RNA polymerase II (Pol II) is the central enzyme that catalyses DNA- directed mRNA synthesis during the transcription of protein-coding genes. Pol II consists of a 10-subunit catalytic core, which alone is capable of elongating the RNA transcript, and a complex of two subunits, Rpb4/7, that is required for transcription initiation. The transcription cycle is divided in three major phases: initiation, elongation, and termination. Transcription initiation include promoter DNA binding, DNA melting, and initial synthesis of short RNA transcripts. The transition from initiation to elongation, is referred to as promoter escape and leads to a stable elongation complex that is characterized by an open DNA region or transcription bubble. The bubble contains the DNA-RNA hybrid, a heteroduplex of eight to nine base pairs. The growing 3-end of the RNA is engaged with the polymerase complex active site. Ultimately transcription terminates and Pol II dissocitates from the template.
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Detailed studies of gene transcription regulation in a wide variety of eukaryotic systems has revealed the general principles and mechanisms by which cell- or tissue-specific regulation of differential gene transcription is mediated (reviewed in Naar, 2001. Kadonaga, 2004, Maston, 2006, Barolo, 2002; Roeder, 2005, Rosenfeld, 2006). Of the three major classes of DNA polymerase involved in eukaryotic gene transcription, Polymerase II generally regulates protein-encoding genes. Figure 1 shows a diagram of the various components involved in cell-specific regulation of Pol-II gene transcription.
Core Promoter: Pol II-regulated genes typically have a Core Promoter where Pol II and a variety of general factors bind to specific DNA motifs: i: the TATA box (TATA DNA sequence), which is bound by the "TATA-binding protein" (TBP). ii: the Initiator motif (INR), where Pol II and certain other core factors bind, is present in many Pol II-regulated genes. iii: the Downstream Promoter Element (DPE), which is present in a subset of Pol II genes, and where additional core factors bind. The core promoter binding factors are generally ubiquitously expressed, although there are exceptions to this.
Proximal Promoter: immediately upstream (5') of the core promoter, Pol II target genes often have a Proximal Promoter region that spans up to 500 base pairs (b.p.), or even to 1000 b.p.. This region contains a number of functional DNA binding sites for a specific set of transcription activator (TA) and transcription repressor (TR) proteins. These TA and TR factors are generally cell- or tissue-specific in expression, rather than ubiquitous, so that the presence of their cognate binding sites in the proximal promoter region programs cell- or tissue-specific expression of the target gene, perhaps in conjunction with TA and TR complexes bound in distal enhancer regions.
Distal Enhancer(s): many or most Pol II regulated genes in higher eukaryotes have one or more distal Enhancer regions which are essential for proper regulation of the gene, often in a cell or tissue-specific pattern. Like the proximal promoter region, each of the distal enhancer regions typically contain a cluster of binding sites for specific TA and/or TR DNA-binding factors, rather than just a single site.
Enhancers generally have three defining characteristics: i: They can be located very long distances from the promoter of the target gene they regulate, sometimes as far as 100 Kb, or more. ii: They can be either upstream (5') or downstream (3') of the target gene, including within introns of that gene. iii: They can function in either orientation in the DNA.
Combinatorial mechanisms of transcription regulation: The specific combination of TA and TR binding sites within the proximal promoter and/or distal enhancer(s) provides a "combinatorial transcription code" that mediates cell- or tissue-specific expression of the associated target gene. Each promoter or enhancer region mediates expression in a specific subset of the overall expression pattern. In at least some cases, each enhancer region functions completely independently of the others, so that the overall expression pattern is a linear combination of the expression patterns of each of the enhancer modules.
Co-Activator and Co-Repressor Complexes: DNA-bound TA and TR proteins typically recruit the assembly of specific Co-Activator (Co-A) and Co-Repressor (Co-R) Complexes, respectively, which are essential for regulating target gene transcription. Both Co-A's and Co-R's are multi-protein complexes that contain several specific protein components.
Co-Activator complexes generally contain at lease one component protein that has Histone Acetyl Transferase (HAT) enzymatic activity. This functions to acetylate Histones and/or other chromatin-associated factors, which typically increases that transcription activation of the target gene. By contrast, Co-Repressor complexes generally contain at lease one component protein that has Histone De-Acetylase (HDAC) enzymatic activity. This functions to de-acetylate Histones and/or other chromatin-associated factors. This typically increases the transcription repression of the target gene.
Adaptor (Mediator) complexes: In addition to the co-activator complexes that assemble on particular cell-specific TA factors, - there are at least two additional transcriptional co-activator complexes common to most cells. One of these is the Mediator complex, which functions as an "adaptor" complex that bridges between the tissue-specific co-activator complexes assembled in the proximal promoter (or distal enhancers). The human Mediator complex has been shown to contain at least 19 protein distinct components. Different combinations of these co-activator proteins are also found to be components of specific transcription Co-Activator complexes, such as the DRIP, TRAP and ARC complexes described below.
TBP/TAF complex: Another large Co-A complex is the "TBP-associated factors" (TAFs) that assemble on TBP (TATA-Binding Protein), which is bound to the TATA box present in many promoters. There are at least 23 human TAF proteins that have been identified. Many of these are ubiquitously expressed, but TAFs can also be expressed in a cell or tissue-specific pattern.
Specific Coactivator Complexes for DNA-binding Transcription Factors.
A number of specific co-activator complexes for DNA-binding transcription factors have been identified, including DRIP, TRAP, and ARC (reviewed in Bourbon, 2004, Blazek, 2005, Conaway, 2005, and Malik, 2005). The DRIP co-activator complex was originally identified and named as a specific complex associated with the Vitamin D Receptor member of the nuclear receptor family of transcription factors (Rachez, 1998). Similarly, the TRAP co-activator complex was originally identified as a complex that associates with the thyroid receptor (Yuan, 1998). It was later determined that all of the components of the DRIP complex are also present in the TRAP complex, and the ARC complex (discussed further below). For example, the DRIP205 and TRAP220 proteins were show to be identical, as were specific pairs of the other components of these complexes (Rachez, 1999).
In addition, these various transcription co-activator proteins identified in mammalian cells were found to be the orthologues or homologues of the Mediator ("adaptor") complex proteins (reviewed in Bourbon, 2004). The Mediator proteins were originally identified in yeast by Kornberg and colleagues, as complexes associated with DNA polymerase (Kelleher, 1990). In higher organisms, Adapter complexes bridge between the basal transcription factors (including Pol II) and tissue-specific transcription factors (TFs) bound to sites within upstream Proximal Promoter regions or distal Enhancer regions (Figure 1). However, many of the Mediator homologues can also be found in complexes associated with specific transcription factors in higher organisms. A unified nomenclature system for these adapter / co-activator proteins now labels them Mediator 1 through Mediator 31 (Bourbon, 2004). For example, the DRIP205 / TRAP220 proteins are now identified as Mediator 1 (Rachez, 1999), based on homology with yeast Mediator 1.
