Molecular biology

VIRION

Enveloped, spherical virion of 85 nm in diameter. The virion has a double capsid structure: Outer capsid a has an icosahedral T=13 symmetry, inner capsid an icosahedral symmetry T=2.
Virion maturation schemes

GENOME

Segmented dsRNA linear genome. Contains 3 segments: Large (L), Medium (M) ans Small (S) coding for 12 proteins. Segments size range from 2.9 kb to 6.4 kb. Genome total size is 13.3 Kb.

GENE EXPRESSION

Each segments encode for a polycystronic mRNA, translated into several proteins by the procaryotic translation machinery.

REPLICATION

CYTOPLASMIC

1. There are two different attachment processes depending on the considered species. Multimeric protein P3 adsorb to host type IV pili which retracts bringing the virion in contact with the host membrane (e.g. Phi6). In other species, the attachment apparatus is a heterodimeric protein assembly that utilizes the rough lipopolysaccharide as a receptor (e.g. Phi8, Phi12 or Phi13).
2. Protein P6 mediates fusion of virus membrane with host outer membrane, releasing the nucleocapsid (NC) into the periplasmic space.
3. The lytic enzyme P5 on the NC surface digests the peptidoglycan layer of the host, allowing the NC to enter the cytoplasm via an endocytic-like route.
4. Genomic dsRNA remains enclosed by the inner capsid, thereby “hiding” from host antiviral mechanism.
5. Polycystronic mRNA are transcribed by the viral RNA-dependent RNA polymerase (RdRp) P2 inside the core particle and released into the cell cytoplasm, so that dsRNA is never exposed to the cytoplasm. Full-length plus-strand transcripts from each of the dsRNA segments are synthesized. These plus-strand transcripts are used as templates for translation.
6. Neo-synthesized proteins assemble into new capsids. The packaging NTPase P4 translocates the three ssRNA(+) genomic segments in the capsids where they are converted into dsRNA by the viral RdRp P2.
7. The capsid is matured and enveloped in the cytoplasm.
8. Mature virions are released from the cell by lysis.