VIRION
Non-enveloped, spherical virion about 23 nm in diameter with T=3 icosahedral symmetry composed of 180 CP proteins, with some percentage containing a readthrough domain (RTD) extension located on the particle’s surface.
GENOME
Monopartite, linear, ssRNA(+) genome of 5.3-5.7 kb in size with a VPg bound at the 5’ end. There is no poly(A) tail or tRNA-like structure at the 3’ end.
GENE EXPRESSION
Gene expression is complex and uses different mechanisms such as subgenomic RNA, ribosomal frameshifting, ribosome leaky scanning, suppression of termination, and polyprotein expression.
The virion RNA is infectious and serves as both the genome and viral messenger RNA. The only proteins translated directly from genomic RNA are ORF0 protein (P0), ORF1 polyprotein (P1) and the polymerase (ORF1-2), which is expressed as a fusion protein by a -1 ribosomal frameshifting near the end of ORF1. Rap1 translation initiates about 1500 nt downstream of the 5’ end of the gRNA by an unusual internal ribosome entry site (IRES).
All other ORFs are translated from subgenomic RNAs (sgRNAs).
Leaky scanning produces CP (ORF3) and movement protein (ORF 4). Suppression of termination of the CP stop codon produces CP-RTD (CP extended), the capsid subunit essential for aphid transmission.
REPLICATION
CYTOPLASMIC
- Virus penetrates into the host cell.
- Uncoating, and release of the viral genomic RNA into the cytoplasm.
- The viral RNA ORF1 and ORF2 are translated to produce the RdRp fusion protein.
- 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.
- Expression of the 3’ coterminal sgRNAs. Traduction of these sgRNAs yields the capsid, CP-RTD and movement proteins.
- Virus assembly.
- Viral movement protein probably mediates virion cell-to-cell transfer.
