What is the structure of transcription factors?

Transcription factors have a modular structure. They are made up of three domains: 

1. DNA-binding domain (DBD), which binds to specific DNA sequences adjacent to regulated genes. 
2. Activation domain (AD), which holds the binding sites for other proteins such as transcription coregulators. 
3. Signal- sensing domain (SSD), which is optional and transmits external signals that it senses to the rest of the transcription complex.