Megabirnavirus (taxid:1285597)

VIRION

Non-enveloped icosahedral capsid, about 50 nm in diameter with T=2* icosahedral symmetry.

GENOME

Segmented linear dsRNA genome: 2 segments encoding potentially for 4 proteins. Segments are about 7 kb and 9 kb.

GENE EXPRESSION

The coding strands of RNA-1 and RNA-2 contain two non-overlapping tandem ORFs. A single major capsid protein of 135 kDa is encoded by RNA-1. Translation is likely initiated by a unique internal ribosome entry site (IRES) element at the 5'-UTR.

ENZYMES

• RNA-directed RNA polymerase [RdRp]
• No capping activity [IRES]?

REPLICATION

CYTOPLASMIC

1. Virus penetrates into the cytoplasm.
2. Transcription of the dsRNA genome by viral polymerase occurs inside the virion, so that dsRNA is never exposed to the cytoplasm. This plus-strand transcript is used as template for translation.
3. (+)RNAs are encapsidated in virion particle, in which they are transcribed to give RNA (-) molecules with which they become base-paired to produce dsRNA genomes.
4. Mature are transmitted to new cell during cell division, sporogenesis or cell fusion.