Promoter-proximal stalling results from the inability to recruit transcription factor IIH to the transcription complex and is a regulated event

Structure of the central core domain of TFIIEbeta with a novel double-stranded DNA-binding surface

Human general transcription factor TFIIE consists of two subunits, TFIIEalpha and TFIIEbeta. Recently, TFIIEbeta has been found to bind to the region where the promoter starts to open to be single-stranded upon transcription initiation by RNA polymerase II. Here, the central core domain of human TFIIEbeta (TFIIEbetac) has been identified by a limited proteolysis. This solution structure has been determined by NMR. It consists of three helices with a beta hairpin at the C-terminus, resembling the winged helix proteins. However, TFIIEbetac shows a novel double-stranded DNA-binding activity where the DNA-binding surface locates on the opposite side to the previously reported winged helix motif by forming a positively charged furrow. A model will be proposed that TFIIE stabilizes the preinitiation complex by binding not only to the general transcription factors together with RNA polymerase II but also to the promoter DNA, where double-stranded DNA starts to open to be single-stranded upon activation of the preinitiation complex.

Rad25p, a DNA helicase subunit of yeast transcription factor TFIIH, is required for promoter escape in vivo

The general transcription factor TFIIH is required for initial DNA unwinding and promoter escape by RNA polymerase II in vitro. We examined whether Rad25p, a DNA helicase subunit of TFIIH, mediates promoter opening and promoter escape in the yeast Saccharomyces cerevisiae. DNA unwinding was probed with an in vivo permanganate reactivity assay, in a temperature-sensitive mutant of RAD25. The consequences of Rad25p inactivation were promoter-specific. Whereas in the TDH2 promoter permanganate reactivity was entirely abolished, the reactivity at the GAL1 and GAL10 promoter regions was only moderately affected. In the GAL genes permanganate reactivity uniformly decreased downstream of the transcription start site, indicating that progression of RNA polymerase II to this region was impaired. Our results suggest that in yeast cells, promoter opening is not sufficient for productive initiation and that Rad25p-mediated promoter escape may be a limiting step in the transcription of some promoters.

The FBP interacting repressor targets TFIIH to inhibit activated transcription

FUSE-binding protein (FBP) binds the single-stranded far upstream element of active c-myc genes, possesses potent transcription activation and repression domains, and is necessary for c-myc expression. A novel 60 kDa protein, the FBP interacting repressor (FIR), blocked activator-dependent, but not basal, transcription through TFIIH. Recruited through FBP's nucleic acid-binding domain, FIR formed a ternary complex with FBP and FUSE. FIR repressed a c-myc reporter via the FUSE. The amino terminus of FIR contained an activator-selective repression domain capable of acting in cis or even in trans in vivo and in vitro. The repression domain of FIR targeted only TFIIH's p89/XPB helicase, required at several stages in transcription, but not factors required for promoter selection. Thus, FIR locks TFIIH in an activation-resistant configuration that still supports basal transcription.

Distinct roles for the helicases of TFIIH in transcript initiation and promoter escape

To provide an explanation of some clinical features observed within rare xeroderma pigmentosum (XP) patients and to further define the role of XPB, XPD, and cdk7, the three enzymatic subunits of TFIIH, in the transcription reaction, we have examined two defined enzymatic steps: phosphodiester bond formation and promoter escape. We provide evidence that the XPB helicase plays a dominant role in initiation, whereas the XPD helicase plays a minor contributing role in this step. The cyclin-activating kinase subcomplex of TFIIH improves the efficiency of initiation, but this involves only the structural contributions of cyclin-activating kinase rather than enzymatic activity. We demonstrate that XPB patient-derived mutants in TFIIH suffer from defects in initiation. Moreover, mutant analysis shows that in addition to its crucial role in initiation, the XPB helicase plays a critical enzymatic role in the promoter escape, whereas XPD plays an important structural role in the promoter escape process. Finally, using patient-derived mutations in TFIIH, we demonstrate deficiencies in promoter escape for both mutants of the class that suffer from combined xeroderma pigmentosum/Cockayne's syndrome.

Dual roles for transcription factor IIF in promoter escape by RNA polymerase II

Transcription factor (TF) IIF is a multifunctional RNA polymerase II transcription factor that has well established roles in both transcription initiation, where it functions as a component of the preinitiation complex and is required for formation of the open complex and synthesis of the first phosphodiester bond of nascent transcripts, and in transcription elongation, where it is capable of interacting directly with the ternary elongation complex and stimulating the rate of transcription. In this report, we present evidence that TFIIF is also required for efficient promoter escape by RNA polymerase II. Our findings argue that TFIIF performs dual roles in this process. We observe (i) that TFIIF suppresses the frequency of abortive transcription by very early RNA polymerase II elongation intermediates by increasing their processivity and (ii) that TFIIF cooperates with TFIIH to prevent premature arrest of early elongation intermediates. In addition, our findings argue that two TFIIF functional domains mediate TFIIF action in promoter escape. First, we observe that a TFIIF mutant selectively lacking elongation activity supports TFIIH action in promoter escape, but is defective in suppressing the frequency of abortive transcription by very early RNA polymerase II elongation intermediates. Second, a TFIIF mutant selectively lacking initiation activity is more active than wild type TFIIF in increasing the processivity of very early elongation intermediates, but is defective in supporting TFIIH action in promoter escape. Taken together, our findings bring to light a function for TFIIF in promoter escape and support a role for TFIIF elongation activity in this process.

