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Lysogeny and Immunity
When lambda lysogenizes a cell, it inserts its DNA into the DNA of the host. There, the lambda DNA is passively replicated by the host just as though it were host DNA. Such a lambda lysogen is highly stable, and lysogeny can be passed on for hundreds of generations. Upon induction of the phage, which may be either spontaneous or caused by exposure to inducing agents such as UV light, lambda excises itself from the chromosome and enters a lytic cycle. About one hundred phage are produced, and the cell lyses.
The advantages to the phage and cell of lysogeny would largely be lost if a lysogenic cell could be lytically infected by another lambda phage. Indeed, lambda lysogenic cells cannot be lytically infected with more phage. The cells are said to possess lambda immunity. The superinfect-ing phage can adsorb to immune cells and inject their DNA into the cell, but the superinfecting phage DNA does little else. This DNA is not replicated or integrated into the host chromosome and therefore is diluted away by cell growth since it is passed to only one of the two daughter cells on cell division.
Lambda repressor protein, the product of the CI gene, confers the immunity. Repressor encoded by the lysogenic phage diffuses through-out the cytoplasm of the cell. When another lambda phage injects its DNA into this cell, the repressor binds to specific sites on the superin-fecting DNA and inactivates the promoters necessary for the first steps of vegetative phage growth. This same repressor activity also prevents the lysogenic phage from initiating its growth. Upon induction of the lysogen however, the repressor is destroyed and the phage can begin a lytic growth cycle.
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