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Enterobacteria phage λ (lambda
Lambda
Lambda is the 11th letter of the Greek alphabet. In the system of Greek numerals it has a value of 30. It was derived from the Phoenician letter Lamed . Letters that arose from Lambda include the Roman L and the Cyrillic letter El . The ancient Grammarians and dramatists attest to a pronunciation...

phage, coliphage λ) is a temperate

Temperate (virology)

In virology, temperate refers to the ability of some bacteriophages to display a lysogenic life cycle. Many temperate phages can integrate their genomes into their host bacterium's chromosome, together becoming a lysogen as the phage genome becomes a prophage...

bacteriophage that infects Escherichia coli
Escherichia coli
Escherichia coli , is a Gram negative bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E...

.

Lambda phage is a virus

Virus

A virus is an infectious agent too small to be seen directly with a light microscope. They are not made of cells and can only replicate inside the cells of another organism . Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

particle consisting of a head, containing double-stranded linear DNA as its genetic material, and a tail that can have tail fibers. The phage particle injects its DNA into its host

Host (biology)

In biology, a host is an organism that harbors a parasite , or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

through the tail, and the phage will then usually enter the lytic pathway where it replicates its DNA, degrades the host DNA and hijacks the cell's replication, transcription and translation mechanisms to produce as many phage particles as cell resources allow. When cell resources are depleted, the phage will lyse (break open) the host cell, releasing the new phage particles. However, under certain conditions, the phage DNA may integrate itself into the host cell chromosome in the lysogenic pathway. In this state, the λ DNA is called a prophage

Prophage

A prophage is a phage genome inserted as part of the linear structure of the DNA chromosome of a bacterium. A temperate phage integrated into the host chromosome or existing as an extrachromosomal plasmid...

and stays resident within the host's genome

Genome

In modern molecular biology the genome refers to all of its hereditary information encoded in DNA .The genome includes both the genes and the non-coding sequences of the DNA. The term was adapted in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany...

without apparent harm to the host, which can be termed a lysogen

Lysogen

A lysogen or lysogenic phage is a phage that can exist as a prophage within its host organism. A prophage is the phage DNA in its dormant state, and is either integrated into the host bacteria's chromosome or more rarely exists as a stable plasmid within the host cell...

when a prophage is present. The prophage is duplicated with every subsequent cell division of the host. The phage genes expressed in this dormant state code for protein

Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...

s that repress expression of other phage genes. These proteins are broken down when the host cell is under stress, resulting in the expression of the repressed phage genes. Stress can be from starvation

Starvation

Starvation is a severe reduction in vitamin, nutrient, and energy intake. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage, and eventually death...

, poisons (like antibiotics), or other factors that can damage or destroy the host. In response to stress, the activated prophage is excised from the DNA of the host cell by one of the newly expressed gene products and enters its lytic pathway.

Lambda phage was discovered by Esther Lederberg

Esther Lederberg

Esther Miriam Zimmer Lederberg was an American microbiologist and immunologist and pioneer of bacterial genetics...

in 1950. It has been used heavily as a model organism

Model organism

A model organism is a species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms...

, and has been a rich source for useful tools in molecular biology

Molecular biology

Molecular biology is the study of biology at a molecular level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

. Uses include its application as a vector for the cloning of recombinant DNA

Recombinant DNA

Recombinant DNA is a form of DNA that does not exist naturally, which is created by combining DNA sequences that would not normally occur together...

, the use of its site specific recombinase, int, for the shuffling of cloned DNAs by the 'Gateway' method, and the application of its Red operon, including the proteins Red alpha (also called 'exo'), beta and gamma in the DNA engineering method called recombineering

Recombineering

Recombineering is a genetic and molecular biology technique based on homologous recombination systems in E. coli to modify DNA. The procedure is widely used in the generation of target vectors for making a conditional mouse knockout. The term was first coined in 2001 by Ellis et al. Recombineering...

.

In the following page, genes will be written in italics and their associated proteins in Roman. For instance, cI refers to the gene, while cI is the resulting protein encoded by that gene.

