Question

(Multi select) For phage lambda to enter the lytic cycle:

-cro must occupy the R1 operator

-cro must occupy the R2 operator

-cro must occupy the R3 operator

-cl must occupy the R1 operator

-cl must occupy the R2 operator

-cl must occupy the R3 operator

Answer 1

Cro must bind to the R3 operator and c1 must bind to the R1 operator.

During lysogenic cycle c1 protein bind to th R1 and R2 operator thus inhibiting Lytic promoter Pr and activating lysogenic promoter Prm. At lower CI levels, R2 becomes free, and CI no longer positively autoregulates Prm, leading to a lower rate of cI expression. Eventually, CI no longer binds to OR1, allowing transcription from PR and expression of cro. For lambda phage to enter into lytic cycle, mode of action of cro repressor is very important. These cro protein inhibit he c1 and int protein which is necessary for the lysogenic cycle, thus inhibit lysogenic cycle and start it's lytic cycle. During lytic cycle cro binds to the R3 operator, cro is an represor protein it bind to operator only in dimer form and bind to the R3 site. This binding activate the lytic cycle and increase the expression of cro protein. When the concentration of cro increase, it bind to R2 region and inhibit expression of cro protein, this is know as negative regalation.