Viral Protein Synthesis

Production of viral mRNA. In a DNA virus infection, cellular polymerases transcribe mRNA in the nucleus of the host cell from one or both DNA strands, whereby the RNA is processed (splicing, polyadenylation, etc.) as with cellular mRNA. An exception to this procedure is the poxviruses, which use their own enzymes to replicate in the cytoplasm.

In viruses with antisense-strand ssRNA and dsRNA the transcription of the genomic RNA into mRNA is carried out by the viral polymerases, usually without further processing of the transcript.

In sense-strand ssRNA viruses, the genome can function directly as mRNA.

Certain viruses (arenaviruses, see p. 462f.) are classified as "ambisense viruses." Part of their genome codes in antisense (-), another part in sense (+) polarity. Proteins are translated separately from subgenomic RNA and the antisense-coded proteins are not translated until the antisense strand has been translated into a sense strand.

Viral mRNA is produced for the translation process, based on both the genome of the invading virus and the nucleic acid already replicated.

3 Fig. 7.5 Schematic diagram of nucleic acid replication.

a Single-stranded RNA viruses with sense-strand genome: the virus-coded RNA polymerase transcribes the viral genome (+) into complementary strands (-) and these into new genomic RNA (+). The latter is then integrated in the viral progeny.

b Single-stranded RNA viruses with antisense-strand genome: the RNA

polymerase in the virion transcribes the viral genome (-) into complementary strands (+), which a virus-coded polymerase then transcribes into new genomic RNA (-).

c Double-stranded RNA viruses: while still in the partially decapsidated virus particle, the virus-coded polymerase transcribes complementary strands (+) from the antisense strand of the (segmented) double-stranded viral genome; these complementary strands are complemented to make the new double-stranded viral genome.

d RNA replication in retroviruses: the reverse transcriptase (RT) carried by the virion transcribes the viral genome (two sense-RNA strands) into complementary DNA (-), which is complemented to produce dsDNA and integrated in the cell genome. The viral RNA is first degraded. Cellular enzymes produce new genomic RNA (+).

e DNA replication in hepadnaviruses: by means of cellular transcription, a sense-strand RNA is made from the viral genome (antisense DNA, partially double-stranded) and integrated in the new virion, where a virus-coded RT produces new genomic DNA (-) and destroys the RNA.

The actual protein synthesis procedure is implemented, coded by the viral mRNA, with the help of cellular components such as tRNA, ribosomes, initiation factors, etc. Two functionally different protein types occur in viruses:

■ The "noncapsid viral proteins" (NCVP) that do not contribute to capsid assembly. These proteins frequently possess enzymatic properties (poly-merases, proteases) and must therefore be produced early on in the replication cycle.

■ The capsid proteins, also known as viral proteins (VP) or structural proteins, appear later in the replication process.

Protein Synthesis Control

Segmented genomes. A separate nucleic acid segment is present for each protein (example: reoviruses).

mRNA splicing. The correct mRNA is cut out of the primary transcript (as in the cell the exon is cut out of the hnRNA) (examples: adenoviruses, retroviruses, etc.).

"Early" and "late" translation. The different mRNA molecules required for assembly of so-called early and late proteins are produced at different times in the infection cycle, possibly from different strands of viral DNA (examples, papovaviruses, herpesviruses).

Posttranslational control. This process involves proteolytic cutting of the primary translation product into functional subunits. Viral proteases that recognize specific amino acid sequences are responsible for this, e.g., the two poliovirus proteases cut between glutamine and glycine or tyrosine and glycine. Such proteases, some of which have been documented in radiocrystallographic images, are potential targets for antiviral chemotherapeutics (example: HIV).

Viral maturation (morphogenesis). In this step, the viral capsid proteins and genomes (present in multiple copies after the replication process) are assembled into new, infectious virus particles. In some viral species these particles are also covered by an envelope (p. 378f.)

Release. The release of viral progeny in some cases correlates closely with viral maturation, whereby envelopes or components of them are acquired when the particles "bud off" of the cytoplasmic membrane and are expelled from the cell (Fig. 7.6). In nonenveloped viruses, release of viral progeny is realized either by means of lysis of the infected cell or more or less continuous exocytosis of the viral particles.

— Release of Retroviruses from an Infected Cell

— Release of Retroviruses from an Infected Cell

Fig. 7.6 Electron microscope image of release of viral progeny. The process takes place in the order, A, B, C.

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