Low resolution models of self-assembled histone fibers from X-ray diffraction studies

Abstract

X-ray diffraction data from self-assembled histone fibers are presented for three systems: H4, H3-H4, and the four core histones H2A, H2B, H3 and H4. These data have been obtained under conditions of high ionic strength and high protein concentration which are thought to promote histone conformation similar to that found in intact chromatin. The low angle equatorial scattering (R less than .05 A-1) is analysed, and, with additional constraints imposed by electron microscopy data, four low resolution fibrillar models are derived. Two features common to all the possible models are a maximum outer diameter of approximately 60 A and a subfibril diameter of approximately 25 A. It is the interference of the protein subfibrils across a central region of low electron density - a 10 A "hole" - which gives rise to the characteristic diffraction peak at 36 A. Possible relationships of the models of the histone fibers to the structure of the histone component of chromatin are suggested.