Anatomy

The virus particle consists of a head and a tail that can have tail fibres. The head contains 48,490 base pairs of double-stranded, linear DNA flanked by 12-base-pair, single-stranded segments that make up the two strands of the cos site. In its circular form in the host cytoplasm, the phage genome therefore is 48,502 base pairs in length. The prophage exists as a linear section of DNA inserted into the host chromosome.

Lifecycle

Infection

Bacteriophage Lambda binds to the target E. coli cell, the J protein in the tail tip interacting with the lamB gene product of E. coli, a porin
Porin
Porin can be:* Porin , a transmembrane protein* Porin , Croatian music award* Porin , the first Croatian opera...

molecule which is part of the maltose
Maltose
Maltose, or malt sugar, is a disaccharide formed from two units of glucose joined with an α linkage. It is the second member of an important biochemical series of glucose chains...

operon.
The linear phage genome is injected past the cell outer membrane.
The DNA passes through a separate sugar transport protein (ptsG) in the inner membrane, and immediately circularises using the cos sites, 12-base G-C rich cohesive "sticky ends". The single-stranded nicks are ligated by host DNA ligase
DNA ligase
In molecular biology, DNA ligase is a special type of ligase that can link together two DNA strands that have double-strand break...

.
Host DNA gyrase
DNA gyrase
DNA gyrase, often referred to simply as gyrase, is an enzyme that supercoils DNA. Many antibiotics work by attacking bacterial DNA gyrase....

puts negative supercoils in the circular chromosome, causing A-T rich regions to unwind and drive transcription.
Transcription starts from the constitutive P_L, P_R and P_R' promoter
Promoter
In genetics, a promoter is a region of DNA that facilitates the transcription of a particular gene. Promoters are typically located near the genes they regulate, on the same strand and upstream .-Overview:...

s producing the 'immediate early' transcripts. Initially these express the N and cro genes, producing N, Cro and a short inactive protein.
Cro binds to OR3 preventing access to the P_RM promoter preventing expression of the cI gene. N binds to the two Nut (N utilisation) sites, one in the N gene in the P_L reading frame, and one in the cro gene in the P_R reading frame.
The N protein is an antiterminator, and functions to extend the reading frames that it is bound to. When RNA polymerase
RNA polymerase
RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...

transcribes these regions, it recruits the N and forms a complex with several host Nus proteins. This complex skips through most termination sequences. The extended transcripts (the 'late early' transcripts) include the N and cro genes along with cII and cIII genes, and xis, int, OP and Q genes discussed later.
The cIII protein acts to protect the cII protein from proteolysis by FtsH (a membrane-bound essential E. coli protease) by acting as a competitive inhibitor. This inhibition can induce a bacteriostatic state, which favours lysogeny. cIII also directly stabilises the cII protein. On initial infection, the stability of cII determines the lifestyle of the phage; stable cII will lead to the lysogenic pathway, whereas if cII is degraded the phage will go into the lytic pathway. Low temperature, starvation of the cells and high multiplicity of infection (MOI) are known to favor lysogeny (see later discussion).

N antitermination

This occurs without the N protein interacting with the DNA; the protein instead binds to the freshly transcribed mRNA. Nut sites contain 3 conserved "boxes," of which only BoxB is essential.

The boxB RNA sequences are located close to the 5' end of the pL and pR transcripts. When transcribed, each sequence forms a hairpin loop structure that the N protein can bind to.
N protein binds to boxB in each transcript, and contacts the transcribing RNA polymerase via RNA looping. The N-RNAP complex is stabilized by subsequent binding of several host Nus (N utilisation substance) proteins (which include transcription termination/antitermination factors and, bizarrely, a ribosome subunit).
The entire complex (including the bound Nut site on the mRNA) continues transcription, and can skip through termination sequences.

Lytic lifestyle

This is the lifecycle that the phage follows following most infections, where the cII protein does not reach a high enough concentration due to degradation, so does not activate its promoters.

