Bacteriophage PR772, a member of the Tectiviridae family, has a 70-nm diameter icosahedral protein capsid that encapsulates a lipid membrane, dsDNA, and various internal proteins. An icosahedrally averaged CryoEM reconstruction of the wild-type virion and a localized reconstruction of the vertex region reveal the composition and the structure of the vertex complex along with new protein conformations that play a vital role in maintaining the capsid architecture of the virion. The overall resolution of the virion is 2.75 Å, while the resolution of the protein capsid is 2.3 Å. The conventional penta-symmetron formed by the capsomeres is replaced by a large vertex complex in the pseudo T=25 capsid. All the vertices contain the host-recognition protein, P5; two of these vertices show the presence of the receptor-binding protein, P2. The 3D structure of the vertex complex shows interactions with the viral membrane, indicating a possible mechanism for viral infection.

Nucleocytoplasmic large DNA viruses (NCLDVs) blur the line between viruses and cells. Melbournevirus (MelV, family Marseilleviridae) belongs to a new family of NCLDVs. Here we present an electron cryo-microscopy structure of the MelV particle, with the large triangulation number T = 309 constructed by 3080 pseudo-hexagonal capsomers. The most distinct feature of the particle is a large and dense body (LDB) consistently found inside all particles. Electron cryo-tomography of 147 particles shows that the LDB is preferentially located in proximity to the probable lipid bilayer. The LDB is 30 nm in size and its density matches that of a genome/protein complex. The observed LDB reinforces the structural complexity of MelV, setting it apart from other NCLDVs.

During the last couple of decades, new discoveries of giant DNA viruses visible under a light microscope and with genome larger than 500 kbp are becoming more and more frequent. Interestingly, about two-thirds of their predicted genes correspond to open reading frames without recognizable database homologs. Herein, we quantitatively investigate viral replication of the newly discovered Lurbovirus to understand what cellular function is retained through the unknown open reading frames. We apply high-resolution soft x-ray microscopy to intact cell systems in their near-native state with high carbon-to-water contrast. New virions produced inside the cell are visible from 12 hours post infection and increase to several hundreds after 48 hours post infection. Due to the large size of the virion, this corresponds to a high bioconversion of 6-12 % from amoebal host to virus. We associate the high bioconversion of large DNA viruses with their large genome that enables complex functionality. The vacuolated structure of the amoebal host disappears when virions are starting to be produced at 12 hours post infection, whereas large circular x-ray-lucent cytoplasmic areas persist that are attributed to viral factories. The nucleus and nucleolus appear unaffected throughout the whole replication cycle, which suggests that nuclear functions are needed for viral replication to occur, whereas other functions are retained in the viral factories in the cytoplasm of the host cell.