How Do Transcription Factors Work

Transcription factors are proteins that bind to DNA in order to regulate the gene expression by promoting or suppressing gene expression. Therefore, they can be either activators that turn “on” the gene expression or repressors that turn “off” the gene expression. The activation of gene expression produces a mRNA copy of the gene in a process known as transcription. It is governed by RNA polymerase. The transcription factor binding sites are found within the regulatory elements of the gene, upstream to the promoter. In brief, the main function of transcription factors is to determine the time, location, and the efficiency of transcription.

Key Areas Covered

1. What are Transcription Factors
     – Definition, Characteristics, Families
2. How Do Transcription Factors Work
     – Activators, Enhancers

Key Terms: Activators, Basal Transcription Factors, Enhancers, Gene Expression, Repressors, Transcription Factors

What are Transcription Factors

A transcription factor is a molecule that regulates the expression of a gene either by activating or repressing the transcription of the gene. RNA polymerase is the enzyme that catalyzes the transcription of a DNA fragment known as a gene into an RNA molecule. Transcription factors are mainly involved in the determination of time, location, and the efficiency of transcription. The key feature of a transcription factor is that they contain at least one DNA binding domain (DBD). Approximately, 10% of the genes in the genome are encoded for transcription factors.

Five families of transcription factors that share structural characteristics can be identified as below.

Transcription Factor Families

In both prokaryotes, transcription factors either facilitate or prevent the binding of RNA polymerase to the promoter. In addition to gene regulation, transcription factors are also involved in chromatin modifications, RNA splicing, and siRNA control mechanisms.

How Do Transcription Factors Work

Transcription factors are the proteins responsible for the regulation of gene expression. Generally, RNA polymerase should recognize and bind to the promoter for the initiation of transcription. Promoter is the region of DNA that initiates the transcription of a particular gene. In prokaryotes, RNA polymerase itself binds to the promoter region. However, in eukaryotes, RNA polymerase binds to the promoter with the help of some other transcription factors called basal (general) transcription factors.

Transcription factors bind to the sequences known as transcription factor binding sites found within the cis-regulator DNA sequences of the gene, upstream to the promoter. Upon binding, they either facilitate or prevent the binding of RNA polymerase to the promoter. The transcription factor binding site is called either as the enhancer or silencer.  The enhancers turn the gene “on” while the silencers turn the gene “off”. The transcription factors that bind to the enhancers and activate the gene expression are known as activators. They help basal transcription factors and/or RNA polymerase to bind to the promoter. The action of activators is shown in figure 1.

The transcription factors that bind to the silencers and repress the gene expression are known as repressors. Repressors prevent the basal transcription factors and/or RNA polymerases from binding to the promoter. Though transcription factor binding sites are apart from the promoter region, the flexibility of the DNA strand allows the joining of both transcription factor binding sites and promoter regions to come together by forming a DNA loop.

Different types of genes are expressed in different types of tissues. This differential gene expression is achieved by means of transcription factors. These genes are composed of several enhancers or silencers.

Conclusion

Gene expression has to be regulated based on the requirements of the cell. Transcription factors are responsible for the regulation of gene expression. They bind either to the enhancer or silencer region, upstream to the promoter of the gene. The transcription factors that bind to the enhancer regions are known as activators, and those that bind to the silencers are known as repressors. Activators facilitate the binding of RNA polymerase to the promoter region while repressors prevent the binding of RNA polymerase to the promoter region.

Reference:

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

Image Courtesy:

1. “Transcription Factors” By Kelvinsong – Own work (CC BY 3.0) via Commons Wikimedia