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Total of 8 families of dsRNA viruses are currently recognized by ICTV. Two most important family members are Reoviruses and Birnaviruses. These two groups contain viruses of medical as well as of veterinary importance. Rotavirus is a major cause of infant diarrhea while blue tongue virus is a big issue for cattle and sheep. Infectious bursal disease virus is another important member of family Birnaviridae which causes an immunosuppressive disease in poultry and are of great economic importance. Another member of family Birnaviridae , Infectious pancreatic necrosis virus causes significant losses to the fisheries industry around the world.
29.1 Expression of viral proteins
The size of dsRNA genome of the viruses is limited by the icosahedral capsid which can accommodate the segmented genome and at the same time actively allow the transcription of the viral messages. The viruses evolve in such a way to conveniently dissociate the total translation products from the genome into several distinct proteins. Sometimes individual genome segments can also be exchanged in the presence of a suitable donor or acceptor which increases the genetic diversity in virus population. The dsRNA transcribes separately to full-length +ve sense RNA. The full-length +ve sense RNA acts as mRNA for viral protein synthesis as well as template for the synthesis of genomic RNA for the progeny virions.
29.2 Replication of the genome
The replication of the genome is divided into several steps as follows:
1. In the first step primary transcription of the viral genome takes place inside viral core in the cytoplasm using viral RNA dependent RNA polymerase (RDRP)
2. Positive sense RNA is then transported into the cytoplasm
3. Positive sense RNA is translated to form viral proteins
4. Positive sense RNA and viral proteins are assembled to form immature virions
5. Positive sense RNA is then transcribed to form dsRNA in virions by viral RDRP
6. dsRNA undergoes secondary transcription
7. Final assembly and maturation of virions