The giant mimivirus 1.2 Mb genome is elegantly organized into a 30-nm diameter helical protein shield

Mimivirus is the prototype of the Mimiviridae family of giant dsDNA viruses. Little is known about the organization of the 1.2 Mb genome inside the membrane-limited nucleoid filling the similar to 0.5 mu m icosahedral capsids. Cryo-electron microscopy, cryo-electron tomography, and proteomics revealed that it is encased into a similar to 30-nm diameter helical protein shell surprisingly composed of two GMC-type oxidoreductases, which also form the glycosylated fibrils decorating the capsid. The genome is arranged in 5- or 6-start left-handed super-helices, with each DNA-strand lining the central channel. This luminal channel of the nucleoprotein fiber is wide enough to accommodate oxidative stress proteins and RNA polymerase subunits identified by proteomics. Such elegant supramolecular organization would represent a remarkable evolutionary strategy for packaging and protecting the genome, in a state ready for immediate transcription upon unwinding in the host cytoplasm. The parsimonious use of the same protein in two unrelated substructures of the virion is unexpected for a giant virus with thousand genes at its disposal.

Automated summary

Mimivirus, the prototype of Mimiviridae family of giant dsDNA viruses, has a unique organizational structure with a protein shell composed of two GMC-type oxidoreductases. The genome is arranged in left-handed super-helices with a central channel for transcription. This supramolecular organization is an efficient evolutionary strategy for packaging and protecting the genome.