Translation, viral mRNA

Figure 38-10. Picornavimses disrupt the 4F complex. The 4E-4G complex (4F) directs the 40S ribosomal subunit to the typical capped mRNA (see text). 4G alone is sufficient for targeting the 40S subunit to the internal ribosomal entry site (IRES) of viral mRNAs. To gain selective advantage, certain viruses (eg, poliovirus) have a protease that cleaves the 4E binding site from the amino terminal end of 4G. This truncated 4G can direct the 40S ribosomal subunit to mRNAs that have an IRES but not to those that have a cap. The widths of the arrows indicate the rate of translation initiation from the AUG codon in each example.

A virus-specific RNA RNA polymerase is needed, since the cell RNA polymerase will generally not copy double-stranded RNA (and ribosomes are not able to translate double-stranded RNA either). A wide variety of modes of viral mRNA synthesis are outlined in Figure. By convention, the chemical sense of the mRNA is considered to be of the plus (+) configuration. The sense of the viral genome nucleic acid is then indicated by a plus if it is the same as the mRNA and a minus if it is of oppposite sense. If the virus has double-stranded DNA (ds DNA), then mRNA synthesis can proceed directly as in uninfected cells. However, if the virus has a singlestranded DNA (ss DNA), then it is first converted to ds DNA and the latter serves as the template for mRNA synthesis with the cell RNA polymerase. 

Intracellular replication of viral particles depends entirely upon successful intracellular transcription of viral genes with subsequent translation of the viral mRNA. Translation of viral or cellular mRNA is dependent upon ribosome formation. Normally, several constituent molecules interact with each other on the mRNA transcript, forming the smaller ribosomal subunit. Subsequent for-mation/attachment of the larger subunit facilitates protein synthesis. 

Translation of Viral mRNA. Once viral mRNA has been formed, translation occurs in the host cytoplasm, using host ribosome to synthesize viral proteins. Viral mRNA, which is usually monocistronic (i.e., has a single coding region) can displace host mRNA from ribosome so that viral products are synthesized preferentially. In the early phase, the proteins produced (enzymes, regulatory molecules) are those that will allow subsequent replication of viral nucleic acids in the later phase, the proteins necessary for the formation of capsid are produced. 

Replication of Viral Nucleic Acid. In addition to producing molecules for the formation of new capsids, the virus must replicate its nucleic acid to provide genetic material for packaging into the capsids. The way in which this is done might vary. In positive-sense, single-strand RNA viruses, a polymerase translated from viral mRNA produces negative-sense RNA from the positive-sense template which is then repeatedly transcribed into more positive strands. 

Budding and release of progeny virus. B. Replicative cycle of an influenza virus, an example of an RNA virus. 1. Attachment. 2. Endocytosis. 3. Influx of H+ through M2 protein. 4. Fusion of the viral envelope with the endosomes membrane, dissociation of the RNP complex, and entry of viral RNA into the nucleus. 5. Synthesis of viral mRNA by viral RNA polymerase. 6. Translation of viral mRNA by host cell s ribosomes. 7. Replication of viral RNA, using viral RNA polymerase, via cRNA replicative form. 8. Assembly of virus particles, and 9. Budding and release of progeny virus. 

Ruiz-Echevarria Ml, Yasenchak JM, Han X, Dinman JD, Peltz SW (1998) The upf3 protein is a component of the surveillance complex that monitors both translation and mRNA turnover and affects viral propagation. Proc Natl Acad Sci USA 95 8721—8726... 

Inhibitors of translation of viral mRNA (antisense oligonucleotides) Inhibitors of posttranslational glycosylation Antagonists of chemokine receptors... 

A number of products exist that are targeted at any of the successive events implicated in the replicative cycle of HIV virus entry, viral adsorption, virus-cell fusion, reverse (RNA —> DNA) transcription, proviral DNA integration, viral (DNA —> RNA) transcription (transactivation), viral (mRNA —> protein) translation, virus release, viral assembly, budding, and maturation... 

Gallie, D. R. (1996) Translational control of cellular and viral mRNAs. Plant Mol. Biol. 32, 145-158. 

Translation of viral mRNA into viral proteins by host ribosome can be affected by myristic acid analogues. 

Another group of cytokines produced by many cell types, including macrophages, in response to viral challenge is the interferons (IFN). In mammalian cells, IFN induces the synthesis of Mx proteins that can inhibit the translation of viral mRNA. Induction of Mx proteins has been used as an indirect measure of IFN activity in fish40,68. Recently, Altmann et aV cloned, sequenced and characterized the first teleost interferon gene from the zebrafish, Danio rerio, so it should soon become feasible to quantify IFN production directly as a measure of anti-viral response. 

The eukaryotic protein-synthesizing machinery begins translation of most cellular mRNAs within about 100 nucleotides of the 5 capped end as just described. However, some cellular mRNAs contain an internal ribosome entry site (IRES) located far downstream of the 5 end. In addition, translation of some viral mRNAs, which lack a 5 cap, is initiated at IRESs by the host-cell machinery of infected eukaryotic cells. Some of the same translation initiation factors that assist in ribosome scanning from a 5 cap are required for locating an internal AUG start codon, but exactly how an IRES is recognized is less clear. Recent results indicate that some IRESs fold into an RNA structure that binds to a third site on the ribosome, the E site, thereby positioning a nearby internal AUG start codon in the P site. 

Repiication— Vlral mRNAs are produced with the aid of the host-cell transcription machinery (DNA viruses) or by viral enzymes (RNA viruses). For both types of viruses, viral mRNAs are translated by the host-cell translation machinery. Production of multiple copies of the viral... 

The lytic cycle Is somewhat more complicated for DNA viruses that Infect eukaryotic cells. In most such viruses, the DNA genome Is transported (with some associated proteins) Into the cell nucleus. Once Inside the nucleus, the viral DNA Is transcribed Into RNA by the host s transcription machinery. Processing of the viral RNA primary transcript by hostcell enzymes 3delds viral mRNA, which Is transported to the cytoplasm and translated Into viral proteins by host-cell ribosomes, tRNA, and translation factors. The viral proteins are then transported back Into the nucleus, where some of them either replicate the viral DNA directly or direct cellular proteins to replicate the viral DNA, as In the case of SV40 discussed In the last section. Assembly of the capsid proteins with the newly replicated viral DNA occurs in the nucleus, 3deldlng hundreds to thousands of progeny virions. 

Puromycin would be useful for treatment of a viral infection, but chloramphenicol would not. Viral mRNAs are translated by eukaryotic translation systems, so one must use an antibiotic active on eukaryotic systems. 

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