Abortive initiation by Saccharomyces cerevisiae RNA polymerase III

Promoter escape can be rate-limiting for transcription by bacterial RNA polymerases and RNA polymerase II of higher eukaryotes. Formation of a productive elongation complex requires disengagement of RNA polymerase from promoter-bound eukaryotic transcription factors or bacterial sigma factors. RNA polymerase III (pol III) stably associates with the TFIIIB-DNA complex even in the absence of localized DNA unwinding associated with the open promoter complex. To explore the role that release of pol III from the TFIIIB-DNA complex plays in limiting the overall rate of transcription, we have examined the early steps of RNA synthesis. We find that, on average, only three rounds of abortive initiation precede the formation of each elongation complex and that nearly all pol III molecules escape the abortive initiation phase of transcription without significant pausing or arrest. However, when elongation is limited to 5 nucleotides, the intrinsic exoribonuclease activity of pol III cleaves 5-mer RNA at a rate considerably faster than product release or reinitiation. This cleavage also occurs in the normal process of forming a productive elongation complex. The possible role of nucleolytic retraction in disengaging pol III from TFIIIB is discussed.

A role for the TFIIH XPB DNA helicase in promoter escape by RNA polymerase II

TFIIH is an RNA polymerase II transcription factor that performs ATP-dependent functions in both transcription initiation, where it catalyzes formation of the open complex, and in promoter escape, where it suppresses arrest of the early elongation complex at promoter-proximal sites. TFIIH possesses three known ATP-dependent activities: a 3' --> 5' DNA helicase catalyzed by its XPB subunit, a 5' --> 3' DNA helicase catalyzed by its XPD subunit, and a carboxyl-terminal domain (CTD) kinase activity catalyzed by its CDK7 subunit. In this report, we exploit TFIIH mutants to investigate the contributions of TFIIH DNA helicase and CTD kinase activities to efficient promoter escape by RNA polymerase II in a minimal transcription system reconstituted with purified polymerase and general initiation factors. Our findings argue that the TFIIH XPB DNA helicase is primarily responsible for preventing premature arrest of early elongation intermediates during exit of polymerase from the promoter.

Ubiquitination of RNA polymerase II large subunit signaled by phosphorylation of carboxyl-terminal domain

A sensitive assay using biotinylated ubiquitin revealed extensive ubiquitination of the large subunit of RNA polymerase II during incubations of transcription reactions in vitro. Phosphorylation of the repetitive carboxyl-terminal domain of the large subunit was a signal for ubiquitination. Specific inhibitors of cyclin-dependent kinase (cdk)-type kinases suppress the ubiquitination reaction. These kinases are components of transcription factors and have been shown to phosphorylate the carboxyl-terminal domain. In both regulation of transcription and DNA repair, phosphorylation of the repetitive carboxyl-terminal domain by kinases might signal degradation of the polymerase.

Initially transcribed sequences strongly affect the extent of abortive initiation by RNA polymerase II

We investigated transcript initiation and early elongation by RNA polymerase II using templates mismatched between -9 and +3 (bubble templates). Highly purified RNA polymerase II alone was able to initiate transcription specifically on these templates in the presence of dinucleotide primers. The length distribution of abortively initiated RNAs was similar for purified RNA polymerase II on bubble templates and polymerase II on double-stranded templates in HeLa nuclear extracts. Increasing the U content in the initial portion of the transcript caused similar increases in abortive initiation for transcription of bubble templates by pure polymerase and double-stranded templates in extracts. Thus, the level of abortive initiation by RNA polymerase II is at least partly determined by interactions of the polymerase with the transcript and/or the template, independent of the general transcription factors. Substitution of 5-bromo-UTP for UTP reduced abortive initiation on bubble templates, consistent with the idea that transcription complex stability during early elongation depends on the strength of the initial RNA-DNA hybrid. Interestingly, transcription of bubble templates in HeLa extracts gave very high levels of abortive initiation, suggesting that inability to reanneal the initially melted template segment inhibits transcript elongation in the presence of the initiation factors.

NELF, a multisubunit complex containing RD, cooperates with DSIF to repress RNA polymerase II elongation

DRB is a classic inhibitor of transcription elongation by RNA polymerase II (pol II). Since DRB generally affects class II genes, factors involved in this process must play fundamental roles in pol II elongation. Recently, two elongation factors essential for DRB action were identified, namely DSIF and P-TEFb. Here we describe the identification and purification from HeLa nuclear extract of a third protein factor required for DRB-sensitive transcription. This factor, termed negative elongation factor (NELF), cooperates with DSIF and strongly represses pol II elongation. This repression is reversed by P-TEFb-dependent phosphorylation of the pol II C-terminal domain. NELF is composed of five polypeptides, the smallest of which is identical to RD, a putative RNA-binding protein of unknown function. This study reveals a molecular mechanism for DRB action and a regulatory network of positive and negative elongation factors.

