ADVERTISEMENTS:
In this article we will discuss about:- 1. Process of Lysogeny 2. Control of Lysogeny 3. Induction 4. Lysogenic Conversion 5. The Genetic Switch 6. Significance.
Process of Lysogeny:
Many bacteriophages (bacterial viruses) such as phage T2, T4, etc. that reproduce within host bacterial cells using a lytic cycle and finally destroy the cell are called virulent bacteriophages.
But, there are some other bacteriophages (e.g., lambda phage) that, in addition to establishing lytic relationship with host cell, are also capable of establishing a different stable genetic relationship with host cell which is called lysogeny.
ADVERTISEMENTS:
In lysogeny the viral genome, after adsorption and penetration, does not take control of its host cell and destroy it. Instead the genome remains within the host cell and is reproduced along with the bacterial chromosome (bacterial DNA) passing from one generation to the next without rendering any harm to the host cell (Fig. 13.9).
Bacteriophages able to establish lysogeny in host cells arc called temperate bacteriophages, and the latent form of temperate bacteriophage present within the host cell without destroying it is called the pro-phage.
The pro-phage usually is integrated into the bacterial chromosome but sometimes exists independently. Under certain conditions, each of the bacterial cells in state of lysogeny can spontaneously produce and release new progeny bacteriophages, and lyse; such a bacterial cell is said to be lysogen or lysogenic.
However, lysogeny is not limited to bacteriophages; there are many animal viruses that establish similar relationships with their hosts.
Control of Lysogeny:
Two events are considered necessary to establish and control lysogeny by lambda phage:
(1) The synthesis of all late proteins should be stopped to prevent phage multiplication and
(2) Lambda genome should integrate into the bacterial chromosome.
1. Prevention of Late Protein Synthesis:
To prevent the synthesis of late proteins, the product of the cl gene must be synthesized. The cl gene product is the lambda repressor protein. The latter, if synthesized, represses the synthesis of all other lambda genome-encoded proteins. The cl gene occurs between PL and PR (promoter left and promoter right) (Fig. 13.10) which are oriented in such a way that neither transcribe the cl gene.
The transcription of cI gene (i.e. the production of mRNA from the cl gene) is promoted by PE promoter (promoter establishment) which is present to the right of the cro gene but facing the direction opposite that of PR. Therefore, transcription is in the direction opposite that promoted by PR (Fig. 13.10 and 13.11).
When PE is activated, it promotes the transcription of cl gene (i.e. the production of mRNA from the cl gene) leading to the synthesis of lambda repressor protein that represses the synthesis of all other lambda genome-encoded proteins and the lysogenic cycle is followed.
The promoter PE is not self-activated rather by an activator protein (cII- protein), the product of ell gene. If the ell-protein is degraded there would be no possibility of lysogenic cycle hence another protein called cIII-proteins (the product of cIII gene) is produced to stabilize the cII- protein.
Lambda repressor protein binds to OL and OR (operator left and operator right) to the sites within these operators. It first binds to site 1, turning off PR (and PL by a similar mechanism). When this occurs, the synthesis of all other lambda proteins is stopped, and the virus fails to enter into the lytic cycle.
However, without the cII-protein PE promoter no longer functions. Therefore, if the lysogenic state is to be maintained, there must be another way to transcribe cI gene. PM (promoter maintenance), which faces towards the cI gene (in the same direction as PE), is responsible for it (Fig. 13.11).
PM is activated when lambda repressor binds the site 1 and is repressed only when lambda repressor is bound to all three sites. Therefore, the lambda repressor is both a repressor and an activator when it binds to site 1, repressing PR and activating PM. This type of regulation continues to take place even after lysogenization. Only the lambda repressor is produced after lambda is integrated as a prophage.
2. Integration:
ADVERTISEMENTS:
Integration takes place by the insertion of lambda genome into the bacterial genome at specific site. This effectively lengthens the bacterial genome by the length of the lambda genome. On injection, the cohesive ends of the linear lambda genome come to each other forming a circle, and it is thus circular genome that integrates into the bacterial genome (Fig. 13.12).
