Non-enveloped, isometric (icosahedral) but slightly flattened, of three types but similar in size (33 nm). Three RNA components, 2 genomic and 1 subgenomic (sgRNA) are distributed among the particles which can therefore be separated on their densities.



Segmented, tripartite, linear ssRNA(+) genome composed of RNA1=5.4 kb, RNA2=2.2 kb and sgRNA3=1 kb. The 3'-terminal regions of the RNAs may form a stem loop structure, and is not polyadenylated.


The virion RNA is infectious and serves as both the genome and viral messenger RNA. RNA1 has a single open reading frame, RNA2 has two open reading frames and RNA3 is the encapsidated subgenomic of RNA2 expressing the capsid protein.



  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 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. Expression of the subgenomic RNA (sgRNA3).
  7. Virus assembly, each segment is encapsidated.
  8. Virus release.