Question

What is the role of the generalized transcription factors TBP and TFIIH in the process of promoter recognition and transcription initiation by RNA Pol II? What further role does the CTD of Pol II play in transcription initiation and elongation?

Answer 1

Answer: Transcription is the process by which DNA is converted into an mRNA. DNA, having the codes for the protein which express each character are converted into mRNA primarily.

The initiation of transcription by RNA polymerase ll need several transcription factors. Generalized transcription factors help RNAP ll to bind to the start site and helps in the separation of DNA strands. They includes TFIIA, TFIIB, TFIIC, etc..

TFIID contains a TATA box-binding protein(TBP) and TBP-associated factors(TAF). The complex containing polymerase II and its general transcription factors called the preinitiation complex(PIC).

TBP is the first protein to bind the TATA box. TBP folds into a saddle-like structure. They interact with the minor groove of the double helix and bends it. Thus TFIID binds to DNA and thus other transcription factors can easily bind.

After the binding of TFIIE, it creates a docking site for TFIIH. TFIIH is a multimeric factor having 10 subunits. The core subunits have helicase activity. They use ATP hydrolysis energy to unwind DNA helix at the start site, thus allowing RNAP II to form an open complex. Thus start site is melted and the template strand is bound at the polymerase active site. Three subunits of TFIIH having kinase activity phosphorylates the CTD of RNAPII for multiple times.

CTD of RNAP II which is phosphorylated multiple times is the docking site for many enzymes which forms the cap structure which is formed at the 5' end of RNA transcribed by RNAPII. CTD has 52 repeats of the amino acid sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser. They act as the scaffold for many nuclear enzymes for capping and RNA splicing. The phosphorylation is reversible and thus after transcription, they get dephosphorylated.