Lambda phage

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Enterobacteria phage λ (lambda phage, coliphage λ) is a temperate bacteriophage that infects Escherichia coli.

Lambda phage is a virus particle consisting of a head, containing double-stranded linear DNA as its genetic material, and a tail that can have tail fibers. 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 many times and the genes for head, tail and lysis proteins are expressed. This leads to assembly of multiple new phage particles within the cell and subsequent cell lysis, releasing the cell contents, including virions that have been assembled, into the environment. However, under certain conditions the phage DNA may integrate itself into the host cell chromosome in the lysogenic pathway. In this state, the λ DNA is called a prophage and stays resident within the host's genome without apparent harm to the host, which can be termed a lysogen when a prophage is present. 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.

Lambda phage was discovered by Esther Lederberg in 1950.[1] It has been used heavily as a model organism, and has been a rich source for useful tools in molecular biology. Uses include its application as a vector for the cloning of recombinant DNA, the use of its site-specific recombinase, int, for the shuffling of cloned DNAs by the 'Gateway' method, and the application of its Red operon, including the proteins Red alpha (also called 'exo'), beta and gamma in the DNA engineering method called recombineering.

In the following page, genes will be written in italics and their associated proteins in Roman. For instance, cI refers to the gene, while cI is the resulting protein encoded by that gene.

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