Example Pathway: Specific Regulation of Target Genes During Notch Signaling:
One well-studied example of cell-specific regulation of gene transcription is selective regulation of target genes during Notch signaling. Notch signaling was first identified in Drosophila, where it has been studied in detail at the genetic, molecular, biochemical and cellular levels (reviewed in Justice, 2002; Bray, 2006; Schweisguth, 2004; Louvri, 2006). In Drosophila, Notch signaling to the nucleus is thought always to be mediated by one specific DNA binding transcription factor, Suppressor of Hairless. In mammals, the homologous genes are called CBF1 (or RBPJkappa), while in worms they are called Lag-1, so that the acronym "CSL" has been given to this conserved transcription factor family. There are at least two human CSL homologues, which are now named RBPJ and RBPJL.
In Drosophila, Su(H) is known to be bifunctional, in that it represses target gene transcription in the absence of Notch signaling, but activates target genes during Notch signaling. At least some of the mammalian CSL homologues are believed also to be bifunctional, and to mediate target gene repression in the absence of Notch signaling, and activation in the presence of Notch signaling.
Notch Co-Activator and Co-Repressor complexes: This repression is mediated by at least one specific co-repressor complexes (Co-R) bound to CSL in the absence of Notch signaling. In Drosophila, this co-repressor complex consists of at least three distinct co-repressor proteins: Hairless, Groucho, and dCtBP (Drosophila C-terminal Binding Protein). Hairless has been show to bind directly to Su(H), and Groucho and dCtBP have been shown to bind directly to Hairless (Barolo, 2002). All three of the co-repressor proteins have been shown to be necessary for proper gene regulation during Notch signaling in vivo (Nagel, 2005).
In mammals, the same general pathway and mechanisms are observed, where CSL proteins are bifunctional DNA binding transcription factors (TFs), that bind to Co-Repressor complexes to mediate repression in the absence of Notch signaling, and bind to Co-Activator complexes to mediate activation in the presence of Notch signaling. However, in mammals, there may be multiple co-repressor complexes, rather than the single Hairless co-repressor complex that has been observed in Drosophila.
During Notch signaling in all systems, the Notch transmembrane receptor is cleaved and the Notch intracellular domain (NICD) translocates to the nucleus, where it there functions as a specific transcription co-activator for CSL proteins. In the nucleus, NICD replaces the Co-R complex bound to CSL, thus resulting in de-repression of Notch target genes in the nucleus (Figure 2). Once bound to CSL, NICD and CSL proteins recruit an additional co-activator protein, Mastermind, to form a CSL-NICD-Mam ternary co-activator (Co-A) complex. This Co-R complex was initially thought to be sufficient to mediate activation of at least some Notch target genes. However, there now is evidence that still other co-activators and additional DNA-binding transcription factors are required in at least some contexts (reviewed in Barolo, 2002).
Thus, CSL is a good example of a bifunctional DNA-binding transcription factor that mediates repression of specific targets genes in one context, but activation of the same targets in another context. This bifunctionality is mediated by the association of specific Co-Repressor complexes vs. specific Co-Activator complexes in different contexts, namely in the absence or presence of Notch signaling.
Small nuclear RNAs (snRNAs) play key roles in splicing and some of them, specifically the U1 and U2 snRNAs, are encoded by multicopy snRNA gene clusters containing tandem arrays of genes, about 30 in the RNU1 cluster (Bernstein et al. 1985) and about 10-20 in the RNU2 cluster (Van Ardsell and Weiner 1984). Whereas U6 snRNA genes are transcribed by RNA polymerase III, U1,U2, U4, U4atac, U5, U11, and U12 genes are transcribed by RNA polymerase II. Transcription of the U1 and U2 genes has been most extensively studied and the other snRNA genes as well as other genes with similar promoter structures, for example the SNORD13 gene, are inferred to be transcribed by similar reactions. The snRNA genes transcribed by RNA polymerase II are distinguished from mRNA-encoding genes by the presence of a proximal sequence element (PSE) rather than a TATA box and the presence of the Integrator complex rather than the Mediator complex (reviewed in Egloff et al. 2008, Jawdeker and Henry 2008). The snRNA genes are among the most rapidly transcribed genes in the genome. The 5' transcribed region of the U2 snRNA gene is largely single-stranded during interphase and metaphase (Pavelitz et al. 2008) and chromatin within the transcribed region is cleared of nucleosomes (O'Reilly et al. 2014). Transcriptional activation of the RNA polymerase II transcribed snRNA genes begins with binding of transcription factors to the distal sequence element (DSE) of the promoter (reviewed in Hernandez 2001, Egloff et al. 2008, Jawdeker and Henry 2008). The factors, which include POU2F1 (Oct-1), POU2F2 (Oct-2), ZNF143 (Staf) and Sp1, promote binding of the SNAPc complex (also known as PTF and PBP) to the PSE. SNAPc helps clear the gene of nucleosomes (O'Reilly et al. 2014) and recruits initiation factors (TFIIA, TFIIB, TFIIE, TFIIF, and snTAFc:TBP) which recruit RNA polymerase II. Phosphorylation of the C-terminal domain (CTD) of RNA polymerase II (reviewed in Egloff and Murphy 2008) by CDK7 recruits RPAP2 and the Integrator complex, which is required for later processing of the 3' end of the pre-snRNA transcript (reviewed in Chen and Wagner 2010, Baillat and Wagner 2015). The Little Elongation Complex (LEC) also appears to bind around the time of transcription initiation (Hu et al. 2013). As transcription proceeds, RPAP2 dephosphorylates serine-5 and P-TEFb phosphorylates serine-2 of the CTD. As transcription reaches the end of the snRNA gene serine-7 of the CTD is phosphorylated. These marks serve to bind protein complexes and are required for 3' processing of the pre-snRNA (reviewed in Egloff and Murphy 2008). After transcription proceeds through the conserved 3' processing sequence of the pre-snRNA the Integrator complex cleaves the pre-snRNA. Transcription then terminates downstream in a less well characterized reaction that requires elements of the polyadenylation system.
DSIF is a heterodimer consisting of hSPT4 (human homolog of yeast Spt4- p14) and hSPT5 (human homolog of yeast Spt5-p160). DSIF association with Pol II may be enabled by Spt5 binding to Pol II creating a scaffold for NELF binding (Wada et al.,1998). Spt5 subunit of DSIF can be phosphorylated by P-TEFb.
DSIF is a heterodimer consisting of hSPT4 (human homolog of yeast Spt4- p14) and hSPT5 (human homolog of yeast Spt5-p160). DSIF association with Pol II may be enabled by Spt5 binding to Pol II creating a scaffold for NELF binding (Wada et al.,1998). Spt5 subunit of DSIF can be phosphorylated by P-TEFb.
The C-terminal domain (CTD) of POLR2A contains about 52 repeats of the consensus heptad YSPTSPS. Serines-2 and 5 of the heptads are phosphorylated in RNA polymerase II initiating transcription of protein coding genes. The exact repeats that are phosphorylated are not known.
Although TBP (TATA box binding factor) is necessary and sufficient for initiation of basal transcription, the other subunits of the general transcription factor TFIID, the TBP-associated factors, are required for response to transcriptional activators. TBP binds to the TATA box (a core promoter element), and bends the DNA 80 degrees toward the major groove. This conformation of TBP-TATA box provides the proper topology for the binding of the general transcription factor TFIIB.
Transcriptional activators function by affecting the kinetics of binding of TBP to the promoter DNA.
