What is the Difference Between Activator Promoter and Repressor

The key difference between activator promoter and repressor is their function. An activator facilitates the upregulation of the transcription process by binding to enhancers, while promoter is the site at which RNA polymerase binds, and transcription initiation takes place, and repressor downregulates transcription by binding to silencers.

Transcription is the process of synthesizing the complementary mRNA using a DNA template. The process takes place in the nucleus in eukaryotes, while in prokaryotes, it takes place in the cytoplasm. Transcription has three main stages: initiation, elongation, and termination. Activators and repressors regulate transcription initiation, while promoters are important in the binding of RNA polymerase. RNA polymerase is the main enzyme that mediates transcription.

CONTENTS

1. Overview and Key Difference
2. What is Activator  
3. What is Promoter 
4. What is Repressor
5. Similarities – Activator Promoter and Repressor
7. Activator vs Promoter vs Repressor in Tabular Form
8. Summary – Activator vs Promoter vs Repressor

What is an Activator?

Activator is a transcriptional factor that binds to the enhancer regions to upregulate transcription. Once bound, it results in a change in the orientation of the template strand, activating or upregulating transcription. Furthermore, it will promote the binding of RNA polymerase to the promoter site to initiate transcription.

Activators are often protein molecules. They promote the anabolic pathway of gene expression. Activator function is predominantly observed in eukaryotic transcriptional regulation.

What is a Promoter?

The promoter region of a gene is the region where the RNA polymerase binding takes place at the initiation of the transcription. Promoter regions in prokaryotes and eukaryotes vary. Promoter activation is an important step in transcriptional control and activation. Prokaryotic promoter regions are present in the -35 and -10 regions upstream of the transcription start site. The -10 promoter region is also called the TATA box or the pribnow box. The sigma factor of bacterial RNA polymerase recognizes the promoter and binds to form the closed promoter complex. The unwinding of the double strands takes place next, resulting in an open promoter complex. Finally, the sigma factor of RNA polymerase leaves starting the transcription.

In eukaryotes, the promoter activation takes place through multiple transcription initiation factors. There are two main types of promoters in eukaryotes. They are TATA-less promoters and TATA promoters.

What is a Repressor?

A repressor is a transcription factor that downregulates the process of transcription by binding to silencer regions. This prevents the binding of RNA polymerase to the promoter. This is the key unit of the cell’s mechanism to stop transcription. Here, the repressor binds to the silencer region and blocks the process of transcription by reducing the rate and completely stopping the process.

Transcriptional repressors respond to external stimuli. This also prevents the binding of activator promoters to promoter regions. Corepressors are a sub-category of repressors and repress transcription initiation through the recruitment of histone deacetylase. Histone deacetylase increases the positive charge on histone protein. This strengthens the interaction between DNA and histones and reduces the accessibility of DNA for transcription.

What are the Similarities Between Activator Promoter and Repressor?

• Activators, promoters, and repressors are involved in the transcription process.
• Their main role is during the initiation of transcription.
• Moreover, they act as transcriptional regulatory factors.
• All facilitate the accurate binding of RNA polymerase.

What is the Difference Between Activator Promoter and Repressor?

Activator facilitates upregulation of the transcription process by binding to enhancers. Meanwhile, RNA polymerase binds with the promoter region during the transcription initiation. Whereas, the repressor downregulates the transcription process by binding to silencers. Thus, this is the key difference between activator promoter and repressor. Moreover, activators and repressors are often proteins that regulate transcription while promoter is an element of DNA present in the upstream areas of the gene. Besides, both activators and promoters enhance the process of transcription while repressors inhibit the process of transcription.

The below infographic presents the differences between activator promoter and repressor in tabular form for side by side comparison.

Summary – Activator vs Promoter vs Repressor

Activator, promoter, and repressor are transcriptional factors that affect the rate of transcription. The key difference between activator promoter and repressor is that an activator facilitates the upregulation of the transcription process by binding to the enhancers, while promoter facilitates the binding of RNA polymerase during the transcription initiation and repressors downregulate transcription by binding to silencers. Activators and repressors are often protein molecules that bind to the regulatory regions in the gene and cause orientation changes in the DNA. Promoters are sites upstream. RNA polymerase binding takes place at the promoter sites. So, this is the difference between activator promoter and repressor.

Reference:

1. “Biology 2e, Genetics, Gene Expression, Prokaryotic Gene Regulation.” OpenEd CUNY.
2. Segel M;Lash B;Song J;Ladha A;Liu CC;Jin X;Mekhedov SL;Macrae RK;Koonin EV;Zhang F; “Mammalian Retrovirus-like Protein peg10 Packages Its Own Mrna and Can Be Pseudotyped for Mrna Delivery.” Science (New York, N.Y.), U.S. National Library of Medicine.

Image Courtesy:

1. “Figure 16 04 01” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Regulation of transcription in mammals” By Bernstein0275 – Own work (CC BY-SA 4.0) via Commons Wikimedia
3. “Figure 16 02 01” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia