Virus particles bacteriophage

Figure 16.1 Viruses vary in size and shape from the simplest satellite viruses (a) that need another virus for their replication to the T-even bacteriophages (d) that have developed sophisticated mechanisms for injecting DNA into bacteria. Four different virus particles are shown to scale.

Viruses are discussed more fully elsewhere (Chapter 3). However, there are certain groups of viruses, called bacteriophages (phages), which can attack bacteria. This attack involves the injechon of viral DNA into baeterial eells which then proceed to make new virus particles and destroy eells. Some viruses, known as temperate viruses, do not cause this catastrophic event when they infect their host, but can pass genetic material from one cell to another. 

The basic problem of virus replication can be simply put the virus must somehow induce a living host cell to synthesize all of the essential components needed to make more virus particles. These components must then be assembled into the proper structure and the new virus particles must escape from the cell and infect other cells. The various phases of this replication process in a bacteriophage can be categorized in seven steps ... 

Bacteriophage T7 Bacteriophage T7 and its close relative T3 are relatively small DNA viruses that infect Escherichia coli. (Some strains of Shigella and Pasteurella are also hosts for phage T7.) The virus particle has an icosahedral head and a very small tail. The virus particle is fairly complex, with S different proteins in the head and 3-6 different proteins in the tail. One tail protein, the tail fiber protein, is the means by which the virus particle attaches to the bacterial cell surface. Only female cells of Escherichia coli can be infected with T7 male cells can be infected but the multiplication process is terminated during the latent period. 

Additional support for DNA as the bearer of genetic information came from studies by Hershey and Chase (1952) in the replication of DNA-containing bacteriophages. Using virus particles isotopically labeled in either the viral DNA or in the viral protein, it was shown that the viral DNA entered the host cell (Escherichia coli), whereas the viral protein did not. Later experiments demonstrated that viral DNA alone was infectious and led to the formation of mature infectious virus particles of the appropriate genotype. The DNA contained all the information required for the synthesis of progeny phage particles. 

The bacteriophage T4 is a complex virus capable of infecting certain bacteria. The virus protein coat (head, tail and tail fibers) contains 40 structural proteins. T4 illustrates well the spatial and temporal control of the stepwise assembly process and the role of external input in regulating that process. A further 13 proteins are required for assembly but do not appear in the completed virus particle. Three of these act as a transient template to promote the formation of the tail baseplate (Fig. 5-3). Another one is a protease which cleaves the major protein of the head (from 55 kDa to 45 kDa) but only after the head has been partially assembled. It is only when the tail reaches its correct length that the cap protein is placed on top allowing the completed head to become attached (Fig. 5-3). Finally, the intact tail fibers are added at the baseplate, and this requires an enzyme-catalyzed reaction to occur. 

Earnshaw, W. C., King,J., and Eiserling, F. A. (1978a). The size of the bacteriophage T4 head in solution with comments about the dimension of virus particles as visualized by electron microscopy./. Mol Biol 122, 247-253. 

Furthermore, as viruses in a sense eat the host cell, and hence may be called phages, for example, bacteriophages, there may be a connection with what is called pleomorphism, in which viruses or microbes are thought to change their form or makeup. That is, if a virus eats or destroys bacteria, then there is nothing left but a virus, or virus particle, called a virion. 

If termination codons are not recognized efficiently by termination factors, synthesis continues past the termination codons and new longer protein chains are made. This read-through translation may sometimes be accidental, but it is also used by cells to form several important proteins. For example, the 14-kDa coat protein of bacteriophage Qp is elongated by read-through during translation of the RNA about 4% of the time. This produces a 38-kDa protein known as Ay which has an extra 200 amino acid residues at the C terminus and is essential for formation of infectious virus particles. 

---