Students aid : Solved, the phages t4 (lytic) and lambda (temperate) share all of the following characteristics except

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Introduction

Bacteriophages—viruses that infect bacteria—are among the most fascinating and widely studied biological entities. Their ability to hijack bacterial cells for reproduction has made them central to many breakthroughs in molecular biology. Two phages in particular, T4 and lambda (λ), have become canonical models. While they both infect Escherichia coli and share many structural and genetic features, they diverge fundamentally in their life strategies.

Phage T4 is a classic example of a lytic phage, one that invariably kills its host. In contrast, phage lambda is a temperate phage capable of either lysing the host or integrating silently into the bacterial genome—a strategic duality with profound biological implications.

This article explores their similarities, fundamental differences, and what these tell us about viral evolution, gene regulation, and therapeutic potential.

What T4 and Lambda Have in Common

Despite their different fates within the host, T4 and lambda share several fundamental biological characteristics:

1. Double-Stranded DNA Genomes

Both phages carry linear double-stranded DNA (dsDNA) genomes. T4’s genome is substantially larger (~169 kilobases), encoding hundreds of genes, while lambda has a more compact genome (~48.5 kb) with approximately 70 genes.

2. Shared Host: Escherichia coli

Both T4 and lambda use E. coli as a host. Their specificity for this bacterium has made them invaluable tools in microbiology and biotechnology.

3. Head-Tail Morphology

Each phage belongs to the order Caudovirales and exhibits a head-tail structure. T4 has a contractile tail, giving it a syringe-like mechanism to puncture the bacterial membrane, while lambda has a non-contractile tail but uses similar baseplate and tail fiber recognition systems.

4. Sequential Gene Expression

In both cases, infection involves ordered gene expression: early genes focus on takeover of host functions, middle genes handle DNA replication, and late genes encode structural components for phage assembly.

5. Use of Host Transcription Machinery

Both phages utilize the host’s RNA polymerase to transcribe their genomes. However, each has evolved distinct strategies to modify or redirect this host machinery for its own needs.

Where They Differ: The Lytic-Lysogenic Decision

The major divergence lies in how each phage handles the post-infection decision:

Phage T4: Irreversibly Lytic

Once T4 injects its DNA, it immediately commits to replication and lysis. There is no regulatory network to delay or redirect this process. The host cell’s DNA is rapidly degraded, and phage replication begins within minutes. After 20–30 minutes, the cell bursts, releasing up to 150 new phages.

T4 encodes nucleases and anti-restriction systems that ensure efficient degradation of host DNA and circumvention of bacterial defenses.

Phage Lambda: Temperate Flexibility

Lambda has a far more sophisticated regulatory mechanism, allowing a decision between:

  • Lytic cycle: Similar to T4, lambda can replicate, assemble, and lyse the cell.

  • Lysogenic cycle: Alternatively, it integrates its genome into the host chromosome as a prophage, remaining latent and replicating with the bacterial genome.

The decision hinges on a genetic switch involving the proteins CI (lambda repressor) and Cro. High CI expression represses lytic genes and promotes lysogeny; high Cro tilts the balance toward lysis. Environmental stress (e.g., UV exposure) can trigger prophage induction, leading the dormant virus to reactivate and destroy its host.

Comparison Table: T4 vs. Lambda

Feature

Phage T4

Phage Lambda (λ)

GenomeLinear dsDNA (~169 kb)Linear dsDNA (~48.5 kb)
HostE. coliE. coli
Tail MorphologyContractile tailNon-contractile tail
Life CycleStrictly lyticLytic or lysogenic (temperate)
LysogenyNot possiblePossible via integration into genome
Genome IntegrationNoYes, integrates into host chromosome
Major Regulatory ProteinsTerminase, anti-host genesCI repressor, Cro, N, Q, Int
Host DNA FateDegraded quicklyPreserved in lysogeny
Release of Progeny~150 phages per cell~50–100 phages per cell (if lytic)
Response to Host StressNone (always lytic)Reactivates from lysogeny
Use in BiotechnologyDNA packaging, recombination modelsGene regulation, recombineering tools


Explaining illustration

Scientific and Practical Implications

Understanding the difference between T4 and lambda is foundational to modern biology.

  • Phage T4 played a pivotal role in the discovery of DNA as the genetic material, as well as in studies of replication and gene expression. Its strictly lytic behavior makes it a model candidate for phage therapy, especially where bacterial eradication is essential.

  • Phage Lambda revolutionized the study of gene regulation, recombination, and genetic circuits. Its ability to integrate into the host genome has been repurposed in genetic engineering, such as the lambda red system used for targeted genome editing in E. coli.

From a therapeutic standpoint, temperate phages like lambda are generally avoided in phage therapy due to risks of horizontal gene transfer and lysogenic conversion, which can spread virulence or antibiotic resistance genes.

Conclusion

T4 and lambda are both bacteriophages that infect the same host and rely on similar early infection mechanisms. But their core divergence—lytic irreversibility in T4 vs. regulatory flexibility in lambda—represents one of the most studied evolutionary forks in the virosphere.

Understanding this dichotomy not only offers insights into virus-host interactions but also continues to fuel innovations in synthetic biology, phage therapy, and genetic engineering.

Sources :

  • Ptashne, M. (2004). A Genetic Switch: Phage Lambda Revisited. Cold Spring Harbor Laboratory Press.

  • Hershey, A.D. & Chase, M. (1952). Independent functions of viral protein and nucleic acid in growth of bacteriophage. J. Gen. Physiol., 36(1), 39–56.

  • Young, R. (1992). Bacteriophage lysis: mechanisms and regulation. Microbiol. Rev., 56(3), 430–481.

  • Brüssow, H. (2002). Phage therapy: the Western perspective. Arch. Virol., 147(1), 1–12.

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