The 'late early' transcripts continue being written, including xis, int, Q and genes for replication of the lambda genome (OP). Cro dominates the repressor site (see "Repressor"), repressing synthesis from the P_RM promoter.
The O and P proteins initiate replication of the phage chromosome (see "Lytic Replication").
Q, another antiterminator, binds to Qut sites.
Transcription from the P_R' promoter can now extend to produce mRNA for the lysis and the head and tail proteins.
Structural proteins and phage genomes self assemble into new phage particles.
Products of the lysis genes R and S, cause cell lysis at high enough concentrations. S is a holin
Holín
Holín is a village and municipality in Jičín District in the Hradec Králové Region of the Czech Republic. -References:...

which makes holes in the membrane. R is an endolysin
Endolysin
Endolysin is a generic term describing an enzyme that degrades the bacterial peptidoglycan cell wall, resulting in lysis of the bacterial cell....

which cleaves the cell wall. Around 100 new phage are released.

Rightward transcription

Rightward transcription expresses the O, P and Q genes. O and P are responsible for initiating replication, and Q is another antiterminator which allows the expression of head, tail and lysis genes from P_R’.

Lytic replication

For the first few replication cycles, the lambda genome undergoes θ replication (circle-to-circle).
This is initiated at the ori site located in the O gene. O protein binds the ori site, and P protein binds the DnaB subunit of the host replication machinery as well as binding O. This effectively commandeers the host DNA polymerase.
Soon, the phage switches to a rolling-circle type of replication similar to that used by phage M13. The DNA is nicked and the 3’ end serves as a primer. Notably, this doesn’t release single copies of the phage genome, but rather one long molecule with many copies of the genome: a concatemer.
These concatemers are cleaved at their cos sites as they are packaged. Packaging cannot occur from circular phage DNA, only from concatomeric DNA.

Q antitermination

Q is similar to N in its effect: Q binds to RNA polymerase in Qut sites and the resulting complex can ignore terminators, however the mechanism is very different; the Q protein first associates with a DNA sequence rather than an mRNA sequence.

The Qut site is very close to the P_R’ promoter, close enough that the σ factor has not been released from the RNA polymerase holoenzyme. Part of the Qut site resembles the -10 Pribnow box
Pribnow box
The Pribnow box is the sequence TATAAT of six nucleotides that is an essential part of a promoter site on DNA for transcription to occur in bacteria...

, causing the holoenzyme to pause.
Q protein then binds and displaces part of the σ factor and transcription re-initiates.
The head and tail proteins are transcribed and self-assemble.

Leftward transcription

Leftward transcription expresses the gam, red, xis and int genes. Gam and red proteins are involved in recombination. Gam is also important in that it inhibits the host RecBCD nuclease from degrading the 3’ ends in rolling circle replication. Int and xis are integration and excision proteins which are vital to lysogeny.

xis and int regulation of insertion and excision

xis and int are found on the same piece of mRNA, so approximately equal concentrations of xis and int proteins are produced. This results (initially) in the excision of any inserted genomes from the host genome.
The mRNA from the P_L promoter forms a stable secondary structure with a bobby pin loop in the sib section of the mRNA. This targets the 3' (sib) end of the mRNA for RNAaseIII degradation, which results in a lower effective concentration of int mRNA than xis mRNA (as the int cistron is nearer to the sib sequence than the xis cistron is to the sib sequence), so a higher concentrations of xis than int is observed.
Higher concentrations of xis than int result in no insertion or excision of phage genomes, the evolutionarily favoured action - leaving any pre-insterted phage genomes inserted (so reducing competition) and preventing the insertion of the phage genome into the genome of a doomed host.

Lysogenic (or lysenogenic) lifestyle

This is the lifecycle that the phage follows after a small number of infections in specific conditions, where the cII protein reaches a high enough concentration due to stabilisation and lack of degradation, and so activates its promoters.

