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Suraj Prakash Sharma | Ekta Chotia


2.10.      Site specific recombination :

Recombination can also takes place when there is a short region of homology is present known as specific region of homology responsible for recombination , this type of recombination called site-specific recombination and visualize during the λ becteriophage integration within E.coli genome. This recombination occur due to the presence of att (attachment site) site on both  λ bacteriophage called attP contain 250 bp along with POP’ and in E.coli called attB contain 23 bp along with BOB’. Both att site has 15 base pair central homologous sequence called ‘O’on to which recombination occur in case of integration but incase of exsition those att site known as attL and allR.

Firstly integration is carried out by lambda integrase which is related to IB topoisomerase family and possess a conserve tyrosine residue thus also called tyrosine recombinase. Integrase require the assistance of IHF (integration host factor), which is a hetrodimer of 20 kDa and responsible to cause bend in DNA by binding the ~20 bp sequence of attP site,in turn  integrase binding site come close, which present on the DNA arm, as a result integration of lambda genome into E.coli genome takes place. Through integration the attachment (att) site generate called attL or BOP’ and attR or POB’ and  excision takes place due to reciprocal recombination takes place between these two att site by the function of integrase, IHF, and Xis which act as excisionase. Xis inhibit Integation but promote excision.

Site specific recombination done by recombinase enzyme family and this event require for the integration of phage genome sequence into E.coli genome and during the excision of phage genome sequence from the E.coli genome. For integration integrase enzyme is required which is known as Int in phage, Cre in phage P1 and FLP in yeast (cause inversion of chromosome). Here we explain Site specific recombination between lambda phage and E.coli. lambda phage possess two type of life cycle mode , first, lytic in which it reside within E.coli as independent chromosomal molecule and second, lysogenic, in which lambda phage DNA integrate in E.coli chromosome called integration as well as release from the E.coli chromosome called excision and thus site specific recombination require to carry out both event.

There are two families of conservative site-specific recombinases: The serine recombinases and the tyrosine recombinases. Fundamental to the mechanism used by both families is that when they cleave the DNA, a covalent protein–DNA intermediate is generated. For the serine recombinases, the side chain of a serine residue within the protein’s active site attacks a specific phosphodiester bond in the recombination site. This reaction introduces a single-strand break in the DNA and simultaneously generates a covalent linkage between the serine and a phosphate at this DNA cleavage site. Likewise, for the tyrosine recombinases, it is the side chain of the activesite tyrosine that attacks and then becomes joined to the DNA.

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