The phage and animal virus growth cycles we have described so far have all involved double-stranded DNA genomes. As you will remember from the start of this chapter, however, many viruses contain RNA instead of DNA as their genetic material, and we now need to consider briefly how these viruses complete their replication cycles.
Replication of RNA viruses occurs in the cytoplasm of the host; depending on whether the RNA is single- or double-stranded, and (+) or ( —) sense, the details differ. The genome of a (+) sense single stranded RNA virus functions directly as an mRNA molecule, producing a giant polyprotein, which is then cleaved into the various structural and functional proteins of the virus. In order for the (+) sense RNA to be replicated, a complementary ( —) sense strand must be made, which acts as a template for the production of more (+) sense RNA (Figure 10.14). The RNA of a ( —) sense RNA virus must first act as a template for the formation of its complementary sequence by a virally encoded RNA polymerase. The (+) sense RNA so formed has two functions: (i) to act
Figure 10.14 Replication in (+) sense single-stranded RNA viruses. On entering the cell, the (+) sense ssRNA genome is able to act directly as mRNA, directing the synthesis of capsid protein and RNA polymerase. In addition, it replicates itself, being converted firstly into a (—) sense ssRNA intermediate. All steps take place outside of the nucleus. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services
Figure 10.14 Replication in (+) sense single-stranded RNA viruses. On entering the cell, the (+) sense ssRNA genome is able to act directly as mRNA, directing the synthesis of capsid protein and RNA polymerase. In addition, it replicates itself, being converted firstly into a (—) sense ssRNA intermediate. All steps take place outside of the nucleus. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services as mRNA and undergo translation into the virus's various proteins, and (ii) to act as template for the production of more genomic ( —) sense RNA (Figure 10.15).
Double-stranded RNA viruses are all segmented. They form separate mRNAs for each of their proteins by transcription of the ( —) strand of their genome. These are each translated, and later form an aggregate (subviral particle) with specific proteins, where they act as templates for the synthesis of a double-stranded RNA genome, ready for incorporation into a new viral particle.
Two final, rather complicated, variations on the viral replication cycles involve the enzyme reverse transcriptase, first discovered in 1970 (see Box 10.3)
These viruses, which include some important human pathogens, have a genome that exists as RNA and DNA at different part of their replication cycle. Retroviruses have a (+) sense ss-RNA genome which is unique among viruses in being diploid. The two copies of the genome serve as templates for the enzyme reverse transcriptase to produce
RNA-dependent RNA polymerase
Figure 10.15 Replication in (—) sense single-stranded RNA viruses. Before it can function as mRNA, the (—) sense ssRNA must be converted to its complementary (+) sense sequence. This serves both as mRNA and as template for the production of new (—) sense ssRNA genomes. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services
Box 10.3 The enzyme that breaks the rules
The discovery in 1970 of an enzyme that could convert an RNA template into DNA caused great surprise in the scientific world. The action of this reverse transcriptase or RNA-dependent DNA polymerase is a startling exception to the 'central dogma' of molecular biology, that the flow of genetic information is unidirectional, from DNA to RNA to protein (see Chapter 11). This in its revised form can be represented:
(The circular arrow to the left denotes DNA's ability to be replicated; the dotted arrow represents the action of reverse transcriptase.)
RNA -► Protein a complementary strand of DNA. The RNA component of this hybrid is then degraded, allowing the synthesis of a second strand of DNA. This proviral DNA passes to the nucleus, where it is incorporated into the host's genome (Figure 10.16). Transcription by means of a host RNA polymerase results in mRNA, which is translated into viral proteins and also serves as genomic material for the new retrovirus particles. The human immunodeficiency virus (HIV), the causative agent of acquired immune deficiency syndrome, is an important example of a retrovirus.
Incorporation of retroviral DNA into the host genome parallels the integration seen in lyso-genic phage growth cycles. Unlike the prophage, however, the provirus is not capable of a separate existence away from the host chromosome.
In two families of viruses (hepadnaviruses and caulimoviruses), DNA and RNA phases again alternate, but their order of appearance is reversed, so that a dsDNA genome is produced. This is made possible by reverse transcriptase occurring at a later stage, during the maturation of the virus particle.
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