Difficulty: Intermediate to Advanced | Prerequisites: Lac and trp operon basics, DNA-protein interactions, general understanding of bacteriophage life cycles.
Tags: lambda phage, bacteriophage lambda, lysogeny, lytic cycle, cI repressor, Cro repressor, cII, cIII, prophage, genetic switch, temperate phage, SOS response, RecA, operator, PRM, PR, PL, lysogen, induction
Lambda (λ) phage is the textbook example of a genetic switch: a single virus that can choose between two completely different lifestyles after infecting a bacterial cell. It either integrates quietly into the host genome (lysogeny) or hijacks the cell's machinery to make new phages and burst the cell open (lytic cycle). The molecular logic behind this decision is one of the most elegant regulatory circuits in biology, built from just a handful of proteins competing for the same stretch of DNA. Understanding lambda phage ties together concepts from operon regulation, cooperative DNA binding, and stress responses.
Lambda phage decides between lysogeny and lysis based on a competition between two repressors, cI and Cro, binding to overlapping operators. High cII protein favours lysogeny by driving cI production. DNA damage (via the SOS response and RecA) destroys the cI repressor and tips the balance toward lysis.
Temperate phage
A bacteriophage capable of both lytic growth (producing new phage particles and lysing the host) and lysogeny (integrating into the host chromosome and replicating passively). Think of it as a virus that can choose to be aggressive or dormant.
Lysogeny
The state in which the phage genome is integrated into the host chromosome as a prophage and is replicated along with the host DNA. The cell is called a lysogen. In simple terms, the phage goes quiet and hitchhikes with the bacterium.
Lytic cycle
The pathway in which the phage replicates its DNA, assembles new phage particles, and lyses (bursts open) the host cell to release them.
cI repressor (λ repressor)
The master regulator of lysogeny. It binds to the operator region, represses lytic genes (including cro), and activates its own transcription, creating a self-sustaining loop that maintains the lysogenic state.
Cro repressor
The master regulator of lytic development. It binds to the same operator sites as cI but with a different order of preference, ultimately shutting down cI transcription and committing the phage to lysis.
Operator region (OR)
A stretch of DNA containing three binding sites, OR1, OR2, and OR3, each of which can be bound by either cI or Cro. The order in which these sites are filled determines which genes are expressed.
PRM (promoter for repressor maintenance)
The promoter that drives transcription of the cI gene. It requires cI bound at OR2 to work efficiently, creating a positive feedback loop.
PR (rightward promoter)
The promoter that drives transcription of cro and downstream lytic genes. It is active when cI is not blocking it.
cII protein
A regulatory protein that, at high concentrations, activates transcription of cI from a separate promoter (PRE), pushing the phage toward lysogeny. It is unstable and easily degraded by host proteases.
cIII protein
A protein that protects cII from degradation by host proteases, thereby supporting the lysogeny decision.
RecA
A host protein activated during the SOS response to DNA damage. Activated RecA stimulates the self-cleavage (autoproteolysis) of the cI repressor, destroying the lysogenic maintenance circuit and triggering the switch to lytic growth.
The lysogeny-vs-lysis decision comes down to a competition between two proteins, cI and Cro, for three operator sites (OR1, OR2, OR3) in the same regulatory region.
These operators sit between two divergent promoters: PRM (leftward, driving cI) and PR (rightward, driving cro and lytic genes).
During lysogeny, cI binds cooperatively to OR1 and OR2.
Binding at OR1 blocks PR, preventing cro transcription and lytic gene expression.
Binding at OR2 contacts RNA polymerase at PRM, stimulating transcription of cI itself. This is a positive autoregulatory loop: the repressor maintains its own production.
At very high cI concentrations, cI also fills OR3, which actually represses PRM and dials back its own production. This negative autoregulation keeps cI levels stable rather than spiralling upward.
If cI levels drop (or Cro is produced first during initial infection), Cro binds the operators in a different priority order. Cro prefers OR3 first.
Cro at OR3 shuts down PRM, blocking cI transcription. Without new cI, the lysogenic circuit cannot be maintained.
Lytic genes are transcribed from PR and PL, leading to phage DNA replication, assembly, and host cell lysis.
When lambda first infects a cell, the outcome depends largely on the cII protein.
High cII levels activate transcription of cI from a special establishment promoter (PRE), flooding the cell with cI and pushing toward lysogeny.
Low cII levels (because cII is degraded by host proteases) mean cI is not produced fast enough, and Cro wins the race, committing the phage to lysis.
cIII helps by inhibiting the host protease that degrades cII. Conditions that favour cIII stability (e.g. high multiplicity of infection, poor host growth) therefore favour lysogeny.
