Alphaflexiviridae

VIRION

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Non-enveloped, flexuous, filamentous, 470-800 nm or more long and 12-13 nm in diameter.

GENOME

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Linear ssRNA(+) genome of 5.4-9 kb in size. The 3' terminus is polyadenylated and, in some genera, the 5' end is capped. Encodes 1 to 6 proteins.

GENE EXPRESSION

The virion RNA is infectious and serves as both the genome and viral messenger RNA. RNA-dependent RNA polymerase (RdRp) is translated directly from the genomic RNA. The other ORFS, when present, are transcribed presumably as monocistronic subgenomic mRNAs (sgRNAs). TGB3 is expressed by leaky scanning of the TGB2 subgenomic mRNA.

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 as a monocistronic mRNA to produce the RdRp (encoded by the 5'-proximal ORF).
  4. Replication occurs in viral factories recruited by TGB1. A dsRNA genome is synthesized from the genomic ssRNA(+).
  5. The dsRNA genome is transcribed/replicated thereby providing viral mRNAs/new ssRNA(+) genomes.
  6. Internal subgenomic promoters are used to transcribe the sgRNAs. Traduction of these sgRNAs yields the capsid and movement proteins.
  7. Assembly of new virus particles.
  8. Triple gene block proteins (TGBp) allow cell-to-cell and long-distance movement.

Host-virus interaction

Suppression of RNA silencing

Potato virus X TGBp1 protein functions as a suppressor of RNA silencing inhibiting the PTGS pathway .