The general transcription factor TFIIB is a single polypeptide of approximately 35 kDa. There is a Zn-binding domain near the N terminus of TFIIB, and the C-terminal domain encompasses two imperfect repeats; between the N and C termini is a phylogenetically conserved region. The C terminus interacts with TBP and RNA Polymerase II, whereas the N terminus interacts with factor TFIIF and RNA polymerase II. TFIIB is a sequence-specific factor, and it interacts with the BRE element within the promoter.
TFIIB interacts with the Rpb1 subunit of RNA polymerase II to define transcription strat sites. Several activators directly bind TFIIB, and stimulate transcription. The N-terminus and the C-terminus can participate in intramolecular interactions, and this can be disrupted by specific activators by causing a conformational change in TFIIB.
TFIIA also binds the preinitiation complex along with TFIIB. However, TFIIA is not required for accurate initiation, but rather functions as a coactivator of transcription.
The general transcription factor TFIIF has a high affinity for the RNA Polymerase II holoenzyme. TFIIF stabilizes the preinitiation complex, and suppresses non-specific binding of RNA Pol II to DNA, and is thus critical for start site recognition.
The binding of TFIIH completes the assembly of the preinitiation complex (PIC) for RNA Polymerase II transcription. Although RNA polymerase binds the TATA box on the promoter DNA, no initiation of transcription occurs until TFIIH is bound to the PIC. TFIIH is the only factor with known enzymatic activities.
RNA polymerase II transcription complexes are susceptible to transcriptional stalling and arrest, when extending nascent transcripts to 30-nt. This susceptibility depends on presence on down-stream DNA, the particular DNA-sequence of the template and presence of transcription factors. Transcription factor TFIIH remains associated to the RNA pol II elongation complex until position +30. At this stage transcription elongation factor TFIIS can rescue stalled transcription elongation complexes. The transcription bubble varies between 13- and 22-nt in size.
Processing is initiated once the U7 snRNP is loaded onto the pre-mRNA. The pre-mRNA HDE makes base-pairing contacts with the 5�² end of U7 snRNA. Binding of the U7 snRNP to the pre-mRNA is stabilized by interactions between a U7 snRNP protein, hZFP100 and other trans-acting factors, including the factor that catalyzes the cleavage reaction, which have yet to be defined. The cleavage occurs in the presence of EDTA as does the cleavage reaction in polyadenylation, it is likely that this reaction is catalyzed by a protein. There may well be additional proteins associated with the U7 snRNP, since the in vitro processing occurs in the absence of SLBP, it is possible that all the other factors required for processing are associated with the active form of the U7 snRNP.
At the beginning of this reaction, 1 molecule of 'FACT complex', 1 molecule of 'Elongin Complex', 1 molecule of 'Early elongation complex with hyperphosphorylated Pol II CTD', 1 molecule of 'TFIIH', 1 molecule of 'RNA polymerase II elongation factor ELL', and 1 molecule of 'TFIIS protein' are present. At the end of this reaction, 1 molecule of 'Elongation complex' is present.
Cdk-9 is the kinase subunit of P-TEFb that phosphorylates Serine 2 on the heptapeptide repeats of Pol II CTD alleviating the negative action of DSIF-NELF complex. This reaction is considered to be a rate limiting step for processive elongation. P-TEFb complex, that has a DRB-sensitive cyclin-dependent kinase activity, is composed of ~43 kDa, Cdk9 kinase (PITALRE), and either Cyclin T1, Cyclin T2a, Cyclin T2b, or Cyclin K. The exact mechanism by which P-TEFb removes the inhibition of elongation by DSIF-NELF is not yet known. P-TEFb is also capable of phosphorylating Spt5 subunit of DSIF complex. A P-TEFb complex (which contains only the Cyclin T1) is implicated in the efficient synthesis of human immunodeficiency virus-1 (HIV-1) transcripts. Cyclin T1 subunit of the P-TEFb(Cyclin T1:Cdk9) complex interacts with HIV-1 encoded Tat protein that binds to the transactivation response (TAR) element in the nascent HIV-1 transcript (reviewed in Price,2000). The mechanism by which DSIF, NELF and P-TEFb or TAK/P-TEFb act together in Pol II-regulated elongation is yet to be fully understood. Various biochemical evidences point to a model in which DSIF and NELF negatively regulate elongation through interactions with polymerase containing a hypophosphorylated CTD. Subsequent phosphorylation of the Pol II CTD by P-TEFb might promote elongation by inhibiting interactions of DSIF and NELF with the elongation complex.
FCP1 dephosphorylates RNAP II in ternary elongation complexes as well as in solution and is thought to function in the recycling of RNAP II during the transcription cycle. Biochemical experiments suggest that human FCP1 targets CTDs that are phosphorylated at serine 2 (CTD-serine 2) and/or CTD-serine 5. It is also observed to stimulate elongation independent of its catalytic activity. Dephosphorylation of Ser2 - phosphorylated Pol II results in hypophosphorylated form that disengages capping enzymes (CE).
High-resolution structures of free, catalytically active yeast Pol II and of an elongating form reveal that Pol II elongation complex includes features like: - RNA-DNA hybrid, an unwound template ahead of 3'-OH terminus of growing transcript and an exit groove at the base of the CTD, possibly for dynamic interaction of processing and transcriptional factors. - a cleft or channel created by Rpb1 and Rpb2 subunits to accommodate DNA template, extending to Mg2+ ion located deep in the enzyme core -a 50 kDa "clamp" with open confirmation in free polymerase, allowing entry of DNA strands but closed in the processive elongation phase. The clamp is composed of portions of Rpb1,Rpb2 and Rpb3 , five loops or "switches" that change from unfolded to well-folded structures stabilizing the elongation complex, and a long "bridging helix" that emanates from Rpb1 subunit, crossing near the Mg2+ ion. The bridging helix is thought to "bend" to push on the base pair at the 3'-end of RNA-DNA hybrid like a ratchet, translocating Pol II along the DNA (Cramer et al.,2001; Gnatt et al.,2001).In addition to its dynamic biochemical potential, Pol II possess a repertoire of functions to serve as a critical platform of recruiting and coordinating the actions of a host of additional enzyme and proteins involved in various pathways.
Recovery from pausing occurs spontaneously after a variable length of time as the enzyme spontaneously slides forward again. This renders the transcript's 3'-OH terminus realigned with the catalytic Mg2+ site of the enzyme. TFIIS is capable of excising the nascent transcript at 2 or 3 nucleotides upstream of the transcript's 3'-end to reinitiate processive elongation (reviewed by Shilatifard et al., 2003).
At the beginning of this reaction, 1 molecule of 'Arrested processive elongation complex' is present. At the end of this reaction, 1 molecule of 'Aborted elongation complex after arrest' is present.
At the beginning of this reaction, 1 molecule of 'Elongation complex prior to separation' is present. At the end of this reaction, 1 molecule of 'Elongation complex with separated and uncleaved transcript' is present.
At the beginning of this reaction, 1 molecule of 'FACT 140 kDa subunit', and 1 molecule of 'FACT 80 kDa subunit' are present. At the end of this reaction, 1 molecule of 'FACT complex' is present.
