Кре-рекомбиназа: фермент и система сайт-специфической рекомбинации.

Genetic recombination enzyme

Cre recombinase is a tyrosine recombinase enzyme derived from the P1 bacteriophage. The enzyme uses a topoisomerase I like mechanism to carry out site specific recombination events. The enzyme (38 kDa) is a member of the integrase family of site specific recombinase and it is known to catalyse the site specific recombination event between two DNA recognition sites (LoxP sites). This 34 base pair (bp) loxP recognition site consists of two 13 bp palindromic sequences which flank an 8bp spacer region. The products of Cre mediated recombination at loxP sites are dependent upon the location and relative orientation of the loxP sites. Two separate DNA species both containing loxP sites can undergo fusion as the result of Cre mediated recombination. DNA sequences found between two loxP sites are said to be "floxed". In this case the products of Cre mediated recombination depends upon the orientation of the loxP sites. DNA found between two loxP sites oriented in the same direction will be excised as a circular loop of DNA whilst intervening DNA between two loxP sites that are opposingly orientated will be inverted. The enzyme requires no additional cofactors (such as ATP) or accessory proteins for its function. The enzyme plays important roles in the life cycle of the P1 bacteriophage, such as cyclization of the linear genome and resolution of dimeric chromosomes that form after DNA replication. Cre recombinase is a widely used tool in the field of molecular biology. The enzyme's unique and specific recombination system is exploited to manipulate genes and chromosomes in a huge range of research, such as gene knock out or knock in studies. The enzyme's ability to operate efficiently in a wide range of cellular environments (including mammals, plants, bacteria, and yeast) enables the Cre Lox recombination system to be used in a vast number of organisms, making it a particularly useful tool in scientific research.

Studies carried out in 1981 by Sternberg and Hamilton demonstrated that the bacteriophage 'P1' had a unique site specific recombination system. EcoRI fragments of the P1 bacteriophage genome were generated and cloned into lambda vectors. A 6.5kb EcoRI fragment (Fragment 7) was found to permit efficient recombination events. The mechanism of these recombination events was known to be unique as they occurred in the absence of bacterial RecA and RecBCD proteins. The components of this recombination system were elucidated using deletion mutagenesis studies. These studies showed that a P1 gene product and a recombination site were both required for efficient recombination events to occur. The P1 gene product was named Cre (causes recombination) and the recombination site was named loxP (locus of crossing (x) over, P1). The effect of the two domain structure is to form a C shaped clamp that grasps the DNA from opposite sides.

Inducible Cre activation is achieved using CreER (estrogen receptor) variant, which is only activated after delivery of tamoxifen. This is done through the fusion of a mutated ligand binding domain of the estrogen receptor to the Cre recombinase, resulting in Cre becoming specifically activated by tamoxifen. In the absence of tamoxifen, CreER will result in the shuttling of the mutated recombinase into the cytoplasm. The protein will stay in this location in its inactivated state until tamoxifen is given. Once tamoxifen is introduced, it is metabolized into 4 hydroxytamoxifen, which then binds to the ER and results in the translocation of the CreER into the nucleus, where it is then able to cleave the lox sites. Importantly, sometimes fluorescent reporters can be activated in the absence of tamoxifen, due to leakage of a few Cre recombinase molecules into the nucleus which, in combination with very sensitive reporters, results in unintended cell labelling. CreER(T2) was developed to minimize tamoxifen independent recombination and maximize tamoxifen sensitivity.

In recent years, Cre recombinase has been improved with conversion to preferred mammalian codons, the removal of reported cryptic splice sites, an altered stop codon, and reduced CpG content to reduce the risk of epigenetic silencing in mammals. A number of mutants with enhanced accuracy have also been identified.