Tol2 FLEX Conditional Gene Expression Vector (Cre-Switch)


The Tol2 FLEX conditional Cre-Switch gene expression vector combines VectorBuilder’s highly efficient Tol2 vector system with the Cre-responsive FLEX conditional gene expression system to help you achieve transfection-mediated permanent integration of FLEX switch into the host genome for Cre-induced switching between the expression of two ORFs. The FLEX Cre-Switch system utilizes two pairs of LoxP-variant recombination sites flanking two antiparallel ORFs in an arrangement which facilitates activation of one gene while repressing the other by Cre-dependent inversion of both ORFs.

The FLEX Cre-Switch system consists of two pairs of heterotypic LoxP-variant recombination sites, namely LoxP, having the wild type sequence and Lox2272, having a mutated sequence flanking a pair of ORFs. Both LoxP variants are recognized by Cre, but only identical pairs of LoxP sites can recombine with each other and not with any other variant. The two ORFs are in an opposite orientation with respect to one-another, such that one ORF is in its proper sense orientation, while the other is in an antisense orientation. The LoxP and Lox2272 sites are organized in an alternating fashion, with an antiparallel orientation for each pair. In the absence of Cre recombinase, while the first ORF is expressed under the control of the user-selected promoter, the second ORF is not expressed due to its antisense orientation. In the presence of Cre, the LoxP and Lox2272 sites undergo recombination with the other LoxP and Lox2272 sites respectively, resulting in the inversion of both ORFs and excision of one from each pair of identical recombination sites. Inversion of the ORFs results in silencing of the first ORF (which will now be in an antisense orientation) and allows expression of the second ORF (which will now be in a sense orientation).

The Tol2 vector system is technically simple, utilizing transfection (rather than viral transduction) to permanently integrate your gene(s) of interest into the host genome. The Tol2 system comprises two components: the transposon plasmid and the transposase. The transposon plasmid contains two inverted terminal repeats (ITRs) bracketing the region to be transposed. The FLEX Cre-Switch described above is cloned into this region. The transposase can be delivered into target cells through two methods. The helper plasmid can be transiently transfected into cells. Alternatively, target cells can be injected with transposase mRNA generated by in vitro transcription from the helper plasmid. When the transposon and helper plasmids are co-transfected into target cells, the transposase produced from the helper plasmid would recognize the two ITRs on the transposon and insert the flanked region including the two ITRs into the host genome. At each insertion site, the Tol2 transposase creates an 8 bp duplication, resulting in identical 8 bp direct repeats flanking each transposon integration site in the genome. Insertion occurs without any significant bias for the insertion site sequence. This is unlike transposon systems which have specific target consensus sites. For example, piggyBac transposons are typically inserted at sites containing the sequence TTAA. Expression of the second ORF in the FLEX Cre-Switch can then be activated while silencing the first ORF in the presence of Cre recombinase, upon Cre-mediated inversion of both ORF sequences. Through both methods of delivering transposase, it is expressed for only a short time. Upon the loss of the helper plasmid or degradation of transposase mRNA, the integration of the transposon into the host genome becomes permanent.

Tol2 is a class II transposon, meaning that it moves in a cut-and-paste manner, hopping from place to place without leaving copies behind. (In contrast, class I transposons move in a copy-and-paste manner.) If Tol2 transposase is reintroduced into the cells, the transposon could get excised from the genome of some cells, resulting in either precise or imprecise excisions with indels created.

While this vector system can be used in tissue culture cells, it is particularly suitable for the generation of transgenic animals. Transgenic animals carrying such a vector originally express the first user-selected ORF, however when crossed to an animal carrying a tissue-specific Cre transgene, expression of the second user-selected ORF will be activated while silencing the first ORF in the progeny animals carrying both types of transgenes, specifically in cells where the tissue-specific Cre is expressed and the user-selected promoter is active.

For further information about this vector system, please refer to the papers below.

