What are transcription factors? Describe the different categories of transcription factors

Transcription factors are special types of proteins that control the process of transcription in eukaryotic cells. Transcription is the first step of gene expression where the DNA is copied into RNA. These factors do not make RNA directly but help the enzyme RNA polymerase to start, stop or control the speed of this process. Transcription factors help the cell know when to turn a gene ON, when to turn it OFF and how much product the gene should make. They play a very important role in development, cell cycle, cell differentiation, stress response and hormone signaling.

Transcription factors are mainly divided into two broad categories:
  1. Based on their function
  2. Based on their structure and DNA-binding domain

1. Types Based on Their Function

There are the following two types:

a) General Transcription Factors

These transcription factors are needed for the transcription of almost all protein-coding genes. They help in forming the transcription initiation complex near the promoter region. They guide the RNA polymerase II to attach at the correct place and begin transcription. These factors are not gene-specific.

Example: TFIID is a general transcription factor. It has a part called TATA-binding protein (TBP) which helps in recognizing the TATA box of the promoter.

b) Specific Transcription Factors

These transcription factors control only certain genes. They work in special conditions such as during stress, hormone action, or development. They bind to enhancer or silencer sequences and either activate or block the transcription of target genes. They are gene-specific and time-specific.

Example: Estrogen receptor is a specific transcription factor that activates genes in the presence of estrogen hormone.

2. Types Based on Their Structure and DNA-Binding Domain

There are the following four major types:

a) Helix-Turn-Helix (HTH)

This type has two alpha helices connected by a short loop or turn. One of the helices fits into the major groove of DNA and helps in DNA binding.

Example: Hox proteins, which are important for body pattern development in animals, are of this type.

b) Zinc Finger

These proteins have finger-like loops that are held together by zinc ions. These loops bind to DNA in a specific way and are very common in eukaryotic transcription factors.

Example: TFIIIA in Xenopus (a frog) was the first discovered zinc finger protein.

c) Leucine Zipper

These proteins have a leucine amino acid at every 7th position which helps in forming a dimer. Two such proteins come together to hold the DNA tightly, like a zip and help in gene regulation.

Example: c-Fos and c-Jun proteins form the AP-1 complex which controls cell growth and division.

d) Basic Helix-Loop-Helix (bHLH)

This type has two helices connected by a loop. They usually form dimers and bind to specific DNA sequences called E-boxes. These are common in developmental genes.

Example: MyoD is a bHLH transcription factor involved in muscle development.





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