Melbournevirus encodes a shorter H2B-H2A doublet histone variant that forms structurally distinct nucleosome...

Area of Science:

• Virology
• Structural Biology
• Molecular Biology

Background:

• Giant viruses possess unique biological features, including the use of histones for genome organization.
• Melbournevirus, a member of the Marseilleviridae family, encodes canonical and variant histone doublets.

Purpose of the Study:

• To characterize the distinct H2B-H2A histone doublet variant in Melbournevirus.
• To elucidate the structural and functional implications of this variant in nucleosome formation and DNA binding.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of reconstituted nucleosome-like particles.
• Biochemical analysis of histone variant abundance and DNA binding capacity.

Main Results:

• Melbournevirus encodes a distinct H2B-H2A histone variant with a truncated H2B and divergent amino acid sequence.
• This variant, less abundant than the main doublet, is conserved in Marseilleviridae and essential for viral fitness.
• Cryo-EM revealed that variant nucleosomes bind significantly less DNA (90 bp) compared to eukaryotic or main viral nucleosomes.
• Structural differences in the variant H2B-H2A doublet lead to reduced DNA binding and lower nucleosome stability.

Conclusions:

• The unique histone variant in Melbournevirus facilitates less stable nucleosomes, potentially enabling rapid genome unpacking for gene expression.
• Giant viruses demonstrate remarkable adaptability in repurposing host or viral components like histones for their unique biological needs.