References Topic
Genome Biol. 8(Suppl 1): S7 (2007) Review of Tol2 vectors
Genetics 174: 639 (2006) Identification of minimal sequences for Tol2 transposable elements
PLoS Genetics 2: e169 (2006) Large cargo-capacity transposition with a minimal Tol2 transposon
Gene. 216:55 (1998) Characterization of LoxP mutants, including Lox2272
Nat Biotechnol. 21:562 (2003) Development of the FLEX switch system
J Neurosci. 28:7025 (2008) Application of a FLEX switch system


The Tol2 FLEX conditional Cre-Switch gene expression vector is designed to achieve Cre-mediated switching between expression of two ORFs in mammalian cells and animals. Expression is under the control of a user-selected promoter and can be permanently switched from one user-selected ORF to another by co-expression of Cre recombinase. 

This vector along with the helper plasmid encoding the Tol2 transposase are optimized for high copy number replication in E. coli, efficient transfection into a wide range of target cells, and high-level expression of the transgene carried on the vector.


Switch-like regulation: Opposite orientation of the two ORFs ensures that while the ORF in the sense orientation is expressed, the ORF in the antisense orientation is repressed without any leaky gene expression.

Permanent integration of vector DNA: Conventional transfection results in almost entirely transient delivery of DNA into host cells due to the loss of DNA over time. This problem is especially prominent in rapidly dividing cells. In contrast, transfection of mammalian cells with the Tol2 transposon plasmid along with the helper plasmid can deliver genes carried on the transposon permanently into host cells due to the integration of the transposon into the host genome.

Technical simplicity: Delivering plasmid vectors into cells by conventional transfection is technically straightforward, and far easier than virus-based vectors which require the packaging of live virus.

Very large cargo space: Our Tol2 FLEX conditional Cre-Switch gene expression vector can accommodate ~11 kb of total DNA. The plasmid backbone, transposon-related sequences and the sequences necessary for Cre-mediated recombination only occupy about 3.1 kb, leaving plenty of room to accommodate the user's sequence of interest.


Limited cell type range: The delivery of Tol2 vectors into cells relies on transfection. The efficiency of transfection can vary greatly from cell type to cell type. Non-dividing cells are often more difficult to transfect than dividing cells, and primary cells are often harder to transfect than immortalized cell lines. Some important cell types, such as neurons and pancreatic β cells, are notoriously difficult to transfect. These issues limit the use of the Tol2 system.

Key components

5' ITR: Tol2 5' terminal repeat. When a DNA sequence is flanked by two ITRs, the Tol2 transposase can recognize them, and insert the flanked region including the two ITRs into the host genome.

Promoter: The promoter driving your gene of interest is placed here.

Lox2272: Recombination site for Cre recombinase. Mutated Lox site with two base substitutions of wild type LoxP. Incompatible with LoxP sites. When Cre is present, the LoxP and LoxP2272 sites will be cut and recombine with compatible sites.

LoxP: Recombination site for Cre recombinase. Incompatible with Lox2272 sites. When Cre is present, the LoxP and Lox2272 sites will be cut and recombine with compatible sites.

Kozak: Kozak consensus sequence. It is placed in front of the start codon of the ORF of interest because it is believed to facilitate translation initiation in eukaryotes.

ORF #1: The open reading frame of a gene of interest is placed here, in a sense orientation. This gene can be expressed without Cre-mediated recombination.

ORF #2: The open reading frame of a gene of interest is placed here, in an antisense orientation. This gene can only be expressed after Cre-mediated recombination.

SV40 late pA: Simian virus 40 late polyadenylation signal. It facilitates transcriptional termination of the upstream ORF.

3' ITR: Tol2 3' terminal repeat.

Ampicillin: Ampicillin resistance gene. It allows the plasmid to be maintained by ampicillin selection in E. coli.

pUC ori: pUC origin of replication. Plasmids carrying this origin exist in high copy numbers in E. coli.

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