Molecular biology
VIRION
Non enveloped, icosahedral virion composed of a 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 totiviruses contain a third small 5’-proximal potential ORF.
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
- Virus either remains intracellular (totivirus) or attaches to host receptors and is endocytosed into vesicles in the host cell (Giardiavirus).
- 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.
- (+)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.
- Mature virions are transmitted to new cell during cell division, sporogenesis, cell fusion (totivirus) or are released from the host cell (Giardiavirus).