Simone Lawson
Gene expression is the process by which a gene's information is utilised to construct a cell's functionality. Constitutive and inducible expression are two types of gene expression. The key difference between the two is that constitutive expression is the expression of a constitutive gene at a constant level, whereas inducible expression is the expression of an inducible gene under certain conditions only. Constitutive genes are always 'on' and are involved in processes vital for cell function and survival. Inducible genes, on the other hand, are expressed only when there is a need for their products, and they are activated by specific stimuli such as chemical agents, temperature, mechanical injury, or light. Inducible gene expression systems are often preferred in research due to their reversible nature, higher efficiency, and fewer side effects.
| Characteristics | Values |
|---|---|
| Constitutive expression | Expression of constitutive genes at a constant level |
| Expression of constitutive genes in an ongoing manner without regulation | |
| Constitutive genes are always on | |
| Constitutive promoters are active in a cell under all circumstances | |
| Constitutive promoters carry out a continuous transcription process | |
| Constitutive genes are mainly housekeeping genes involved in processes vital for cell function and survival | |
| Inducible expression | Expression of inducible genes under certain conditions only |
| Expression of inducible genes under certain conditions when there is a need for their products | |
| Inducible promoters can be switched from an off to an on state | |
| Inducible promoters are regulated by positive or negative control | |
| Inducible gene expression is mostly reversible and thus more flexible to use | |
| Inducible gene expression generally exhibits higher efficiency and has fewer side effects |
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What You'll Learn
- Constitutive expression is continuous, inducible expression is conditional
- Inducible expression is reversible, flexible, and efficient
- Inducible promoters can be regulated by positive or negative control
- Constitutive promoters are always active
- Chemical agents, temperature, mechanical injury, and light can act as stimuli for inducible promoters
Constitutive expression is continuous, inducible expression is conditional
Gene expression takes place via two steps: transcription and translation. Constitutive expression is the continuous expression of a constitutive gene, which remains "on" all the time, producing products at a constant rate. These genes are involved in processes vital for cell function and the survival of organisms, such as glycolysis, the citric acid cycle, transcription, and translation. Constitutive promoters are active in a cell under all circumstances, carrying out a continuous transcription process.
On the other hand, inducible expression is conditional. It is the expression of inducible genes, which are only expressed under certain conditions when there is a necessity for their products. For example, when a particular substrate is present and needs to be metabolized, inducible genes will express the required products for metabolism. Inducible promoters are activated only when they receive a specific stimulus, such as chemical agents, temperature changes, mechanical injury, or light. After activation, they initiate the transcription process by binding to the RNA polymerase and transcriptional factors.
Constitutive promoters, such as CMV, EF1A, and SV40 promoters, are always active. In contrast, inducible promoters like the ones found in bacteria can be switched from an "OFF" to an "ON" state. Temperature-sensitive expression systems, for instance, remain inactive at regular temperatures but can be induced by heat or cold exposure. Light-sensitive inducible promoters are also available, where the absence or presence of light activates or inhibits gene expression.
Inducible gene expression systems are often favored over stable expression systems in research areas like functional genomics, gene therapy, and tissue engineering due to their reversible nature, higher efficiency, and fewer side effects.
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Inducible expression is reversible, flexible, and efficient
Inducible expression is a highly flexible and efficient process that can be reversed. Inducible promoters can be switched from an OFF to an ON state, and they are only active under specific circumstances. This means that inducible gene expression can be activated or suppressed temporarily and quantitatively, depending on the requirements of the experiment.
Inducible promoters are activated only when they receive a specific stimulus, such as chemical agents, temperature changes, mechanical injury, or light. For example, the Hsp70 promoter is only expressed following exposure to a brief heat shock. In the case of bacterial expression vectors, the presence of lactose or lactose analog IPTG acts as a stimulus, causing the lac repressor to undergo a conformational change that removes it from the promoter and initiates gene expression.
The Tet-on and Tet-off configurations of the cumate system are commonly used inducible systems. In the Tet-off configuration, the addition of tetracycline disrupts the association between TetR and TetO, triggering gene expression. The cumate operator (CuO) and its repressor (CymR) are also used in three configurations, with the repressor configuration suppressing downstream gene expression.
Inducible expression is often favored over stable expression systems due to its reversibility and flexibility. It generally exhibits higher efficiency and has fewer side effects, such as cell death and delayed growth or development.
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Inducible promoters can be regulated by positive or negative control
Positive control of inducible promoters can be observed in the case of the heat shock-inducible Hsp70 or Hsp90-derived promoters. A brief heat shock induces the expression of the gene of choice. This heat shock releases heat shock factor 1 (HSF-1), which binds to the promoter, activating transcription. The red flame plasmid pDawn contains the blue-light-sensing protein YFI, which induces transcription of the phage repressor cI in the absence of light. This repressor inhibits transcription from the phage promoter pR, preventing the expression of a reporter gene.
