Lysogen

Certain strains of Escherichia coli can be stimulated by irradiation with a moderate dose of ultraviolet (UV) light to stop normal growth and start producing bacteriophages that eventually lyse the bacterium. Bacteria of these so-called lysogenic strains carry the DNA of the phage integrated into their own... 

Diphtheria toxin, an exotoxin of Corynebacterium diphtheriae infected with a specific lysogenic phage, catalyzes the ADP-ribosylation of EF-2 on the unique amino acid diphthamide in mammalian cells. This modification inactivates EF-2 and thereby specifically inhibits mammalian protein synthesis. Many animals (eg, mice) are resistant to diphtheria toxin. This resistance is due to inability of diphtheria toxin to cross the cell membrane rather than to insensitivity of mouse EF-2 to diphtheria toxin-catalyzed ADP-ribosylation by NAD. 

When a temperate phage is mixed with sensitive indicator bacteria and plated as described above, the reaction at each focus of infection is generally a combination of lytic and lysogenic responses. Some bacteria will be lysed and produce phage, others will survive as lysogenic cells, and the plaque becomes visible as a partial area of clearing in the bacterial lawn. It is possible to pick off cells from the central areas of these plaques and demonstrate that they carry prophage. 

The essential features of lysogenic cells and the phenomenon of lysogeny are listed... 

In a normally growing culture of lysogenic bacteria, the majority of bacteria manage to keep their prophages in a dormant state. In a very small minority of cells, however, the prophage genes express themselves. This results in the multiplieation of the virus, lysis of the cells and liberation of infectious particles into the medium. 

Exposure of lysogenic cultures to certain chemical and physical agents, e. g. hydrogen peroxide, mitomycin C and ultraviolet light, results in mass lysis and the production of high titres of phage. This process is called induction. 

The induction of a lysogenic culture to produce infectious phages, followed by lysogenization of a second strain of the bacterial species by these phages, results in the... 

Studies on the transformahon of tissue cultures with DNA-containing vimses have shown that, although complete vims particles cannot be found in the infected, transformed cells, viral DNA is present and is bound to the transformed cell DNA as provirus, analogous to the prophage of lysogenic bacteria. 

Although bacteria have a single essential chromosome, one DNA molecule responsible for their genetic material, they may also contain extrachromosomal DNA. One variety of such DNA is viral DNA. This DNA can become incorporated into the chromosome and reproduced with it (lysogenic bacteria) or can become virulent — produced independently from the chromosome DNA — when it will cause lysis i.e. cell breakdown. The compound cis-[Pt(NH3)2Cl2] causes the change from... 

The effect of platinum in a bacterial cell is to act in a very selective way — on cell division or causing lysis of lysogenic bacteria. It is likely that these changes are due to site specific attack on particular proteins or on particular bases in RNA or in DNA. It is necessary now to describe this attack in detail and to develop new probes for following the site in vivo. This exercise can be followed by a parallel examination of how cis- [Pt (NH3) 2CI2] acts as an anti-tumour agent. Here we only point to some interesting observations. 

The effective compounds of platinum and the dialkylating agents both cause lysis in lysogenic bacteria, section I. 

There is evidence that both classes of anti-tumour agent act by inhibiting DNA synthesis as opposed to RNA or protein synthesis. Many agents which inhibit DNA synthesis cause lysis of lysogenic bacteria-X-rays, ultra-violet light, many carcinogens (3, 90). 

With most temperate phages, if the host simply makes a copy of the viral DNA, lysis does not occur but if complete virion particles are produced, then the host cell lyses. In a lysogenic bacterial culture at any one time, a small fraction of the cells, 0.1 to 0.0001 percent,... 

The temperate virus does not exist in its mature, infectious state inside the cell, but rather in a latent form, called the provirus or prophage state. In considering virulent viruses we learned that the DNA of the virulent virus contains information for the synthesis of a number of enzymes and other proteins essential to virus reproduction. The prophage of the temperate virus carries similar information, but in the lysogenic cell this information remains dormant because the expression of the virus genes is blocked through the action of a specific repressor coded for by the virus. As a result of a genetic switch, the repressor is inactivated, virus reproduction occurs, the cell lyses, and virus particles are released. 

A lysogenic culture can be treated so that most or all of the cells produce virus and lyse. Such treatment, called induction, usually involves the use of agents such as ultraviolet radiation, nitrogen mustards, or X rays, known to damage DNA and activate the SOS system. However, not all prophages are inducible in some temperate viruses, prophage expression occurs only by natural events. 

Although a lysogenic bacterium may be susceptible to infection by other viruses, it cannot be infected by virus particles of the type for which it is lysogenic. This immunity, which is characteristic of lysogenized cells, is conferred by the intracellular repression mechanism under the control of virus genes. 

It is sometimes possible to eliminate the lysogenic virus (to cure the strain) by heavy irradiation or treatment with nitrogen mustards. Among the few survivors may be some cells that have been cured. Presumably the treatment causes the prophage to detach from the host chromosome and be lost during subsequent cell growth. Such a cured strain is no longer immune to the virus and can serve as a suitable host for study of virus replication. 

Figure 5.21 Consequences of infection by a temperate bacteriophage. The alternatives upon infection are integration of the virus DNA into the host DNA (lysogenization) or replication and release of mature virus (lysis). The lysogenic cell can also be induced to produce mature virus and lyse.

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