This reaction takes place in the 'nucleus' (Kamakaka et al.1993, Orphanides et al.1998).
At the beginning of this reaction, 1 molecule of 'Cdk 9 protein', 1 molecule of 'Cyclin T1', and 1 molecule of 'Cyclin T2' are present. At the end of this reaction, 1 molecule of 'P-TEFb complex' is present.
At the beginning of this reaction, 1 molecule of 'SUPT5H protein', and 1 molecule of 'SPT4H1 protein' are present. At the end of this reaction, 1 molecule of 'DSIF complex' is present (Wada et al. 1998).
At the beginning of this reaction, 1 molecule of 'Elongin B protein', and 1 molecule of 'Elongin C protein' are present. At the end of this reaction, 1 molecule of 'Elongin B:C complex' is present. This reaction takes place in the 'nucleus' (Aso et al., 1995; Duan et al., 1995b; Kibel et al., 1995).
At the beginning of this reaction, 1 molecule of 'Elongin A1 protein', and 1 molecule of 'Elongin B:C complex' are present. At the end of this reaction, 1 molecule of 'Elongin Complex' is present.
This reaction takes place in the 'nucleus' (Kibel et al. 1995, Aso et al. 1995, Duan et al. 1995).
At the beginning of this reaction, 1 molecule of 'NELF-A protein', 1 molecule of 'RD protein', 1 molecule of 'NELF-B protein', and 1 molecule of 'NELF-C/D protein' are present. At the end of this reaction, 1 molecule of 'NELF complex' is present(Yamaguchi et al.1999).
NELF complex is a ~ 300 kDa multiprotein complex composed of 5 peptides (A - E): ~66,61,59,58 and 46 kDa. All these peptides are required for NELF-mediated inhibition of Pol II elongation. NELF complex has been reported to bind to the pre-formed DSIF:RNA Pol II complex that may act as a scaffold for its binding. NELF-A is suspected to be involved in Wolf-Hirschhorn syndrome. Binding of DSIF:NELF to RNA Pol II CTD results in abortive termination of early elongation steps by the growing transcripts.
DSIF is a heterodimer consisting of hSPT4 (human homolog of yeast Spt4- p14) and hSPT5 (human homolog of yeast Spt5-p160). DSIF association with Pol II may be enabled by Spt5 binding to Pol II creating a scaffold for NELF binding (Wada et al.,1998). Spt5 subunit of DSIF can be phosphorylated by P-TEFb.
In the early elongation phase, shorter transcripts typically of ~30 nt in length are generated due to random termination of elongating nascent transcripts. This abortive cessation of elongation has been observed mainly in the presence of DSIF-NELF bound to Pol II complex. (Reviewed in Conaway et al.,2000; Shilatifard et al., 2003 ).
Pol II pausing is believed to result from reversible backtracking of the Pol II enzyme complex by ~2 to 4 nucleotides. This leads to misaligned 3'-OH terminus that is unable to be an acceptor for the incoming NTPs in synthesis of next phosphodiester bond (reviewed by Shilatifard et al., 2003).
At the beginning of this reaction, 1 molecule of 'Processive elongation complex', and 1 molecule of 'NTP' are present. At the end of this reaction, 1 molecule of 'Elongation complex prior to separation', and 1 molecule of 'NTP' are present.
TFIIS reactivates arrested RNA Pol II directly interacting with the enzyme resulting in endonucleolytic excision of nascent transcript ~7-14 nucleotides upstream of the 3' end. This reaction is catalyzed by the catalytic site and results in the generation of a new 3'-OH terminus that could be used for re-extension from the correctly base paired site (reviewed by Shilatifard et al., 2003).
RNA Pol II arrest is believed to be a result of irreversible backsliding of the enzyme by ~7-14 nucleotides. It is suggested that, arrest leads to extrusion of displaced transcripts 3'-end through the small pore near the Mg2+ ion. Pol II arrest may lead to abortive termination of elongation due to irreversible trapping of the 3'-end of the displaced transcript in the pore (reviewed by Shilatifard et al., 2003).
At the beginning of this reaction, 1 molecule of 'Processive elongation complex' is present. At the end of this reaction, 1 molecule of 'DSIF complex', 1 molecule of 'FACT complex', 1 molecule of 'RNA Polymerase II holoenzyme complex (hyperphosphorylated)', 1 molecule of 'damaged DNA substrate:nascent mRNA hybrid', 1 molecule of 'Elongin Complex', 1 molecule of 'FCP1P protein', 1 molecule of 'P-TEFb complex', 1 molecule of 'NELF complex', 1 molecule of 'RNA polymerase II elongation factor ELL', 1 molecule of 'NTP', 1 molecule of 'TFIIS protein', and 1 molecule of 'TFIIF' are present.
This reaction takes place in the 'nucleus' (Woudstra et al. 2002).
At the beginning of this reaction, 1 molecule of 'Pol II transcription complex containing transcript to +30' is present. At the end of this reaction, 1 molecule of 'Pol II transcription complex containing extruded transcript to +30' is present.
This reaction takes place in the 'nucleus' (Buratowski 2009).
Endonucleolytic cleavage separates the pre-mRNA into an upstream fragment destined to become the mature mRNA, and a downstream fragment that is rapidly degraded. Cleavage depends on two signals in the RNA, a highly conserved hexanucleotide, AAUAAA, 10 to 30 nucleotides upstream of the cleavage site, and a poorly conserved GU- or U-rich downstream element. Additional sequences, often upstream of AAUAAA, can enhance the efficiency of the reaction. Cleavage occurs most often after a CA dinucleotide. A single gene can have more than one 3' processing site.
Cleavage is preceded by the assembly of a large processing complex, the composition of which is poorly defined. ATP, but not its hydrolysis, is required for assembly. Cleavage at the 3'-end of mRNAs depends on a number of protein factors. CPSF, a heterotetramer, binds specifically to the AAUAAA sequence. The heterotrimer CstF binds the downstream element. CF I, which appears to be composed of two subunits, one of several related larger polypeptides and a common smaller one, also binds RNA, but with unknown specificity. RNA recognition by these proteins is cooperative. Cleavage also requires CF II, composed of at least two subunits, and poly(A) polymerase, the enzyme synthesizing the poly(A) tail in the second step of the reaction. The polypeptide catalyzing the hydrolysis of the phosphodiester bond remains to be identified.
Cleavage produces a 3'-OH on the upstream fragment and a 5'-phosphate on the downstream fragment. At some unknown point after cleavage, the downstream RNA fragment, CstF, CF I and CF II are thought to be released, whereas CPSF and poly(A) polymerase remain to carry out polyadenylation.
At the beginning of this reaction, 1 molecule of 'pol II transcription complex' is present. At the end of this reaction, 1 molecule of 'TFIIA', 1 molecule of 'TFIIH', 1 molecule of 'TFIIE', 1 molecule of 'TFIID', 1 molecule of 'TFIIB', 1 molecule of 'RNA Polymerase II (unphosphorylated):TFIIF complex', and 1 molecule of 'template DNA with first transcript dinucleotide, opened to +8 position' are present.
Factor TFIIE enters the preinitiation complex after TFIIF recruits RNA Polymerase II. TFIIE is composed of two subunits of 56 kDA and 34 kDa. TFIIE facilitates the recruitment of factor TFIIH to the preinitiation complex, and it also stimulates the phosphorylation of the RNA Polymerase II CTD by TFIIH.
