RNA polymerase II-associated protein (RAP) 74 binds transcription factor (TF) IIB and blocks TFIIB-RAP30 binding

Mechanism of mammalian transcriptional repression by noncoding RNA

Transcription by RNA polymerase II (Pol II) can be repressed by noncoding RNA, including the human RNA Alu. However, the mechanism by which endogenous RNAs repress transcription remains unclear. Here we present cryogenic-electron microscopy structures of Pol II bound to Alu RNA, which reveal that Alu RNA mimics how DNA and RNA bind to Pol II during transcription elongation. Further, we show how distinct domains of the general transcription factor TFIIF control repressive activity. Together, we reveal how a noncoding RNA can regulate mammalian gene expression.

Diverse and conserved roles of the protein Ssu72 in eukaryotes: from yeast to higher organisms

Gene transcription is a complex biological process that involves a set of factors, enzymes and nucleotides. Ssu72 plays a crucial role in every step of gene transcription. RNA polymerase II (RNAPII) occupies an important position in the synthesis of mRNAs. The largest subunit of RNAPII, Rpb1, harbors its C-terminal domain (CTD), which participates in the initiation, elongation and termination of transcription. The CTD consists of heptad repeats of the consensus motif Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7 and is highly conserved among different species. The CTD is flexible in structure and undergoes conformational changes in response to serine phosphorylation and proline isomerization, which are regulated by specific kinases/phosphatases and isomerases, respectively. Ssu72 is a CTD phosphatase with catalytic activity against phosphorylated Ser5 and Ser7. The isomerization of Pro6 affects the binding of Ssu72 to its substrate. Ssu72 can also indirectly change the phosphorylation status of Ser2. In addition, Ssu72 is a member of the 3'-end cleavage and polyadenylation factor (CPF) complex. Together with other CPF components, Ssu72 regulates the 3'-end processing of premature mRNA. Recent studies have revealed other roles of Ssu72, including its roles in balancing phosphate homeostasis and controlling chromosome behaviors, which should be further explored. In conclusion, the protein Ssu72 is an enzyme worthy of attention, not confined to its role in gene transcription.

Structural and binding studies of the C-terminal domains of yeast TFIIF subunits Tfg1 and Tfg2

The general transcription factor TFIIF plays essential roles at several steps during eukaryotic transcription. While several studies have offered insights into the structure/function relationship in human TFIIF, much less is known about the yeast system. Here, we describe the first NMR structural and binding studies of the C-terminal domains (CTDs) of Tfg1 and Tfg2 subunits of Saccharomyces cerevisiae TFIIF. We used the program CS-ROSETTA to determine the three-dimensional folds of these domains in solution, and performed binding studies with DNA and protein targets. CS-ROSETTA models indicate that the Tfg1 and Tfg2 C-terminal domains have winged-helix architectures, similar to the human homologs. We showed that both Tfg1 and Tfg2 CTDs interact with double-stranded DNA oligonucleotides, and mapped the DNA binding interfaces using solution NMR. Tfg1-CTD, but not Tfg2-CTD, also binds to yeast FCP1, an RNA polymerase II-specific phosphatase, and we delineated the interaction surface with the CTD of FCP1. Our results provide insights into the structural basis of yeast TFIIF function and the differential roles of Tfg1 and Tfg2 subunits during transcription.

The YEATS family member GAS41 interacts with the general transcription factor TFIIF

Background

In eukaryotes the transcription initiation by RNA polymerase II requires numerous general and regulatory factors including general transcription factors. The general transcription factor TFIIF controls the activity of the RNA polymerase II both at the initiation and elongation stages. The glioma amplified sequence 41 (GAS41) has been associated with TFIIF via its YEATS domain.

Results

Using GST pull-down assays, we demonstrated that GAS41 binds to both, the small subunit (RAP30) and the large subunit (RAP74) of TFIIF in vitro. The in vivo interaction of GAS41 and endogenous RAP30 and RAP74 was confirmed by co-immunoprecipitation. GAS41 binds to two non-overlapping regions of the C-terminus of RAP30. There is also an ionic component to the binding between GAS41 and RAP30. There was no evidence for a direct interaction between GAS41 and TBP or between GAS41 and RNA polymerase II.

