Virus-encoded histone doublets are essential and form nucleosome-like structures

The organization of genomic DNA into defined nucleosomes has long been viewed as a hallmark of eukaryotes. This paradigm has been challenged by the identification of minimalist'' histones in archaea and more recently by the discovery of genes that encode fused remote homologs of the four eukaryotic histones in Marseilleviridae, a subfamily of giant viruses that infect amoebae. We demonstrate that viral doublet histones are essential for viral infectivity, localize to cytoplasmic viral factories after virus infection, and ultimately are found in the mature virions. Cryogenic electron microscopy (cryo-EM) structures of viral nucleosome-like particles show strong similarities to eukaryotic nucleosomes despite the limited sequence identify. The unique connectors that link the histone chains contribute to the observed instability of viral nucleosomes, and some histone tails assume structural roles. Our results further expand the range of organisms'' that require nucleosomes and suggest a specialized function of histones in the biology of these unusual viruses.

Automated summary

Recent studies have challenged the traditional view of nucleosome organization in eukaryotes by identifying minimalist histones in archaea and discovering genes encoding remote homologs of eukaryotic histones in giant viruses. The presence of viral doublet histones, essential for viral infectivity, suggests a specialized function of histones in the biology of these unusual viruses. Cryo-EM structures of viral nucleosome-like particles show similarities to eukaryotic nucleosomes despite limited sequence identity, further expanding the range of organisms that require nucleosomes.