The 'late early' transcripts continue being written, including xis, int, Q and genes for replication of the lambda genome.
The stabilized cII acts to promote transcription from the P_RE, P_I and P_antiq promoters.
The P_antiq promoter produces antisense mRNA to the Q gene message of the P_R promoter transcript, thereby switching off Q production. The P_RE promoter produces antisense mRNA to the cro section of the P_R promoter transcript, turning down cro production, and has a transcript of the cI gene. This is expressed, turning on cI repressor production. The P_I promoter expresses the int gene, resulting in high concentrations of int protein. This int protein integrates the phage DNA into the host chromosome (see "Prophage Integration").
No Q results in no extension of the P_R' promoter's reading frame, so no lytic or structural proteins are made. Elevated levels of int (much higher than that of xis) result in the insertion of the lambda genome into the hosts genome (see diagram). Production of cI leads to the binding of cI to the OR1 and OR2 sites in the P_R promoter, turning off cro and other early gene expression. cI also binds to the P_L promoter, turning off transcription there too.
Lack of cro leaves the OR3 site unbound, so transcription from the P_RM promoter may occur, maintaining levels of cI.
Lack of transcription from the P_L and P_R promoters leads to no further production of cII and cIII.
As cII and cIII concentrations decrease, transcription from the P_antiq, P_RE and P_I stop being promoted since they are no longer needed.
Only the P_RM and P_R' promoters are left active, the former producing cI protein and the latter a short inactive transcript. The genome remains inserted into the host genome in a dormant state.

Prophage integration

The integration of phage λ takes place at a special attachment site in the bacterial and phage genomes, called att^λ. The sequence of the bacterial att site is called attB, between the gal and bio operons, and consists of the parts B-O-B', whereas the complementary sequence in the circular phage genome is called attP and consists of the parts P-O-P'. The integration itself is a sequential exchange (see genetic recombination

Genetic recombination

Genetic recombination is the process by which a strand of genetic material is broken and then joined to a different DNA molecule. In eukaryotes recombination occurs in mitosis as a common mechanism of DNA repair and in meiosis as a way of facilitating chromosomal crossover...

) via a Holliday junction

Holliday junction

A Holliday junction is a mobile junction between four strands of DNA. The structure is named after Robin Holliday, who proposed it in 1964 to account for a particular type of exchange of genetic information he observed in yeast known as homologous recombination...

and requires both the phage protein

Protein

Int and the bacterial protein IHF (integration host factor). Both Int and IHF bind to attP and form an intasome, a DNA-protein-complex designed for site-specific recombination

Site-specific recombination

Site-specific recombination, also known as conservative site-specific recombination, is a type of genetic recombination in which DNA strand exchange takes place between segments possessing only a limited degree of sequence homology...

of the phage and host DNA. The original B-O-B' sequence is changed by the integration to B-O-P'-phage DNA-P-O-B'. The phage DNA is now part of the host's genome.

Maintenance of lysogeny

Lysogeny is maintained solely by cI. cI represses transcription from P_L and P_R while upregulating and controlling its own expression from P_RM. It is therefore the only protein expressed by lysogenic phage.
This is coordinated by the P_L and P_R operators. Both operators have three binding sites for cI: OL1, OL2, and OL3 for P_L, and OR1, OR2 and OR3 for P_R.
cI binds most favorably to OR1; binding here inhibits transcription from P_R. As cI easily dimerises, the binding of cI to OR1 greatly increases the affinity of the binding of cI to OR2, and this happens almost immediately after OR1 binding. This activates transcription in the other direction from P_RM, as the N terminal domain of cI on OR2 tightens the binding of RNA polymerase to P_RM and hence cI stimulates its own transcription. When it is present at a much higher concentration, it also binds to OR3, inhibiting transcription from P_RM, thus regulating its own levels in a negative feedback loop.
cI binding to the P_L operator is very similar, except that it has no direct effect on cI transcription. As an additional repression of its own expression, however, cI dimers bound to OR3 and OL3 bend the DNA between them to tetramerise.
The presence of cI causes immunity to superinfection by other lambda phages, as it will inhibit their P_L and P_R promoters.

