Totiviridae

VIRION

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


Virion structure at ViperDB

GENOME

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Linear dsRNA 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 to 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, non-fused polypeptide and may be expressed through a termination-reinitiation mechanism.

REPLICATION

CYTOPLASMIC

  1. Virus either remains intracellular (totivirus) or attachement to host receptors mediates endocytosis of virus into host cell (Giardiavirus).
  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. (+)RNAs are encapsidated in a sub-viral particle, in which they are transcribed to give RNA (-) molecules with which they become base-paired to produce dsRNA genomes.
  4. Mature virions are transmitted to new cell during cell division, sporogenesis, cell fusion (totivirus) or are released from the host cell (Giardiavirus).