Fig. 6: A schematic of a tilted balance model for in vivo assembly of canonical octasomes and (H3-H4)-only octasomes that possibly represent a ‘fossil complex’ before emergence of the eukaryotic nucleosomes.
The dynamic process of nucleosome assembly is initiated with action of (H3-H4)2-tetramer being deposited onto a DNA template, creating a tetrasome platform that resembles their archaeal ancestors. The subsequent loading of H2A-H2B dimers leads to formation of the canonical octasome (eukaryotic core nucleosome). In the absence of H2A-H2B dimer, the non-canonical (H3-H4)4 octasomes may form, which represent a ‘fossil complex’ that marks the intermediate before emergence of the eukaryotic nucleosomes. The H2A-H2B dimers showed a greater tendency to bind (H3-H4)2 tetrasomes to form the eukaryotic core nucleosome, tilting evolution to eukaryotic structures.
