Site-specific recombination

（重定向自Site specific recombination）

Figure 1:Tyr-Recombinases: Details of the crossover step
Top: Traditional view including strand-exchange followed by branch-migration (proofreading). The mechanism occurs in the framework of a synaptic complex (1) including both DNA-sites in parallel orientation. While branch-migration explains the specific homology requirements and the reversibility of the process in a straightforward manner, it could not be reconciled with the motions recombinase subunits have to undergo in three dimensions.

Bottom: Current view. Two simultaneous strand-swaps, each depending on the complementarity of three successive bases at (or close to) the edges of the 8 bp spacer (dashed lines indicate base-pairing). Didactic complications arise from the fact that, in this model, the synaptic complex must accommodate both substrates in an anti-parallel orientation.

This synaptic complex (1) arises from the association of two individual recombinase subunits (

Figure 2: Ser-Recombinases: The (essentially irreversible) subunit-rotation pathway.
Contrary to Tyr-recombinases, the four participating DNA-strands are cut in synchrony at points staggered by only 2 bp (leaving little room for proofreading). Subunit-rotation (180°) permits the exchange of strands while covalently linked to the protein partner. The intermediate exposure of double-strand breaks bears risks of triggering illegitimate recombination and thereby secondary reactions.

Here, the synaptic complex arises from the association of pre-formed recombinase dimers with the respective target sites (CTD/NTD, C-/N-terminal domain). Like for Tyr-recombinases, each site contains two arms, each accommodating one protomer. As both arms are structured slightly differently, the subunits become conformationally tuned and thereby prepared for their respective role in the recombination cycle. Contrary to members of the Tyr-class the recombination pathway converts two different substrate sites (attP and attB) to site-hybrids (attL and attR). This explains the irreversible nature of this particular recombination pathway, which can only be overcome by auxiliary

Figure 3B: Inversion by the Cre-lox system

Site-specific recombination, also known as conservative site-specific recombination, is a type of genetic recombination in which DNA strand exchange takes place between segments possessing at least a certain degree of sequence homology. Site-specific recombinases (SSRs) perform rearrangements of DNA segments by recognizing and binding to short DNA sequences (sites), at which they cleave the DNA backbone, exchange the two DNA helices involved and rejoin the DNA strands. While in some site-specific recombination systems just a recombinase enzyme and the recombination sites is enough to perform all these reactions, in other systems a number of accessory proteins and/or accessory sites are also needed. Multiple genome modification strategies, among these recombinase-mediated cassette exchange (RMCE), an advanced approach for the targeted introduction of transcription units into predetermined genomic loci, rely on the capacities of SSRs.

单词	Site specific recombination
释义	Site specific recombination 英语百科 Site-specific recombination （重定向自Site specific recombination） Site-specific recombination, also known as conservative site-specific recombination, is a type of genetic recombination in which DNA strand exchange takes place between segments possessing at least a certain degree of sequence homology. Site-specific recombinases (SSRs) perform rearrangements of DNA segments by recognizing and binding to short DNA sequences (sites), at which they cleave the DNA backbone, exchange the two DNA helices involved and rejoin the DNA strands. While in some site-specific recombination systems just a recombinase enzyme and the recombination sites is enough to perform all these reactions, in other systems a number of accessory proteins and/or accessory sites are also needed. Multiple genome modification strategies, among these recombinase-mediated cassette exchange (RMCE), an advanced approach for the targeted introduction of transcription units into predetermined genomic loci, rely on the capacities of SSRs.
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