Tobravirus

VIRION

Non-enveloped, rod shaped with helical symmetry. The virion is composed of two segments 180-215 and 46-115 nm in length, and 22 nm in diameter.

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GENOME

Segmented, linear, ssRNA(+) genome. The two segments are 6.8 and 4.5 kb in size respectively. The genomic RNAs are capped, not polyadenylated in 3'.

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GENE EXPRESSION

The virion RNA is infectious and serves as both the genome and viral messenger RNA. The RNA-1 5'proximal ORFs are directly translated to produce the viral constituents of the replicase complex. RdRp is translated through suppression of termination at the end of ORF1. Subgenomic RNAs may be synthesized during replication of RNA-1 and RNA-2 and give rise to the movement protein and viral suppressor of RNA silencing.

REPLICATION

CYTOPLASMIC

  1. Virus penetrates into the host cell.
  2. Uncoating, and release of the viral genomic RNA into the cytoplasm.
  3. The viral RNA is translated to produce the two proteins necessary for RNA synthesis (replication and transcription).
  4. Replication takes place in cytoplasmic viral factories. A dsRNA genome is synthesized from the genomic ssRNA(+).
  5. The dsRNA genome is transcribed/replicated thereby providing viral mRNAs/new ssRNA(+) genomes.
  6. The RdRp recognizes internal subgenomic promoters on the negative-sense RNA to transcribe the 3'co-terminal subgenomic RNAs that will generate the movement protein and viral suppressor of RNA silencing. The capsid protein is directly expressed from RNA-2.
  7. Virus assembly in the cytoplasm. Each segment (if multipartite) is encapsidated.
  8. Viral movement protein probably mediates virion cell-to-cell transfer.

Host-virus interaction

Suppression of RNA silencing

The cysteine-rich protein of tobacco rattle virus act as suppressor of RNA silencing in roots, but not in leaves .