Negative control of inducible promoters can be observed in the case of the pBad promoter, a popular prokaryotic promoter used for bacterial protein purification. When arabinose is absent, the regulatory protein AraC binds to sites upstream of pBad, blocking transcription. The addition of arabinose causes AraC to bind to different sites, allowing transcription to begin. Another example of negative control is the pLac promoter, which requires the removal of the lac repressor (lacI protein) for transcription to be activated. In the presence of lactose or lactose analogue IPTG, the lac repressor undergoes a conformational change, removing it from the promoter and ceasing repression of the target gene.
Inducible promoters are useful tools in research, allowing for the activation of specific genes only when needed.
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Constitutive promoters are always active
The main difference between constitutive and inducible expression is that constitutive expression occurs at a constant level, while inducible expression only occurs under certain conditions. Constitutive genes are always active and are expressed continuously in the cell, producing their products all the time. These genes are involved in processes that are vital for cell function and the survival of organisms, such as glycolysis, the citric acid cycle, transcription, and translation.
Constitutive promoters are a type of regulatory promoter that helps in the effective expression of the desired gene. Unlike inducible promoters, which require a specific stimulus to activate, constitutive promoters are always active and do not require transcriptional factors to initiate the transcription process. They carry out a continuous transcription process in the desired gene. Some common promoters that are always active and are thus considered constitutive include CMV, EF1A, and SV40 promoters.
The constant activity of constitutive promoters is distinct from inducible promoters, which can switch from an "OFF" to an "ON" state. Inducible promoters require the presence of specific regulatory substances, such as inducers or activators, to initiate gene expression. For example, the lac operon in bacteria is an inducible gene that is expressed when a particular substrate needs to be metabolized.
The choice between using constitutive or inducible promoters depends on the specific experimental requirements and designs. Inducible gene expression systems are often favored due to their reversibility, higher efficiency, and reduced side effects. However, constitutive promoters may be preferred in certain cases, such as when using minimal promoters for the induction of a specific gene.
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Chemical agents, temperature, mechanical injury, and light can act as stimuli for inducible promoters
The key difference between constitutive and inducible expression is that constitutive expression involves the expression of a constitutive gene at a constant level, while inducible expression involves the expression of an inducible gene under certain conditions only.
Constitutive genes are always active and are expressed continuously in the cell, producing their products all the time at a constant rate. They are mainly housekeeping genes involved in processes vital for cell function and the survival of organisms.
Inducible genes, on the other hand, are expressed only under specific circumstances when their products are needed. For example, certain substrates may be present and need to be metabolized, or there may be an insufficient amount of a particular molecule within the cell.
Inducible promoters can be switched from an OFF to an ON state. In the OFF state, the promoter is inactive because a bound repressor protein actively prevents transcription. Inducible promoters can be activated by various stimuli, including chemical agents, temperature, mechanical injury, and light.
Chemical Agents as Stimuli
Chemically regulated promoters are among the most common inducible promoters. The positive inducible tetracycline ON (Tet-On) system is a versatile tool developed for use in prokaryotes and eukaryotes. In this system, the activator rtTA (reverse tetracycline-controlled transactivator) is normally inactive and cannot bind to the tetracycline response elements (TRE) in a promoter. Once an inducer binds the repressor protein, the repressor protein is removed from the DNA, allowing transcription to turn ON. Other examples of chemically induced promoters include positive inducible alcohol and steroid-regulated promoters commonly used in plant research.
Temperature as a Stimulus
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 Hsp90-derived promoters, in which a gene of choice is only expressed following exposure to a brief heat shock.
Light as a Stimulus
Light is another way to activate gene expression. Two-component systems used in synthetic biology use light to regulate transcription. Red flame plasmid pDawn contains the blue-light-sensing protein YFI. When light is absent, YFI phosphorylates FixJ, which binds to the FixK2 promoter to induce transcription of the phage repressor cI. 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.
Mechanical Injury as a Stimulus
While not discussed in the sources provided, mechanical injury is also mentioned as a stimulus for inducible promoters.
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Frequently asked questions
Constitutive expression refers to the expression of constitutive genes at a constant rate in a cell, regardless of the environmental conditions. In contrast, inducible expression refers to the expression of inducible genes under certain conditions.
A constitutive gene is a gene that is expressed continuously in the cell and produces its products all the time at a constant rate. These genes are mainly housekeeping genes involved in processes vital for cell function and the survival of organisms.
An inducible gene is a gene expressed under certain conditions when there is a need for its products. For example, when a particular substrate is present, inducible genes express and produce the required products to metabolize it.
Usually, chemical agents, temperature, mechanical injury, and even light act as stimuli for inducible promoters.
Inducible gene expression systems are favored over stable expression systems because they are mostly reversible and thus more flexible to use. They also generally exhibit higher efficiency and have fewer side effects, such as cell death and delayed growth or development.
