Bacteriophage $$\lambda$$ proteins, CI and Cro are crucial regulators of the lysogeny and lytic cycles of the bacteriophage. These proteins bind to the rightward operator region consisting of OR1, OR2, and OR3 (shown below).

which one of the following statements about the regulation by CI and Cro proteins is CORRECT?

Correct option is D

In the bacteriophage λ (lambda) system, the CI and Cro proteins are involved in regulating the lysogenic and lytic cycles. The interaction of these proteins with the operator regions (OR1, OR2, and OR3) controls whether the bacteriophage enters the lytic cycle (where the virus replicates and causes cell death) or the lysogenic cycle (where the virus integrates into the host genome and remains dormant).

Let's go through the options and evaluate which one is correct.

Key Points:

• CI (repressor) protein: The CI protein binds to the operator sites and represses the lytic cycle, promoting the lysogenic cycle. It binds specifically to OR1 and OR2.

• Cro protein: The Cro protein promotes the lytic cycle by binding to OR3, inhibiting the expression of CI.

Now, let's analyze each statement:

Option 1: "Cro binding to OR3 activates expression of cl."

• Incorrect: Cro binding to OR3 actually represses the expression of CI (not activates it). This allows the virus to enter the lytic cycle. Therefore, this option is incorrect.

Option 2: "CI binding to OR3 activates expression of cl and represses the expression of cro."

• Incorrect: The CI protein typically binds to OR1 and OR2, not OR3. When CI binds to OR1 and OR2, it promotes CI expression and represses Cro. CI binding to OR3 does not activate cl; instead, CI mainly binds OR1 and OR2 for its repressive role.

Option 3: "CI binding to OR1 and OR2 leads to repression of cl and cro."

• Incorrect: CI binding to OR1 and OR2 actually promotes the expression of CI (which leads to repression of the lytic cycle, including Cro expression). Therefore, this statement is incorrect because CI binding to OR1 and OR2 results in activation of CI and repression of Cro, not both being repressed.

Option 4: "CI binding to OR1 and OR2 leads to higher expression of cl and repression of cro."

• Correct: This statement is correct. CI binds to OR1 and OR2, which activates CI expression (the repressor) and represses the expression of Cro. This is the key mechanism for maintaining the lysogenic cycle. CI represses the lytic cycle by preventing Cro expression, ensuring the virus remains dormant.

Conclusion:

The correct answer is Option 4: CI binding to OR1 and OR2 leads to higher expression of CI and repression of Cro.