Tat-associated kinase (P-TEFb): a component of transcription preinitiation and elongation complexes

Human immunodeficiency virus, type 1 (HIV-1) Tat protein activates transcription from the HIV-1 long terminal repeat. Tat interacts with TFIIH and Tat-associated kinase (a transcription elongation factor P-TEFb) and requires the carboxyl-terminal domain of the largest subunit of RNA polymerase II (pol II) for transactivation. We developed a stepwise RNA pol II walking approach and used Western blotting to determine the role of TFIIH and P-TEFb in HIV-1 transcription elongation. Our results demonstrate the new findings that P-TEFb is a component of the preinitiation complex and travels with the elongating RNA pol II, whereas TFIIH is released from the elongation complexes before the trans-activation responsive region RNA is synthesized. Our results suggest that TFIIH and P-TEFb are involved in the clearance of promoter-proximal pausing of RNA pol II on the HIV-1 long terminal repeat at different stages.

The general transcription factors IIA, IIB, IIF, and IIE are required for RNA polymerase II transcription from the human U1 small nuclear RNA promoter

RNA polymerase II transcribes the mRNA-encoding genes and the majority of the small nuclear RNA (snRNA) genes. The formation of a minimal functional transcription initiation complex on a TATA-box-containing mRNA promoter has been well characterized and involves the ordered assembly of a number of general transcription factors (GTFs), all of which have been either cloned or purified to near homogeneity. In the human RNA polymerase II snRNA promoters, a single element, the proximal sequence element (PSE), is sufficient to direct basal levels of transcription in vitro. The PSE is recognized by the basal transcription complex SNAPc. SNAPc, which is not required for transcription from mRNA-type RNA polymerase II promoters such as the adenovirus type 2 major late (Ad2ML) promoter, is thought to recruit TATA binding protein (TBP) and nucleate the assembly of the snRNA transcription initiation complex, but little is known about which GTFs other than TBP are required. Here we show that the GTFs IIA, IIB, IIF, and IIE are required for efficient RNA polymerase II transcription from snRNA promoters. Thus, although the factors that recognize the core elements of RNA polymerase II mRNA and snRNA-type promoters differ, they mediate the recruitment of many common GTFs.

Immunoaffinity purification and functional characterization of human transcription factor IIH and RNA polymerase II from clonal cell lines that conditionally express epitope-tagged subunits of the multiprotein complexes

Purification of multiprotein complexes such as transcription factor (TF) IIH and RNA polymerase II (pol II) has been a tedious task by conventional chromatography. To facilitate the purification, we have developed an effective scheme that allows human TFIIH and pol II to be isolated from HeLa-derived cell lines that conditionally express the FLAG-tagged p62 subunit of human TFIIH and the RPB9 subunit of human pol II, respectively. An approximate 2000-fold enrichment of FLAG-tagged TFIIH and a 1000-fold enhancement of total pol II are achieved by a one-step immunoaffinity purification. The purified complexes are functional in mediating basal and activated transcription, regardless of whether TATA-binding protein or TFIID is used as the TATA-binding factor. Interestingly, repression of basal transcription by the positive cofactor PC4 is alleviated by increasing amounts of TFIID, TFIIH, and pol II holoenzyme, suggesting that phosphorylation of PC4 by these proteins may cause a conformational change in the structure of PC4 that allows for preinitiation complex formation and initiation of transcription. Furthermore, pol II complexes with different phosphorylation states on the carboxyl-terminal domain of the largest subunit are selectively purified from the inducible pol II cell line, making it possible to dissect the role of carboxyl-terminal domain phosphorylation in the transcription process in a highly defined in vitro transcription system.

The molecular mechanism of mitotic inhibition of TFIIH is mediated by phosphorylation of CDK7

TFIIH is a multisubunit complex, containing ATPase, helicases, and kinase activities. Functionally, TFIIH has been implicated in transcription by RNA polymerase II (RNAPII) and in nucleotide excision repair. A member of the cyclin-dependent kinase family, CDK7, is the kinase subunit of TFIIH. Genetically, CDK7 homologues have been implicated in transcription in Saccharomyces cerevisiae, and in mitotic regulation in Schizosaccharomyces pombe. Here we show that in mitosis the CDK7 subunit of TFIIH and the largest subunit of RNAPII become hyperphosphorylated. MPF-induced phosphorylation of CDK7 results in inhibition of the TFIIH-associated kinase and transcription activities. Negative and positive regulation of TFIIH requires phosphorylation within the T-loop of CDK7. Our data establishes TFIIH and its subunit CDK7 as a direct link between the regulation of transcription and the cell cycle.