Editing Lambda phage (section)

==Repressor==
[[File:Lambda phage temperate controls.png|thumbnail|right|Protein interactions that lead to either Lytic or Lysogenic cycles for Lambda phage]]
The [[repressor]] found in the phage lambda is a notable example of the level of control possible over gene expression by a very simple system. As discovered by [[Barbara J. Meyer]],<ref>{{cite web | url = http://www.hhmi.org/biointeractive/gender/meyer.html | title = Barbara J. Meyer | work = [[HHMI]] Interactive }}</ref> it forms a 'binary switch' with two genes under mutually exclusive expression. 

[[File:LambdaPhage Repressor Cooperativity.jpg|thumb|right|upright=1.75|Visual representation of repressor tetramer/octamer binding to phage lambda L and R operator sites (stable lysogenic state)]]

The lambda repressor gene system consists of (from left to right on the chromosome):
* ''cI'' gene
* O<sub>R</sub>3
* O<sub>R</sub>2
* O<sub>R</sub>1
* ''cro'' gene

The lambda repressor is a self assembling dimer also known as the cI protein.<ref name="pmid8031775">{{cite journal | vauthors = Burz DS, Beckett D, Benson N, Ackers GK | title = Self-assembly of bacteriophage lambda cI repressor: effects of single-site mutations on the monomer-dimer equilibrium | journal = Biochemistry | volume = 33 | issue = 28 | pages = 8399–8405 | date = July 1994 | pmid = 8031775 | doi = 10.1021/bi00194a003 }}</ref> It binds DNA in the helix-turn-helix binding motif. It regulates the transcription of the cI protein and the Cro protein.

The life cycle of lambda phages is controlled by cI and Cro proteins. The lambda phage will remain in the lysogenic state if cI proteins predominate, but will be transformed into the lytic cycle if cro proteins predominate.

The cI dimer may bind to any of three operators, O<sub>R</sub>1, O<sub>R</sub>2, and O<sub>R</sub>3, in the order O<sub>R</sub>1 > O<sub>R</sub>2 > O<sub>R</sub>3.
Binding of a cI dimer to O<sub>R</sub>1 enhances binding of a second cI dimer to O<sub>R</sub>2, an effect called [[cooperativity]]. Thus,  O<sub>R</sub>1 and O<sub>R</sub>2 are almost always simultaneously occupied by cI. However, this does not increase the affinity between cI and O<sub>R</sub>3, which will be occupied only when the cI concentration is high.

At high concentrations of cI, the dimers will also bind to operators O<sub>L</sub>1 and O<sub>L</sub>2 (which are over 2 kb downstream from the R operators). When cI dimers are bound to O<sub>L</sub>1, O<sub>L</sub>2, O<sub>R</sub>1, and O<sub>R</sub>2 a loop is induced in the DNA, allowing these dimers to bind together to form an octamer. This is a phenomenon called ''long-range cooperativity''. Upon formation of the octamer, cI dimers may cooperatively bind to O<sub>L</sub>3 and O<sub>R</sub>3, repressing transcription of cI. This ''autonegative'' regulation ensures a stable minimum concentration of the repressor molecule and, should SOS signals arise, allows for more efficient prophage induction.<ref>{{cite book | vauthors = Ptashne M |title=A genetic switch: phage lambda revisited |date=2004 |publisher=Cold Spring Harbor Laboratory Press |location=Cold Spring Harbor, NY |isbn=978-0-87969-716-7 |edition=3rd | page = 112 }}</ref>
* In the absence of cI proteins, the ''cro'' gene may be transcribed.
* In the presence of cI proteins, only the ''cI'' gene may be transcribed.
* At high concentration of cI, transcriptions of both genes are repressed.

<gallery>
File:Viral DNA setup.svg|Some base pairs with serve a dual function with promoter and operator for either cl and cro proteins.
File:Polymerase ON.svg|Protein cl turned ON, with repressor bound to OR2 polymerase binding is increased and turn OFF OR1.
File:Repressor concentration.svg|Lysogen repression all 3 sites bound is a low occurrence due to OR3 weak binding affinity.  OR1 repression increases binding affinity to OR2 due to repressor-repressor interaction.  Increased concentrations of repressor increase binding.
</gallery>