Viruses bacterial

Valegard, K., et al. The three-dimensional structure of the bacterial virus MS2. Nature 345 36-41, 1990. 

One year later, 22-year-old James Watson completed his Ph.D. studies on bacterial viruses at Indiana University and began postdoctoral research in biochemistry in Copenhagen. After a year at Copenhagen, Watson decided Cambridge was the place to be. 

Bacteriophage very small bacterial virus) 25 4,700,000 ... 

Like some eukaryotic viruses (eg, herpes simplex, HIV), some bacterial viruses can either reside in a dormant state within the host chromosomes or can rephcate... 

There is a third structural group comprising the poxviruses and many bacterial viruses, in which a number of major structural components can be identified and the overall geometry of the particles is complex. 

The long incubation times of many human virus diseases indicate that they replicate slowly in host cells. In tissue culture systems it has been shown that most human viruses take from 4 to 24 hours to complete a single replication cycle, contrasting with the 30 or so minutes for many bacterial viruses. 

Viruses are classified initially on the basis of the hosts they infect. Thus we have animal viruses, plant viruses, and bacterial viruses. Bacterial viruses, sometimes called bacteriophages (or phage for short, from the Greek phago meaning to eat), have been studied primarily as convenient model systems for research on the molecular biology and genetics of virus reproduction. Many of the basic concepts of... 

When a virus multiplies, the genome becomes released from the coat. This process occurs during the infection process. The present chapter is divided into three parts. The first part deals with basic concepts of virus structure and function. The second part deals with the nature and manner of multiplication of the bacterial viruses (bacteriophages). In this part we introduce the basic molecular biology of virus multiplication. The third part deals with important groups of animal viruses, with emphasis on molecular aspects of animal virus multiplication. 

The complete complex of nucleic acid and protein, packaged in the virus particle, is called the virus nucleocapsid. Although the virus structure just described is frequently the total structure of a virus particle, a number of animal viruses (and a few bacterial viruses) have more complex structures. These viruses are enveloped viruses, in which the nucleocapsid is enclosed in a membrane. Virus membranes are generally lipid bilayer membranes, but associated with these membranes are often virus-specific proteins. Inside the virion are often one or more virus-specific enzymes. Such enzymes usually play roles during the infection and replication process. 

Enveloped viruses Many viruses have complex membranous structures surrounding the nucleocapsid. Enveloped viruses are common in the animal world (for example, influenza virus), but some enveloped bacterial viruses are also known. The virus envelope consists of a lipid bilayer with proteins, usually glycoproteins, embedded in it. Although the glycoproteins of the virus membrane are encoded by the virus, the lipids are derived from the membranes of the host cell. The symmetry of enveloped viruses is expressed not in terms of the virion as a whole but in terms of the nucleocapsid present inside the virus membrane. 

Complex viruses Some virions are even more complex, being composed of several separate parts, with separate shapes and symmetries. The most complicated viruses in terms of structure are some of the bacterial viruses, which possess not only icosahedral heads but helical tails. In some bacterial viruses, such as the T4 virus of Escherichia coli, the tail itself is a complex structure. For instance, T4 has almost 20 separate proteins in the tail, and the T4 head has several more proteins. In such complex viruses, assembly is also complex. For instance, in T4 the complete tail is formed as a subassembly, and then the tail is added to the DNA-containing head. Finally, tail fibers formed from another protein are added to make the mature, infectious virus particle. 

As we have noted, viruses can be classified into broad groups depending on their hosts. For instance, there are plant viruses, animal viruses, and bacterial viruses. A number of viruses infecting insects are also known and although viruses are known for fungi, protozoa, and algae, these viruses have been so little studied that no classification has been developed. In the present chapter, we discuss only the animal (primarily mammalian) and bacterial viruses, and we discuss here briefly how these two groups of viruses are classified. 

Classification of bacterial viruses In the bacterial viruses, a formal classification scheme is rarely used. Rather, each bacterial virus is designated in terms of its principal bacterial host, followed by an arbitrary alphanumeric. Thus, we speak of T4 virus of Escherichia coli or P22 virus of Salmonella typhimurium. An overview of some of the major types of bacterial viruses is given later. We should note, however, that although a bacterial virus may be designated in reference to its principal host, the actual host range of the virus may be broader. Thus, bacteriophage Mu, generally studied with Escherichia coli, also infects Citrobacter and Salmonella. 

Because viruses only replicate inside living cells, research on viruses requires use of appropriate hosts. For the study of bacterial viruses, pure cultures are used either in liquid or on semi-solid (agar) medium. Because bacteria are so easy to culture, it is quite easy to study bacterial viruses and this is why such detailed knowledge of bacterial virus reproduction is available. 

Plaques are essentially windows in the lawn of confluent cell growth. With bacterial viruses, plaques may be obtained when virus particles are mixed into a thin layer of host bacteria which is spread out as an agar overlay on the surface of an agar medium. During incubation of the culture, the bacteria grow and form a turbid layer which is visible... 

Figure 5.8 Quantification of bacterial virus by plaque assay using the agar overlay technique.

Figure 5.9 The replication cycle of a bacterial virus. The general stages of virus replication are indicated.

The eclipse is the period during which the stages of virus multiplication occur. This is called the latent period, because no infectious virus particles are evident. Finally, maturation begins as the newly synthesized nucleic acid molecules become assembled inside protein coats. During the maturation phase, the titer of active virus particles inside the cell rises dramatically. At the end of maturation, release of mature virus particles occurs, either as a result of cell lysis or because of some budding or excretion process. The number of virus particles released, called the burst size, will vary with the particular virus and the particular host cell, and can range from a few to a few thousand. The timing of this overall virus replication cycle varies from 20-30 minutes in many bacterial viruses to 8-40 hours in most animal viruses. We now consider each of the steps of the virus multiplication cycle in more detail. 

Most of the bacterial viruses which have been studied in any detail infect bacteria of the enteric group, such as Escherichia coli and Salmonella typhimurium. However, viruses are known that infect a variety of procaryotes, both eubacteria and archaebacteria. A few bacterial viruses have lipid envelopes but most do not. However,... 

A number of bacterial viruses have RNA genomes. The best-known bacterial RNA viruses have single-stranded RNA. Interestingly, the bacterial RNA viruses known in the enteric bacteria group infect only bacterial cells which behave as gene donors (males) in genetic recombination. This restriction to male bacterial cells arises because these viruses infect bacteria by attaching to male-specific pili. Since such pili are absent on female cells, these RNA viruses are unable to attach to the females, and hence do not initiate infection in females. 

Figure 5.12 Schematic representations of the main types of bacterial viruses. Those discussed in detail are fd, M13,

Many bacterial viruses have genomes containing double-stranded DNA. Such viruses were the first bacterial viruses discovered, and have been the most extensively studied. With such a range of double-stranded DNA viruses, a wide variety of replication systems are present. In the present section, we discuss the best studied and most representative of the group, T4 and T7. The simpler, T7, will be discussed first. 

We thus see that T7 has a much more complex replication scheme than that seen for the other bacterial viruses discussed earlier. 

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