Cre recombinase | |||||||
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Identifiers | |||||||
Organism | |||||||
Symbol | cre | ||||||
Entrez | 2777477 | ||||||
RefSeq (Prot) | YP_006472.1 | ||||||
UniProt | P06956 | ||||||
Other data | |||||||
EC number | 2.7.7.- | ||||||
Chromosome | genome: 0 - 0 Mb | ||||||
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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. [1] The enzyme requires no additional cofactors (such as ATP) or accessory proteins for its function. [2]
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. [3]
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. [4]
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. [5] 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 (cyclization recombination) and the recombination site was named loxP (locus of crossing (x) over, P1). [5] The Cre protein was purified in 1983 and was found to be a 35,000 Da protein. [2] No high energy cofactors such as ATP or accessory proteins are required for the recombinase activity of the purified protein. [2] Early studies also demonstrated that Cre binds to non specific DNA sequences whilst having a 20 fold higher affinity for loxP sequences and results of early DNA footprinting studies also suggested that Cre molecules bind loxP sites as dimers. [2]
Tyrosine recombinase family members [3] |
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S.cerevisiae Flp recombinase |
Bacterial XerC recombinase |
Bacterial XerD recombinase |
λ integrase protein |
HP1 integrase protein |
Cre recombinase consists of 343 amino acids that form two distinct domains. The amino terminal domain encompasses residues 20–129 and this domain contains 5 alpha helical segments linked by a series of short loops. Helices A & E are involved in the formation of the recombinase tetramer with the C terminus region of helix E known to form contacts with the C terminal domain of adjacent subunits. Helices B & D form direct contacts with the major groove of the loxP DNA. These two helices are thought to make three direct contacts to DNA bases at the loxP site. The carboxy terminal domain of the enzyme consists of amino acids 132–341 and it harbours the active site of the enzyme. The overall structure of this domain shares a great deal of structural resemblance to the catalytic domain of other enzymes of the same family such as λ Integrase and HP1 Integrase. This domain is predominantly helical in structure with 9 distinct helices (F−N). The terminal helix (N) protrudes from the main body of the carboxy domain and this helix is reputed to play a role in mediating interactions with other subunits. Crystal structures demonstrate that this terminal N helix buries its hydrophobic surface into an acceptor pocket of an adjacent Cre subunit. [6]
The effect of the two-domain structure is to form a C-shaped clamp that grasps the DNA from opposite sides. [3]
The active site of the Cre enzyme consists of the conserved catalytic triad residues Arg 173, His 289, Arg 292 as well as the conserved nucleophilic residues Tyr 324 and Trp 315. Unlike some recombinase enzymes such as Flp recombinase, Cre does not form a shared active site between separate subunits and all the residues that contribute to the active site are found on a single subunit. Consequently, when two Cre molecules bind at a single loxP site two active sites are present. Cre mediated recombination requires the formation of a synapse in which two Cre-LoxP complexes associate to form what is known as the synapse tetramer in which 4 distinct active sites are present. [6] Tyr 324 acts as a nucleophile to form a covalent 3’-phosphotyrosine linkage to the DNA substrate. The scissile phosphate (phosphate targeted for nucleophilic attack at the cleavage site) is coordinated by the side chains of the 3 amino acid residues of the catalytic triad (Arg 173, His 289 & Trp 315). The indole nitrogen of tryptophan 315 also forms a hydrogen bond to this scissile phosphate. (n.b A Histidine occupies this site in other tyrosine recombinase family members and performs the same function). This reaction cleaves the DNA and frees a 5’ hydroxyl group. This process occurs in the active site of two out of the four recombinase subunits present at the synapse tetramer. If the 5’ hydroxyl groups attack the 3’-phosphotyrosine linkage one pair of the DNA strands will exchange to form a Holliday junction intermediate. [3]
Cre recombinase plays important roles in the life cycle of the P1 bacteriophage. Upon infection of a cell the Cre-loxP system is used to cause circularization of the P1 DNA. In addition to this Cre is also used to resolve dimeric lysogenic P1 DNA that forms during the cell division of the phage. [7]
Inducible Cre activation is achieved using CreER (estrogen receptor) variant, which is only activated after delivery of tamoxifen. [8] 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. [9] 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. [10] 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. [11] A number of mutants with enhanced accuracy have also been identified. [12]
Retroviral integrase (IN) is an enzyme produced by a retrovirus that integrates its genetic information into that of the host cell it infects. Retroviral INs are not to be confused with phage integrases (recombinases) used in biotechnology, such as λ phage integrase, as discussed in site-specific recombination.
A DNA-binding domain (DBD) is an independently folded protein domain that contains at least one structural motif that recognizes double- or single-stranded DNA. A DBD can recognize a specific DNA sequence or have a general affinity to DNA. Some DNA-binding domains may also include nucleic acids in their folded structure.
Cre-Lox recombination is a site-specific recombinase technology, used to carry out deletions, insertions, translocations and inversions at specific sites in the DNA of cells. It allows the DNA modification to be targeted to a specific cell type or be triggered by a specific external stimulus. It is implemented both in eukaryotic and prokaryotic systems. The Cre-lox recombination system has been particularly useful to help neuroscientists to study the brain in which complex cell types and neural circuits come together to generate cognition and behaviors. NIH Blueprint for Neuroscience Research has created several hundreds of Cre driver mouse lines which are currently used by the worldwide neuroscience community.
Site-specific recombinase technologies are genome engineering tools that depend on recombinase enzymes to replace targeted sections of DNA.
Recombinases are genetic recombination enzymes.
