Cryo-electron microscopy structure of the H3-H4 octasome without histones H2A and H2B

Abstract

The canonical nucleosome, which represents the predominant packaging unit in eukaryotic chromatin, has an octameric core made up of two histone H2A-H2B and H3-H4 dimers with ~147 base-pair (bp) DNA wrapping around it. Non-nucleosome particles with alterative histone stoichiometries and DNA wrapping configurations have been found, and they could profoundly influence genome architecture and function. Here we solved the structure of the H3-H4 octasome, which is a nucleosome-like particle with a core made up of four H3-H4 dimers. Two conformations, open and closed, are determined at 3.9 [A] and 3.6 [A] resolutions by cryo-electron microscopy, respectively. The H3-H4 octasome, made up of a di-tetrameric core, is wrapped by ~120 bp DNA in 1.5 negative superhelical turns. The symmetrical halves are connected by a unique H4-H4 interface along the dyad axis. In vivo crosslinking of cysteine probes placed at another unique H3-H3 interface demonstrated the existence of the H3-H4 octasome in cells.