Questa fase viene definita lisogena. Se la cellula ospite il batterio si trova in una situazione di stress, sottoposta a raggi UV, antibiotici , agenti mutageni o altri fattori che la danneggino, il profago si riattiva, il dna fagico exciso dal genoma batterico e viene attivato il ciclo litico. Di seguito sono riportati i sette trascritti, catalogati in base al nome del promotore, di fago lambda con i geni contenuti. Alcune aree di trascrizione dipendono dai fattori N e Q. Alcuni trascritti sono sovrapposti.
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Anatomy[ edit ] Bacteriophage lambda virion schematic. Protein names and their copy numbers in the virion particle are shown. The presence of the L and M proteins in the virion is still unclear. The whole particle consists of 12—14 different proteins with more than protein molecules total and one DNA molecule located in the phage head. However, it is still not entirely clear whether the L and M proteins are part of the virion.
The cos site circularizes the DNA in the host cytoplasm. In its circular form, the phage genome, therefore, is 48, base pairs in length. See section below for details. It must instead use an existing pathway to invade the host cell, having evolved the tip of its tail to interact with a specific pore to allow entry of its DNA to the hosts.
Bacteriophage Lambda binds to an E. The J protein interacts with the maltose outer membrane porin the product of the lamB gene of E. The linear phage genome is injected through the outer membrane. The DNA passes through the mannose permease complex in the inner membrane   encoded by the manXYZ genes and immediately circularises using the cos sites, base G-C-rich cohesive "sticky ends".
Lambda phage DNA injection into the cell membrane using Mannose PTS permease a sugar transporting system as a mechanism of entry into the cytoplasm. Host DNA gyrase puts negative supercoils in the circular chromosome, causing A-T-rich regions to unwind and drive transcription. At first, these express the N and cro genes, producing N, Cro and a short inactive protein.
N binds to the two Nut N utilisation sites, one in the N gene in the PL reading frame, and one in the cro gene in the PR reading frame. The N protein is an antiterminator , and functions by engaging the transcribing RNA polymerase at specific sites of the nascently transcribed mRNA. When RNA polymerase transcribes these regions, it recruits N and forms a complex with several host Nus proteins.
This complex skips through most termination sequences. This inhibition can induce a bacteriostatic state, which favours lysogeny. Low temperature, starvation of the cells and high multiplicity of infection MOI are known to favor lysogeny see later discussion. N antitermination[ edit ] N Antitermination requires the assembly of a large ribonucleoprotein complex to effectively prolong the anti-termination process, without the full complex the RNA polymerase is able to bypass only a single terminator  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. When transcribed, each sequence forms a hairpin loop structure that the N protein can bind to. The entire complex including the bound Nut site on the mRNA continues transcription, and can skip through termination sequences.
Lytic life cycle[ edit ] Main article: Lytic cycle 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. Cro dominates the repressor site see "Repressor" section , repressing synthesis from the PRM promoter which is a promoter of the lysogenic cycle. The O and P proteins initiate replication of the phage chromosome see "Lytic Replication". Q, another antiterminator , binds to Qut sites.
Structural proteins and phage genomes self-assemble into new phage particles. Products of the genes S,R, Rz and Rz1 cause cell lysis. S is a holin , a small membrane protein that, at a time determined by the sequence of the protein, suddenly makes holes in the membrane. R is an endolysin , an enzyme that escapes through the S holes and cleaves the cell wall. Rz and Rz1 are membrane proteins that form a complex that somehow destroys the outer membrane, after the endolysin has degraded the cell wall.
For wild-type lambda, lysis occurs at about 50 minutes after the start of infection and releases around virions. Rightward transcription[ edit ] Rightward transcription expresses the O, P and Q genes. 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 replication similar to that used by phage M Note that this does not 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. Note that NusA can stimulate the activity of the Q protein. Part of the Qut site resembles the Pribnow box , causing the holoenzyme to pause. The head and tail genes are transcribed and the corresponding proteins self-assemble. Leftward transcription[ edit ] Diagram showing the retro-regulation process that yields a higher concentration of xis compared to int.
Leftward transcription expresses the gam, red, xis, and int genes. Gam and red proteins are involved in recombination. Int and xis are integration and excision proteins vital to lysogeny. This results initially in the excision of any inserted genomes from the host genome. 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 life cycle[ edit ] Main article: Lysogenic cycle The lysogenic lifecycle begins once the cI protein reaches a high enough concentration to activate its promoters, after a small number of infections. This is expressed, turning on cI repressor production. The PI promoter expresses the int gene, resulting in high concentrations of Int protein. Elevated levels of int much higher than that of xis result in the insertion of the lambda genome into the hosts genome see diagram.
The genome remains inserted into the host genome in a dormant state. The prophage is duplicated with every subsequent cell division of the host. The phage genes expressed in this dormant state code for proteins that repress expression of other phage genes such as the structural and lysis genes in order to prevent entry into the lytic cycle. These repressive 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 , 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. The integration itself is a sequential exchange see genetic recombination via a Holliday junction and requires both the phage protein Int and the bacterial protein IHF integration host factor.
Lysogeny is maintained solely by cI. It is therefore the only protein expressed by lysogenic phage. This is coordinated by the PL and PR operators.
When it is present at a much higher concentration, it also binds to OR3, inhibiting transcription from PRM, thus regulating its own levels in a negative feedback loop. The presence of cI causes immunity to superinfection by other lambda phages, as it will inhibit their PL and PR promoters. Induction[ edit ] Transcriptional state of the PRM and PR promoter regions during a lysogenic state vs induced, early lytic state.
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. RecA a cellular protein detects DNA damage and becomes activated.
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 PR and PL 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.
There is however one notable difference. LexA expression leads to inhibition of various genes including LexA. Control of phage genome excision in induction[ edit ] The phage genome is still inserted in the host genome and needs excision for DNA replication to occur. The sib section beyond the normal PL promoter transcript is, however, no longer included in this reading frame see diagram.
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. Multiplicity reactivation and prophage reactivation[ edit ] Multiplicity reactivation MR is the process by which multiple viral genomes, each containing inactivating genome damage, interact within an infected cell to form a viable viral genome.
Repressor[ edit ] Protein interactions that lead to either Lytic or Lysogenic cycles for Lambda phage The repressor found in the phage lambda is a notable example of the level of control possible over gene expression by a very simple system.
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Anatomy[ edit ] Bacteriophage lambda virion schematic. Protein names and their copy numbers in the virion particle are shown. The presence of the L and M proteins in the virion is still unclear. The whole particle consists of 12—14 different proteins with more than protein molecules total and one DNA molecule located in the phage head. However, it is still not entirely clear whether the L and M proteins are part of the virion. The cos site circularizes the DNA in the host cytoplasm. In its circular form, the phage genome, therefore, is 48, base pairs in length.