Molecular biology

VIRION

Non enveloped, icosahedral virion composed of a 60 asymmetric dimers of the single capsid protein (CP), about 40 nm in diameter. The capsid has a T=2 symmetry.

GENOME

dsRNA linear genome of 4.6-6.7 kb. Contains 2 overlapping ORFs gag and pol, respectively encoding CP and RdRp.
Some isolates contain additional satellite dsRNAs which encode “killer” proteins and are encapsidated separately. Contains 2 usually overlapping ORFs gag and pol, respectively encoding CP and RdRp.

GENE EXPRESSION

The dsRNA genome is never completely uncoated, to prevent activation of antiviral state by the cell in response of dsRNA. The viral polymerase synthesizes a mRNA, which is translocated to the cell cytoplasm where it is translated.
Translation is initiated on a unique internal ribosome entry site (IRES) element situated at the 5’-UTR.
In most totiviruses, the plus-strand viral transcript is flanked by a 5’untranslated region (5’-UTR) and a 3’-UTR and directs the translation of a major CP (Gag) and a minor fusion protein CP-RdRP (Gag-Pol) via a -1 ribosomal frameshift.
In other totiviruses (e.g. Hv190SV), RDRP is detected only as a separate, nonfused polypeptide and may be expressed through a termination-reinitiation mechanism.

REPLICATION

CYTOPLASMIC

1. Virus remains intracellular.
2. Transcription of the dsRNA genome by viral polymerase occurs inside the virion, so that dsRNA is never exposed to the cytoplasm. This plus-strand transcript is used as template for translation.
3. The 5’ termini of the transcripts are diphosphorylated.
4. (+)RNAs are encapsidated in virion particle, inside which they are transcribed to give RNA (-) molecules with which they become base-paired to produce dsRNA genomes.
5. Mature virions are transmitted to new cell during cell division, sporogenesis or cell fusion.