21. Bacteriophage λ proteins, Cl 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 Cl and Cro proteins is CORRECT?
(1) Cro binding to OR3 activates expression of cl.
(2) Cl binding to OR3 activates expression of cl and represses the expression of cro.
(3) Cl binding to OR1 and OR2 leads to repression of cl and cro.
(4) Cl binding to OR1 and OR2 leads to higher expression of cl and repression of cro.
Bacteriophage lambda (λ phage) is a model system for studying gene regulation, especially the switch between lysogenic and lytic life cycles. Central to this regulation are two proteins, CI (lambda repressor) and Cro, which bind to the right operator (OR) region consisting of three adjacent sites: OR1, OR2, and OR3. The binding of these proteins to specific operator sites controls transcription from the divergent promoters P_R (for cro and lytic genes) and P_RM (for cI repressor).
Binding Preferences and Regulatory Roles of CI and Cro
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CI protein binds cooperatively with highest affinity to OR1, then OR2, and lastly OR3.
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Cro protein has the highest affinity for OR3, followed by OR1 and OR2 with lower affinity.
This differential binding leads to distinct regulatory outcomes:
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CI Binding to OR1 and OR2
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Binding at OR1 represses transcription from the P_R promoter, blocking cro and lytic gene expression.
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Binding at OR2 activates transcription from the P_RM promoter, enhancing cI expression (positive autoregulation).
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Together, CI binding at OR1 and OR2 represses cro and promotes its own synthesis, maintaining lysogeny.
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CI Binding to OR3
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At high CI concentrations, binding to OR3 represses P_RM, providing negative feedback to limit cI expression.
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Cro Binding to OR3
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Cro binding at OR3 represses transcription from P_RM, thereby repressing cI expression and favoring the lytic cycle.
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Cro binding to OR1 and OR2 represses P_R and P_RM promoters but with lower affinity.
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Evaluating the Given Statements
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Cro binding to OR3 activates expression of cI.
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This is incorrect. Cro binding at OR3 represses cI transcription by blocking P_RM.
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CI binding to OR3 activates expression of cI and represses cro.
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This is incorrect. CI binding at OR3 represses P_RM (cI promoter), thus repressing cI expression at high concentrations. Activation of cI occurs via CI binding at OR2, not OR3.
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CI binding to OR1 and OR2 leads to repression of cI and cro.
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This is incorrect. CI binding at OR1 and OR2 represses cro but activates cI expression (via P_RM).
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CI binding to OR1 and OR2 leads to higher expression of cI and repression of cro.
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This is correct. CI binding at OR1 represses cro, while binding at OR2 activates cI expression, maintaining lysogeny.
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Conclusion
The correct statement about regulation by CI and Cro proteins at the OR operator is:
(4) CI binding to OR1 and OR2 leads to higher expression of cI and repression of cro.
Summary Table of CI and Cro Binding Effects at OR Sites
| Binding Site | CI Binding Effect | Cro Binding Effect |
|---|---|---|
| OR1 | Represses cro (P_R promoter) | Represses P_R and P_RM (less affinity) |
| OR2 | Activates cI (P_RM promoter) | Represses P_R and P_RM (less affinity) |
| OR3 | Represses cI (P_RM promoter) at high CI | Represses cI (P_RM promoter) (high affinity) |
Biological Significance
This finely tuned regulatory system allows lambda phage to maintain lysogeny when CI predominates by repressing lytic genes and promoting its own synthesis. When Cro predominates, it represses cI expression, leading to lytic growth and host cell lysis. The cooperative and competitive binding of these proteins to OR1, OR2, and OR3 forms the molecular basis of the phage’s developmental switch.
