Hydrodynamic Studies on Defined Heterochromatin Fragments Support a 30-nm Fiber Having Six Nucleosomes per Turn

We have compared the physical properties of a 15.51-kb constitutive heterochromatin segment and a 16.17-kb facultative heterochromatin segment that form part of the chicken β-globin locus. These segments were excised from an avian erythroleukemia cell line by restriction enzyme digestion and released from the nucleus, thus allowing measurement of the sedimentation coefficients by use of calibrated sucrose gradients. A determination of the buoyant density of the cross-linked particle in CsCl led to the total mass of the particles and their frictional coefficients, f. Despite the slight differences in nucleosome density, the measured value of f for both fragments was consistent with a rodlike particle having a diameter of 33–45 nm and a length corresponding to approximately six to seven nucleosomes per 11-nm turn. At higher ionic strengths we found no evidence of any abrupt conformational change, demonstrating that these chromatin fragments released from the nucleus did not assume the more compact conformations recently described for some reconstituted structures.