The site of the cohesion of two ends of lambda genome is called cos site. Genes cl and int (encoding integrase enzymes) must be expressed to establish lysogenic cycle. The integration process requires integrase enzyme which catalyzes recombination of the phage and bacterial attachment sites labelled as att. The int gene has a promoter which, like PE (promoter establishment), is activated by the cII protein.
When the cell grows, the lambda repression system stops the expression of the integrated lambda genes except for the gene cI which codes for the lambda repressor protein (as discussed in the prevention of late protein synthesis). During replication of bacterial genome, the integrated lambda genome is replicated along with the bacterial genome and transmitted progeny to progeny.
Induction of Lysogeny:
Lysogeny is an indefinite process is not true. The phage genome (pro-phage) can dissociate from the bacterial DNA and start progeny production to result in the lysis of the bacterial cell (i.e., lytic cycle). Dissociation of pro-phage from bacterial DNA, its conversion into virulent phage, and initiation of lytic cycle is called induction (Fig. 13.13).
Induction may occur spontaneously at a low frequency in about one out of 102 to 105 cells, or may be due to external agents including ultraviolet irradiation, X-rays, and DNA-damaging chemicals such as the nitrogen mustards. 
The process of induction is triggered by a drop in the level of lambda repressor protein. The recA gene of E. coli synthesizes recA protein, which normally plays role in genetic recombination. The recA protein acts as a protease (proteolytic enzyme) and cleaves the lambda repressor protein chain between its two domains.
The separated domains fail to assemble to form the normal active repressor protein and, as a result, the inactive form of lambda repressor protein can no longer bind to the promoter inhibiting transcription of the lytic genes and the lytic genes become active again.
ADVERTISEMENTS:
There is some recent evidence that recA protein may not directly clear lambda repressor protein; it may instead bind to the lambda repressor protein and stimulate it to proteolyticallv clear itself.
However, axis gene located next to the int gene codes for the synthesis of an excisionase protein that binds to the integrase enzyme and enables it to reserve the integration process and free the pro-phage. In this condition, the synthesis of another protein, called the cro-protein, by cro genes is initiated. The cro-protein switches the process of synthesis of viral components and the lytic cycle then proceeds normally.
Lysogenic Conversion:
When a temperate bacteriophage lysogenizes a bacterial cell and its genome integrates into the bacterial DNA as pro-phage, it may induce a change in the phenotype of its host cell that is not directly related to completion of its life cycle. Such a phenotypic change in the host cell brought about through lysogenization by the temperate bacteriophage is called lysogenic conversion or phage conversion.
Two cases of lysogenic conversion have been especially well studied, and are as follows:
1. When Salmonella anatum is lysogenized by an epsilon bacteriophage (bacteriophage ԑ15), the structure of its outer lipopolysaccharide layer on the cell surface is modified. The bacteriophage changes the activities of various enzymes involved in the synthesis of the lipopolysaccharide carbohydrate component and thus alters the antigenic properties of the host.
These alterations induced by epsilon bacteriophage appear to eliminate surface-receptors for bacteriophage and prevent infection of the bacterial cell by another epsilon bacteriophage, i.e., the bacterial cell becomes immune to further infection by the epsilon bacteriophage.
ADVERTISEMENTS:
2. When nontoxin producing strains of Corynebacterium diphtheriae (the bacterium that causes diphtheria) are lysogenized by bacteriophage β, they are converted to toxin-producing strains. The toxin-producing strains of C. diphtheriae are pathogenic because they produce the powerful exotoxin called diphtheria toxin, which inhibits eukaryotic protein synthesis and thus kills infected cells of the host.
In both of the above mentioned lysogenic conversions, the genes encoding the necessary molecules required to bring alternations are an integral part of the bacteriophage DNA and hence are automatically (and exclusively) transferred upon infection by the bacteriophage and lysogenization.
Choice between Lysogeny and Lytic Cycle: The Genetic Switch:
The lambda phages (also some other temperate phages) possess ability to follow the route of either lysogeny or lytic cycle. What controls whether the phage will take the lysogenic or lytic route?
