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. Author manuscript; available in PMC: 2010 Mar 1.
Published in final edited form as: Crit Rev Biochem Mol Biol. 2009 Jun;44(2-3):117–141. doi: 10.1080/10409230902858785

Figure 5. Assembly of the pre-initiation complex.

Figure 5

Two forms of pre-initiation complexes and an early elongation complex are shown: A) Partial PIC (Zanton and Pugh, 2006), B) Poised PIC (Martens et al., 2001; Radonjic et al., 2005; Sekinger and Gross, 2001), and C) Paused Pol II complex (Lee et al., 2008; Muse et al., 2007; Zeitlinger et al., 2007).

(A) A partial PIC contains GTFs assembled in the context of resident nucleosomes, but is relatively depleted of TFIIH and Pol II.

(B) A poised PIC contains Pol II and TFIIH in addition to the GTFs and exists in the context of an evicted -1 nucleosome. The poised PIC has not yet cleared the promoter. In vivo, such complexes may be undergoing abortive initiation events where very short transcripts are released and degraded.

(C) A paused Pol II complex typically occurs 30–50 nucleotides after the TSS. Negative elongation factor (NELF) and other factors (not shown) bound to Pol II help create the paused state. The +1 nucleosomes might also contribute to pausing by creating a barrier. Many initiation and regulatory factors may be retained at the promoter after Pol II has cleared the area, which might promote subsequent rounds of transcription (not shown).