Induction

The classic induction of a lysogen involved irradiating the infected cells with UV light. Any situation where a lysogen undergoes DNA damage or the SOS response of the host is otherwise stimulated leads to induction.

The host cell, containing a dormant phage genome, experiences DNA damage due to a high stress environment, and starts to undergo the SOS response
SOS response
The SOS response is a postreplication DNA repair system that allows DNA replication to bypass lesions or errors in the DNA. The SOS uses the RecA protein. The RecA protein, stimulated by single-stranded DNA, is involved in the inactivation of the LexA repressor thereby inducing the response...

.
RecA (a cellular protein) detects DNA damage and becomes activated. It is now RecA*, a highly specific co-protease.
Normally RecA* binds LexA (a transcription
Transcription (genetics)
Transcription, or RNA synthesis, is the process of creating an equivalent RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA in the presence of the correct enzymes...

repressor), activating LexA auto-protease activity,which destroys LexA repressor allowing production of DNA repair
DNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and Radiation can cause DNA damage, resulting in as many as 1...

proteins. In lysogenic cells this response is hijacked, and RecA* stimulates cI autocleavage. This is because cI mimics the structure of LexA at the autocleavage site.
Cleaved cI can no longer dimerise, and loses its affinity for DNA binding.
The P_R and P_L promoters are no longer repressed and switch on, and the cell returns to the lytic sequence of expression events (note that cII is not stable in cells undergoing the SOS response
SOS response
The SOS response is a postreplication DNA repair system that allows DNA replication to bypass lesions or errors in the DNA. The SOS uses the RecA protein. The RecA protein, stimulated by single-stranded DNA, is involved in the inactivation of the LexA repressor thereby inducing the response...

). There is however one notable difference.

Control of phage genome excision in induction

The phage genome is still inserted in the host genome and needs excision for DNA replication to occur. The sib section beyond the normal P_L promoter transcript is, however, no longer included in this reading frame (see diagram).
No sib domain on the P_L promoter mRNA results in no hairpin loop on the 3' end, and the transcript is no longer targeted for RNAaseIII degradation.
The new intact transcript has one copy of both xis and int, so approximately equal concentrations of xis and int proteins are produced.
Equal concentrations of xis and int result in the excision of the inserted genome from the host genome for replication and later phage production.

Protein function overview

cro; (Control of Repressor's Operator) Transcription inhibitor, binds OR3, OR2 and OR1 (affinity OR3 > OR2 = OR1, ie. preferentially binds OR3). At low concentrations blocks the pRM promoter (preventing cI production). At high concentrations downregulates its own production through OR2 and OR1 binding. No cooperative binding (c.f. below for cI binding)

cI; (Clear 1) Transcription inhibitor, binds OR1, OR2 and OR3 (affinity OR1 > OR2 = OR3, ie. preferentially binds OR1). At low concentrations blocks the pR promoter (preventing cro production). At high concentrations downregulates its own production through OR3 binding. Binding of cI at OR1 stimulates an almost simultaneous cI binding to OR2 via cooperative binding (via cI C terminal domain interactions) N terminal domain of cI on OR2 tightens the binding of RNA polymerase to pRM and hence stimulate its own transcription. Repressor also inhibits transcription from the pL promoter. Susceptible to cleavage by RecA* in cells undergoing the SOS response

SOS response

The SOS response is a postreplication DNA repair system that allows DNA replication to bypass lesions or errors in the DNA. The SOS uses the RecA protein. The RecA protein, stimulated by single-stranded DNA, is involved in the inactivation of the LexA repressor thereby inducing the response...

.

cII; (Clear 2) Transcription activator. Activates transcription from the pAQ, pRE and pI promoters. Low stability due to susceptibility to cellular HflB (FtsH) proteases (especially in healthy cells and cells undergoing the SOS response

SOS response

).

cIII;(Clear 3) HflB (FtsH) binding protein, protects cII from degradation by proteases.

N; (aNtiterminator) RNA binding protein and RNA polymerase

RNA polymerase

RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...

cofactor

Cofactor

Cofactor may refer to any of the following:* Cofactor , the signed minor of a matrix* Minor , an alternative name for the determinant of a smaller matrix than that which it describes...

