Lambda Life Cycle

Lambda Lysogenic cycle

However Lambda Bacteriophage undergoes temperate/lysogenic replication. This is where the viral DNA site specifically integrates into the host DNA, and is replicated at the same rate as the host genome without causing cell death. That is until there is a releasing event of the viral genome, which then feeds into the lytic life cycle.

1. Adsorption – Differs from T4 bacteriophage in that Lambda uses the outer membrane protein necessary for maltodextrin transport. [4]

 2. Lysogeny/Genome Replication – The genetic structure of λ is structurally simple but functionally complex. The figure below represents the genetic structure of λ, in it’s replicative form, after it has been injected into the host-cell. Early transcripts are involved in replication, integration, and excision. Late transcripts are involved with capsid protein assembly and cell lysis. The cos site is the point at which the linear ends of the genome come together to form a circular genome structure within the host-cell. The att site is involved with host-cell genome interactions, the ori site is the origin of circular replication, and the cI site is involved with controlling gene expression.

Following infection, the host RNA polymerase binds to the viral promoters pR and pL and replication occurs in both directions. The transcription of early genes is stopped because RNA polymerase stops transcribing at the p-dependent early sites tR and tL. RNA polymerase can proceed once protein N is translated because protein N antagonizes the p-dependent early sites and allows the RNA polymerase to read through. The next genes to be transcribed are cIII and Q which are late gene transcriptional activators. At this point the virus can either proceed to activate genes which are involved in the lytic or lysogenic cycles. The mechanism which underlies the fate of the virus is not fully understood. However, it is speculated that intracellular conditions of the bacterium help decide the lytic or lysogenic fate of the virus.

Lytic genes, controlled by Q, are found on the right side of the genome. Lysogenic genes, controlled by cIII, are on the left. Lytic growth only requires one transcript (pQ) and immediately follows replication. Transcription of pQ regulates late proteins involved in capsid formation and cell lysis. On the other hand, lysogenic growth occurs from a more complex interaction of various gene products. Lysogeny is essentially entered by repression of genes which promote replication, phage assembly and lysis. The cro protein is encoded on the right side of the genome and is an early transcript. Cro protein is responsible for the repression of sites OR and pR by competitive inhibition of RNA polymerase for those promoting sites. Binding of cro prevents transcription of pR and pL and the virus begins to enter lysogeny.

Lysogeny is maintained by a single gene product, cI, which encodes the λ repressor protein. The λ repressor protein binds to OR and OL sites to block the synthesis of genes which promote lytic development. Interestingly, the λ repressor protein acts to promote its own transcription by also binding to the PM site which is a promoter for cI. [16]

The λ genome integrates within the host-cell chromosome during lysogeny. The phage genome integrates into the host-cell genome through site-specific integration during recombination and this is catalyzed by a virus-encoded intragrase (int). The int protein simultaneously binds directly to the PP’ site on the viral genome and the BB’ site on the bacterial genome to drive integration. Due to the fact that there is site specific integration, a host-cell can normally only have one integrated copy of λ.

Following integration, the λ genes can now be replicated along with the host-cell genome during cellular division. However, the lysogenic state is not stable and has a tendency to break down once every 104 cell divisions. Excision of the virus genome is mediated by the viral encoded protein excisionase (xis). Once lysogeny breaks down, the phage enters the lytic state and propagation of progeny virus through replication, lyses, and release will occur.

 Replication of λ occurs through rolling circle replication, in bidirectional manner starting at the ori site. One strand of circular DNA is cut and the exposed 3` end acts as a primer. The first several replication cycles occur in circular form as seen in the figure below. Then, sigma replication leads to the elongation of the viral genome creating concatamers. Elongation of the phage occurs using several different host proteins including: DnaB helicase, DnaG primase, Pol III holoenzyme, and the host RNA polymerase. [16]

Packaging: Concatamers of replication are simultaneously cut at cos sites between each complete genome using the viral encoded protein terminase. The λ phage packages two proteins which help activate the origin of replication and also directs host replicative enzymes. [16]

3. Rescue from Lysogeny – Stability of lysogeny soon breaks down, spontaneously or by DNA damaging agents. Excisionase binds to genome, bends the DNA and interacts with integrase, and reverses the integration process by excising the phage DNA from the host DNA. [4]

4.  Lytic development/Genome Replication– Follows similar Lytic cycle of T4 bacteriophage.

5. Assembly – Concatamers of replication are simultaneously cut at cos sites between each complete genome using the viral encoded protein terminase. The λ phage packages two proteins which help activate the origin of replication and direct host replicative enzymes. [16]

6. Release – Lambda is released through lysis of the host-cell which is mediated by two viral encoded proteins. One of the proteins is a holoenzyme (S107) which becomes embedded in the cytoplasmic membrane. When activated, the holoenzyme creates a hole in the cytoplasmic wall and allows the release of lysozymes into the periplasm. Lysozymes break apart the peptidoglycan cell wall which leads to cell lysis. [4, 16]

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