The structure of a virus-encoded nucleosome

Certain large DNA viruses, including those in the Marseilleviridae family, encode histones. Here we show that fused histone pairs Hβ-Hα and Hδ-Hγ from Marseillevirus are structurally analogous to the eukaryotic histone pairs H2B-H2A and H4-H3. These viral histones form ‘forced’ heterodimers, and a heterotetramer of four such heterodimers assembles DNA to form structures virtually identical to canonical eukaryotic nucleosomes. The cryo-EM structure of DNA-assembled histone pairs Hβ-Hα and Hδ-Hγ from Marseillevirus, a nucleocytoplasmic large DNA virus, reveals that these proteins form viral nucleosomes with highly conserved features when compared to canonical eukaryotic nucleosomes.