MoBio Regulation of lambda Repressor (cI) Chapter 4

The life cycle of λ phages is controlled by cI and Cro proteins. The λ phage will remain in the lysogenic state if cI proteins predominate, but will be transformed into the lytic cycle if Cro proteins predominate. Transcription of the two proteins is regulated by the cI protein (λ repressor) itself.

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Figure 4-H-2. Regulation of the expression of cI and Cro proteins by cI. The cI dimer may bind to any of three operators, OR1, OR2, and OR3, in the order OR1 > OR2 > OR3. Binding of a cI dimer to OR1 enhances binding of a second cI dimer to OR2. Thus, OR1 and OR2 are almost always simultaneously occupied by cI. However, this does not increase the affinity between cI and OR3, which will be occupied only when the cI concentration is high.
(a) In the absence of cI proteins, the cro gene may be transcribed.
(b) In the presence of cI proteins, only the cI gene may be transcribed.
(c) At high concentration of cI, transcriptions of both genes are repressed.

When the host DNA is damaged (e.g., under UV irradiation), the cI protein may be cleaved by certain protease promoted by the RecA protein. Cleaved cI proteins cannot bind to the operators. Thus, the Cro proteins can be produced to transform the λ phage into the lytic cycle.