Effect of the B--Z transition in poly(dG-m5dC) . poly(dG-m5dC) on nucleosome formation.

We have studied the properties of complexes formed between histones and the methylated synthetic polydeoxynucleotide poly(dG-m5dC). poly(dG-m5dC). This polymer undergoes the transition from B DNA to left-handed Z DNA at moderate ionic strength. When the polymer is in the Z form it will bind histones, but nucleosomes are not detected. When the polymer in the B form is combined with equimolar quantities of the four core histones and digested with micrococcal nuclease, particles are formed which behave in all respects as normal nucleosome cores. When these core particles are placed in solvents that would result in conversion of the protein-free polymer to the Z form, no transition is observed. The formation of a nucleosome core particle thus stabilizes the B form, whereas the presence of the Z form prevents nucleosome formation. The results suggest that if Z DNA is present in eukaryotic nuclei, it will serve to disrupt the normal chromatin structure.