Organization of nucleosomes and spacer DNA in chromatin fibres

In this paper we analyse in detail the orientation of X-ray diffraction diagrams obtained from the following materials: nucleosome cores, whole nuclei and the sodium and thallium salts of H1-depleted nucleohistone and of briefly digested chromatin. Our analysis indicates that spacer DNA is organized in bundles of parallel segments which contribute to the equatorial maxima in the diagrams. Several models are compatible with this organization, in particular a modified solenoid model in which the central part is filled with such a bundle of spacer DNA segments parallel to the axis of the fibre. It is also shown that spacer DNA is covered by histones, probably the N-terminal regions. This observation indicates that the differential activity of nucleases on chromatin is strongly influenced by conformational features of DNA. An analysis of the orientation of the low angle rings found in the X-ray diffraction patterns of H1-depleted nucleohistone shows that the 11 nm peak has maxima which are ～ 0.007 nm^?1 off the meridian. The 5.5 and 8 nm peaks have a meridional maximum plus two side maxima which occur at spacings between 0.02 and 0.055 nm^?1 from the meridian, depending on the conditions. A comparison of these results with those reported by Finch et al.¹ for crystals indicates that in fibres the nucleosome cores are arranged with their short axis perpendicular to the axis of the fibre. Some evidence on the path of DNA in the nucleosome cores is also obtained.