Tfii F Tfiib

DNA TFII A TFII D RNA poly II Gene

Startpoint

Binding of tfiie, tfiih, tfii j

Tfiih phosphorylates the TAIL of RNA Pol II

Phosphate groups

with RNA polymerases I and III. The largest subunit of RNA polymerase II is related to the b' subunit of bacterial RNA polymerase and possesses the CTD tail. In addition, the assorted TFII complexes each consist of several polypeptide chains. Thus the initiation complex for RNA polymerase II includes over 20 polypeptides.

Upstream elements close to the promoter bind a range of specific transcription factors.

Upstream Elements Increase the Efficiency of RNA Polymerase II Binding

RNA polymerase II can bind and initiate transcription at minimal promoter consisting of just an initiator and TATA boxes. However, this is extremely inefficient unless upstream elements are also present. There are many different upstream elements. They are typically five to ten base pairs long and located from 50 to 200 bases upstream of the start site. There may be more than one upstream element in a given promoter and the same upstream element may be found at different places in different promoters.

The TFII proteins are general transcription factors because they are always required. In contrast, specific transcription factors affect only certain genes and are involved in regulating gene expression in response to a variety of signals (Fig. 6.24). The upstream elements are the recognition sites for specific transcription factors. These usually make contact with the transcription apparatus via TFIID, TFIIB or TFIIA, not by directly touching RNA polymerase II itself. Most commonly, binding is to TFIID. Binding of the specific transcription factors helps assembly of the transcription apparatus and therefore increases the frequency of initiation.

FIGURE 6.24 Upstream Elements Facilitate Transcription

The upstream elements make contact with one domain of an activator protein. The activator also binds to the transcription apparatus near the start site.

DNA uPstream element

TATA box

Initiator box

Start site

Start site

FIGURE 6.24 Upstream Elements Facilitate Transcription

The upstream elements make contact with one domain of an activator protein. The activator also binds to the transcription apparatus near the start site.

Was this article helpful?

0 0

Post a comment