Lambda phage

The lambda phage ( phage λ ) is a virus of the order Caudovirales who has the bacterium Escherichia coli to host; so it belongs to the bacteriophage. He has in the history of virus research and molecular genetics as well as in genetic engineering processes is of great importance.

Morphology

The virion of the lambda phage is constructed, as with all Siphoviridae of a capsid containing the dsDNA, and a non- contractile tail. The capsid has an isometric icosahedral symmetry and is about 60 nm in diameter in size; It consists of 72 capsomers (60 hexamers, pentamers, 12, T = 7). Because of the gap for the approach of the tail but not the capsid consist of 420 (= 60 × 6 12 × 5) molecules of the two major capsid proteins E and D, but only from 415

The tail part is flexible but non- contractile and 8x150 nm in size. There are four long fibers, may disappear in vitro cultured strains at its end.

Linear, double-stranded DNA genome of the lambda phage is 48,502 bp in size and encodes about 70 proteins. The complete genome sequence was determined by Frederick Sanger in 1982. The linear DNA having at both ends a short single-stranded portions which are complementary to each other and serve for recircularization of the phage DNA by injection into the host. These ends are also called cohesive ends ( cos), homologous binding ends or sticky ends ( engl. sticky ends ) called.

Integration

The phage λ is a temperate bacteriophage renter. Its linear phage DNA will be closed after the infection of the host cell by the DNA ligase of the bacterium to a circular DNA. The circular DNA can be incorporated by the viral integrase into the bacterial chromosome. The integration of the phage DNA into the host genome is a site-specific recombination and can lead to specialized transduction. In this state, the lambda phage is a prophage and is integrated, without expressing their genes into the host genome.

Expression

The expression of the prophage genes are suppressed by a repressor protein. The only gene that is active, the repressor cI is itself That is, the prophage DNA is also duplicated with each division of the host cell, but its genes are not transcribed. This process is also known as lysogenic cycle. The termination of these inactive lysogeny can be induced by stress factors such as antibiotics, ultraviolet radiation, nutrient deficiency, and particularly DNA damage. In the latter case, by damaging the genetic information for the activation of RecA, an inducer of DNA repair processes, the SOS response. However, RecA cleaves after activation not only his actual substrate but also the lambda repressor cI. This leads to the cessation of repression and thus the expression of the phage genome. Now may be the lambda phage in the lytic cycle in which the actual replication of the phages is taking place. After the synthesis of the components and assembly of phage lysis of the cells and thus the release of the mature, infectious phage is performed.

Applications in genetic engineering

Lambda phage are used in genetic engineering as insertional vectors. However, some modifications are necessary before use as a vector. First supernumerary restriction sites should be removed. For insertion of an additional gene segment (Insert) original phage sequences must be removed, otherwise can be due to the too large, artificial genome, no packaging in the capsid. These are not required for replication of phage genes and the genes are removed for the lysogenic cycle. This creates space for a large approx 8kb insert. Bacteriophage lambda is used as a cloning vector for the production of cDNA libraries, as well as a platform for phage display.