Molecular biology

VIRION

Non-enveloped, T=3 icosahedric particles of 25-30 nm diameter, with 180 copies of the capsid protein (CP).

GENOME

Linear, ssRNA(+) genome of 5.3-5.7 kb in size. Poleroviruses and enamoviruses have 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, frameshift, leaky ribosome scanning, termination suppression, and polyprotein.

The virion RNA is infectious and serves as both the genome and viral messenger RNA. The only proteins translated directly from genomic RNA are the polymerase (ORF2), which is expressed as a fusion protein by -1 translational frameshift at the end of ORF1, and ORF1 protein. All other ORFs are translated from subgenomic RNAs (sgRNAs).

There are two or three ORFs at the 3’end of the CP depending on the genus. Leaky scanning produces CP (ORF3) and MOVP (ORF 4). Readthrough of the CP stop codon produces CP-ORF5 (CP extended), the capsid subunit essential for aphid transmission.

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 ORF1 and ORF2 are translated to produce the RdRp fusion protein.
4. A negative-sense complementary ssRNA is synthesized using the genomic RNA as a template.
5. New genomic RNA is synthesized using the negative-sense RNA as a template.
6. The negative-sense complementary ssRNA also serves as template for the synthesis of 3’ coterminal sgRNAs. Traduction of these sgRNAs yields the capsid (and extended CP) and movement proteins.
7. Formation of new virus particles.