nucleosomes into a two-start double su perhelix with densely packed nucleosomes. Linkers are arranged inside the superhelix, which is bent or folded into a loop, depending on its size.