Marseillevirus G648 Genome Assembly

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. Marseillevirus G648 Genome Assembly G648_26-12-14_Genome_vD.fasta病毒粒子内的巨型病毒基因组通过稳定的核小体（nucleosome）实现致密包装。马赛病毒（Marseillevirus）的两种双联组蛋白与真核生物的四类核心组蛋白亲缘关系较远，可缠绕121个碱基对（basepair）的DNA，形成结构极为相似的核小体。通过透化处理马赛病毒粒子，并开展全基因组范围的核酸酶消化、化学裂解与质谱（mass spectrometry）检测实验，我们发现马赛病毒染色质的高级组织结构与真核生物存在本质差异。马赛病毒的核小体可完全保护病毒粒子内的DNA，其DNA缠绕核心区域紧密邻接，长度为121bp，无连接DNA，也不沿基因呈现周期性排布。同样地，我们观察到以核小体定位序列的多拷贝串联重复序列重构得到的核小体同样呈致密排列。这种由完全包裹的核小体所进行的致密且无特定排布模式的包装，而非带有基因分隔特征的‘串珠状结构（beads-on-a-string）’，代表了组蛋白介导DNA包装的一种全新方式。我们推测，双联组蛋白的演化是为了实现病毒基因组的保护，其可能类似于组蛋白分化形成真核八聚体核小体的早期阶段。马赛病毒G648基因组组装序列：G648_26-12-14_Genome_vD.fasta