The main difference between inducible and repressible operons is that the inducible operons are turned off under normal conditions while the repressible operons are turned on under normal conditions. Furthermore, the binding of the inducer to the active repressor of inducible operons causes the inactivation of the repressor and the binding of RNA polymerase to the promoter region while binding of the co-repressor to the inactive repressor of the repressible operons causes the activation of the repressor, which prevents the binding of the RNA polymerase to the promoter region.

Inducible and repressible operons are two types of operons in the prokaryotic genome. An operon is a cluster of functionally-related genes regulated under a common promoter. Moreover, lac operon is such an inducible operon while trp operon is a repressible operon.

Key Areas Covered

1. What are Inducible Operons
     – Definition, Characteristics, Examples
2. What are Repressible Operons
     – Definition, Characteristics, Examples
3. What are the Similarities Between Inducible and Repressible Operons
     – Outline of Common Features
4. What is the Difference Between Inducible and Repressible Operons
     – Comparison of Key Differences

Key Terms

Co-Repressor, Inducer, Inducible Operons, Prokaryotic Gene Structure, Repressible Operons, Repressor

What are Inducible Operons

Inducible operons are a type of operons in prokaryotes, which turn on with the binding of an effector molecule called the inducer to the repressor region of the operon. Generally, this type of operons are kept turned off, and the activation of the repressor occurs with the binding of the inducer. Hence, inducible operons become active in the presence of the substrate.

Figure 1: The Lac Operon

The lac operon of prokaryotes is such that an inducible operon kept turned off in the presence of glucose. It occurs by the binding of the repressor region to the operator region of the operon. However, in the absence of glucose, the allolactose, a converted form of a lactose, serves as the inducer, which binds to the repressor region. 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.

What are Repressible Operons

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.

Figure 2: The Trp Operon

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.

Similarities Between Inducible and Repressible Operons

• Inducible and repressible operons are two types of operons in the prokaryotic gene structure.
• Both contain functionally-related genes in a contiguous manner, in the genome.
• Also, the regulation of the genes in both operons are under common regulatory elements.
• Furthermore, their differential regulation is by the type of effector molecule, which binds to the repressor. When activated, the repressor binds to the operator region of the promoter, making the RNA polymerase bind with the promoter region.

Difference Between Inducible and Repressible Operons

Definition

Inducible operons refer to the gene system, which encodes a coordinated group of enzymes responsible for catabolic pathways. An early metabolite in the pathway causes activation by interacting of a repressor of the transcription. In contrast, repressible operons refer to the gene system responsible for the synthesis of a coordinated group of enzymes responsible in a single synthetic pathway. The excess quantities of the end product of the pathway leads to cessation of transcription. Thus, this explains the main difference between inducible and repressible operons.

Effector Molecule

Another difference between inducible and repressible operons is that, in inducible operons, inducer binds to the repressor while in repressible operons, co-repressor binds to the repressor.

Repressor

The repressor of the inducible operons is active under normal conditions while the repressor of the repressible operons is inactive under normal conditions. It is a major difference between inducible and repressible operons. 

Effect on Transcription

Another difference between inducible and repressible operons is that the inducer turns on the transcription of the inducible operon while the co-repressor turns off the transcription of the repressible operon.

Type of Metabolic Pathway

Furthermore, the anabolic pathways employ the inducible operons while, the catabolic pathways employ the repressible operons.

Importance

Inducible operons turn on only in the presence of their substrate while the end product of repressible operons serves as a feedback inhibitor of the operon. It is another important difference between inducible and repressible operons.

Examples

The lac operon is an example of an inducible operon while the trp operon is an example of a repressible operon.

Conclusion

Inducible operons are generally turned off by binding a repressor to the operator region of the promoter. The binding of the inducer, which is an early metabolite in the pathway of the gene products of the operon, is responsible for the inactivation of the repressor, allowing the transcription of the operon. On the other hand, repressible operons are generally turned on, and their repressor remains inactive. Therefore, the operator region of these operons remain free, and their transcription occurs under normal conditions. But with the binding of the co-repressor, which is the end product of the pathway, the repressor becomes active and binds to the operator region, preventing the binding of the RNA polymerase to the promoter region. Therefore, the main difference between inducible and repressible operons is the type of activity of the operons.

Reference:

1. “Prokaryotic Gene Regulation.” Lumen|Boundless Biology, Lumen Candela, Available Here

Image Courtesy:

1. “The lac Operon: An Inducer Operon” By OpenStax CNX (CC BY 3.0) via OpenStax Collage
2. “The trp Operon: A Repressor Operon” By OpenStax CNX (CC BY 3.0) via OpenStax Collage

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