The Divalent Cations Ca2+ and Mg2+ Play Specific Roles in Stabilizing Histone–DNA Interactions within Nucleosomes That Are Partially Redundant with the Core Histone Tail Domains

We previously reported that reconstituted nucleosomes undergo sequence-dependent translational repositioning upon removal of the core histone tail domains under physiological conditions, indicating that the tails influence the choice of position. We report here that removal of the core histone tail domains increases the exposure of the DNA backbone in nucleosomes to hydroxyl radicals, a nonbiased chemical cleavage reagent, indicative of an increase in the motility of the DNA on the histone surface. Moreover, we demonstrate that the divalent cations Mg2+ and Ca2+ can replace the role of the tail domains with regard to stabilization of histone–DNA interactions within the nucleosome core and restrict repositioning of nucleosomes upon tail removal. However, when nucleosomes were incubated with Mg2+ after tail removal, the original distribution of translational positions was not re-established, indicating that divalent cations increase the energy barrier between translational positions rather than altering the...