Widom 601 arrays
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A giant virus genome is densely packaged by stable nucleosomes within virions. The two doublet histones of Marseillevirus are distantly related to the four eukaryotic core histones and wrap 121 basepairs of DNA to form remarkably similar nucleosomes. By permeabilizing Marseillevirus virions and performing genome-wide nuclease digestion, chemical cleavage and mass spectrometry assays, we find that the higher-order organization of Marseillevirus chromatin fundamentally differs from that of eukaryotes. Marseillevirus nucleosomes fully protect DNA within virions as closely abutted 121-bp DNA wrapped cores without linker DNA or phasing along genes. Likewise, we observed that nucleosomes reconstituted onto multi-copy tandem repeats of a nucleosome positioning sequence are tightly packed. Dense promiscuous packing of fully wrapped nucleosomes rather than “beads-on-a-string” with genic punctuation represents a new mode of DNA packaging by histones. We suggest that doublet histones have evolved for viral genome protection and may resemble an early stage of histone differentiation leading to the eukaryotic octameric nucleosome. DNA sequences of Widom 601 multi-copy arrays used for nucleosome reconstitutions.
病毒粒子（virion）内，巨型病毒基因组由稳定的核小体（nucleosome）紧密包装。马赛病毒（Marseillevirus）的两种双联体组蛋白（doublet histones）与四种真核核心组蛋白（eukaryotic core histones）亲缘关系较远，可包裹121碱基对的DNA，形成结构极为相似的核小体。通过透化马赛病毒粒子，并开展全基因组范围的核酸酶消化（nuclease digestion）、化学裂解（chemical cleavage）与质谱分析（mass spectrometry）实验，我们发现马赛病毒染色质（chromatin）的高级组织结构与真核生物存在根本性差异。马赛病毒的核小体可在病毒粒子内完全保护DNA，以紧密邻接的121 bp DNA包裹核心结构存在，无连接DNA（linker DNA），也不沿基因呈现相位规律。同理，我们观察到，将核小体重组装至核小体定位序列（nucleosome positioning sequence）的多拷贝串联重复序列上时，核小体呈现紧密堆积状态。由完全包裹DNA的核小体形成的密集、混杂的堆积模式，而非带有基因边界分隔的“串珠状”结构，代表了组蛋白介导DNA包装的一种全新方式。我们推测，双联体组蛋白的演化是为了保护病毒基因组，其可能代表了组蛋白分化早期的状态，该分化过程最终形成了真核生物的八聚体核小体（octameric nucleosome）。本研究中用于核小体重组装的Widom 601多拷贝阵列的DNA序列。
