Figure 5.

Model for transcriptional activation involving interplay between TFIID, TFIIH, and PC4. Steps I and II outline the initial events in the assembly of the activated PIC. The complex in step I, formed in the absence of PC4, represents an unstable complex and leads to minimal transcription. Activators (shown here as an array of polypeptides) bound to the upstream activator sequence (UAS) mediate recruitment of the general transcription factors to the promoter. This process entails binding of TFIID (cross-hatched) to TATA elements followed by interactions of other factors either in a step-wise fashion or in the form of a pol II-associated complex. In step II, PC4 (solid) is proposed to stabilize the complex, possibly through its ability to engage in multiple interactions with the complex (see text). However, the resulting complex cannot initiate transcription unless a mechanism such as phosphorylation of PC4 by TAF_II250 or TFIIH dislodges PC4 and/or an ATP-hydrolysis dependent TFIIH helicase activity frees the pol II (step III). Only the first possibility (phosphorylation) is shown. In step II, a chaperone-like function of PC4 is imagined. Therefore, after its dissociation from the complex, the stabilizing influences of PC4 may be sustained through changes in DNA topology or rearrangements of the PIC polypeptides (not highlighted). Stoichiometry and spatial localization of PC4 is for illustrative purposes only.