Lecture 28 Site Specific Recombination
Homework: What limits transposon mobility in humans?
λ
A sitespecific recombination reaction. The reaction is for a common class of sitespecific recombinases called integraseclass recombinases (named after bacteriiophage λ integrase, the first one characterized). The reaction is carried out within a tetramer of identical subunits. Recombinase subunits bind to a specific sequence, often called simply the recombination site. ① One strand in each DNA is cleaved at particular points within the sequence. The nucleophile is the OH group of an activesite Tyr residue, and the product is a covalent phosphotyrosine link between protein and DNA. ② The cleaved strands join to new partners, producing a Holliday intermediate. Steps ③ and ④ complete the reaction by a process similar to the first two steps. The original sequence of the recombination site is regenerated after recombining the DNA flanking the site. These steps occur within a complex of multiple recombinase subunits that sometimes includes other proteins not shown here.
A sitespecific recombination reaction. A surface contour model of a foursubunit integraseclass recombinase called the Cre recombinase, bound to a Holliday structure recombination intermediate shown with light blue and dark blue helix strands. The protein has been rendered transparent so that the bound DNA is visible.
Phase variation
(site specific recombination)
Inverted Repeats
Inverted Repeats
(prokaryotic)
in the generation of antibody diversity