What is the difference between an inducible and repressible promoter

Chemically regulated promoters are among the most common inducible promoters. The positive inducible tetracycline ON Tet-On system, a versatile tool developed for use in prokaryotes and eukaryotes, works via direct activation. In this system, the activator rtTA reverse tetracycline-controlled transactivator is normally inactive and cannot bind the tetracycline response elements TRE in a promoter. Tetracycline and its derivatives serve as inducing agents to allow promoter activation.

One of the most commonly used prokaryotic promoters is the negative inducible pLac promoter. This promoter requires removal of the lac repressor lacI protein for transcription to be activated. In the presence of lactose or lactose analog IPTG, the lac repressor undergoes a conformational change that removes it from lacO sites within the promoter and ceases repression of the target gene. A simplified lac inducible system is found in many bacterial expression vectors. Negative inducible promoter pBad is another popular prokaryotic promoter often used for bacterial protein purification.

When arabinose is absent, regulatory protein AraC binds O and I1 sites upstream of pBad, blocking transcription. The addition of arabinose causes AraC to bind I1 and I2 sites, allowing transcription to begin. Supplementing cell growth media with glucose decreases cAMP and represses pBad, decreasing promoter leakiness.

Other examples of chemically induced promoters include positive inducible alcohol and steroid regulated promoters commonly used in plant research. Temperature sensitive expression systems are typically less leaky than chemically induced promoters; they show near-zero expression at regular temperatures but can be induced by heat or cold exposure. Examples include the heat shock-inducible Hsp70 or Hspderived promoters, in which a gene of choice is only expressed following exposure to a brief heat shock.

In the case of Hsp70, the heat shock releases heat shock factor 1 HSF-1 , which subsequently binds to heat shock elements in the promoter, thereby activating transcription. Addgene depositors have developed heat shock-inducible Cre and Cas9 for easy genome engineering in species like C. Light is another way to activate gene expression, and two-component systems used in synthetic biology use light to regulate transcription.

Red flame plasmid pDawn contains the blue-light sensing protein YFI. Repressor cI inhibits transcription from phage promoter pR, preventing expression of a reporter gene.

When light is present, YFI is inactive, preventing repressor cI synthesis and allowing reporter gene transcription to take place. The Tet-Off system, a positive repressible promoter, was engineered from the bacterial tet operon. In the native system, the tetracycline repressor TetR can bind to the tetracycline operator sequences TetO , preventing transcription.

To turn this inducible system into a repressible system, Gossen and Bujard created the tetracycline-controlled transactivator tTA by fusing TetR with the transcriptional activation domain VP When tetracycline or one of its derivatives is added, it binds tTA, removing it from the promoter and turning transcription OFF. Despite their bacterial origins, Tet systems function well in mammalian cells, and TRE-containing promoters can be used in the repressible manner described above, as well as the inducible manner detailed in our previous post.

For more information on Addgene Tet plasmids, see our Tetracycline resource page. The ADH1 negative repressible promoter is commonly used in yeast. As ethanol accumulates, it binds to the repressor, enabling it to bind the promoter and repress pADH1 activity. Binary systems permit exquisite control of gene expression and tracing of gene expression across development.

In this way, you can interrogate the activity of uncharacterized promoters. Similarly, placing UAS upstream of a transgene permits directed expression of that gene in cells that also express GAL4.

Potter et al. QF-mediated repression is reversible by the addition of quinic acid to Drosophila cells. Work by Wei et al. Subsequent work by the Luo and Potter labs has refined the Q system and introduced the second generation QF2 activators. Also, this binding changes the conformation of the repressor, detaching it from the operator. Furthermore, it allows the binding of the RNA polymerase to the promoter region. Hence, the lac operon turns on its transcription.

Here, the lac operon encodes for the enzymes required by the break down of lactose into glucose and galactose. Repressible operons are the other type of operons in prokaryotes, which turn off with the binding of the effector molecule called the co-repressor to the repressor region of the operon. Repressible operons are kept turned on.

Hence, the repressor is inactive under normal conditions. The binding of the co-repressor to the repressor causes the activation and the binding of the repressor with the operator site of the repressible operon. Thus, this results in the turn off of the transcription of this type of operons. The trp operon of prokaryotes is an example of such repressible operons usually kept turned on. The gene products of the trp operon are responsible for the biosynthesis of tryptophan, an amino acid inside the cell starting from chorismate.

However, when the cell has an excess amount, tryptophan binds to the inactive repressor, activating it. The activated repressor binds to the operator region of the trp operon, preventing the binding of the RNA polymerase to the promoter region. In turn, this will turn off the transcription of the operon. That means; the end product of the repressible operon serves as the feedback inhibitor for the transcription of the operon. Inducible operons refer to the gene system, which encodes a coordinated group of enzymes responsible for catabolic pathways.