In genetics, Flp-FRT recombination is a site-directed recombination technology, increasingly used to manipulate an organism's DNA under controlled conditions in vivo. It is analogous to Cre-lox recombination but involves the recombination of sequences between short flippase recognition target (FRT) sites by the recombinase flippase (Flp) derived from the 2 μ plasmid of baker's yeast Saccharomyces cerevisiae.
P1 is a temperate bacteriophage that infects Escherichia coli and some other bacteria. When undergoing a lysogenic cycle the phage genome exists as a plasmid in the bacterium unlike other phages that integrate into the host DNA. P1 has an icosahedral head containing the DNA attached to a contractile tail with six tail fibers. The P1 phage has gained research interest because it can be used to transfer DNA from one bacterial cell to another in a process known as transduction. As it replicates during its lytic cycle it captures fragments of the host chromosome. If the resulting viral particles are used to infect a different host the captured DNA fragments can be integrated into the new host's genome. This method of in vivo genetic engineering was widely used for many years and is still used today, though to a lesser extent. P1 can also be used to create the P1-derived artificial chromosome cloning vector which can carry relatively large fragments of DNA. P1 encodes a site-specific recombinase, Cre, that is widely used to carry out cell-specific or time-specific DNA recombination by flanking the target DNA with loxP sites.
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. Enzymes known as site-specific recombinases (SSRs) perform rearrangements of DNA segments by recognizing and binding to short, specific DNA sequences (sites), at which they cleave the DNA backbone, exchange the two DNA helices involved, and rejoin the DNA strands. In some cases the presence of a recombinase enzyme and the recombination sites is sufficient for the reaction to proceed; in other systems a number of accessory proteins and/or accessory sites are required. Many different 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 SSRs.
The Tn3 transposon is a 4957 base pair mobile genetic element, found in prokaryotes. It encodes three proteins:
Conditional gene knockout is a technique used to eliminate a specific gene in a certain tissue, such as the liver. This technique is useful to study the role of individual genes in living organisms. It differs from traditional gene knockout because it targets specific genes at specific times rather than being deleted from beginning of life. Using the conditional gene knockout technique eliminates many of the side effects from traditional gene knockout. In traditional gene knockout, embryonic death from a gene mutation can occur, and this prevents scientists from studying the gene in adults. Some tissues cannot be studied properly in isolation, so the gene must be inactive in a certain tissue while remaining active in others. With this technology, scientists are able to knockout genes at a specific stage in development and study how the knockout of a gene in one tissue affects the same gene in other tissues.
Tre recombinase is an experimental enzyme that in lab tests has removed DNA inserted by HIV from infected cells. Through selective mutation, Cre recombinase which recognizes loxP sites are modified to identify HIV long terminal repeats (loxLTR) instead. As a result, instead of performing Cre-Lox recombination, the new enzyme performs recombination at HIV provirus sites.
RMCE is a procedure in reverse genetics allowing the systematic, repeated modification of higher eukaryotic genomes by targeted integration, based on the features of site-specific recombination processes (SSRs). For RMCE, this is achieved by the clean exchange of a preexisting gene cassette for an analogous cassette carrying the "gene of interest" (GOI).
Cystathionine beta-lyase, also commonly referred to as CBL or β-cystathionase, is an enzyme that primarily catalyzes the following α,β-elimination reaction
A P1-derived artificial chromosome, or PAC, is a DNA construct derived from the DNA of P1 bacteriophages and Bacterial artificial chromosome. It can carry large amounts of other sequences for a variety of bioengineering purposes in bacteria. It is one type of the efficient cloning vector used to clone DNA fragments in Escherichia coli cells.
In genetic engineering, floxing refers to the insertion of a DNA sequence between two LoxP sequences, creating an artificial gene cassette which can then be conditionally deleted, translocated, or inverted in a process called Cre-Lox recombination. Recombination between LoxP sites is catalysed by Cre recombinase. The term "floxing" is a portmanteau constructed from the phrase "flanking/flanked by LoxP".
In molecular biology, excisionase is a bacteriophage protein encoded by the Xis gene. It is involved in excisive recombination by regulating the assembly of the excisive intasome and by inhibiting viral integration. It adopts an unusual winged-helix structure in which two alpha helices are packed against two extended strands. Also present in the structure is a two-stranded anti-parallel beta-sheet, whose strands are connected by a four-residue wing. During interaction with DNA, helix alpha2 is thought to insert into the major groove, while the wing contacts the adjacent minor groove or phosphodiester backbone. The C-terminal region of excisionase is involved in interaction with phage-encoded integrase (Int), and a putative C-terminal alpha helix may fold upon interaction with Int and/or DNA.
In molecular biology, the YqaJ refers to the YqaJ/K domain from the skin prophage of the bacterium, Bacillus subtilis. This protein domain, often found in bacterial species, is actually of viral origin. The protein forms an oligomer and functions as an alkaline exonuclease, or in simpler terms, an enzyme that digests double-stranded DNA. It is a reaction which is dependent on Magnesium. It has a preference for 5'-phosphorylated DNA ends. It thus forms part of the two-component SynExo viral recombinase functional unit.
Lysine carboxypeptidase is an enzyme. This enzyme catalyses the following chemical reaction:
EcoRI is a restriction endonuclease enzyme isolated from species E. coli. It is a restriction enzyme that cleaves DNA double helices into fragments at specific sites, and is also a part of the restriction modification system. The Eco part of the enzyme's name originates from the species from which it was isolated - "E" denotes generic name which is "Escherichia" and "co" denotes species name, "coli" - while the R represents the particular strain, in this case RY13, and the I denotes that it was the first enzyme isolated from this strain.
Nat L. Sternberg was an American molecular biologist and bacteriophage researcher, particularly known for his work on DNA recombination and the phage P1.