The Mimivirus 1.2 Mb dsDNA genome is elegantly organized into a nuclear-like weapon

Abstract Mimivirus is the prototype of the Mimiviridae family of giant dsDNA viruses, initially isolated in Acanthamoeba1. Little is known about the organization of the viral genome inside the membrane limited nucleoid2 and whether unpacking or other rearrangements are required prior to transcription and replication. Here we show that opening of its large icosahedral capsid in vitro leads to the release of electron dense, 30 nm diameter rod-shaped objects that appear to be expelled from the particles and unwinding. We developed a purification procedure and characterized the detailed structure at various stages of decompaction using cryo-electron microscopy single particle analyses and its composition by proteomics. This revealed that the viral genome is encased into a helical protein shell surprisingly made of the two GMC-type oxydoreductases that are also the major components of the glycosylated fibrils surrounding the capsid3. The 1.2 Mb genome is folded to follow a 5- or 6-start left-handed helix, depending on the nature of the GMC-oxydoreductase, with each helical strand lining the interior of the protein shell. Proteomic analyses of the purified genomic fibre revealed the presence of several RNA polymerase subunits as well as additional proteins involved in genome compaction that can fit into the central channel of the protein shield. Such an elegant supramolecular organization represents a remarkable evolutionary solution for packaging while protecting the viral genome, in a state ready for immediate transcription upon unwinding in the host cytoplasm. We expect that a dedicated energy-driven machinery is required for the assembly of this rod-shaped giant viral chromosome and its further compaction in the membrane limited electron dense nucleoid, characteristic of the mature Mimivirus particles2,4,5.The parsimonious implication of the same protein in two functionally unrelated substructures of the virion is also unexpected for a giant virus with a thousand genes at its disposal.