A repeating unit of higher order chromatin structure in chick red blood cell nuclei.

The organization of nucleosomes in higher order chromatin structures has been studied by electron microscopy of chick red blood cell nuclei. Chromatin appears as a thick fiber with an average diameter of approximately 300 A when prepared for electron microscopy in buffers which approximate physiological ionic strength. Progressive steps of disassembly of the thick fiber into individual nucleosomes could be induced either by ionic strength reduction or by tRNA treatment (which removes histone H1 and some non-histone chromosomal proteins). When disassembly was induced by ionic strength reduction in the presence of Mg++ (or Ca++), the lengths of the intermediate disassembly products were found to be multiples of 330 A. The diameter of these structures was estimated to be 275 A. This intermediate in the disassembly process is not observed if thick fiber disassembly is induced by ionic strength reduction in the absence of divalent cations. To investigate whether the higher order structural unit is present in the thick fiber at physiological ionic strengths, tRNA treatment was used to induce thick fiber disassembly under physiological monovalent ionic conditions. In this case, either with or without divalent cations, a supranucleosomal unit was found with dimensions similar to those given above. This data provides evidence for a slightly oblong supranucleosomal structure (330 x 275 A) whick forms a repeating unit in the chromatin thick fiber.