Viral genomic mRNAs

Most viruses are heavily dependent upon the cellular machinery for transcription of viral genomes, mRNA splicing, translation, and protein transport. Unlike genome replication, uniquely viral proteins are not involved in these processes and to date none have been utilized as targets for antiviral therapy. 

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. 

The life history of a retroviras is described in chapter 17 (see Figure 17.45). A summary is presented here. The genome of a retrovirus is composed of RNA not DNA but, when a retrovirus infects a host cell its RNA is transcribed into DNA, catalysed by the enzyme, reverse transcriptase. This DNA is then incorporated into the genome of the host. On transcription of the host DNA, during cell division, viral mRNA and viral genomic RNA are produced. The... 

When viral replication occurs, the DNA segment corresponding to the viral genome is first transcribed by host cell enzymes (6). This gives rise not only to viral ssRNA, but also to transcription of mRNAs for precursors of the viral proteins (7). These precursors are integrated into the plasma membrane (8, 9) before undergoing proteolytic modification (10). The cycle is completed by the release of new virus particles (11). 

Viral genomes consist of RNA or DNA, which can be single- or double-stranded, and may consist of one or more fragments. During viral replication, both DNA and RNA viruses synthesize protein by translation of messenger RNA. The mRNA is then translated by the cell into the viral proteins that will constitute the viral particles. 

Transcription, production of viral mRNA from the viral genome."... 

The way in which the viral genome is replicated depends entirely on the nature of the nucleic acid carried by the virus. Positive strand RNA viruses (e.g. poliovirus) can use the parent RNA directly as mRNA, after the acquisition from the host cell of a terminal sequence enabling immediate translation. With negative strand RNA viruses (e.g. influenza virus), a positive RNA strand complementary in base sequence to the parent RNA has to be transcribed using an RNA-dependent RNA polymerase carried by the virus, as eukaryotic cells do not possess such enzymes. 

In cells, DNA is transcribed into mRNA, which is translated into proteins. An infecting virus insinuates its genetic information into cellular DNA, so that the cell synthesizes viral proteins as specified by viral genes. The proteins synthesize many copies of the viral genome and execute any other instructions encoded by the viral genes. In some instances the instructions include a command that converts the host cell to a cancerous state. 

Within 2 minutes after the injection of T4 phage DNA into an E. coli cell, synthesis of host DNA, RNA, and protein stops and phage mRNA synthesis begins. Phage mRNA codes for the synthesis of capsid proteins and some of the enzymes required for the replication of the viral genome and the assembly of virion components. In addition, other enzymes are synthesized that weaken the host cell s cell wall, so that new phage can be released for new rounds of infection. Approximately 22 minutes after viral DNA (vDNA) is injected, the host cell, now filled with several hundred new virions, lyses. Upon release, the virions attach to nearby bacteria, thus initiating new infections. 

Most animal and plant DNA viruses require host-cell nuclear enzymes to carry out transcription of the viral genome Into mRNA and production of progeny genomes. In contrast, most RNA viruses encode enzymes that can transcribe the RNA genome Into viral mRNA and produce new copies of the RNA genome. 

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