Enterobacteria phage λ (lambda phage) is a temperate phage that lives in E. coli. Once the phage is inside its host, it may integrate itself into the host's DNA. In this state, λ is called a prophage and stays resident within the host's genome, without causing it much harm. This way, the prophage gets duplicated with every cell division of the host. The DNA of the prophage that is expressed in that state codes for proteins that look out for signs of stress in the host cell. Stress can be a result of starvation, poisons (like antibiotics), and other factors that can damage or destroy the host. At that point, the prophage becomes active again, excises itself from the DNA of the host cell and enters its lytic cycle. The reactivated phage takes apart the host's DNA and "reprograms" its "protein factory" to produce new phages in multiple copies. When all resources of the host are depleted from building new phages, the cell is lysed (the cell membrane is broken down), and the new phages are released.
The integration of phage λ takes place at a special attachment site in the bacterial genome, called attλ. The sequence of the att site is called attB 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) via a Holliday structure and requires both the phage protein int and the bacterial protein IHF (integration host factor). Both int and IHF bind to attP and built an intrasome , a DNA-protein-complex designed for site-specific recombination of the phage and host DNA. The original BOB' secuanes 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.
In the following, the convention that genes are italicized, while protein products are not, is followed, i.e. cI refers to the gene, whilst cI is the resulting protein encoded by that gene. The lambda repressor gene system consists of (from left to right on the chromosome):
The lambda repressor is a dimer also known as the cI protein. 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. 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.