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There are twelve families of viruses with linear dsRNA genomes. The number of dsRNA segments in the virus particle may be 1 (Totiviridae, Hypoviridae, and Endornaviridae, viruses of fungi, protozoa, and plants); 2 (Partitiviridae, Birnaviridae, and Megabirnaviridae, viruses of fungi, plants, insects, fish, and chickens); 3 (Cystoviridae, viruses of Pseudomonas bacteria); 4 (Chrysoviridae, viruses of fungi); or 10 to 12 (Reoviridae, viruses of protozoa, fungi, invertebrates, plants, and vertebrates). While dsRNA contains a (+) strand, it cannot be translated to synthesize viral proteins as part of a duplex. The (–) strand of the genomic dsRNA is first copied into mRNAs by a viral RNA-dependent RNA polymerase. Newly synthesized mRNAs are encapsidated and then copied to produce dsRNAs.

Figure 3.4 Structure and expression of viral single-stranded DNA genomes. (A) Synthesis of genomes, mRNA, and protein. (B and C) Genome configurations.

Figure 3.5 Structure and expression of viral double-stranded RNA genomes. (A) Synthesis of genomes, mRNA, and protein. (B) Genome configuration.