Question

Discuss in detail the structural features of transcription factors and how these features relate to function.

Answer 1

Transcription:- It is a process in which the m- RNA molecule is synthesized on the DNA template strand. This process is catalyzed by an enzyme RNA polymerase II. There are different transcriptional factors also, which are participated in the process along with m- RNA. According to the prokaryotic and eukaryotic organism, the factors are listed out below with there proper role in this process. The biochemical nature of the transcriptional factor is protein. On DNA template strand the factors are attached to the promotor region or enhancer region.  

prokaryotic transcription factors:- It is called DNA dependent RNA polymerase. It was reported by two scientists in 1960 'Samuel B. Weiss and Jerard Hurwitz'. It is made up of five different subunits which perform different functions.

1. alfa subunit:- helps in assembly of the core enzyme and plays a role of promotor recognition. It plays a role in the interaction of RNA polymerase with some regulatory factors.  
2. beta and beta dash subunits:- both together make up the catalytic center. Beta subunit, however, involves in chain elongation.
3. omega subunit :- facilitate assembly of RNA polymerase and stabilized assembled RNA polymerase.
4. sigma factor :- concern with the promotor recognition and transcription initiation.

Eukaryotic transcription factors:- In eukaryotes three different types of RNA polymerase are used for transcription of three different RNA molecule .

• RNA polymerase I for r- RNA
• RNA polymerase II for m- RNA
• RNA polymerase III for t- RNA

Each RNA polymerase is a multi-subunit protein. [8-12 subunits].

In eukaryotes, the transcription process initiation requires different transcription factors along with RNA polymerase. There are two categories of transcription factors,

• regulatory transcription factors:- bind to the regulatory sequences like enhancer or silencer.
• basal or general transcription factors:- attached to the gene promoters

following are some examples of regulatory transcription factors and their functions and binding sites .-

1. CBF :- it is a CAAT motif bind factor binds to CAAT box.
2. C/EBP :- Also binds to CAAT box and acts as an enhancer-binding protein.
3. CREB:- It acts as a c-AMP response element-binding protein
4. Sp1:- SV40 early and late promotor binding protein 1 which binds to GC box, regulates DNA sequences and stimulates transcription.

following are some examples of general or basal transcription factors and their functions [with respect to human].

1. TFIID [TBP component] :- recognize TATA box and Inr sequence on template strand and provide platform for binding of factor TFIIB.
2. TFIID [TAFs] :- it recognise core promotor region and helps in regulation of binding of Tata binding protein
3. TFIIA :- helps in stabilization of TAF and TBP binding.
4. TFIIB:- it acts as an intermediate in the recruitment of RNA polymerase II and influences the selection of start point of transcription.

Transcription factors are also often classified based on the structural motifs that constitute their DNA-binding domains-

1. HELIX -TURN- HELIX MOTIF :- participates in sequence - specific recognition of DNA.
2. HELIX -LOOP- HELIX MOTIF :- it involves in both DNA binding and protein dimerization.
3. LEUCIN -ZIPPER MOTIF:- it is a mediator of both DNA binding and protein dimerization.
4. ZINC - FINGER MOTIF:- Regulates the transcription of genes for S - rRNA.