Question

Many molecules are involved in providing stability to the Pre-Initiation Complex (PIC) in eukaryotes. Describe:

a) The steps involved in stimulating initiation of transcription from a silent gene

b) How the molecules interact to provide stability to the PIC

c) How the polymerase escapes from this PIC to allow the transcript to elongate

Your discussion needs to be illustrated with well-labelled diagrams

Answer 1

(a) A silent gene is stimulated to initiate transcription with the help of RNA polymerase II enzyme and six transcription factors: TFIIA, TFIIB, TFIID, TFIIE, TFIIF and TFIIH. Various co-factors and chromatin remodelling complexes also help to initiate transcription by the formation of Pre-Initiation Complex (PIC). TATA box, a promoter sequence indiactes the location from which transcription begins.

(b) A stable RNA polymerase II pre-initiation complex is formed through the following steps:

(i) TATA binding prodtein (TBP) binds to TATA box and specifies the location from which transcription will begin.

(ii) Transcription factor TFIIB also binds along with the complex between polymerase II and TFIIF. These chemical species provide stability to the pre-initiation complex.

(iii) TFIIE binds to TFIIF and polymerase complex and this provides stability to the complex (PIC) and regulates transcription.

(iv) Finally, TFIIH binds to the assembly and completes the formation of a stable Pre-Initiation Complex.

(c) TFIIH factor from the pre-initiation complex melts DNA chain at the promoter region and polymerase II starts the transcription of mRNA. Another unit of TFIIH phosphorylates C-terminal domain of polymerase enzyme and this enhances the processivity of the enzyme polymerase. As a result, the polymerase escapes from the PIC and allows elongation of the transcription.