The PiggyBac inducible gene expression vector combines VectorBuilder’s highly efficient PiggyBac vector system and the Tet-On inducible gene expression system to help you achieve transfection-based permanent integration of tetracycline-inducible gene expression cassettes into the host genome.

The Tet-On inducible gene expression system is a powerful tool to control the timing of expression of the gene(s) of interest in mammalian cells. Our Tet-On inducible gene expression vectors are designed to achieve nearly complete silencing of the gene(s) of interest in the absence of tetracycline and its analogs (e.g. doxycycline), and strong, rapid expression in response to the addition of tetracycline or one of its analogs (e.g. doxycycline). This is achieved through a multicomponent system that incorporates active silencing by the tTS protein in the absence of tetracycline and strong activation by the rtTA protein in the presence of tetracycline. In the absence of tetracycline, the tTS protein derived from the fusion of TetR (Tet repressor protein) with KRAB-AB (the transcriptional repressor domain of Kid-1 protein) binds to the TRE promoter leading to the active suppression of gene transcription. The rtTA protein, on the other hand, derived from the fusion of a mutant Tet repressor to VP16 (the transcription activator domain of virion protein 16 of herpes simplex virus), binds to the TRE promoter to activate gene transcription only in the presence of tetracycline.

While our standard piggyBac inducible gene expression vector expresses tTS and rtTA as a fusion protein, which acts as a gene activation switch, the low leak version of our piggyBac Tet-On vector is designed to allow tissue-specific induction of target transgenes in the presence of tetracycline while minimizing leaky expression in non-target tissues in the absence of tetracycline. There are three expression cassettes in this vector: 1) the GOI driven by the TRE promoter, 2) the tTS gene driven by a ubiquitous promoter, and 3) the rtTA gene driven by a user-selected tissue-specific promoter. In the absence of tetracycline, the tTS protein, ubiquitously expressed in all tissues, binds to the TRE promoter with high affinity thereby suppressing the GOI expression in all tissues. In the presence of tetracycline, the rtTA protein, which is specifically expressed in the target tissue, can bind to the TRE promoter to activate the GOI expression only in the target tissue.

Our piggyBac inducible gene expression system comprises two components: the transposon plasmid and the transposase (helper PBase). The transposon plasmid contains two terminal repeats (TRs) bracketing the region to be transposed. For the low leak version of this vector, all three expression cassettes consisting of the GOI, the tTS gene, and the rtTA gene described above are 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 helper PBase and the piggyBac transposon vector are co-introduced into target cells, the transposase produced from the helper would recognize the two TRs on the transposon and insert the flanked region 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. Gene expression can then be turned on in the presence of tetracycline. 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.

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.) If the transposase is reintroduced into the cells, 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.

Our Tet-On inducible gene expression vectors are designed to achieve nearly complete silencing of the gene(s) of interest in the absence of tetracycline, and strong, rapid expression in response to the addition of tetracycline. The low leak version of our piggyBac Tet-On vector is an improved version that helps to achieve tissue-specific induction of target transgenes in the presence of tetracycline, while minimizing leaky expression in non-target tissues. The piggyBac inducible gene expression vector along with the helper plasmid 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 gene activation: Unlike rtTA only Tet-On systems that usually have significant leaky expression in the absence of induction, our Tet-On gene expression vectors act as true tetracycline-regulated on-and-off switch for controlling gene expression, which can minimize the background expression without induction and result in high sensitivity and high dynamic range of the tetracycline induction.

Minimized leaky expression in non-target tissues: The low leak version of our piggyBac Tet-on vector incorporates a ubiquitous CBh promoter to drive the expression of the tTS protein, which binds to TRE in the absence of tetracycline thereby inhibiting transgene expression in non-target tissues.

Tissue-specific induction: The low leak version of our piggyBac Tet-on vector utilizes a user-selected tissue-specific promoter for driving tetracycline-induced gene expression specifically in the target tissues of interest.

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: The piggyBac inducible gene expression vector can be introduced into mammalian cells by conventional transfection. 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, transposon-related sequences and the Tet-On components occupy only about 6.5 kb, leaving plenty of room to accommodate the user's GOI and promoter for driving the rtTA protein.

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. Additionally, plasmid transfection is largely limited to in vitro applications and rarely used in vivo. 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. 

TRE: Tetracycline-responsive element promoter (2nd generation). This element can be regulated by a class of transcription factors (e.g. tTA, rtTA and tTS) whose activities are dependent on tetracycline or its analogs (e.g. doxycycline).

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

ORF: The open reading frame of your gene of interest is placed here.

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

Promoter: The promoter chosen to drive expression of the rtTA protein.

rtTA: Reverse tetracycline responsive transcriptional activator M2 (2nd generation). This protein binds to TRE promoter to activate gene transcription only in the presence of tetracycline or its analogs (e.g. doxycycline). It has higher sensitivity to the inducing drug and lower leaky activity in the absence of the drug compared to its predecessor.

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

CBh promoter: CMV early enhancer fused to modified chicken β-actin promoter. This drives the expression of the downstream tTS protein.

tTS: Tetracycline-controlled transcriptional silencer. This protein binds to TRE promoter to actively suppress gene transcription only in the absence of tetracycline and its analogs (e.g. doxycycline).

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

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.