Formation of the second phosphodiester bond creates a 3-nt product. This short transcript is still loosely associated with the RNA polymerase II initiation complex and can dissociate to yield abortive products, which are not further extended. The transcription complex still requires continued ATP-hydrolysis by TFIIH and remains sensitive to single-stranded oligo-nucleotide inhibition.
The open region (“transcription bubble�) expands concomitant with the site of RNA-extension. In this case this region spans positions -9 to +3.
At the beginning of this reaction, 1 molecule of 'Pol II Promoter Escape Complex' is present. At the end of this reaction, 1 molecule of 'TFIIA', 1 molecule of 'TFIIH', 1 molecule of 'TFIIE', 1 molecule of 'TFIID', 1 molecule of 'TFIIB', 1 molecule of 'RNA Polymerase II (unphosphorylated):TFIIF complex', and 1 molecule of 'DNA containing Pol II promoter with transcript with 2 or 3 nucleotides' are present.
At the beginning of this reaction, 1 molecule of 'pol II open pre-initiation complex', and 2 molecules of 'NTP' are present. At the end of this reaction, 1 molecule of 'Pol II initiation complex' is present.
At the beginning of this reaction, 1 molecule of 'pol II open pre-initiation complex' is present. At the end of this reaction, 1 molecule of 'pol II closed pre-initiation complex' is present.
At the beginning of this reaction, 1 molecule of 'Pol II Initiation complex with phosphodiester-PPi intermediate' is present. At the end of this reaction, 1 molecule of 'pyrophosphate', and 1 molecule of 'pol II transcription complex' are present.
At the beginning of this reaction, 1 molecule of 'Pol II initiation complex' is present. At the end of this reaction, 1 molecule of 'Pol II Initiation complex with phosphodiester-PPi intermediate' is present.
Formation of the third phosphodiester bond creates a 4-nt product. This commits the initiation complex to promoter escape. The short 4-nt transcript is still loosely associated with the RNA polymerase II initiation complex and can dissociate to yield abortive products, which are not further extended. Inhibition of ATP-hydrolysis by TFIIH does not lead to collapse of the open region any longer. The transcription complex has lost the sensitivity to single-stranded oligo-nucleotide inhibition. However, ATP-hydrolysis and TFIIH are required for efficient promoter escape. The open region ("transcription bubble") expands concomitant with the site of RNA-extension. In this case this region spans positions -9 to +4.
Formation of the second phosphodiester bond creates a 3-nt product. This transcript is still loosely associated with the RNA polymerase II initiation complex and can dissociate to yield abortive products, which are not further extended. At this stage pausing by RNA polymerase II may result in repeated slippage and reextension of the nascent RNA. The transcription complex still requires continued ATP-hydrolysis by TFIIH for efficient promoter escape. Basal transcription factor TFIIE dissociates from the initiation complex before position +10.
Basal transcription factor TFIIF may reassociate and can stimulate transcription elongation at multiple stages. The open region (“transcription bubble�) expands concomitant with the site of RNA-extension, eventually reaching an open region from -9 to +9.
At the beginning of this reaction, 1 molecule of 'pol II transcription complex containing 4-9 nucleotide long transcript' is present. At the end of this reaction, 1 molecule of 'template DNA:4-9 nucleotide transcript hybrid', 1 molecule of 'TFIIH', 1 molecule of 'TFIIE', and 1 molecule of 'RNA Polymerase II (unphosphorylated):TFIIF complex' are present.
After assembly of the complete RNA polymerase II-preinitiation complex, the next step is separation of the two DNA strands. This isomerization step is known as the closed-to-open complex transition and occurs prior to the initiation of mRNA synthesis. In the RNA polymerase II system this step requires the hydrolysis of ATP or dATP into Pi and ADP or dADP (in contrast to the other RNA polymerase systems) and is catalyzed by the XPB subunit of TFIIH. The region of the promoter, which becomes single-stranded , spans from –10 to +2 relative to the transcription start site.
Negative supercoiling in the promoter region probably induces transient opening events and can alleviate requirement of TFIIE, TFIIH and ATP-hydrolysis for open complex formation. ATP is also used in this step by the cdk7-subunit of TFIIH to phosphorylate the heptad repeats of the C-terminal domain of the largest subunit of RNA polymerase II (RPB1) on serine-2
Formation of phosphodiester bonds nine and ten creates RNA products, which do not dissociate from the RNA pol II initiation complex. The transcription complex has enter the productive elongation phase. TFIIH and ATP-hydrolysis are required for efficient promoter escape. The open region (“transcription bubble�) expands concomitant with the site of RNA-extension. The region upstream from the transcription start site (-9 to -3) collapses to the double-stranded state. TFIIH remains associated to the RNA pol II initiation complex.
At the beginning of this reaction, 1 molecule of 'RNA Pol II with phosphorylated CTD: CE complex' is present. At the end of this reaction, 1 molecule of 'RNA Pol II with phosphorylated CTD: CE complex with activated GT' is present.
At the beginning of this reaction, 1 molecule of 'mRNA capping enzyme', and 1 molecule of 'Pol II transcription complex with (ser5) phosphorylated CTD containing extruded transcript to +30' are present. At the end of this reaction, 1 molecule of 'RNA Pol II with phosphorylated CTD: CE complex' is present.
Phosphorylation of serine 5 residue at the CTD of pol II largest subunit is an important step signaling the end of initiation and escape into processive elongation processes. Cdk7 protein subunit of TFIIH phosphorylates RNA Pol II CTD serine 5 residues on its heptad repeats (Buratowski 2009).
The capping enzyme interacts with the Spt5 subunit of transcription elongation factor DSIF. This interaction may couple the capping reaction with promoter escape or elongation, thereby acting as a "checkpoint" to assure that capping has occurred before the polymerase proceeds to make the rest of the transcript (Gonatopoulos-Pournatzis et al.2011).
Processing is initiated once the SLBP (bound to the stem loop) and the U7 snRNP (bound to the HDE) are both loaded onto the pre-mRNA. The pre-mRNA HDE makes base-pairing contacts with the 5�² end of U7 snRNA. Binding of the U7 snRNP to the pre-mRNA is stabilized by interactions between a U7 snRNP protein, hZFP100 and SLBP. It should be noted that there must be other trans-acting factors, including the factor that catalyzes the cleavage reaction, which have yet to be defined. The cleavage occurs in the presence of EDTA as does the cleavage reaction in polyadenylation, it is likely that this reaction is catalyzed by a protein. There may well be additional proteins associated with the U7 snRNP, and since in some conditions in vitro processing occurs in the absence of SLBP, it is possible that all the other factors required for processing are associated with the active form of the U7 snRNP.
The polypeptide catalyzing the hydrolysis of the phosphodiester bond remains to be identified. Cleavage produces a 3'-OH on the upstream fragment and a 5'-phosphate on the downstream fragment. At some unknown point after cleavage, the downstream fragment, CstF, CF I and CF II are thought to be released, whereas CPSF and poly(A) polymerase remain to carry out polyadenylation.
