Giant variations in giant virus genome packaging.

Nucleocytoviricota have a common ancestral morphogenesis module with an internal membrane, a double jelly roll major capsid protein that forms icosahedral capsids, and a FtsK-HerA family ATPase that translocates DNA into the capsid, but nevertheless exhibit variations in genome packaging. Vaccinia virus and Mollivirus use their double jelly roll capsid protein to shape their internal membranes but lack an icosahedral capsid. Mimivirus encloses its genome in a sheath made of nonessential glucose–methanol–choline oxidoreductases. Marseillevirus uses histone doublets to package its genomes in a dense nucleosome fiber without linkers or genic phasing that resembles a model of a columnar telomeric nucleosome fiber. Medusaviruses and pandoraviruses encode multiple packaging ATPases of unknown function, including doublet ATPases. Nucleoproteins from phycodnaviruses bind DNA with a similar affinity to histones, but their function remains unexplored. Giant viruses (Nucleocytoviricota) have a largely conserved lifecycle, yet how they cram their large genomes into viral capsids is mostly unknown. The major capsid protein and the packaging ATPase (pATPase) comprise a highly conserved morphogenesis module in giant viruses, yet some giant viruses dispense with an icosahedral capsid, and others encode multiple versions of pATPases, including conjoined ATPase doublets, or encode none. Some giant viruses have acquired DNA-condensing proteins to compact their genomes, including sheath-like structures encasing folded DNA or densely packed viral nucleosomes that show a resemblance to eukaryotic nucleosomes at the telomeres. Here, we review what is known and unknown about these ATPases and condensing proteins, and place these variations in the context of viral lifecycles. [ABSTRACT FROM AUTHOR]