, binds RNA (at Nut sites) and transfers onto the nascent RNApol that just transcribed the nut site. This RNApol modification prevents its recognition of termination sites, so normal RNA polymerase termination signals are ignored and RNA synthesis continues into distal phage genes.

Q; DNA binding protein and RNApol cofactor, binds DNA (at Qut sites) and transfers onto the initiating RNApol. This RNApol modification alters its recognition of termination sequences, so normal ones are ignored; special Q termination sequences some 20,000 bp away are effective.

xis; (eXcISion) excisionase and Int protein regulator, manages excision and insertion of phage genome into the host's genome.

int; (INTegration) Int protein, manages insertion of phage genome into the host's genome. In Conditions of low int concentration there is no effect. If xis is low in concentration and int high then this leads to the insertion of the phage genome. If xis and int have high (and approximately equal) concentrations this leads to the excision of phage genomes from the host's genome.

A, B, C, D, E, F, Z, U, V, G, T, H, M, L, K, I, J [Shown on diagram as head and tail, A-F code for phage head genes, Z-J code for phage tail genes. The order shown here is as found on the genome, reading in a clockwise direction]; structural proteins, self assemble with the phage genome into daughter phage particles.

S, R [Shown on diagram as lysis. The order shown here is as found on the genome, reading in a clockwise direction]; cause the host cell to undergo lysis at high enough concentrations.

OP [Shown on diagram as O replication P]; DNA replication functions, promotes the specific replication of only the phage genome.

sib [not a protein, but a vital conserved DNA sequence]; Forms a stable hairpin loop structure in transcribed mRNA beyond int. Attracts degradation of mRNA by RNAaseIII.

attP [not a protein, but a conserved DNA sequence]; point of action of Int and Xis in integration and excision of the phage genome into the host's genome. Corresponding attB found in the host's genome at the point of insertion.

Repressor

The repressor

Repressor

A repressor is a DNA-binding protein that regulates the expression of one or more genes by decreasing the rate of transcription. This blocking of expression is called repression....

found in the phage lambda is a notable example of the level of control possible over gene expression by a very simple system. It forms a 'binary switch' with two genes under mutually exclusive expression, as discovered by Barbara J. Meyer

Barbara J. Meyer

Barbara J. Meyer is a biologist, noted for her pioneering research on lambda phage, a virus that infects bacteria; discovery of the master control gene involved in sex determination; and studies of gene regulation, particularly dosage compensation.-Biography:...

.

The lambda repressor gene system consists of (from left to right on the chromosome):

cI gene
OR3
OR2
OR1
cro gene

The lambda repressor is a dimer also known as the cI protein

CI protein

cI protein , also known as the lambda repressor, is the sole protein expressed in the lysogenic state of Lambda phage. cI turns off transcription at the phage's L and R promoters as well as any other invading lambda phages into the host cell. It is a transcription factor and contains the common...

. It regulates the transcription of the cI protein and the Cro protein.

The life cycle of lambda phages is controlled by cI and Cro proteins. The lambda phage will remain in the lysogenic state if cI proteins predominate, but will be transformed into the lytic cycle if cro proteins predominate.

The cI dimer may bind to any of three operators, OR1, OR2, and OR3, in the order OR1 = OR2 > OR3.
Binding of a cI dimer to OR1 enhances binding of a second cI dimer to OR2, an effect called cooperativity

Cooperativity

Cooperativity is a phenomenon in biology displayed by enzymes or receptors that have multiple binding sites. This is referred to as cooperative binding. We also see cooperativity in large chain molecules made of many identical subunits , when such molecules undergo phase transitions such as...

. Thus, OR1 and OR2 are almost always simultaneously occupied by cI. However, this does not increase the affinity between cI and OR3, which will be occupied only when the cI concentration is high. This cooperative action is shown by the relative affinity of the repressor for the native sequences individually, which is OR1 > OR2 = OR3; different from the actual order of binding.

