PiggyBac FLEX Conditional Gene Expression Vector (Cre-Switch)

The piggyBac FLEX conditional Cre-Switch gene expression vector combines VectorBuilder’s highly efficient piggyBac 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 piggyBac vector system is technically simple, utilizing plasmid transfection (rather than viral transduction) to permanently integrate your gene(s) of interest into the host genome. The piggyBac FLEX conditional Cre-Switch gene expression system contains two vectors, both engineered as E. coli plasmids. One vector, referred to as the helper plasmid, encodes the transposase. The other vector, referred to as the transposon plasmid, contains two terminal repeats (TRs) bracketing the region to be transposed. The FLEX Cre-Switch described above is cloned into this region.

When the helper and transposon plasmids are co-transfected into target cells, the transposase produced from the helper would recognize the two TRs on the transposon and insert the flanked FLEX Cre-on switch including the two TRs into the host genome. Insertion typically occurs at host chromosomal sites that contain the TTAA sequence, which is duplicated on the two flanks of the integrated fragment. 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.

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.

PiggyBac 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.) Because the helper plasmid is only transiently transfected into host cells, it will get lost over time. With the loss of the helper plasmid, the integration of the transposon in the genome of host cells becomes permanent. If these cells are transfected with the helper plasmid again, the transposon could get excised from the genome of some cells, footprint free.

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

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 piggyBac 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 transposon vector can accommodate ~30 kb of total DNA. The plasmid backbone and transposon-related sequences only occupies about 3.1 kb, leaving plenty of room to accommodate the user's sequence of interest.

Limited cell type range: The delivery of piggyBac 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 piggyBac system.

5' ITR: 5' inverted terminal repeat. When a DNA sequence is flanked by two ITRs, the piggyBac transpose 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.

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.

rBG pA:  Rabbit beta-globin polyadenylation signal. It facilitates transcriptional termination of the upstream ORF.

CMV promoter: Human cytomegalovirus immediate early promoter. It drives the ubiquitous expression of the downstream marker gene.

Marker: A drug selection gene (such as neomycin resistance), a visually detectable gene (such as EGFP), or a dual-reporter gene (such as EGFP/Neo). This allows cells transduced with the vector to be selected and/or visualized.

BGH pA: Bovine growth hormone polyadenylation signal. It facilitates transcriptional termination of the upstream ORF.

3' ITR: 3' inverted 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.