Cryo-electron tomography reveals the multiplex anatomy of condensed native chromatin and its unfolding by histone citrullination

Nucleosome chains fold and self-associate to form higher order structures whose internal organization is unknown. Here, cryo-electron tomography (cryo-ET) of native human chromatin reveals novel folding motifs such as 1) non-uniform nucleosome stacking, 2) intermittent parallel and perpendicular orientations of adjacent nucleosome planes, and 3) an inverse zigzag nucleosome chain path, which deviates from the direct zigzag topology seen in reconstituted nucleosomal arrays. By examining these self-associated structures, we observed prominent nucleosome stacking in-cis and anti-parallel nucleosome interactions in-trans, which are consistent with partial nucleosome interdigitation. Histone citrullination strongly inhibits nucleosome stacking and self-association with a modest effect on chromatin folding, while the reconstituted arrays showed a zigzag topology which undergoes a dramatic unfolding induced by histone citrullination. This study sheds light on the internal structure of compact chromatin nanoparticles and suggests a novel mechanism for how epigenetic changes in chromatin are retained across both open and condensed forms of chromatin.