PIC Assembly Pathways
Reference work entry
There are two possible PIC ( pre- initiation complex) assembly pathways in the RNAPII system (Lee and Young 2000; Thomas and Chiang 2006) (Fig. 1). In the sequential assembly pathway (Model 1), TFIID binds to the core promoter, and then the binary TFIID-DNA complex incorporates TFIIA, TFIIB, RNAPII/TFIIF, TFIIE, and TFIIH in a stepwise manner. In the holoenzyme recruitment pathway (Model 2), a huge RNAPII complex, the so-called holoenzyme containing not only GTFs (TFIIB, E, F, and H) but also Mediator and/or some ATP-dependent chromatin remodelers is incorporated at once into the TFIID-TFIIA-DNA complex that is pre-formed in the same manner as in the sequential assembly pathway. Although both pathways are plausible and supported by the evidence obtained in vitro, it is difficult to determine which pathway operates on the promoters of individual genes in living cells. Note that two similar pathways are also...
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