17. Bacteriophage λ, has two modes in its life cycle. Lytic and lysogenic. In the lysogenic mode, the expression of all the phage genes are repressed while the expression of repressor gene switches between on and offposition depending on the concentration of repressor
The following statements are made:
A. Repressor may act both as a positive regulator and negative regulator
B. Expression of repressor gene, cl is independent of the expression of cll and clll genes.
C. Mutation of cl gene will cause it to it to form clear plaques on both wild type E. coli and E. coli (λ⁺)
D. Mutation at operators, O_L and O_R will allow the phage to act as virulent phage.
The correct statements are
(1) A and B       (2) B and C
(3) C and D      (4) D and A

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Bacteriophage lambda (λ phage) is a temperate virus with two distinct life cycles: the lysogenic cycle, where the viral genome integrates into the host chromosome and remains dormant, and the lytic cycle, where the virus replicates and lyses the host cell. The switch between these cycles is tightly regulated by the lambda repressor protein (encoded by the cI gene) and operator DNA sequences (O_L and O_R).

Evaluating the Statements on Lambda Phage Regulation

Let’s analyze the given statements about lambda phage gene regulation:

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A. Repressor may act both as a positive regulator and negative regulator

• The lambda repressor (CI protein) is a classic example of a bifunctional regulator.

• It represses transcription of lytic genes by binding operator sites O_R and O_L, preventing RNA polymerase from initiating transcription at lytic promoters (P_R and P_L).

• Simultaneously, it activates its own transcription from the promoter P_RM, ensuring maintenance of lysogeny.

• This dual role is well-documented and essential for stable prophage maintenance.

• Statement A is correct.

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B. Expression of repressor gene, cI, is independent of the expression of cII and cIII genes

• The cII and cIII proteins are upstream regulators that activate transcription of cI from the P_RE promoter during lysogeny establishment.

• cII acts as a transcriptional activator of P_RE, and cIII protects cII from degradation.

• Therefore, cI expression depends on cII and cIII during lysogeny establishment.

• However, once lysogeny is established, cI autoregulates its own expression independently.

• Given the context, statement B is incorrect if interpreted as cI expression being independent during establishment.

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C. Mutation of cI gene will cause it to form clear plaques on both wild type E. coli and E. coli (λ+)

• The cI gene encodes the lambda repressor necessary for lysogeny.

• Mutations in cI disable repressor function, preventing lysogeny and forcing the phage into the lytic cycle.

• This results in clear plaques because all infected cells lyse without forming lysogens.

• This phenotype occurs regardless of whether the host is wild type or lysogenic.

• Statement C is correct.

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D. Mutation at operators, O_L and O_R, will allow the phage to act as virulent phage

• The operator sites O_L and O_R are binding sites for the lambda repressor.

• Mutations in these operators prevent repressor binding, disabling repression of lytic promoters.

• This leads to constitutive lytic gene expression, making the phage behave as a virulent phage that cannot establish lysogeny.

• Statement D is correct.

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Correct Combination of Statements

From the above analysis:

• A is correct (repressor acts as both positive and negative regulator).

• C is correct (cI mutants form clear plaques).

• D is correct (operator mutations lead to virulent phage behavior).

• B is incorrect due to dependence of cI expression on cII and cIII during lysogeny establishment.

Therefore, the correct combination is:

(4) D and A

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Molecular Basis of These Regulatory Features

• The lambda repressor binds cooperatively to operator sites O_R1, O_R2, and O_R3, with different affinities.

• Binding to O_R1 and O_R2 represses lytic promoters and activates P_RM, maintaining repressor synthesis.

• At higher concentrations, binding to O_R3 represses P_RM, providing negative feedback.

• Mutations in cI or operators disrupt this balance, leading to loss of lysogeny and clear plaques.

Biological Significance

• The dual regulatory role of the lambda repressor ensures stable maintenance of lysogeny while allowing rapid induction of the lytic cycle when needed.

• Operator mutations that prevent repressor binding convert lambda into a virulent phage, unable to establish lysogeny, which can be advantageous under certain conditions.

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Conclusion

Bacteriophage lambda’s life cycle regulation hinges on the lambda repressor’s ability to function as both a positive and negative regulator and the integrity of operator sites O_L and O_R. Mutations in the cI gene or operator sequences disrupt lysogeny, resulting in clear plaques and virulent phage behavior. Understanding these mechanisms provides deep insights into viral gene regulation and the molecular basis of genetic switches.