7. Among the following in which mutant gene of l phages will form clear bacterial plaques
(1) cl^–            (2) cro-
(3) INT^–         (4) XIS^–

Explore how mutations in the lambda phage cI gene lead to clear bacterial plaques, indicating a failure to lysogenize. Learn about the genetic basis of plaque clarity in lambda phage mutants and its biological significance.

Bacteriophage lambda (λ phage) is a well-studied virus that infects Escherichia coli and exhibits a dual lifestyle: it can either enter a lysogenic cycle, integrating its genome into the host chromosome, or proceed through a lytic cycle, producing new phage particles and lysing the host cell. The nature of the plaques formed by lambda phage on bacterial lawns—whether clear or turbid—provides important clues about the phage’s genetic state and lifecycle.

What Are Clear and Turbid Plaques?

• Turbid plaques are cloudy due to the presence of lysogenized bacteria that survive infection and continue to grow, forming microcolonies within the plaque.

• Clear plaques are transparent and sharply defined because the phage kills all infected bacteria, with no lysogens forming.

The formation of clear plaques is typically linked to mutations in genes that regulate the lysogenic cycle.

Which Lambda Phage Mutant Produces Clear Plaques?

Among the options:

1. cI- (cI mutant)

2. cro- (cro mutant)

3. int- (int mutant)

4. xis- (xis mutant)

The cI- mutant is the one known to produce clear plaques.

Why Does the cI Mutant Form Clear Plaques?

The cI gene encodes the lambda repressor protein (CI), which is essential for establishing and maintaining lysogeny. The CI repressor binds to operator sites on the phage DNA to repress lytic genes, allowing the phage genome to integrate and remain dormant within the host.

• In cI mutants, the repressor is defective or absent.

• Without a functional CI repressor, the phage cannot maintain lysogeny.

• The phage immediately enters the lytic cycle, killing all infected bacteria.

• This results in clear plaques, as no lysogens survive to form turbid microcolonies.

What About Other Mutants?

• cro- mutants fail to repress the lysogenic promoters, often leading to a bias toward lysogeny and turbid plaques.

• int- mutants lack integrase, an enzyme necessary for phage genome integration into the host chromosome. While this affects lysogen formation, it does not necessarily produce clear plaques because other factors influence plaque clarity.

• xis- mutants lack excisionase, required for prophage excision during induction, but this gene does not affect initial lysogeny establishment or plaque clarity.

Supporting Evidence

Research shows that clear plaque mutants of lambda phage predominantly carry mutations in the cI gene. These mutants are unable to lysogenize host cells, resulting in plaques where all bacteria are lysed. Mutations in other genes like cII and cIII can also affect lysogeny efficiency but are less commonly associated with completely clear plaques.

Biological Significance of Clear Plaques

Clear plaques indicate a virulent phenotype, where the phage strictly follows the lytic cycle. This contrasts with temperate phages, which can switch between lysogenic and lytic cycles. The ability to form clear plaques is often used as a genetic marker to identify mutants defective in lysogeny.

Summary Table: Lambda Phage Mutants and Plaque Phenotypes

Conclusion

Among the mutant genes of lambda phage, the cI- mutant is responsible for producing clear bacterial plaques. This is due to the loss of function of the CI repressor protein, which prevents the phage from entering or maintaining lysogeny. As a result, the phage follows a strictly lytic pathway, killing all infected bacteria and forming clear plaques. Understanding the genetic basis of plaque clarity helps researchers study phage biology, gene regulation, and the balance between lysogenic and lytic cycles.