Pomovirus

VIRION

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Non-enveloped, rod shaped with helical symmetry. The virion is composed of two segments 190 and 245 nm in length, and 21 nm in diameter.

GENOME

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Segmented, linear, ssRNA(+) genome. The three segments are 6, 3.5 and 3 kb in size respectively. The genomic RNAs are capped, and display a tRNA-like structure in 3'.

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. Suppression of termination in RNA-2 produces an extended capsid protein. Subgenomic RNAs may be synthesized during replication of RNA-3. TGB1, TGB2 and TGB3 function as movement proteins. TGB2 and TGB3 are produced by leaky scanning.

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 capsid protein, movement protein and viral suppressor of RNA silencing (VSR).
  7. Virus assembly in the cytoplasm. Each segment (if multipartite) is encapsidated.
  8. Viral movement proteins probably mediate virion cell-to-cell transfer.