The human BTF2 basic transcription factor (also called TFIIH), is required for class1 gene transcription of the second round of transcripts. TFIIH has an adenosine triphosphate-dependent DNA helicase activity. The helicase activity is closely associated with the multi-subunit BTF2/TFIIH transcription factor which also has a CTD protein kinase activity.
The human BTF2 basic transcription factor (also called TFIIH), is required for class1 gene transcription of transcripts. TFIIH has an adenosine triphosphate-dependent DNA helicase activity. The helicase activity is closely associated with the multi-subunit BTF2/TFIIH transcription factor which also has a CTD protein kinase activity.
RNA
Pol
II
(hypophosphorylated) complex bound to DSIF protein
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DataNodes
ligated exon
containing complexcomplex after
arrestIntronless Histone
pre-mRNA:CBP80:CBP20:SLBP:ZFP100 ComplexHistone pre-mRNA:CBC:ZFP100
ComplexII promoter with transcript with 2
or 3 nucleotidesPolymerase II
promotercomplex with hyperphosphorylated
Pol II CTDwith separated and uncleaved
transcriptTranscription
PathwayDetailed studies of gene transcription regulation in a wide variety of eukaryotic systems has revealed the general principles and mechanisms by which cell- or tissue-specific regulation of differential gene transcription is mediated (reviewed in Naar, 2001. Kadonaga, 2004, Maston, 2006, Barolo, 2002; Roeder, 2005, Rosenfeld, 2006). Of the three major classes of DNA polymerase involved in eukaryotic gene transcription, Polymerase II generally regulates protein-encoding genes. Figure 1 shows a diagram of the various components involved in cell-specific regulation of Pol-II gene transcription.
Core Promoter: Pol II-regulated genes typically have a Core Promoter where Pol II and a variety of general factors bind to specific DNA motifs:
i: the TATA box (TATA DNA sequence), which is bound by the "TATA-binding protein" (TBP).
ii: the Initiator motif (INR), where Pol II and certain other core factors bind, is present in many Pol II-regulated genes.
iii: the Downstream Promoter Element (DPE), which is present in a subset of Pol II genes, and where additional core factors bind.
The core promoter binding factors are generally ubiquitously expressed, although there are exceptions to this.
Proximal Promoter: immediately upstream (5') of the core promoter, Pol II target genes often have a Proximal Promoter region that spans up to 500 base pairs (b.p.), or even to 1000 b.p.. This region contains a number of functional DNA binding sites for a specific set of transcription activator (TA) and transcription repressor (TR) proteins. These TA and TR factors are generally cell- or tissue-specific in expression, rather than ubiquitous, so that the presence of their cognate binding sites in the proximal promoter region programs cell- or tissue-specific expression of the target gene, perhaps in conjunction with TA and TR complexes bound in distal enhancer regions.
Distal Enhancer(s): many or most Pol II regulated genes in higher eukaryotes have one or more distal Enhancer regions which are essential for proper regulation of the gene, often in a cell or tissue-specific pattern. Like the proximal promoter region, each of the distal enhancer regions typically contain a cluster of binding sites for specific TA and/or TR DNA-binding factors, rather than just a single site.
Enhancers generally have three defining characteristics:
i: They can be located very long distances from the promoter of the target gene they regulate, sometimes as far as 100 Kb, or more.
ii: They can be either upstream (5') or downstream (3') of the target gene, including within introns of that gene.
iii: They can function in either orientation in the DNA.
Combinatorial mechanisms of transcription regulation: The specific combination of TA and TR binding sites within the proximal promoter and/or distal enhancer(s) provides a "combinatorial transcription code" that mediates cell- or tissue-specific expression of the associated target gene. Each promoter or enhancer region mediates expression in a specific subset of the overall expression pattern. In at least some cases, each enhancer region functions completely independently of the others, so that the overall expression pattern is a linear combination of the expression patterns of each of the enhancer modules.
Co-Activator and Co-Repressor Complexes: DNA-bound TA and TR proteins typically recruit the assembly of specific Co-Activator (Co-A) and Co-Repressor (Co-R) Complexes, respectively, which are essential for regulating target gene transcription. Both Co-A's and Co-R's are multi-protein complexes that contain several specific protein components.
Co-Activator complexes generally contain at lease one component protein that has Histone Acetyl Transferase (HAT) enzymatic activity. This functions to acetylate Histones and/or other chromatin-associated factors, which typically increases that transcription activation of the target gene. By contrast, Co-Repressor complexes generally contain at lease one component protein that has Histone De-Acetylase (HDAC) enzymatic activity. This functions to de-acetylate Histones and/or other chromatin-associated factors. This typically increases the transcription repression of the target gene.
Adaptor (Mediator) complexes: In addition to the co-activator complexes that assemble on particular cell-specific TA factors, - there are at least two additional transcriptional co-activator complexes common to most cells. One of these is the Mediator complex, which functions as an "adaptor" complex that bridges between the tissue-specific co-activator complexes assembled in the proximal promoter (or distal enhancers). The human Mediator complex has been shown to contain at least 19 protein distinct components. Different combinations of these co-activator proteins are also found to be components of specific transcription Co-Activator complexes, such as the DRIP, TRAP and ARC complexes described below.
TBP/TAF complex: Another large Co-A complex is the "TBP-associated factors" (TAFs) that assemble on TBP (TATA-Binding Protein), which is bound to the TATA box present in many promoters. There are at least 23 human TAF proteins that have been identified. Many of these are ubiquitously expressed, but TAFs can also be expressed in a cell or tissue-specific pattern.
Specific Coactivator Complexes for DNA-binding Transcription Factors.
A number of specific co-activator complexes for DNA-binding transcription factors have been identified, including DRIP, TRAP, and ARC (reviewed in Bourbon, 2004, Blazek, 2005, Conaway, 2005, and Malik, 2005). The DRIP co-activator complex was originally identified and named as a specific complex associated with the Vitamin D Receptor member of the nuclear receptor family of transcription factors (Rachez, 1998). Similarly, the TRAP co-activator complex was originally identified as a complex that associates with the thyroid receptor (Yuan, 1998). It was later determined that all of the components of the DRIP complex are also present in the TRAP complex, and the ARC complex (discussed further below). For example, the DRIP205 and TRAP220 proteins were show to be identical, as were specific pairs of the other components of these complexes (Rachez, 1999).
In addition, these various transcription co-activator proteins identified in mammalian cells were found to be the orthologues or homologues of the Mediator ("adaptor") complex proteins (reviewed in Bourbon, 2004). The Mediator proteins were originally identified in yeast by Kornberg and colleagues, as complexes associated with DNA polymerase (Kelleher, 1990). In higher organisms, Adapter complexes bridge between the basal transcription factors (including Pol II) and tissue-specific transcription factors (TFs) bound to sites within upstream Proximal Promoter regions or distal Enhancer regions (Figure 1). However, many of the Mediator homologues can also be found in complexes associated with specific transcription factors in higher organisms. A unified nomenclature system for these adapter / co-activator proteins now labels them Mediator 1 through Mediator 31 (Bourbon, 2004). For example, the DRIP205 / TRAP220 proteins are now identified as Mediator 1 (Rachez, 1999), based on homology with yeast Mediator 1.
Example Pathway: Specific Regulation of Target Genes During Notch Signaling:One well-studied example of cell-specific regulation of gene transcription is selective regulation of target genes during Notch signaling. Notch signaling was first identified in Drosophila, where it has been studied in detail at the genetic, molecular, biochemical and cellular levels (reviewed in Justice, 2002; Bray, 2006; Schweisguth, 2004; Louvri, 2006). In Drosophila, Notch signaling to the nucleus is thought always to be mediated by one specific DNA binding transcription factor, Suppressor of Hairless. In mammals, the homologous genes are called CBF1 (or RBPJkappa), while in worms they are called Lag-1, so that the acronym "CSL" has been given to this conserved transcription factor family. There are at least two human CSL homologues, which are now named RBPJ and RBPJL.
In Drosophila, Su(H) is known to be bifunctional, in that it represses target gene transcription in the absence of Notch signaling, but activates target genes during Notch signaling. At least some of the mammalian CSL homologues are believed also to be bifunctional, and to mediate target gene repression in the absence of Notch signaling, and activation in the presence of Notch signaling.
Notch Co-Activator and Co-Repressor complexes: This repression is mediated by at least one specific co-repressor complexes (Co-R) bound to CSL in the absence of Notch signaling. In Drosophila, this co-repressor complex consists of at least three distinct co-repressor proteins: Hairless, Groucho, and dCtBP (Drosophila C-terminal Binding Protein). Hairless has been show to bind directly to Su(H), and Groucho and dCtBP have been shown to bind directly to Hairless (Barolo, 2002). All three of the co-repressor proteins have been shown to be necessary for proper gene regulation during Notch signaling in vivo (Nagel, 2005).
In mammals, the same general pathway and mechanisms are observed, where CSL proteins are bifunctional DNA binding transcription factors (TFs), that bind to Co-Repressor complexes to mediate repression in the absence of Notch signaling, and bind to Co-Activator complexes to mediate activation in the presence of Notch signaling. However, in mammals, there may be multiple co-repressor complexes, rather than the single Hairless co-repressor complex that has been observed in Drosophila.
During Notch signaling in all systems, the Notch transmembrane receptor is cleaved and the Notch intracellular domain (NICD) translocates to the nucleus, where it there functions as a specific transcription co-activator for CSL proteins. In the nucleus, NICD replaces the Co-R complex bound to CSL, thus resulting in de-repression of Notch target genes in the nucleus (Figure 2). Once bound to CSL, NICD and CSL proteins recruit an additional co-activator protein, Mastermind, to form a CSL-NICD-Mam ternary co-activator (Co-A) complex. This Co-R complex was initially thought to be sufficient to mediate activation of at least some Notch target genes. However, there now is evidence that still other co-activators and additional DNA-binding transcription factors are required in at least some contexts (reviewed in Barolo, 2002).
Thus, CSL is a good example of a bifunctional DNA-binding transcription factor that mediates repression of specific targets genes in one context, but activation of the same targets in another context. This bifunctionality is mediated by the association of specific Co-Repressor complexes vs. specific Co-Activator complexes in different contexts, namely in the absence or presence of Notch signaling.
Intronless transcript derived Histone
mRNA:SLBP:CBP80:CBP20transcript derived Histone pre-mRNA:CBC
complexcomplex with phosphodiester-PPi
intermediatecomplex containing extruded transcript
to +30complex containing
transcript to +30complex with (ser5) phosphorylated CTD containing extruded
transcript to +30Pol II
(hypophosphorylated) complex bound to DSIF proteinPol II
(hypophosphorylated):capped pre-mRNA complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II holoenzyme complex
(hyperphosphorylated)phosphorylated CTD: CE complex with
activated GTphosphorylated CTD:
CE complextranscribes snRNA
genesThe snRNA genes are among the most rapidly transcribed genes in the genome. The 5' transcribed region of the U2 snRNA gene is largely single-stranded during interphase and metaphase (Pavelitz et al. 2008) and chromatin within the transcribed region is cleared of nucleosomes (O'Reilly et al. 2014). Transcriptional activation of the RNA polymerase II transcribed snRNA genes begins with binding of transcription factors to the distal sequence element (DSE) of the promoter (reviewed in Hernandez 2001, Egloff et al. 2008, Jawdeker and Henry 2008). The factors, which include POU2F1 (Oct-1), POU2F2 (Oct-2), ZNF143 (Staf) and Sp1, promote binding of the SNAPc complex (also known as PTF and PBP) to the PSE. SNAPc helps clear the gene of nucleosomes (O'Reilly et al. 2014) and recruits initiation factors (TFIIA, TFIIB, TFIIE, TFIIF, and snTAFc:TBP) which recruit RNA polymerase II. Phosphorylation of the C-terminal domain (CTD) of RNA polymerase II (reviewed in Egloff and Murphy 2008) by CDK7 recruits RPAP2 and the Integrator complex, which is required for later processing of the 3' end of the pre-snRNA transcript (reviewed in Chen and Wagner 2010, Baillat and Wagner 2015). The Little Elongation Complex (LEC) also appears to bind around the time of transcription initiation (Hu et al. 2013). As transcription proceeds, RPAP2 dephosphorylates serine-5 and P-TEFb phosphorylates serine-2 of the CTD. As transcription reaches the end of the snRNA gene serine-7 of the CTD is phosphorylated. These marks serve to bind protein complexes and are required for 3' processing of the pre-snRNA (reviewed in Egloff and Murphy 2008). After transcription proceeds through the conserved 3' processing sequence of the pre-snRNA the Integrator complex cleaves the pre-snRNA. Transcription then terminates downstream in a less well characterized reaction that requires elements of the polyadenylation system.
pre-mRNA:CBC:RNA Pol II (phosphorylated)
complexsubstrate:nascent
mRNA hybridintronless mRNA
fragmentII
promoter:TFIID:TFIIA:TFIIB complexII
promoter:TFIID:TFIIA:TFIIB:Pol II:TFIIF complexII
promoter:TFIID:TFIIA:TFIIB:Pol II:TFIIF:TFIIE complexpromoter:TFIID
complexpre-initiation
complexpre-initiation
complexcomplex containing 11 nucleotide long
transcriptcomplex containing 3 Nucleotide long
transcriptcomplex containing 4 nucleotide long
transcriptcomplex containing 4-9 nucleotide long
transcriptcomplex containing 9 nucleotide long
transcriptfirst transcript dinucleotide, opened to +8
positionnucleotide
transcript hybridmRNA
fragment:CPSF:PAP:PABPN1 complexAnnotated Interactions
ligated exon
containing complexcomplex after
arrestIntronless Histone
pre-mRNA:CBP80:CBP20:SLBP:ZFP100 ComplexHistone pre-mRNA:CBC:ZFP100
ComplexII promoter with transcript with 2
or 3 nucleotidesPolymerase II
promotercomplex with hyperphosphorylated
Pol II CTDcomplex with hyperphosphorylated
Pol II CTDwith separated and uncleaved
transcriptIntronless transcript derived Histone
mRNA:SLBP:CBP80:CBP20transcript derived Histone pre-mRNA:CBC
complexcomplex with phosphodiester-PPi
intermediatecomplex with phosphodiester-PPi
intermediatecomplex containing extruded transcript
to +30complex containing extruded transcript
to +30complex containing extruded transcript
to +30complex containing
transcript to +30complex containing
transcript to +30complex with (ser5) phosphorylated CTD containing extruded
transcript to +30complex with (ser5) phosphorylated CTD containing extruded
transcript to +30Transcriptional activators function by affecting the kinetics of binding of TBP to the promoter DNA.