It is considered that the lambda and such other temperate phages have a genetic switch that controls which route will be followed. This genetic switch mainly consists of two proteins, namely, cro-protein (by cro gene) and lambda repressor protein (by cI gene). Both these proteins are repressor proteins and are simultaneously produced.
Since both proteins can block the transcription by each other, there is a race between the production of cro-protein and lambda repressor protein. If the lambda repressor protein wins, the race the linear lambda genome becomes circular, integrates into the bacterial genome, and the lysogeny is followed. But, if the concentration of cro- protein increases so that it wins the race, the lytic cycle is initiated.
The cro-protein binds to OR (operator right) and OL (operator left), turns off the transcription of repressor gene (as well as inhibiting the expression of ether early genes), and represses PM (promoter maintenance) function to switch off the lysogenic cycle. The mechanism of cro-protein action (Fig. 13.11) is as follows.
ADVERTISEMENTS:
There are three similar but non-identical sites at OR where the cro-protein can bind. It does so first at site 3, and then site 2 and only when these two sites are filled, at site 1.
This 3-site binding of cro-protein blocks PR. Once PR and PL are blocked, no more cII or cIII proteins are produced. These proteins are required to enter the lysogenic cycle, and so when cro-protein concentration goes high the lambda phage initiates the lytic cycle.
Significance of Lysogeny:
ADVERTISEMENTS:
Lysogeny is probably of major significance to temperate bacteriophages because most bacteria isolated from natural habitats are lysogenic for one or more bacteriophages.
1. Lysogeny confers immunity to lysogens to infection by the same type of phage:
The temperate bacteriophage does not exist free in the lysogen’s cytoplasm. Instead, it remains integrated into the bacterial DNA and replicates along with it as long as its lytic cycle genes are not expressed. Typically, it is the phage repressor protein that maintains this control.
The phage repressor protein, which is encoded by phage gene, not only controls the lytic cycle genes situated in pro-phage but also prevents the expression of any incoming genes of the same type of phage. This results in the lysogens having immunity to infection by the same type of phage.
2. Lysogeny is advantageous to phage in nutrient deficiency:
Bacteria enter dormancy in a phage- infected culture that becomes nutrient-deficient, and they degrade their own mRNA and protein. In this condition, the phage reproduction is confined only in actively metabolizing bacterial cells and is usually permanently interrupted in mRNA and protein degrading bacterial cells.
This difficult situation can be avoided if the phage turns to be dormant (lysogenic) simultaneously with the host, i.e., the nutrient deficiency docs favour lysogeny.
3. A high multiplicity of infection (MOI) stimulates lysogeny:
Temperate bacteriophages also ate advantageous in situations when each bacterial cell is subjected to infection by many viruses, i.e., there is a high multiplicity of infection (MOI). In this situation when every cell becomes infected, the last round of replication will destroy all host cells.
Thus there is a risk that the phages may be left without a host and directly exposed to environmental hazards. Lysogeny therefore comes forward to avoid this prospect; some bacteria that become lysogenized manage to survive, carry the phage genome, and increase their population by reproduction.
When bacterial population increases, the phage genome may dissociate from the bacterial DNA and may enter into lytic cycle to reproduce its own progeny. Therefore, not surprisingly, a high multiplicity of infection (MOI) does stimulate lysogeny.
4. Lysogeny can also confer new properties on the bacterial cell:
Lysogeny may induce a change in the phenotypic property of the host cell employing lysogenic conversion and thus confers a new property on it.
There are several examples of pathogenic bacteria such as Corynebacterium diphtheriae (the cause of diphtheria), Clostridium botulinum (the cause of botulism), Vibrio cholerae (the cause of cholera), and streptococci (the cause of scarlet fever), whose virulence is due, at least in part, to the lysogenic bacteriophage they harbor.
For convenience, when non-toxin producing strains of C. diphtheriae are lysogenized by bacteriophage β, they are converted to toxin producing strains. These strains produce the powerful exotoxin called diphtheria toxin, which inhibits eukaryotic protein synthesis and thus kills infected cells of the host.
