Closteroviridae

VIRION

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Non-enveloped, flexuous and exceptionally long, filamentous particles about 950-2200 nm in length and 10-13 nm in diameter. The virion body is assembled by the major capsid protein (CP) and the tail by the minor capsid protein (CPm).

GENOME

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Mono-(Ampelovirus, Closterovirus) or bi-partite (Crinivirus), linear ssRNA(+) genome of 15 to 20 kb. 3' terminus has no poly(A) tract and the 5' terminus probably has a methylated nucleotide cap.

GENE EXPRESSION

The virion RNA is infectious and serves as both the genome and viral messenger RNA. ORF1a and ORF1ab protein (containing RdRp) are produced by ribosomal frameshifting. Other ORFs are translated from a set of nested 3' co-terminal 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 into a processed ORF1 polyprotein to yield the replication proteins.
  4. Replication occurs in 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. Subgenomic RNA translation gives rise to the other viral proteins.
  7. Assembly of new virus particles.
  8. Viral movement proteins mediate virion cell-to-cell transfer.

Host-virus interaction

Suppression of RNA silencing

Citrus tristeza virus p25, p20 and p23 and Beet yellows virus p21 and function as suppressors of RNA silencing

Sweet potato chlorotic stunt virus and Tomato chlorosis virus p22 proteins also function as suppressors of RNA silencing