Helical repeat of DNA in the nucleosome core particle

tAlthough the crystal structure of nucleosome core particle is essentially symmetrical in the vicinity of the dyad, the linker histone binds asymmetri cally in this region to select a single high-affinity site from potentially two equivalent sites. To try to resolve this apparent paradox we mapped to basepair resolution the dyads and rotational settings of nucleosome core p articles reassembled on synhetic tandemly repeating 20 bp DNA sequences. In agreement with previous observations, we observed(1) that the helical repe at on each side of the dyad cluster is 10 bp maintaining register with the sequence repeat and (2) that this register changes by 2 bp in the vicinity of the dyad. The additional 2 bp required to effect the change in the rotat ional settings is accommodated by an adjustment immediately adjacent to the dyad. At the dyad the hydroxyl radical cleavage is asymmetric and we sugge st that the inferred structural asymmetry could direct the binding of the l inker histone to a single preferred site.