Is totally independent of host cell enzymes.

* ss, single-stranded; ds, double-stranded.

352 Chapter 14 Viruses, Prions, and Viroids: Infectious Agents of Animals and Plants capsid structure that surrounds the viral genome. The maturation process and multistep formation of the viral coat involve the same general principles in all kinds of viruses. In animal viruses, maturation takes place in a variety of organelles such as the nucleus and microtubules, depending on the virus. This process has already been discussed for bacteriophage T4. ■ assembly, p. 328

The maturation of a tobacco mosaic virus (TMV), a cylin-drically shaped plant virus, has been studied extensively and serves as a model for both animal and plant viruses (figure 14.10). For TMV many identical protein structural subunits, the capsomeres, are first formed and then are added one by one to the growing coat structure that surrounds the viral RNA. The coat elongates in both directions, starting from a specific site on the single-stranded viral RNA. The RNA interacts with each protein disc as it is added, and when the end of the long RNA molecule is reached, the discs are no longer added. Enzymes are not required for the process, since the coat self assembles. Recall that the maturation of bacteriophage T4 is also, in part, a self-assembly process. It is a far more complicated process in T4 since this virion has many more different parts than the coat of TMV.

Assembly of many animal viruses takes place at specific regions of the membrane which have embedded specific protein components. These regions are termed lipid rafts.

Step 7: Release

Depending on where the virion is assembled, most non-enveloped viruses accumulate within the cytoplasm or nucleus following their assembly. Unlike virulent phages, animal viral nucleic acid does not code for enzymes that lyse the host cells. Infected cells often die because viral DNA and proteins rather than host cell material are synthesized. Thus, functions required for cell survival are not carried out, and cells die. Cell degradation and lysis may also result from the release of degradative enzymes normally contained in cellular lysosomes. This degradation and release of the virions give rise to cytopathic effects. The dead cells lyse, releasing the virions, which may then invade any healthy cells in the vicinity. ■ lysosomes, p. 77

Another mechanism for release is by budding from the plasma membrane (figure 14.11). This process is frequently associated with persistent infections, but the process may kill infected cells. An example of the latter is the killing by HIV as it buds from cells of the immune system. This process involves a number of steps. First, the region of the host cell plasma membrane where budding is going to take place acquires the protein spikes coded by the viruses, which eventually are attached to the outside of the virion. These are at the lipid raft regions. Then, the inside of the plasma membrane becomes coated with the matrix protein of the virus. In the next step, the nucleocapsid becomes completely enclosed by the lipid raft region of the plasma membrane into which the spikes and matrix protein are embedded. Most enveloped viruses obtain their envelopes as they exit the cell through the plasma membrane. Some viruses, however, bud through the Golgi apparatus or rough endoplasmic reticulum. Vesicles containing the virus then migrate to the plasma membrane, with which they fuse. The virions are released by exocytosis. Thousands of virions can be released over hours or days, often without significant cell damage. For all enveloped viruses, budding is part of the maturation process. The process of budding may not lead to cell death, because the plasma membrane can be repaired following budding. As discussed in chapter 13, filamentous phages also are released from bacterial cells by budding or extrusion, without killing the bacterial cells. ■ filamentous phage, p. 331 ■ Golgi apparatus, p. 77 ■ rough endoplasmic reticulum, p. 76

Virus RNA

Virus RNA

Protein subunits (capsomeres)

Figure 14.10 Tobacco Mosaic Virus Assembly The capsomeres are added, one by one, to the coat structure to enclose the viral nucleic acid (RNA).

Protein subunits (capsomeres)

Figure 14.10 Tobacco Mosaic Virus Assembly The capsomeres are added, one by one, to the coat structure to enclose the viral nucleic acid (RNA).

Step 8: Shedding

To be maintained in nature, infectious virions must exit or be shed from the host. Shedding usually occurs from the same openings or surfaces that viruses use to gain entry. These include mucus or saliva from the respiratory tract during coughing, or sneezing, feces, urine, skin, genital secretions and blood.

Step 9: Transmission

Once an infectious virion has been shed from a host, it must be transmitted to another host, whether the same or another species. It enters into the new host and begins the infection cycle again. As previously discussed, human viruses can be classified based on their route of transmission (see table 14.4).

Differences in the various steps in the replication cycle of virulent animal viruses and phages are listed in table 14.6.

Persistent Infections

In persistent infections, the viruses are continually present in the body and are released from infected cells by budding. Persistent infections can be divided conveniently into four categories. These are (a) late complications following an acute infection, (b) latent infections, (c) chronic infections, and (d) slow infections. The categories are distinguished from one another

Nester-Anderson-Roberts: Microbiology, A Human Perspective, Fourth Edition

II. The Microbial World

14. Viruses, Prions, and Viroids: Infectious Agents of Animals and Plants

© The McGraw-H Companies, 2003

14.3 Interactions of Animal Viruses with Their Hosts 353

Capsid Protein protein spikes

Capsid Protein protein spikes

Nucleic acid

Host plasma membrane

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