Question

B3. Describe four types of transcription factor with example (10 points).

Answer 1

Transcriptional factors can be classified on the basis of their action mechanism , function and structure

• On the basis of mechanism, it can be classified into two

a) General transcription factors : They help in the formation of preinitiation complex. They interact with promoter region

Eg: TFIIA, TFIIB, TFIID etc

b) Upstream transcription factors : They stimulate or repress the transcrption by binding at any point upsteam to the initiation site.

Eg: Heat shock factor, SP1 , AP-1 etc

• Based on the regulatory function, it can be classified into two.

a) Constitutively active: They are present in every cell throughout the lifetime of a cell.

Eg: SP1, NF-1 etc

b) Conditionally active : This type of factors require activation. It can be further divided into two

Developemental: It require no additional factors once expressed. Example: GATA, HNF etc

Signal dependant: They require external signal activation for their functioning.It can be further divided into, extra cellular ligand (eg : nuclear receptors), intracellular ligand (eg:SREBP, p53 ), cell membrane receptor dependent ( eg: resident nuclear facors- CREB, AP-1 , latent cytoplasmic factors- STAT, Notch etc ).

• On the basis of structure it can be divided into

a) 1 superclass : They are basic domains : It includes 6 classes.Some examples are, CREB, SREBP, USF NF-1 RF-X

b) 2 Superclass : They are Zinc- coordinating DNA binding domains. It consists of 5 classes. Some examples include, TFIIIA, Sp1, Alternating composition of Zinc fingers etc.

c) 3 Superclass: They are helix-turn-helix factors. it consists of 6 class. Some examples include, Homeo domain, Fork head etc.

d) 4 Superclass : They are beta-Scaffold Factors with minor Minor Groove Contacts. It consists of 11 classes. Some of the examples include SRY, Cold-shock domains etc.

e) 0 Superclass : These are other transcription factors. It consists of 5 classes. Some examples are, HMGA1, AP2 etc.