title: The superstructure of chromatin and its condensation mechanism. I. Synchrotron radiation X-ray scattering results.
abstract: Synchroton radiation X-ray scattering experiments have been performed on chicken erythrocyte chromatin fibres over a wide range of ionic conditions and on various states of the fibres (i.e. "native" in solution, in gels and in whole nuclei; chromatin depleted of the H1 (H5) histones and chromatin with bound ethidium bromide). A correlation between the results obtained with the various chromatin preparations provides evidence for a model according to which at low ionic strength the chromatin fibre already possesses a helical superstructure, with a diameter comparable to that of condensed chromatin, held together by the H1(H5) histone. The most significant structural modification undergone upon an increase of the ionic strength is a reduction of the helix pitch, this leads to condensation in a manner similar to the folding of an accordion. The details of this process depend on whether monovalent or divalent cations are used to raise the ionic strength, the latter producing a much higher degree of condensation. Measurements of the relative increase of the mass per unit length indicate that the most condensed state is a helical structure with a pitch around 3.0-4.0 nm. In this paper we give a detailed presentation of the experimental evidence obtained from static and time-resolved scattering experiments, which led to this model.
mesh_terms: D000818:Animals; D002467:Cell Nucleus / Q000648:ultrastructure; D002645:Chickens; D002843:Chromatin / Q000648:ultrastructure*; D004247:DNA / Q000097:blood; D004912:Erythrocytes / Q000648:ultrastructure; D006657:Histones / Q000097:blood; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D002843:Chromatin; D006657:Histones; D004247:DNA