Stability and reversibility of higher ordered structure of interphase chromatin: continuity of deoxyribonucleic acid is not required for maintenance of folded structure

Publisher Website
Google Scholar
Abstract
The organization of the higher order structure of chromatin was examined in chicken erythrocyte. Chromatin solubilized during the time course of a gentle micrococcal nuclease digestion of nuclei shows a continuous variation in the distribution of molecular weights. EM studies of large chromatin fragments solubilized at physiological ionic strength (0.14 M NaCl or KCl) suggest that the polynucleosome chain is folded in continuous compact structures of an average diameter of 23 nm in which the individual nucleosomes are difficult to distinguish. This compact structure is destabilized even at intermediate ionic strengths (e.g., 40 mM NaCl), resulting in looser fibers of similar diameter. At 5 mM NaCl the fiber is unraveled into a continuous filament of 10 nm diameter. These conformational changes are reversible as determined by hydrodynamic and biochemical parameters. The 10 nm .fwdarw. 23 nm transition of chromatin appears to be a cooperative process requiring the full complement of histones H1 and H5. Micrococcal nuclease cleaves the DNA in the compact chromatin structure to an apparent limit of digestion corresponding to an average of 8-9 nucleosomes with little effect on the size of the fiber. Thus, the continuity of the DNA apparently is not required for the stability of the folded chromatin fiber. Histones H1 and H5 exhibit a binding preference to larger chromatin fragments regardless of the length of the DNA. This behavior is not observed with relaxed chromatin, suggesting that multiple stabilizing interactions involving H1 (H5) are possible only in the compact configuration.