Describe the steps in formation of the transcription initiat

Describe the steps in formation of the transcription initiation complex in Eukaryotes. Make sure to mention the roles of the consensus sequences.

Solution

Steps in formation of the transcription initiation complex:

The prokaryotic RNA polymerase that can bind to a DNA template on its own where as eukaryotes require several other proteins(factors), called transcription factors, to first bind to the promoter region and then help recruit the appropriate polymerase. The completed assembly of transcription factors and RNA polymerase bind to the promoter, forming a transcription pre-initiation complex (PIC).

The most-extensively studied core promoter element in eukaryotes is a short DNA sequence known as a TATA box, found 25-30 base pairs upstream from the start site of transcription. Only about 10-15% of mammalian genes contain TATA boxes, while the rest contain other core promoter elements, but the mechanisms by which transcription is initiated at promoters with TATA boxes is well characterized.

The TATA box, as a core promoter element, is the binding site for a transcription factor known as TATA-binding protein (TBP), which is itself a subunit of another transcription factor: Transcription Factor II D (TFIID). After TFIID binds to the TATA box via the TBP, five more transcription factors and RNA polymerase combine around the TATA box in a series of stages to form a pre-initiation complex. One transcription factor, Transcription Factor II H (TFIIH), is involved in separating opposing strands of double-stranded DNA to provide the RNA Polymerase access to a single-stranded DNA template. However, only a low, or basal, rate of transcription is driven by the pre-initiation complex alone. Other proteins known as activators and repressors, along with any associated coactivators or corepressors, are responsible for modulating transcription rate. Activator proteins increase the transcription rate, and repressor proteins decrease the transcription rate.

Consensus sequences.

TATA element (TATAAA) : Which is recognized by the transcription factor - TATA-binding protein (TBP).
B recognition element (BRE) : which is recognized by the general transcription factor TFIIB
E-box (CACGTG) : which binds transcription factors in the basic helix-loop-helix (bHLH) family