TFIIB interacts with the Rpb1 subunit of RNA polymerase II to define transcription strat sites. Several activators directly bind TFIIB, and stimulate transcription. The N-terminus and the C-terminus can participate in intramolecular interactions, and this can be disrupted by specific activators by causing a conformational change in TFIIB.
TFIIA also binds the preinitiation complex along with TFIIB. However, TFIIA is not required for accurate initiation, but rather functions as a coactivator of transcription.
This reaction takes place in the 'nucleus'.
A P-TEFb complex (which contains only the Cyclin T1) is implicated in the efficient synthesis of human immunodeficiency virus-1 (HIV-1) transcripts. Cyclin T1 subunit of the P-TEFb(Cyclin T1:Cdk9) complex interacts with HIV-1 encoded Tat protein that binds to the transactivation response (TAR) element in the nascent HIV-1 transcript (reviewed in Price,2000).
The mechanism by which DSIF, NELF and P-TEFb or TAK/P-TEFb act together in Pol II-regulated elongation is yet to be fully understood. Various biochemical evidences point to a model in which DSIF and NELF negatively regulate elongation through interactions with polymerase containing a hypophosphorylated CTD. Subsequent phosphorylation of the Pol II CTD by P-TEFb might promote elongation by inhibiting interactions of DSIF and NELF with the elongation complex.
- RNA-DNA hybrid, an unwound template ahead of 3'-OH terminus of growing transcript and an exit groove at the base of the CTD, possibly for dynamic interaction of processing and transcriptional factors.
- a cleft or channel created by Rpb1 and Rpb2 subunits to accommodate DNA template, extending to Mg2+ ion located deep in the enzyme core
-a 50 kDa "clamp" with open confirmation in free polymerase, allowing entry of DNA strands but closed in the processive elongation phase.
The clamp is composed of portions of Rpb1,Rpb2 and Rpb3 , five loops or "switches" that change from unfolded to well-folded structures stabilizing the elongation complex, and a long "bridging helix" that emanates from Rpb1 subunit, crossing near the Mg2+ ion. The bridging helix is thought to "bend" to push on the base pair at the 3'-end of RNA-DNA hybrid like a ratchet, translocating Pol II along the DNA (Cramer et al.,2001; Gnatt et al.,2001).In addition to its dynamic biochemical potential, Pol II possess a repertoire of functions to serve as a critical platform of recruiting and coordinating the actions of a host of additional enzyme and proteins involved in various pathways.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus' (Kamakaka et al.1993, Orphanides et al.1998).
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus' (Kibel et al. 1995, Aso et al. 1995, Duan et al. 1995).
This reaction takes place in the 'nucleus'.
Binding of DSIF:NELF to RNA Pol II CTD results in abortive termination of early elongation steps by the growing transcripts.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus' (Woudstra et al. 2002).
This reaction takes place in the 'nucleus' (Buratowski 2009).
Cleavage is preceded by the assembly of a large processing complex, the composition of which is poorly defined. ATP, but not its hydrolysis, is required for assembly. Cleavage at the 3'-end of mRNAs depends on a number of protein factors. CPSF, a heterotetramer, binds specifically to the AAUAAA sequence. The heterotrimer CstF binds the downstream element. CF I, which appears to be composed of two subunits, one of several related larger polypeptides and a common smaller one, also binds RNA, but with unknown specificity. RNA recognition by these proteins is cooperative. Cleavage also requires CF II, composed of at least two subunits, and poly(A) polymerase, the enzyme synthesizing the poly(A) tail in the second step of the reaction. The polypeptide catalyzing the hydrolysis of the phosphodiester bond remains to be identified.
Cleavage produces a 3'-OH on the upstream fragment and a 5'-phosphate on the downstream fragment. At some unknown point after cleavage, the downstream RNA fragment, CstF, CF I and CF II are thought to be released, whereas CPSF and poly(A) polymerase remain to carry out polyadenylation.
This reaction takes place in the 'nucleus'.
The open region (“transcription bubble�) expands concomitant with the site of RNA-extension. In this case this region spans positions -9 to +3.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
Basal transcription factor TFIIF may reassociate and can stimulate transcription elongation at multiple stages. The open region (“transcription bubble�) expands concomitant with the site of RNA-extension, eventually reaching an open region from -9 to +9.
This reaction takes place in the 'nucleus'.
Negative supercoiling in the promoter region probably induces transient opening events and can alleviate requirement of TFIIE, TFIIH and ATP-hydrolysis for open complex formation. ATP is also used in this step by the cdk7-subunit of TFIIH to phosphorylate the heptad repeats of the C-terminal domain of the largest subunit of RNA polymerase II (RPB1) on serine-2
This reaction takes place in the 'nucleus'.
This reaction takes place in the 'nucleus'.
Pol II
(hypophosphorylated) complex bound to DSIF proteinPol II
(hypophosphorylated) complex bound to DSIF proteinPol II
(hypophosphorylated):capped pre-mRNA complexPol II
(hypophosphorylated):capped pre-mRNA complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II
(unphosphorylated):TFIIF complexPolymerase II holoenzyme complex
(hyperphosphorylated)phosphorylated CTD: CE complex with
activated GTphosphorylated CTD: CE complex with
activated GTphosphorylated CTD:
CE complexphosphorylated CTD:
CE complexpre-mRNA:CBC:RNA Pol II (phosphorylated)
complexsubstrate:nascent
mRNA hybridintronless mRNA
fragmentII
promoter:TFIID:TFIIA:TFIIB complexII
promoter:TFIID:TFIIA:TFIIB complexII
promoter:TFIID:TFIIA:TFIIB:Pol II:TFIIF complexII
promoter:TFIID:TFIIA:TFIIB:Pol II:TFIIF complexII
promoter:TFIID:TFIIA:TFIIB:Pol II:TFIIF:TFIIE complexII
promoter:TFIID:TFIIA:TFIIB:Pol II:TFIIF:TFIIE complexpromoter:TFIID
complexpromoter:TFIID
complexpre-initiation
complexpre-initiation
complexpre-initiation
complexpre-initiation
complexpre-initiation
complexpre-initiation
complexcomplex containing 11 nucleotide long
transcriptcomplex containing 11 nucleotide long
transcriptcomplex containing 3 Nucleotide long
transcriptcomplex containing 3 Nucleotide long
transcriptcomplex containing 3 Nucleotide long
transcriptcomplex containing 3 Nucleotide long
transcriptcomplex containing 4 nucleotide long
transcriptcomplex containing 4 nucleotide long
transcriptcomplex containing 4-9 nucleotide long
transcriptcomplex containing 9 nucleotide long
transcriptcomplex containing 9 nucleotide long
transcriptfirst transcript dinucleotide, opened to +8
positionnucleotide
transcript hybridmRNA
fragment:CPSF:PAP:PABPN1 complex