Virgaviridae

VIRION

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Non-enveloped, rigid helical rods with a helical symmetry, about 20-25 nm in diameter with a central "canal". There may be one, two or three segments depending on the considered genus.

GENOME

Monopartite or segmented (depending on the considered genus), linear, ssRNA(+) genome. Genomic RNA displays a tRNA-like structure in 3', but no poly-A tail, except for some hordeiviruses which have an internal poly-A tract between the coding sequence and the tRNA-like structure.

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 in most genera. The downstream genes encoding the movement protein, viral suppressor of RNA silencing (VSR) and capsid proteins are usually translated from subgenomic RNAs.

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 protein probably mediate virion cell-to-cell transfer.

Host-virus interaction

Suppression of RNA silencing

Soilborne wheat mosaic virus (furovirus) 19K protein suppresses RNA silencing .

P15 of peanut clump virus (pecluvirus) suppresses RNA silencing .

Barley stripe mosaic virus (hordeivirus)gamma-b protein suppresses RNA silencing .

Tobacco mosaic tobamovirus (tobamovirus)126-kDa small replicase subunit suppresses RNA silencing .

Tobacco rattle virus (tobravirus)16K protein suppresses RNA silencing .