Prokaryotie RNA Polymerase Holoenzyme

Fig.2.2 Assembly of transcription preinitiation complexes. Preinitiation complex (PIC) formation may occur via an ordered addition, as outlined in the sequential assembly pathway (A), or via a preassembled RNA polymerase II holoenzyme complex and TFIID, as depicted in the two-component pathway (B). The two-component pathway depicted here resembles the prokaryotic RNA polymerase holoenzyme system where a dissociable a factor directs the entry of the bacterial RNA polymerase core enzyme which is comprised of ct2pp'.

D: The Pol II Holoenzyme Pathway

An alternative pathway for PIC formation was uncovered when several laboratories found that pol II could be purified as a preassembled holoenzyme complex containing pol II, a subset of GTFs, SRBs (suppressors of RNA polymerase B mutations) (Kim et al, 1994; Koleske and Young, 1994), and other proteins involved in chromatin remodeling, DNA repair, and mRNA processing (Ossipow et al., 1995; Chao et al., 1996; Maldonado et al., 1996; Wilson et al., 1996; Yuryev et al., 1996; McCracken et al., 1997; Nakajima et al., 1997; Cho et al., 1998; Wu and Chiang, 1998; Wu et al., 1999). Although the composition of pol II holoenzymes isolated from different laboratories varies according to the methods of purification and the source of materials, the human pol II holoenzyme complex isolated in our laboratory contains pol II, TFIIB, TFIIE, TFIIF, TFIIH, GCN5 histone acetyltransferase, SWI/SNF chromatin remodeling factor, and SRBs, but is devoid of TFIID and TFIIA (Wu and Chiang, 1998; Wu et al., 1999; see Fig. 2.2B). The identification of a TFIID-deficient pol II holoenzyme complex suggests that TFIID, as a core promoter-binding factor, may facilitate the entry of pol II holoenzyme to the promoter region, a scenario analogous to the prokaryotic RNA polymerase system where a dissociable a factor can recruit core polymerase (a2pfV ) to the promoter region for PIC formation (Fig. 2.2B). However, there are still unresolved issues whether eukaryotic pol II indeed exists as a holoenzyme complex in the cell or whether PIC assembly occurs in a series of steps at the promoter. It is likely that both assembly pathways exist in vivo and, depending on signaling molecules involved and the promoter context, either pathway may be selectively used in responding to environmental cues. Indeed, evidence supporting both models has been reported for different regulatory systems (Orphanides et al, 1996; Hampsey, 1998; Parvin and Young, 1998; Lee and Young, 2000; Lemon and Tjian, 2000).

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