page_title: Lambda phage
section_title: Infection
hierarchical_section_title: Lambda phage / Life cycle / Infection
caption_attribution_description: English: Step 10 in the mannose PTS permease transportation complex
context_page_description: Enterobacteria phage λ is a bacterial virus, or bacteriophage, that infects the bacterial species Escherichia coli. It was discovered by Esther Lederberg in 1950. The wild type of this virus has a temperate life cycle that allows it to either reside within the genome of its host through lysogeny or enter into a lytic phase, during which it kills and lyses the cell to produce offspring. Lambda strains, mutated at specific sites, are unable to lysogenize cells; instead, they grow and enter the lytic cycle after superinfecting an already lysogenized cell.
  The phage particle consists of a head, a tail, and tail fibers. The head contains the phage's double-strand linear DNA genome. During infection, the phage particle recognizes and binds to its host, E. coli, causing DNA in the head of the phage to be ejected through the tail into the cytoplasm of the bacterial cell. Usually, a "lytic cycle" ensues, where the lambda DNA is replicated and new phage particles are produced within the cell. This is followed by cell lysis, releasing the cell contents, including virions that have been assembled, into the environment.
context_section_description: Lambda phage is a non-contractile tailed phage, meaning during an infection event it cannot 'force' its DNA through a bacterial cell membrane. 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. coli cell by means of its J protein in the tail tip. The J protein interacts with the maltose outer membrane porin (the product of the lamB gene) of E. coli, a porin molecule, which is part of the maltose operon.
  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, 12-base G-C-rich cohesive "sticky ends". The single-strand viral DNA ends are ligated by host DNA ligase.
  Host DNA gyrase puts negative supercoils in the circular chromosome, causing A-T-rich regions to unwind and drive transcription.
  Transcription starts from the constitutive PL, PR and PR' promoters producing the 'immediate early' transcripts. At first, these express the N and cro genes, producing N, Cro and a short inactive protein.
  Cro binds to OR3, preventing access to the PRM promoter, preventing expression of the cI gene. 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. The extended transcripts (the 'late early' transcripts) include the N and cro genes along with cII and cIII genes, and xis, int, O, P 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).
caption: Lambda phage DNA injection into the cell membrane using Mannose PTS permease (a sugar transporting system) as a mechanism of entry into the cytoplasm.
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