Eukaryotic RNA PolII Initiation

The three RNA Polymerases in Eukaryotic cells have slightly different processes of transcription initiation; both the promoters and associated proteins are different. What is discussed here is initiation for RNA PolII which transcribes pre-mRNAs in the nucleus. In particular, what we cover here is initiation at a Type I promoter. Make sure you are familiar with the structure of a Eukaryotic Promoter and the idea of a Type I promoter.

RNA PolII does not bind directly to the promoter. Instead, a complex of proteins form and then RNA PolII binds to this complex and initiates transcription. These proteins collectively are referred to as General (or Basal) Transcription Factors and are referred to as TFIIX proteins since they are associated with transcription by polII. The X is a letter that differs for each specific protein as you will see below.

The diagrams below show the formation of the general transcription factors at the promoter. The binding of the GTFs results in a Pre-Initiation Complex (PIC). Once the RNA PolII has been bound and activated it is ready for elongation.

The role of TFIID: As you will see in the figures below the initial step in the formation of the PIC is the binding of the basal transcription factor TFIID. The process shown below is for a Type I promoter with a TATA Box, with TFIID biding to the TATA Box, but initiation at Type I promoters that lack a TATA Box follows the same basic process. This is because TFIID can also bind the Inr and DPE Core Promoter elements. TFIID is a complex protein composed of over a dozen different subunits. One subunit is called TBP (TATA Binding Protein) and the others are all referred to as TAFs (TBP Associated Factors). TFIID can bind different elements because a different subunit recognizes each element as shown here:

• TBP (TATA Binding Protein: The subunit of TFIID that binds the TATA Box
• TAF2: The subunit of TFIID that binds the Inr element
• TAF6: The subunit of TFIID that binds DPE

Therefore, regardless of which of these elements is present (or absent) in any given Type I promoter, TFIID can bind. The following figures show the formation of the PIC and then activation of RNA PolII so that initiation can occur.

Here are some videos that illustrate the process: