How Do Activators and Repressors Affect Transcription

Activators and repressors are the two types of transcription factors involved in the regulation of gene expression at the transcription level. Activators bind to the enhancer regions in the DNA and facilitate the binding of RNA polymerase to the promoter. Repressors bind to the silencer regions and prevent the binding of RNA polymerase to the promoter.

The regulation of transcription is vital in the regulation of gene expression in both prokaryotes and eukaryotes. Transcription factors are the proteins responsible for the regulation of gene expression at the transcriptional level. They bind to the transcription control elements in DNA. Depending on the function, transcription factors can be categorized either as activator or repressors. The role of activators and repressors during the transcription of both eukaryotes and prokaryotes is described in this article.

Key Areas Covered

1. What are Activators and Repressors
     – Definition, Characteristics
2. How Do Activators and Repressors Affect Transcription
    – Role of Activators and Repressors in Transcription

Key Terms: Activators, Enhancers, Repressor, Silencers, Transcription, Transcription Factors

What are Activators and Repressors

Activators and repressors are two types of transcription factors that regulate the gene expression at the transcriptional level. Transcription factors are trans-acting regulatory proteins, determining the time, location, and the efficiency of transcription. The mechanism of action of transcription factors is to promote or prevent the binding of RNA polymerase to the promoter sequence of the gene. RNA polymerase is the enzyme responsible for the synthesis of a mRNA molecule by transcribing the coding region of a gene. Activators facilitate the binding of RNA polymerase to the promoter while repressors prevent the binding of the enzyme to the promoter.

How Do Activators and Repressors Affect Transcription

The proteins that are responsible for the regulation of gene expression are transcription factors. RNA polymerase should recognize the promoter of the gene and bind to it for the initiation of transcription. Transcription factors binding sites in DNA are known as cis-regulatory sequences; these are located upstream to the promoter. Upon binding, transcription factors either facilitate or prevent the binding of RNA polymerase to the promoter.

1. RNA polymerase itself binds to the promoter region in prokaryotes while transcription factors aid the binding of RNA polymerase to the promoter in eukaryotes. This type of transcription factor is known as basal (general) transcription factors.
2. Transcription binding sites are known either as enhancers or silencers based on the effect of the bound transcription factor on transcription. Enhancers turn the genes “on” upon binding of transcription factors while silencers turn the genes “off” upon binding transcription factors.
3. Activators are the transcription factors that bind to the enhancer regions, activating the transcription by facilitating the binding of RNA polymerase and/or basal transcription factors to the promoter. The action of activators is shown in figure 1.

Figure 1: Activators

4. Repressors are the transcription factors that bind to the silencer regions, repressing the transcription by preventing the binding of RNA polymerase and/or basal transcription factors to the promoter.

Generally, the enhancer or silencer regions are located far from the promoter region. However, the basal transcription factors join the two regions of DNA forming a loop. The differential gene expression in different types of tissues of the body is achieved by the regulation of gene expression at the transcriptional level by transcription factors.

Conclusion

Activators and repressors are the two types of transcription factors involved in the regulation of gene expression at the transcriptional level. Activators bind to the enhancer regions in the DNA and facilitate the binding of RNA polymerase to the promoter. Repressors bind to the silencer regions and prevent the binding of RNA polymerase to the promoter.

Reference:

1. “Transcription Factors.” Khan Academy, Available here.

Image Courtesy:

1. “Role of transcription factor in gene expression regulation” By Philippe Hupé – Emmanuel Barillot, Laurence Calzone, Philippe Hupé, Jean-Philippe Vert, Andrei Zinovyev, Computational Systems Biology of Cancer Chapman & Hall/CRC Mathematical & Computational Biology , 2012 (CC BY-SA 3.0) via Commons Wikimedia