Question

Can bacteriophage lambda tell the difference between integration and excision? If so, how?

Answer 1

Solution

Bacteriophage lambda can tell the difference between integration amd excision by following mechanism

• Bacteriophage lambda site-specific recombination comprises two overall reactions, integration into and excision.
• During excision, excisionase (Xis) helps Int to bind DNA and introduces a bend in the DNA that facilitates formation of the proper excisive nucleoprotein complex.
• Lambda integrase (Int) carries out both reactions on genetic material.
• The carboxyl-terminal α-helix of Xis interact with Int through direct protein-protein interactions between them.
• The gel mobility shift assay showing that the amino-terminal domain of Int maintaining cooperative interactions with Xis.
• This results in to the amino-terminal arm-type DNA binding domain of Int interacts with Xis.
• The site-specific recombination system encoded by bacteriophage lambda provides a idea of a reaction controlled by a higher-order multiple-protein-DNA complex.
• The lambda recombination complex is composed of specific DNA sites in the phage and bacterial chromosomes.
• it utilizes host-encoded and phage-encoded proteins.
• Integrative recombination between specific attachment sites, attP on the phage DNA and attB on the bacterial chromosome, generates recombinant attR and attL sites flanking the prophage DNA.
• Both reactions are catalyzed by the phage-encoded protein integrase (Int) and are assisted by accessory proteins.
• The host-encoded integration host factor is required for both reactions.
• Excision requires an additional phage-encoded protein called excisionase (Xis).
• Excision is stimulated by the factor for inversion stimulation, which is supplied by the host.
• Int plays a central role in recombination. To carry out the cleavage, strand exchange, and resealing of the DNA attachment site, Int recognizes two distinct DNA sequences.
• The two classes of Int binding sequences are called core-type and arm-type sites.
• The core-type sites consist of imperfect inverted repeats that flank the sites of strand exchange during recombination.
• Five arm-type sites occur outside the region of strand exchange on attP, attR, and attL.
• Three contiguous sites called P′1, P′2, and P′3 are located on attP and attL. Two sites called P1 and P2 are both on attP and on attR.