Transcription Factor: Definition & Identification

Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

What is a transcription factor? Transcription factors are one of several ways cells regulate genetic expression. In this lesson we will cover how transcription factors were first identified and how they function.

Identification of the First Transcription Factors

Transcription factors were first identified in 1983 by two separate groups of researchers. Both groups were trying to determine how fruit fly development works. What they found was a group of genes that determined how a fruit fly develops from an embryo to an adult. These proteins were named homeobox proteins, and they were the first transcription factors to be identified.

Definition of a Transcription Factor

Genes contain all the information a cell needs to carry out daily processes and are comprised of DNA. Since genes cannot leave the nucleus, they must be copied through a process called transcription. The copy, called mRNA, is then turned into a gene product. However, not every gene needs to be expressed at the same time or in the same amounts and sometimes we want to repress gene expression. It is the job of transcription factors to regulate gene expression. By definition, a transcription factor is a protein capable of binding DNA at specific sequences and controlling the expression of a target gene.

Transcription Factor Structure

All transcription factors have two domains that are necessary for their function. The first is a DNA binding domain (DBD). DBDs attach to specific DNA sequences that are upstream to a regulated gene. This specific region is called a promoter or response element. The transactivation domain (TAD) is where other proteins (co-regulatory proteins) bind to the transcription factor. A third element is sometimes present and called a signal-sensing domain (SSD). This region, when present, allows signaling molecules to bind the transcription factor. The first image shows a representative transcription factor.

The structure of a transcription factor
Image of the structure of a transcription factor

Transcription factors can either stimulate or repress transcription. Transcription factors that promote transcription are called activators. Transcription factors that inhibit transcription are called repressors. Some genes have only activators, others have only repressors, and some complicated regulatory systems have both.

Transcription factors do not bind to just any gene. Instead, they recognize a specific gene or a specific group of genes. What allows a transcription factor to be highly specific yet recognize more than one gene? The transcription factor DBD. The DBD has a specific sequence that will recognize only one promoter. When a gene is not part of a group of related genes, then it has a unique promoter. In contrast, when a group of related genes needs to be turned on at the same time they all have the same promoter.

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