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Template:Taxobox begin Template:Taxobox begin placement virus Template:Taxobox group i entry Template:Taxobox ordo entry Template:Taxobox familia entry Template:Taxobox genus entry Template:Taxobox species entry Template:Taxobox end placement Template:Taxobox end 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 large 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 att. The sequence of the att site is called att and consists of the parts B-O-B', whereas the complementary sequence in the circular phage genome is called att 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 att 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.

Repressor

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):

• cI gene
• OR3
• OR2
• OR1
• cro gene

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.

• 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.

Lifecycle

(detailed lifecycle coming soon)

1. Bacteriophage Lambda binds to the taget E.Coli cell, the tail fibres binding to maltose receptors.
2. The linear phage genome is injected into the cell, and immediately circularises.
3. Transcription starts, from the L, R and R' promoters producing the 'immediate early' transcripts.
4. Initially these produce N, Cro and a short inactive transcript.
5. Cro binds to OR1 preventing access to the RM promoter preventing production of cI. N binds to the two Nut sites, on in the N gene, and one in the Cro gene.
6. The N bound in the L and R open reading frames extends the reading frames. The early translation products of these transcripts are more N, Cro and cII and cIII.
7. cIII binds to cII partially preventing protease vulnerability. The stability of cII determines the lifestyle of the phage. In unstressed cells with abundant nutrients protease activity is high, and cII unstable. This leads to the lytic lifestyle. In stressed cells with limited nutrients protease activity is low, and cII stable. This leads to the lysrnogenic lifestyle.

Lytic Lifestyle (coming soon)

Lysenogenic Lifestyle (coming soon)

Induction (coming soon)

Protein Function Overview (coming soon)

• Caudovirales
• Model organisms