VIRION
Non-enveloped, rigid rod-shaped, 20-25 nm in diameter and of three lengths: 150-160 nm (B), 126 nm (M) and 109 nm (T) depending on which RNA is encapsidated. The nucleocapsid is longitudinally striated.
GENOME
Segmented, tri- (or possibly quadri-) partite linear ssRNA(+) genome composed of RNA-alpha=3.8 kb, RNA-beta=3.3 kb and RNA-gamma=2.9-3.3. The 3' terminus of each RNA contains a tRNA-like structure. The 5' terminus is capped (m7G5pppGUA). There are a total of 7 open reading frames.
GENE EXPRESSION
The virion RNA is infectious and serves as both the genome and viral messenger RNA. Three proteins (replicase, CP and RdRp) are translated directly from the genomic RNAs.
The subgenomic RNAβ1 directs synthesis of the triple gene block 1 (TGB1) protein. TGB2' minor translational readthrough protein is translated through suppression of termination. TGB3 is expressed through ribosomal leaky scanning. TGB1, TGB2 and TGB3 function as movement proteins. A third sgRNA, sgRNAγ, is required for expression of the γ-B protein, which is a viral suppressor of RNA silencing and a movement protein.
ENZYMES
REPLICATION
CYTOPLASMIC
- Virus penetrates into the host cell.
- Uncoating, and release of the viral genomic RNA into the cytoplasm.
- The viral RNA is translated to produce the two proteins necessary for RNA synthesis (replication and transcription).
- Replication takes place in cytoplasmic viral factories. A dsRNA genome is synthesized from the genomic ssRNA(+).
- The dsRNA genome is transcribed/replicated thereby providing viral mRNAs/new ssRNA(+) genomes.
- The RdRp recognizes internal subgenomic promoters on the negative-sense RNA to transcribe the 3'co-terminal subgenomic RNAs that will generate the movement proteins and viral suppressor of RNA silencing (VSR). The capsid protein is directly expressed from RNA-2.
- Virus assembly in the cytoplasm. Each segment (if multipartite) is encapsidated.
- Viral movement proteins probably mediate virion cell-to-cell transfer.
