RNA Polymerase II and TAFs Undergo a Slow Isomerization after the Polymerase Is Recruited to Promoter-Bound TFIID

Transcription of mRNA genes requires that RNA polymerase II (Pol II) and the general transcription factors assemble on promoter DNA to form an organized complex capable of initiating transcription. Biochemical studies have shown that Pol II and TFIID (transcription factor IID) contact overlapping regions of the promoter, leading to the question of how these large factors reconcile their promoter interactions during complex assembly. To investigate how the TAP (TATA-binding protein-associated factor) subunits of THID alter the kinetic mechanism by which complexes assemble on promoters, we used a highly purified human transcription system We found that TAFs sharply decrease the rate at which Pol II, TFIIB, and TRIF assemble on promoter-bound TRID THIA. Interestingly, the slow step in this process is not recruitment of these factors to the DNA, but rather a postrecruitment isomerization of protein DNA contacts that occurs throughout the core promoter Our findings support a model in which Pol II and the general transcription factors rapidly bind promoter-bound TFIID TFIIA, after which complexes undergo a slow isomerization in which the TAFs reorganize their contacts with the promoter to allow Pol to properly engage the DNA. In this manner, TAFs kinetically repress basal transcription. (C) 2010 Elsevier Ltd All rights reserved