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- RNA polymerase II and the integration of nuclear events

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Figure 1.   Linking pre-mRNA processing to the RNAP II transcription cycle. (A) The general transcription factors (A-H) and srb/mediator (SrbMed) complex, represented by orange squares, form the preinitiation initiation complex with RNAP IIA at the promoter (for review, see Orphanides et al. 1996). The polyadenylation factor CPSF can also be found in this complex. (B) Shortly after transcription initiation, capping enzyme (CE) is recruited to and activated by the phosphorylated CTD of RNAP IIO. SCAF proteins can also associate with the phosphorylated CTD and may mediate the recruitment of SR proteins to RNAP IIO. However, interactions between SCAFs and SR proteins (indicated by a red double-headed arrow) and a role for SCAFs in splicing have not been experimentally demonstrated. Specific, apparently functional interactions between certain transcription factors (blue squares) and/or CE or SR proteins are indicated by double-headed arrows (see text for details). (C) Elongating RNAP IIO is associated with transcription elongation factors (TEFs: blue square; for review, see Reines et al. 1999) and helps in the recruitment of the splicing machinery (SR proteins, snRNPs) to splice sites in the pre-mRNA to facilitate efficient excision of introns (purple line). CBC represents the cap-binding complex, which has been suggested to be capable of stimulating both splicing and polyadenylation (Flaherty et al. 1997 and references therein). (D) After transcribing the poly(A) signal (AATAAA), polyadenylation factors (green ovals) associated with the CTD form a functional complex on the pre-mRNA to catalyze endonucleolytic cleavage (indicated by a purple arrow). Pin1 may stimulate a conformational change of the phosphorylated CTD, enhancing the efficiency of 3'-end formation and/or subsequent transcription termination. Whether or not TFIIF is present in termination complexes is unknown.

Figure 1

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Figure 2.   Allosteric activation of guanylyltransferase activity by the phosphorylated CTD. Distinctions in the mechanism by which guanylylation activity of the capping enzyme is enhanced by the CTD in mammals and yeast are shown in the top and bottom of the figure, respectively. CTD repeats phosphorylated on serine 5 stimulate guanylylation in mammals by interacting directly with the capping enzyme (mCE). Guanylylation can also be stimulated by the transcription factor hSPT5. In yeast, interaction between the guanylyltransferase subunit of CE (Ceg1) and phosphorylated CTD is, by itself, inhibitory, but is stimulatory in the presence of the triphosphosphatase subunit, Cet1. See text for details. Phosphorylation of serine position 5 specifically by the TFIIH component Kin28 (red arrow) enhances interaction between the CTD and Ceg1. Covalent linkage between CE and GMP is indicated by a single bar. Double-headed arrows indicate the physical interactions. (For simplicity, the 7-methyltransferase is not shown.)

Figure 2

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Figure 3.   Effects of RNAP II on mammalian pre-mRNA splicing. Possible interactions between RNAP IIO and the splicing machinery during splicing complex formation are indicated by arrows. However, exactly how RNAP II0 stimulates splicing is unknown, as reflected by the question marks. Splicing complexes formed on the pre-mRNA splicing substrate in a stepwise manner (from E to B) are indicated on the right. The five snRNPs (U1, U2, U4/U6, and U5) are represented by ovals. U2AF indicates U2 snRNP auxiliary factor, which binds to the polypyrimidine tract (Py) near the 3' splice site (AG). SR proteins, which have multiple functions in spliceosome assembly, are shown. Thick solid line indicates the intron and "A" in the intron indicates the branch point adenosine. The arrows show stimulation or stabilization of complex assembly by RNAP IIO, while inhibition or disruption of a complex A by RNAP IIA is indicated by the line with crossbar. See text for details.

Figure 3

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Figure 4.   Coupling polyadenylation and transcription. After RNAP IIO passes the poly(A) site (AATAAA), polyadenylation factors (green ovals) and the CTD form a functional complex on the pre-mRNA to catalyze endonucleolytic cleavage (indicated by a purple arrow), possibly with the help of Pin1. In many cases, there may be lag in processing until RNAP II encounters a pause site (line with crossbar), which facilitates 3' cleavage and in turn signals the polymerase to terminate and the complex to dissociate. Given that the CTD is required for the cleavage reaction, the transcribed pre-mRNA may form a large loop (see text for details). FCP1, the apparent CTD phosphatase (Cho et al. 1999), dephosphorylates the CTD to allow reinitiation and the next round of transcription. The timing of CTD dephosphorylation, and whether this contributes to polyadenylation/termination, is not known.

Figure 4

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Source: Genes and Development, Vol. 14, No. 12, pp. 1415-1429, June 2000

 

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