Sunday, 21 February 2021

Lysogenic Cycle of bacteriophage

Lysogeny or lysogenic cycle is one of type of viral reproduction or life cycle.
Lysogenic cycle is characterized by
(1) integration of the bacteriophage nucleic acid into host bacterial genome by site specific recommendation and replication with bacterial genome e.g. λ phage in E.coli.
(2) integration and formation of circular replicon in bacterial cytoplasm that replicates as extra-chromosomal plasmid e.g. P1 in E.coli.
  Lysogenic virus (temperate phage) can remain in this stage for various replications of host cell DNA until it excises it self from cell DNA and undergoes a lytic life cycle.

Genetic material of temperate phages that is inserted into the DNA of host is called pro-phase.
A cell that contain a Prophase is known as lysogen.

Phage conversion :

  When a cell become lysogenised, occassionally extra genes carried by the phage get expressed in the cell and change the properties of the bacterial cell. This process is called lysogenic or phage conversion.
- Examples :
1) lysogenic conversion has shown to enable biofilm formation in Bacillus anthacis.

2) Lysogenic conversion of Bacillus subtilis and Bacillus cereus has shown an enhanced rate or extent of sporulation that produces endospore that are resistant to temperature, ionising radiation, desiccation, antibiotics and disinfectants. 

3) Virulence genes carried within Prophase as autonomous genetic elements morons that confers advantage to bacteria through enhanced lysogen survival.

4) non-virulent bacteria are transformed to virulent pathogen.
e.g. Cornebacterium diphtheriae produces diptheria toxin only when infected by Phage-B.

Stages(steps) of lysogenic cycle :

1. Attachment
2. Penetration
3. Integration of phage genetic material to bacterium
4. Replication of genetic material
5. Cell division.

1). Attachment -

The cycle begins with the attached of bacteriophage to specific host cell receptor on bacterial cell wall by adsorption.

2). Penetration -

a) injection of linear phage DNA into bacterium.
b) circularisation of phage DNA
c) The virus genetic material is known as Prophase, while it is in the dormant stage.
Dormant stage :- if the virus does not start multiplying or replicating after infecting the cell it is said to be dormant.

3). Integration of phage/ lysogenization -

The integration of the circular λ genome involves the presence of attachment sites in the genome of both bacteriophage and bacterium.
The bacterial attachment site, called attB Consists of a specific DNA sequence composed of 3 domain (B,O,B').
The attachment site of bacteriophage called attp, consist of P,O,P' domains.
The O domain has a sequence common to both attB and attP.
The O domain is also known as core sequence.

4). Replication of genetic material-

a) lysogenised cell will follow its regular metabolic activities and eventually prepare for cell division.
b) The nucleic acid replicates and nucleus divides into two parts.

5) cell division -

Division of cell body results in two daughter cells each having viruses genetic information incorporated to their genetic material.
The cells (host bacterial cell) remain normal until virus is triggered or induced.

Lysogeny is maintained by a repressor protein encoded. The DNA of λ Prophase called λ repressor. It bloks the expression of other genes responsible for phage replication and synthesis of phage proteins.

Immunity : The lysogens are immune to superinfecting phage. The resistance of a λ lysogen to superinfection by λ phage is called immunity.
  λ repressor encoded by CI gene. These protein inhibitor of transcription from PL and PR.
In lysogen CI encoded λ repressor is continuously when a phage infects a lysogen excess repressor present in cytoplasm bind to two promoters PL and PR before RNA polymerase bind.

Induction - Under some condition, the Prophase initiates synthesis of phage proteins, that leads to lysis of cells and release of new phages. This process is called induction.
Induction occur because cro protein express and inhibits Transcription of CI gene.

Induction is defined into mainly two ways.
1. UV Induction - if UV light damage host DNA ensuring pro-phase induction.
2. Zygotic induction - when Hfr lysogen mates with f- recipient (non lysogen) transcription of pro-phase begins and a non lysogenic recipient receiving a pro-phase will lyse ensuring lytic cycle.


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