Virus dsRNA containing

FIGURE 10.10 RNAi sequence. First the vims approaches and attaches itself to the cell wall (A). The virus injects double-stranded RNA (B) into the cell cytoplasm. The dicer attacks the dsRNA, breaking it into smaller units (C and D). These smaller units are then acted on by RISC, forming single-stranded viral RNA (E), which are rendered incapable of forming their own protein by attachment to a complement contained within the cell s own single-stranded RNA (F). 

While most plant viruses contain dsRNA, the geminiviruses,458 which cause a number of plant diseases, contain single-stranded DNA. The virus particle consists of a fused pair of incomplete icosahedra, evidently containing a single 2500-bp DNA strand. Replication may require coinfection with two virus particles of differing sequence. 

The binding of interferons to their receptors induces a rapid increase in the transcription of particular genes and synthesis of corresponding proteins.196 202 One of the proteins induced is a double-stranded RNA-activated 2 -5 A synthase, which polymerizes ATP to a series of 2 -5 linked oligonucleotides containing triphosphates at the 5 termini.202-204 Double-stranded RNA is uncommon except in replicating viruses, and it is thought that the activation by dsRNA is related to establishment of an antiviral state. Another interferon-induced enzyme is the small subunit of eukaryotic protein synthesis initiation factor eIF-2. 

The protein kinase PKR (RNA-specific eIF-2 kinase) is regulated by binding of dsRNA and by interferon on the level of expression. PKR contains two dsRNA binding sites and it is thought that dsRNA binding disrupts inhibitory interactions in PKR leading to its activation. The activation of PKR by dsRNA and its induction by interferon identify PKR as a component of the cellular anit-viral defense. Consistent with this notion, a large number of viruses express inhibitors of PKR. 

Each genus differs in morphology and phy-siochemical details. In general, virions are naked, polyhedral, dsRNA (10-12 segments). Synthesis and maturation take place in the host cell cytoplasm. Viruses are released via cell lysis. Virions contain ribosomes. 

It is well known that synthetic or natural dsRNAs can serve as potent inhibitors of protein synthesis in various in vitro translation systems derived from mammalian cells (Ehrenfeld and Hunt, 1971 Kaempfer and Kaufman, 1973). A variety of other viral dsRNAs inhibit cell-free protein synthesis. The presence of dsRNA has also been detected by crosslinking experiments in HeLa cells infected with en-cephalomyocarditis virus or the t G114(I) mutant of VSV (Nilsen et al., 1981). Reovirus mRNA and polyadenylated vaccinia RNA transcribed in vitro have both been shown to contain dsRNAs which inhibited protein synthesis in cell-free systems (Baglioni et al., 1978 McDowell et al., 1972). Studies such as these with viruses other than rhabdoviruses are discussed in other chapters of this volume. The question that must be kept in mind is whether the in vitro inhibition of translation initiation by VSV dsRNA or any other viral dsRNA truly represents the sequence of events that takes place in the virus-infected cell. 

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