Bacteriophage Reproduction: Lysogenic Cycle.

The bacteriophages are the viruses that infect the bacteria. There are two different types of bacteriophages based on the mode of reproduction: Temperate bacteriophages and Virulent bacteriophages.

The virulent bacteriophage reproduces through lytic cycle (last post). The temperate phages can reproduce through both the lysogenic and the lytic cycle.

Both a lytic and lysogenic state require expression of the immediate early and delayed early genes of the phage.

Fig 1: A temperate phage & the two mode of phage reproduction

It depends on the presence of two different types of repressors as to which cycle will be followed. These two types of repressors are Cro protein and the cI protein.

The Cro repressor favours the lytic cycle and the cI repressor favours the lysogenic infection. To maintain lysogeny, cI represses cro protein and to initiate lytic cycle, cro must repress cI.

cI repressor = Lysogeny

cro protein = Lytic cycle.

As mentioned before, when a temperate phage DNA enters a bacterial cell, the immediate and delayed early genes are expressed. One such early gene is int gene. This gene codes for the enzyme integrase, which brings about the integration of the phage nucleic acid into the host genome. Depending on the phage and the host the integration may occur at the specific site of the host and the phage genomes or at any random site. In this integrated state, the phage DNA is known as the ‘prophage‘. The phage DNA is replicated with the host genome.

This lysogenic state can be maintained for many generations of the host cell, with help of a repressor, a product of cI gene.

The gene for this cI repressor is located near Cro repressor gene. The genes for two repressors are separated by an operator region OR. Hope you all know that operator is a region where repressor binds. These operator region is divided into three sub regions: OR1, OR2 and OR3.

OR1 and OR2 overlap the promoter PR* (for Cro protein) and part of OR2 and OR3 overlap promoter PRM* (for cI repressor). Again remember that promoter is the region where RNA polymerase binds to initiate the transcription of a gene.

Now as soon as the phage DNA is injected, the cI gene is transcribed. The cI repressor binds OR1 and OR2 more strongly and block PR, promoter for Cro gene. Hence binding of cI repressor at PR region blocks the transcription of the Cro gene (Refer fig 4).

Fig 4: The switch during lysogenic cycle.

cI repressors also binds to another promoter *PL and block the transcription of a gene called N gene. The product of N gene is responsible for the modifications in host RNA polymerase, which enables it to transcribe late genes, i.e. the structural genes for the viral particles and results into lysis of the host cell.

Hence if the synthesis of cI protein would stop, the structural proteins would be synthesized and the lytic cycle would begin. The synthesis of cI repressor can be blocked by the cro repressor, but requires induction to occur.

Induction is any DNA damaging event, such as exposure to certain radiations (UV) or chemicals ( hydrogen peroxide). During such an event the SOS response is initiated wherein the RecA protein cleaves various proteins including the cI repressor proteins.

As soon as the cI repressors are cleaved, the PR is available for the RNA polymerase to bind and transcribe the cro gene. The Cro protein binds to the operator OR3, which overlaps with PRM, the promoter for cI repressor gene. Hence the repressor protein synthesis is blocked (Fig 5).

In absence of the cI repressor from the OL, transcription of N gene, also begins, which enables the production of structural genes and the lytic cycle initiates .

Hence the temperate phage can follow any of the mode of reproduction but the choice depends on the competition between the two repressors.

(*L, R and RM are subscript in PL, PR and PRM).

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