Viruses with RNA genomes must overcome two specific problems that arise from the need to replicate the viral genome and to produce a number of viral proteins in eukaryotic host cells. First, there is no host cell RNA polymerase that can use the viral parental RNA as a template for synthesis of complementary RNA strands. Second, translation of eukaryotic mRNAs begins at only a single initiation site, and they are, therefore, translated into only a single polypeptide. However, RNA viruses, which frequently contain only a single molecule of RNA, must express the genetic information for at least two proteins: an RNA-dependent RNA polymerase and a minimum of one type of capsid protein. Although the replication of each RNA virus family has unique features, the mechanisms evolved to sur mount these restrictions can be grouped into four broad patterns (or “types”) of replication.

1. Type I—RNA viruses with a single-stranded genome (ssRNA) of (+) polarity that replicates via a complementary (–) strand intermediate: In Type I viral replication, the infecting parental RNA molecule serves both as mRNA and, later, as a template for syn thesis of the complementary (–) strand (Figure.1).

Fig1. Type I virus with a ssRNA genome of (+) polarity replicates via a complementary (–) strand intermediate.

a. Role of (+) ssRNA as mRNA: Because the parental RNA genome is of (+), or messenger, polarity, it can be translated directly upon uncoating and associating with cellular ribosomes. The product is usually a single polyprotein from which individual polypeptides, such as RNA-dependent RNA polymerase and various proteins of the virion, are cleaved by a series of proteolytic processing events carried out by a pro tease domain of the polyprotein (see Figure.1).

b. Role of (+) ssRNA as the template for complementary (–) strand synthesis: The viral (+) ssRNA functions early in infection, not only as mRNA for translation of polyproteins but also as a template for virus-encoded RNA-dependent RNA polymerase to synthesize complementary (–) ssRNA (see Figure.1). The progeny (–) strands, in turn, serve as templates for synthesis of progeny (+) strands, which can serve as additional mRNAs, amplifying the capacity to produce virion proteins for progeny virus. When a sufficient quantity of capsid proteins has accumulated later in the infection, progeny (+) ssRNAs begin to be assembled into newly formed nucleocapsids.

2. Type II—viruses with a ssRNA genome of (–) polarity that replicate via a complementary (+) strand intermediate: Viral genomes with (–) polarity, similar to the (+) strand genomes, also have two functions: 1) to provide information for protein synthesis and 2) to serve as templates for replication. Unlike (+) strand genomes, however, the (–) strand genomes cannot accomplish these goals without prior construction of a complementary (+) strand intermediate (Figure 2).

Fig2. Type II virus with an ssRNA genome of (–) polarity that replicates via a complementary (+) strand inter mediate.

a. Mechanism of replication of viral ssRNA with (–) polarity: The replication problems for these viruses are twofold. First, the (–) strand genome cannot be translated, and, therefore, the required viral RNA polymerase cannot be synthesized immediately following infection. Second, the host cell has no enzyme capable of transcribing the (–) strand RNA genome into (+) strand RNAs capable of being translated. The solution to these problems is for the infecting virus particle to contain viral RNA-dependent RNA polymerase and to bring this enzyme into the host cell along with the viral genome. As a consequence, the first synthetic event after infection is generation of (+) strand mRNAs from the parental viral (–) strand RNA template.

b. Mechanisms for multiple viral protein synthesis in Type II viruses: The synthesis of multiple proteins is achieved in one of two ways among the (–) strand virus families: 1) The viral genome may be transcribed into a number of individual mRNAs, each specifying a single, polypeptide. 2) Alternatively, the (–) strand viral genome may be segmented (that is, com posed of a number of different RNA molecules, most of which code for a single polypeptide).

c. Production of infectious virus particles: Although the details differ, the flow of information in both segmented and unsegmented genome viruses is basically the same. In the Type II replication scheme, an important control point is the shift from synthesis of (+) strand mRNA to progeny (–) strand RNA molecules that can be packaged in the virions. This shift is not a result of activity of a different polymerase, but rather a result of interaction of (+) strand RNA molecules with one or more newly synthesized proteins. This enhances the availability of the (+) strands as templates for the synthesis of genomic (–) strands.

3. Type III—viruses with a dsRNA genome: The dsRNA genome is segmented, with each segment coding for one polypeptide (Figure 3). However, eukaryotic cells do not have an enzyme capable of transcribing dsRNA. Type III viral mRNA transcripts are, therefore, produced by virus-coded, RNA-dependent RNA polymerase (transcriptase) located in a subviral core particle. This particle consists of the dsRNA genome and associated virion proteins, including the transcriptase. The mechanism of replication of the dsRNA is unique, in that the (+) RNA transcripts are not only used for translation but also as templates for complementary (–) strand synthesis, resulting in the formation of dsRNA progeny.

Fig3. Type III virus with a dsRNA genome.

4. Type IV—viruses with a genome of ssRNA of (+) polarity that is replicated via a DNA intermediate: The conversion of a (+) strand RNA to a double-stranded DNA is accomplished by an RNA dependent DNA polymerase, commonly referred to as a “reverse transcriptase,” that is contained in the virion. The resulting dsDNA becomes integrated into the cell genome by the action of a viral “integrase.” Viral mRNAs and progeny (+) strand RNA genomes are transcribed from this integrated DNA by the host cell RNA polymerase (Figure 4).

Fig4. Type IV virus with a ssRNA genome of (+) polarity that replicates via a DNA intermediate.

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مواضيع ذات صلة

Clinical Syndromes of Adenovirus

JCPyV and progressive multifocal leukoencephalopathy (PML)

Polyomavirus-associated nephropathy (PVAN)

Epidemiology Human Papillomavirus

Pathogenesis of Human Papillomavirus

Clinical Syndromes of EBV

EBV Life Cycle

Epstein–Barr virus

Treatment of Cytomegalovirus

Diagnosis of Cytomegalovirus

Clinical Manifestations of Cytomegalovirus

The Viral Genome