Hello Friends
To continue with the Lysogenic and Lytic cycle of Lambda Phage..
PRM transcribes only cI and it is a weak promoter and efficient only when activator is bound just upstream.
PL and PR are stong promoter and do not require any activator.

For lytic phase PL and PR remains on PRM is off and for lysogeny PRM is on and  PL and PR are off.
cI encodes Lambda repressor which is dumbbell shaped and join to DNA as a dimer.
Lambda repressor and cro can each bind with 6 operators. 3 operator on left and 3 on right side of cI.
OR1 OR2 OR3 are three operators on the right side of cI. PR overlaps OR1 and OR2 and PRM overlaps OR2 and OR3.

For lytic cycle single cro dimer bind to OR3 which overlap PRM and repress it. No repressor and no cro on OR1 and OR2 therefore PR binds RNA polymerase and transcribes lytic genes. so does  PL.
For lysogeny PRM is on. Repressor is bound cooperatively at OR1 and OR2. RNA Polymerase bind at PR and its transcription is repressed.

Induction to lytic phase.

Eg. E. coli sense and respond to DNA damage by activating Rec A protein. Rec A cleaves Lex A protein during DNA damage repair mechanism. Lambda repressor resemble Lex A hence Lambda repressor is also cleaved by Rec A protein.The cleavage of repressor removes C terminal domain of repressor and its dimerization and cooperativity is lost.
Loss of dimer triggers transcription PR and PL which induces lytic phase.
I will continue with it in the next blog.
Thank you for reading.