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.

长期以来，将基因组 DNA 组织成确定的核小体一直被视为真核生物的标志。这种范式受到古细菌中“极简主义”组蛋白的鉴定以及最近发现的编码马赛病毒科四种真核组蛋白的融合远程同源物的基因的挑战，一种感染变形虫的巨型病毒亚科。我们证明病毒双联组蛋白对于病毒感染性至关重要，在病毒感染后定位于细胞质病毒工厂，并最终在成熟的病毒粒子中发现。尽管序列识别有限，但病毒核小体样颗粒的低温电子显微镜 (cryo-EM) 结构显示出与真核核小体的强烈相似性。连接组蛋白链的独特连接器有助于观察到病毒核小体的不稳定性，并且一些组蛋白尾部承担结构性角色。我们的结果进一步扩大了需要核小体的“生物体”的范围，并表明组蛋白在这些不寻常病毒的生物学中具有特殊的功能。