Bryson_MOLECULAR-CELL-D-22-00555_Figure_7D

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.
Assembly of Marseillevirus histones onto Widom 601 arrays produces 147-bp nucleosomes. Tapestation gel analysis of DNA from Marseillevirus tetramers and Xenopus octamers assembled using a 3-copy Widom 601 array with zero linker (441 bp) after 5 min MNase digestion and DNA purification. Histone:DNA ratios are indicated for each lane 0.25, 0.5, 1, 1.5. Drosophila melanogaster S2 cell chromatin was digested in parallel as a control (D.m.).