In the absence of cI proteins, the cro gene may be transcribed.
In the presence of cI proteins, only the cI gene may be transcribed.
At high concentration of cI, transcriptions of both genes are repressed.

Lytic or lysogenic?

The gene

Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...

regulatory circuitry of phage λ is among the best-understood circuits at the mechanistic level. This circuitry involves several interesting regulatory behaviors. An infected cell undergoes a decision between two alternative pathways, the lytic and lysogenic pathways. If the latter is followed, the lysogenic state is established and maintained. While this state is highly stable, it can switch to the lytic pathway in the process of prophage

Prophage

induction, which occurs when the host SOS response is triggered by DNA damage..

An important distinction here is that between the two decisions; lysogeny and lysis on infection, and continuing lysogeny or lysis from a prophage. The latter is determined solely by the activation of RecA in the SOS response of the cell, as detailed in the section on induction. The former will also be affected by this; a cell undergoing an SOS response will always by lysed, as no cI protein will be allowed to build up. However, the initial lytic/lysogenic decision on infection is also dependent on the cII and cIII proteins.

Simplistically, in cells with abundant nutrients, protease activity is high, which breaks down cII. This leads to the lytic lifestyle. In cells with limited nutrients, protease activity is low, making cII stable. This leads to the lysogenic lifestyle. cIII appears to stabilise cII, both directly and by acting as a competitive inhibitor to the relevant proteases. This means that a cell "in trouble", i.e. lacking in nutrients and in a more dormant state, is more likely to lysogenise. This would be selected for because the phage can now lie dormant in the bacterium until it falls on better times, and so the phage can create more copies of itself with the additional resources available and with the more likely proximity of further infectable cells.

Realistically, a full biophysical model for lambda's lysis-lysogeny decision remains to be developed. Computer modeling and simulation suggest that random processes during infection drive the selection of lysis or lysogeny within individual cells (Arkin et al.). However, recent experiments suggest that physical differences among cells, that exist prior to infection, predetermine whether a cell will lyse or become a lysogen (St-Pierre & Endy)

Basic overviews

Brussow, H. and Hendrix, R.W. 2002 "Small is beautiful.", Cell, 108, 13-16
Dodd, J.B. Shearwin, K.E. and Egan, J.B. 2005 "Revisited gene regulation in phage lambda.", Curr Opin Genet Dev, 15, 145-152
Friedman, D.I. and Court, D.L. 2001 "Bacteriophage lambda; alive and well and still doing its thing.", Current Opinion in Microbiology, 4, 201-207
Gottesman, M. and Weisberg, R.A. 2004 "Little lambda - who made thee?", Micro and Mol Biol Revs, 68, 796-813 (available online at Microbiology and Molecular Biology Reviews, American Society for Microbiology
American Society for Microbiology
The American Society for Microbiology is a scientific organization, based in the United States although with over 43,000 members throughout the world...

)
Hendrix, R.W. et al. 1999 "All the world's a phage.", Proc Nat Acad Sc USA, 96, 2192-2197
Kitano, R. 2002 "Systems biology: a brief overview.", Science, 295, 1662-1664
Ptashne, M. "A Genetic Switch: Phage Lambda Revisited", 3rd edition 2003
Ptashne, M. 2005 "Regulation of transcription: from lambda to eukaryotes.", Trends in Biochem Sci, 30, 275-279
Snyder, L. and Champness, W. "Molecular Genetics of Bacteria", 3rd edition 2007 (Contains an informative and well illustrated overview of bacteriophage lambda)

Lambda phage

Anatomy

Lifecycle

Infection

N antitermination

Lytic lifestyle

Rightward transcription

Lytic replication

Q antitermination

Leftward transcription

xis and int regulation of insertion and excision

Lysogenic (or lysenogenic) lifestyle

Prophage integration

Maintenance of lysogeny

Induction

Control of phage genome excision in induction

Protein function overview

Repressor

Lytic or lysogenic?

See also

Significant papers

Basic overviews