A model for chromatin sub-structure incorporating symmetry considerations of histone oligomers

Symmetry considerations of the kind of structures which can be generated when dimers of histones f2al-f3 and f2a2-f2b interact lead to the following conclusions: chromatin subunits based on closed-shell structures give rise to discrete, non-interacting nucleoprotein subunits with the histones arranged at random along the DNA chain; open structures based on infinite helices give rise to highly ordered, regular arrangements of dimers. A model is proposed in which helical polymers of f2al-f3 and f2b-f2a2 form a central core with the DNA helically arranged around it. The helical repeat contains 9.6 turns of B-form DNA and one molecule each of f2al, f2a2, f2b, f3 and f1. The pitch of the helix is 53Angstrom and the other diameter 130Angstrom. The protein molecular repeat is 106Angstrom.