Betaflexiviridae (taxid:675068)

VIRION

Non-enveloped, flexuous, filamentous, 600-1000 nm or more long and 12-13 nm in diameter.

GENOME

Linear ssRNA(+) genome of 6.5-9 kb in size. The 3' terminus is polyadenylated and, in some genera, the 5' end is capped. Encodes 2 to 6 proteins.

GENE EXPRESSION

The virion RNA is infectious and serves as both the genome and viral messenger RNA. RNA-dependent RNA polymerase (RdRp) is translated directly from the genomic RNA. The other ORFs are transcribed presumably as monocistronic subgenomic mRNAs (sgRNAs).

ENZYMES

• RNA-dependent RNA polymerase [Polyprotein]
• Alpha-type capping [Polyprotein]
• Polyprotein major protease (Peptidase C23) [Polyprotein]
• Fe2OG dioxygenase [Polyprotein]

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 as a monocistronic mRNA to produce the RdRp (encoded by the 5'-proximal ORF).
4. Replication occurs in 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. Internal subgenomic promoters are used to transcribe the sgRNAs. Traduction of these sgRNAs yields the capsid and movement proteins.
7. Assembly of new virus particles.
8. Triple gene block proteins (TGBp) allow cell-to-cell and long-distance movement.

Host-virus interaction

Suppression of RNA silencing

Potato virus M cysteine-rich protein (ORF6) and TGBp1 proteins act as suppressors of RNA silencing, the former inhibiting both local and systemic silencing, whereas the latter inhibits systemic silencing .

Grapevine virus A ORF5 p10 suppresses local and systemic RNA silencing .

Citrus leaf blotch virus movement protein acts as suppressor of RNA silencing .

Apple chlorotic leaf spot virus 50 kDa movement protein acts as a systemic suppressor of RNA silencing through phloem .