Team:USTC-China/background

Some description of the picture

Entcrobacteria phage λ (lambda phage, coliphage λ) is a bacterial virus, or bacteriophage, that infects the bacterial species Escherichia coli. This virus is temperate and may reside within the genome of its host through lysogeny or amplify its number and lyse the host through lytic life cycle.

Usually, a "lytic cycle" ensues, where the lambda DNA is replicated many times and the genes for head, tail and lysis proteins are expressed. This leads to assembly of multiple new phage particles within the cell and subsequent cell lysis, releasing the cell contents, including virions that have been assembled, into the environment

However, under certain conditions the phage DNA may integrate itself into the host cell chromosome in the lysogenic pathway. In this state, the λ DNA is called a prophage and stays resident within the host's genome without apparent harm to the host. The host can be termed a lysogen when a prophage is present.

Genome structure of the toggle switch in lambda phage

Promoter pR:Initializes the rightward transcription. If the pR is not repressed, these proteins will be expressed and the phage will transfer into lytic cycle and plenty of new phage particles will be assembled.

Promoter pRM:Initializes the transcription of repressor CI. It transcripts leftwards. OR1, OR2, OR3: Three protein binding regions on the genome of lambda phage. They can be bound by protein CI dimmers(CI2) and Cro dimmers(Cro2). The sites of these three regions on DNA are shown in the picture below.

Function overview of the proteins involve in the toggle switch

Cro: Transcription inhibitor. The protein Cro can automatically form diamer Cro2 and then binds OR3, OR2 and OR1 (affinity OR3 > OR2 = OR1, i.e. preferentially binds OR3) (See the free energy of CI2 or Cro2 interacting with the three regions)..At low concentrations the Cro2 blocks the pRM promoter, preventing cI production afterwards. At high concentrations the Cro2 downregulates its own production through OR2 and OR1 binding. No cooperative binding (c.f. below for cI binding)

CI: Transcription inhibitor. The protein CI can also automatically form diamer CI2(similar to Cro2) and then binds OR1, OR2 and OR3 (affinity OR1 > OR2 = OR3, i.e. preferentially binds OR1). At low concentrations the CI2 blocks the pR promoter, preventing cro production afterwards. At high concentrations the CI2 downregulates its own production through OR3 binding. N terminal domain of cI on OR2 tightens the binding of RNA polymerase holoenzyme complex to pRM and hence stimulate its own transcription. Repressor also inhibits transcription from the pL promoter.

Maintenance of lysogeny

Lysogeny is maintained solely by cI. cI represses transcription from pL and pR while upregulating and controlling its own expression from pRM. It is therefore the only protein expressed by lysogenic phage.

Transformation into Lytic life cycle

Cro is responsible for preventing the synthesis of the repressor CI and this action shuts off the possibility of establishing lysogeny. It has two effects:

• It prevents the synthesis of repressor via the maintenance circuit; that is, it prevents transcription via pRM .
• It also inhibits the expression of early genes from both pL and pR Cro achieves its function by binding to the same operators as (el) repressor protein.

Lytic or lysogenic?

The gene regulatory circuitry of phage λ is among the best-understood circuits at the mechanistic level. This circuitry involves several interesting regulatory behaviors. An infected cell undergoes a decision between two alternative pathways, the lytic and lysogenic pathways, just like two states controlled by a toggle switch.

Simplistically, in cells with sufficient nutrients, protease activity is high, which breaks down cI.[10] This leads to the lytic lifestyle. In cells with limited nutrients, protease activity is low, making cI stable. This leads to the lysogenic lifestyle. This means that a cell "in trouble", i.e. lacking in nutrients and in a more dormant state, is more likely to lysogenise. This would be selected for because the phage can now lie dormant in the bacterium until it falls on better times, and so the phage can create more copies of itself with the additional resources available and with the more likely proximity of further infectable cells.