Tombusviridae (taxid:39738)

VIRION

Non-enveloped, spherical, icosahedral, about 28-34 nm in diameter, capsid with a T=3 icosahedral symmetry. 30 hexameric capsomers per nucleocapsid (virion composed of 180 protein subunits).

GENOME

Monopartite (except Dianthovirus which is bipartite), linear, ssRNA(+) genome of 4-5.4 kb, which lacks a cap structure and a poly(A)tail.

GENE EXPRESSION

The virion RNA is infectious and serves as both the genome and viral messenger RNA. Two 5'-proximal ORFs are directly translated to produce the viral constituents of the replicase complex. RdRp is translated through suppression of termination at the end of ORF1 (tombusvirus-like and carmovirus-like), or ribosomal frameshifting (Dianthovirus and umbravirus). The downstream genes encoding the movement protein, and capsid proteins are translated from subgenomic RNAs. viral suppressor of RNA silencing (VSRs) is expressed by leaky scanning of the MVP subgenomic mRNA.
Cap-independent translation is mediated possibly through the interaction between a 3'-cap-independent translation element and a 5'UTR stem-loop.

ENZYMES

• RNA-dependent RNA polymerase [RdRp]

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 to produce the two 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. The RdRp recognizes internal subgenomic promoters on the negative-sense RNA to transcribe the 3'co-terminal subgenomic RNAs that will generate the capsid and movement protein.
7. Virus assembly in the cytoplasm.
8. Viral movement protein probably mediates virion cell-to-cell transfer.

Host-virus interaction

Suppression of RNA silencing

Viruses of the Tombusviridae family encode proteins acting as suppressors of RNA silencing. Examples are tombusvirus P19/P20 , aureusvirus P14 , dianthovirus movement protein , carmovirus capsid protein P38 .