A lysogen can be induced to enter the lytic cycle if the host cell suffers DNA damage.
DNA damage activates the host SOS response. RecA protein, activated by single-stranded DNA, stimulates the autocleavage of the cI repressor.
Once cI is destroyed, PR and PL are no longer blocked. Cro and lytic genes are expressed, and the phage excises from the chromosome and enters lytic growth.
This is a survival strategy: if the host is dying, the phage bails out.
The lambda genetic switch is a foundational model in synthetic biology. Researchers use its toggle-switch architecture to design synthetic gene circuits in engineered bacteria. Lambda-derived vectors are also widely used in molecular cloning (e.g. lambda phage libraries), and the concept of prophage induction is clinically relevant because many bacterial pathogens carry phage-encoded toxin genes that are expressed when the prophage switches to lytic mode.
Students often think cI and Cro bind to different DNA sequences. They bind to the same three operator sites (OR1, OR2, OR3), just with different affinities and in different orders of preference.
Lysogeny is not a dead end. The prophage can be reactivated (induced) at any time if cI is destroyed, typically through the SOS response.
cII does not maintain lysogeny. It establishes it by driving initial cI production. Once lysogeny is established, cI maintains itself through the PRM autoregulatory loop.
RecA does not directly cleave cI. It stimulates the cI repressor's own self-cleavage activity (autoproteolysis).
⚠️ Be able to explain the roles of OR1, OR2, and OR3 and what happens when each is occupied by cI versus Cro. Diagrams of the operator region are common exam material.
⚠️ Know the difference between establishment of lysogeny (cII-driven, PRE) and maintenance of lysogeny (cI autoregulation, PRM).
⚠️ Understand the link between the SOS response, RecA activation, and prophage induction. This ties lambda phage regulation to bacterial DNA repair.
⚠️ Questions may ask you to predict the outcome if a specific protein (cI, Cro, cII) is mutated or absent.
True or false: Cro protein activates its own transcription through a positive feedback loop like cI does.
Fill in the blank: The protein that stimulates self-cleavage of the cI repressor during the SOS response is ______.
True or false: High levels of cII favour the lytic cycle.
Fill in the blank: cI binds cooperatively to ______ and ______, blocking PR and activating PRM.
True or false: cIII protects cII from degradation by host proteases.
Answers: 1. False (Cro does not have a comparable positive autoregulatory loop). 2. RecA. 3. False (high cII favours lysogeny). 4. OR1 and OR2. 5. True.
Q: A lambda phage carries a mutation in the cI gene that makes the cI protein resistant to RecA-stimulated cleavage. What is the expected phenotype?
A: The phage would be locked in lysogeny and unable to undergo prophage induction in response to DNA damage, because the cI repressor could not be destroyed. The lysogenic state would be irreversible under normal SOS-inducing conditions.
Q: Explain why a high multiplicity of infection (many phages infecting one cell) favours lysogeny.
A: Multiple phage genomes produce more cII and cIII protein. Higher cII levels, protected from degradation by abundant cIII, drive strong transcription of cI from PRE. The resulting high cI concentration overwhelms Cro and establishes lysogeny.
Q: What is the functional significance of cI repressing its own transcription at very high concentrations (via OR3)?
A: It provides negative autoregulation, preventing cI from accumulating to excessively high levels. This keeps the system responsive: if cI levels were too high, the phage would be very difficult to induce, and the escape-to-lytic strategy during DNA damage would fail.
Q: Compare the roles of cI and Cro in determining the lambda phage life cycle.
A: cI is the lysogeny repressor. It blocks lytic genes (by occupying OR1) and activates its own transcription (via OR2), maintaining the quiescent lysogenic state. Cro is the lytic repressor. It preferentially binds OR3, shutting down cI production and allowing lytic gene expression. The outcome depends on which protein dominates the operator region first.
Lambda phage regulation connects to the SOS response and DNA repair mechanisms in bacteria. RecA, the same protein that triggers prophage induction, is also central to recombinational repair and the SOS regulon (controlled by LexA). The operator-binding logic of cI and Cro parallels the repressor-operator interactions seen in the lac and trp operons, but adds cooperative binding and autoregulation as extra layers of complexity.
lambda phage, bacteriophage lambda, genetic switch, lysogeny, lytic cycle, temperate phage, cI repressor, lambda repressor, Cro repressor, cII, cIII, prophage, prophage induction, SOS response, RecA, autoproteolysis, operator OR1 OR2 OR3, PRM, PR, PL, PRE, cooperative binding, toggle switch, gene regulation, molecular biology