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

Area of Science:

• Molecular Biology
• Virology
• Structural Biology

Background:

• Genomic DNA organization into nucleosomes is a eukaryotic hallmark.
• Minimalist histones in archaea and fused histone homologs in giant viruses challenge this paradigm.
• Marseilleviridae, a giant virus subfamily, encodes fused homologs of eukaryotic histones.

Purpose of the Study:

• To investigate the role and structure of viral doublet histones in Marseilleviridae.
• To determine the necessity of these histones for viral infectivity and lifecycle.
• To elucidate the structural characteristics of viral nucleosome-like particles.

Main Methods:

• Demonstration of viral doublet histone essentiality for infectivity.
• Localization studies within infected host cells and mature virions.
• Cryogenic electron microscopy (cryo-EM) for structural analysis of viral nucleosome-like particles.

Main Results:

• Viral doublet histones are crucial for viral infectivity.
• These histones localize to cytoplasmic viral factories and are present in mature virions.
• Cryo-EM structures reveal eukaryotic nucleosome-like particles with unique histone connectors and structural histone tails.

Conclusions:

• Viral nucleosomes share structural similarities with eukaryotic counterparts despite sequence divergence.
• Unique histone connectors influence viral nucleosome stability and histone tail function.
• These findings broaden the scope of organisms requiring nucleosomes and suggest specialized viral functions for histones.