Inducible Gene Expression Solutions

Tetracycline-inducible gene expression

The tetracycline (Tet) inducible expression system allows precise temporal control of expression of your gene of interest (GOI), shRNA, or other ORF in target cells. This system is especially useful when studying toxic genes, genes whose expression must be tightly regulated for normal cellular function, or genes that are otherwise not well-tolerated by the cell. In Tet-inducible systems, the ORF is placed under the control of a Tet-responsive promoter (Tet-responsive element, or TRE). Binding of Tet regulatory proteins to this promoter, and therefore expression of the ORF, is controlled by the presence or absence of tetracycline or its analogs, resulting in reversible and dose-dependent regulation of gene expression, as illustrated in Figure 1. Tetracycline analogs such as doxycycline (dox) are commonly used as inducers due to their stability and potency. Tetracycline and its analogs can be used to either turn gene expression on (Tet-On) or off (Tet-off). 

Tet-On vector systems (Figure 1, left) are designed to exhibit strong silencing of an ORF in the absence of tetracycline and strong expression in response to the addition of tetracycline. This is achieved through a multicomponent system that incorporates the tetracycline-controlled transcriptional silencer (tTS) and the reverse tetracycline transactivator (rtTA). In the absence of tetracycline, the tTS protein derived from the fusion of TetR (Tet repressor protein) and KRAB-AB (transcriptional repressor domain of Kid-1 protein) binds to the TRE promoter, leading to the active suppression of ORF transcription. The rtTA protein, on the other hand, derived from the fusion of rTetR (a mutant Tet repressor) and VP16 (transcriptional activator domain of virion protein 16 of HSV), binds to the TRE promoter to activate gene transcription only in the presence of tetracycline. Unlike Tet-On systems relying only on rtTA, which usually have significant leaky expression without induction, our Tet-On vector system expressing tTS/rtTA acts as a true tetracycline-regulated on-and-off switch for controlling gene expression, minimizing background expression without induction and ensuring robust, dynamic, and scalable induction by tetracycline (Figure 2).

The Tet-Off vector system (Figure 1, right) inhibits target gene expression in the presence of tetracycline. In the absence of tetracycline, tetracycline-controlled transactivator (tTA) binds to the TRE promoter resulting in transcriptional activation. When tetracycline is added, it binds to tTA, resulting in a conformational change that prevents tTA from binding to TRE and causes transcriptional repression.

Figure 1. Mechanisms of Tet-regulated gene expression in Tet-On and Tet-Off systems. Dox: tetracycline analog doxycycline.

All-in-one vs. dual vector

To achieve Tet-regulated induction or inhibition of your ORF, it is essential for the target cells to co-express the ORF driven by the TRE promoter and the Tet regulatory proteins (tTS and rtTA or tTA) at the same time. This can be accomplished by either expressing both the ORF and the Tet regulatory protein(s) from the same vector (using an all-in-one vector) or by using separate vectors: one expressing the ORF driven by TRE and one expressing Tet regulatory protein(s) (dual vectors). All-in-one vectors provide a system that is technically straightforward and ideal for difficult-to-transfect cells. Conversely, using separate vectors to co-express the ORF and the Tet regulatory proteins requires co-transfection or co-transduction of the target cells with two separate plasmids or viruses, which can be technically challenging. An alternative approach to using dual vectors is to first create cells or organisms stably expressing the Tet regulatory proteins, then introduce a vector expressing the ORF. However, this method can be considerably time consuming and labor intensive. Despite its challenges, the dual vector system is sometimes preferred since it offers the flexibility of using the same Tet regulatory protein expressing plasmid, virus, or cell line with multiple different overexpression constructs.

Tissue-specific Tet-inducible gene expression vector

VectorBuilder offers non-viral all-in-one (Tet-On) vectors in two formats, standard and low-leak, to help you achieve effective induction of your GOI, particularly when requiring tissue-specific induction. While the standard Tet-On vector system expresses tTS and rtTA as a fusion protein driven by a user-selected promoter, the low-leak Tet-On vector system utilizes a ubiquitous promoter to drive tTS expression and a user-selected (e.g. tissue-specific) promoter to drive rtTA expression. By repressing GOI expression in all transduced cells while restricting activator expression to only target cells, the low-leak Tet-On design minimizes expression in non-target tissues in both the presence and absence of tetracycline and ensures robust, specific activation in target tissues when tetracycline is present.

One of the key factors underlying the design of any successful gene delivery experiment is the choice of the vector system used for delivering the components for Tet-inducible expression into target cells. Given the variety of viral and non-viral vector options available, it is important to consider several factors when choosing the ideal delivery system, such as the ease of transfection of the target cells, whether transient expression or stable genomic integration is desired, the intended application in cell culture or in vivo, and the size of the ORF and other components.

The table below lists the commonly used vector systems and key considerations for selecting the system most suitable for your experimental design.

IPTG-inducible systems

In addition to the Tet system, the IPTG (isopropyl-β-D-thiogalactoside) inducible system is another widely used method for temporally-controlled gene expression. The IPTG-inducible vector utilizes the interaction between LacI (repressor) protein and LacO operator sequence, derived from the bacterial lactose operon to regulate gene expression in the presence or absence of the lactose analogue, IPTG.

In this system, one or more operator sequences (LacO) are present either within or adjacent to the ORF promoter. In the absence of IPTG, the LacI protein binds to the operator sequence and blocks RNA polymerase from initiating transcription. When IPTG is present, it binds to LacI and causes a conformational change, releasing it from the operator region. This removal of repression allows RNA polymerase to initiate transcription of the target gene, enabling tight regulation of gene expression by minimizing background promoter activity in the absence of IPTG.

Other inducible systems

Beyond Tet and IPTG, several other methods including drug-inducible systems, RNA-based switches, and physical stimulants can be used for inducible gene expression. In drug-inducible systems, transcriptional activity is typically controlled by a ligand-binding domain/receptor fused to a regulatory protein, to provide tight control of GOI expression only in the presence of the ligand; for example, estrogen receptor- or glucocorticoid receptor-based systems can be activated by tamoxifen or glucocorticoids such as dexamethasone, respectively. Drug-inducible systems are often combined with conditional expression systems to provide tight temporal and spatial control, for instance using CreERT2. Alternatively, riboswitches are RNA-based, cis-acting regulatory elements that control gene expression at the RNA level upon binding to a ligand, while physical induction systems use external stimuli like light or oxygen levels (e.g. using hypoxia responsive elements) or temperature shifts (using heat shock protein promoters) associated with transcriptional regulatory elements to turn ORF expression on or off.

The Tet-inducible gene expression system has gained popularity over other available inducible gene expression systems since it offers several advantages, as summarized in the table below:

Visit Vector Academy for educational resources to help you successfully plan, execute, and troubleshoot your Tet-inducible gene expression experiments. Additionally, you can find helpful guides on Tet-inducible vector systems here and Tet-inducible vector components here.