Roles of histone chaperone Nap1 and histone acetylation in regulating phase-separation of nucleosome arrays

Chromatin condensation is dynamically regulated throughout the cell cycle and plays key roles in modulating gene accessibility. The DNA-histone dynamics in the nucleosome are central to the regulation mechanisms of chromatin condensation, which remain poorly understood. Employing fluorescence recovery after photobleaching, optical super-resolution imaging, and microrheology with optical tweezers, we investigated the roles of various parameters in regulating phase-separation of 12-mer nucleosome arrays. Here, we show that histone H4 tail lysine residues are the main drivers of liquid-liquid phase separation of nucleosome arrays. We also show that the condensed liquid-like droplets comprise a mobile fraction and a relatively immobile structural scaffold. Histone chaperone Nap1 and histone H3 tail acetylation enhance DNA-histone dynamics within this scaffold, thereby lowering the overall viscosity of the droplets. These results suggest that histone chaperone and histone H3/H4 tails play critical roles in regulating chromatin condensation and gene accessibility in condensed chromatin.