Lysogenic infection

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. 

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 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. 

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.

Lytic growth of Mu can occur either upon initial infection, if the c gene repressor is not formed, or by induction of a lysogen. In either case, replication of Mu DNA involves repeated transposition of Mu to multiple sites on the host genome. Initially, transcription of only the early genes of Mu occurs, but after gene C protein, a positive activator of late RNA synthesis, is expressed, the synthesis of the Mu head and tail proteins occurs. Eventually, expression of the lytic function occurs and mature phage particles are released. 

In lysogenic cytlos. replication of the bacteriophage lambda occurs without lysis of the infected tell. 

In one mutated form of lambda the repressor protein is inactive at temperatures above 37 °C but active at lower temperatures. When this lambda is used as a vector the infected E. coli are grown first at 32 °C to allow replication of the DNA in the lysogenic cycle. The temperature is then increased to 37 °C to inactivate the repressor. This results in the excision of the lambda genome and release of lambda particles by lysis. 

The exonuclease synthesized after induction of A lysogens or after infection with virulent mutants of this phage has received a great deal... 

About half of the time when A infects a cell it adopts a dormant lysogenic state in which the virus is linearly integrated into the host genome. This state is maintained by moderate amounts of the A-encoded cl repressor. The cl repressor prevents the lytic cycle from developing by inhibiting two promoters the PL promoter for early leftward transcription and PR the promoter for early rightward transcrip-... 

Thus far we have considered the events that take place after infection or after activation of the prophage that lead to phage replication and ultimately lysis. As already indicated, when A infects a cell, the cells can follow this lytic pathway, or alternatively they can follow the lysogenic pathway, in... 

Lysogenic virus. A virus that can adopt an inactive (lysogenic) state, in which it maintains its genome within a cell instead of entering the lytic cycle. The circumstances that determine whether a lysogenic (temperate) virus adopts an inactive state or an active lytic state are often subtle and depend on the physiological state of the infected cell. 

S. Vasilukova, nee Reslova, a young Czechoslovakian microbiologist, and an ex-student of J. Drobnik who contributed much to our microbial experiments, worked with strains of E. coli bacteria that had been previously infected with a bacterial virus (A-bacteriophage). In these lysogenic bacteria, the genetic information of the virus has been incorporated into the cell, but it is repressed so that it is not normally detectable. It replicates during cell division along with the bacterial DNA and so is not lost or diluted out after many divisions. 

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