Conclusions

Our results demonstrate binding between endogenous GAS41 and the endogenous TFIIF subunits (RAP30 and RAP74). Since we did not find evidence for a binding of GAS41 to TBP or RNA polymerase II, GAS41 seems to preferentially bind to TFIIF. GAS41 that does not contain a DNA-binding domain appears to be a co-factor of TFIIF.

The increase in the number of subunits in eukaryotic RNA polymerase III relative to RNA polymerase II is due to the permanent recruitment of general transcription factors

The number of subunits of RNA polymerases (RNAPs) increases during evolution from 5 in eubacteria to 12 in archaea. In eukaryotes, which have at least three RNAPs, the number of subunits has expanded from 12 in RNA polymerase II (RNAPII) to 14 in RNA polymerase I (RNAPI) and to 17 in RNA polymerase III (RNAPIII). It was recently demonstrated that the two additional subunits found in RNAPI relative to RNAPII are homologous to TFIIF, a dimeric general transcription factor of RNAPII. Here, we extend this finding by demonstrating that four of the five RNAPIII-specific subunits are also homologous to transcription factors of RNAPII. We use the available evidence to propose an evolutionary history of the eukaryotic RNAPs and argue that the increases in the number of subunits that occurred in RNAPs I and III are due to the permanent recruitment of preexisting transcription factors.

Functional characterization of the native NH2-terminal transactivation domain of the human androgen receptor: binding kinetics for interactions with TFIIF and SRC-1a

The androgen receptor (AR) is a ligand-activated transcription factor that mediates the actions of the steroid hormones testosterone and dihydrotestosterone at the level of gene transcription. The main transactivation function is modular in structure, maps to the N-terminal domain (NTD), and is termed AF1. This region of the AR is structurally flexible and functions in multiple protein-protein interactions with coregulatory proteins and components of the general transcription machinery. Using surface plasmon resonance, the binding kinetics for the interaction of AR-AF1 with the large subunit of the general transcription factor TFIIF, termed RAP74, and the coactivator SRC-1a were measured. AR-AF1 interacts with both the NTD and CTD of RAP74 and the CTD of SRC-1a. The dissociation constants ( Kd) for the binding of polypeptides derived from RAP74 are in the submicromolar range, while a peptide from SRC-1a bound with a Kd of 14 microM. Significantly, the individual NTD and CTD of RAP74 interacted with AR-AF1 with distinct binding kinetics, with the NTD exhibiting slower on and off rates. TFIIF is involved in transcription initiation and elongation, and the CTD of RAP74 binds to the RNA polymerase II enzyme, the general transcription factor TFIIB, and a CTD phosphatase, FCP1. We have mutated hydrophobic residues in the RAP74-CTD structure to disrupt secondary structure elements and show that binding of AR-AF1 depends upon helix 3 in the winged-helix domain of the RAP74-CTD polypeptide. Altogether, a model is suggested for AR-AF1-dependent transactivation of receptor-target genes.

Structural characterization of human general transcription factor TFIIF in solution

Human general transcription factor IIF (TFIIF), a component of the transcription pre-initiation complex (PIC) associated with RNA polymerase II (Pol II), was characterized by size-exclusion chromatography (SEC), electrospray ionization mass spectrometry (ESI-MS), and chemical cross-linking. Recombinant TFIIF, composed of an equimolar ratio of alpha and beta subunits, was bacterially expressed, purified to homogeneity, and found to have a transcription activity similar to a natural one in the human in vitro transcription system. SEC of purified TFIIF, as previously reported, suggested that this protein has a size >200 kDa. In contrast, ESI-MS of the purified sample gave a molecular size of 87 kDa, indicating that TFIIF is an alphabeta heterodimer, which was confirmed by matrix-assisted laser desorption/ionization (MALDI) MS of the cross-linked TFIIF components. Recent electron microscopy (EM) and photo-cross-linking studies showed that the yeast TFIIF homolog containing Tfg1 and Tfg2, corresponding to the human alpha and beta subunits, exists as a heterodimer in the PIC, so the human TFIIF is also likely to exist as a heterodimer even in the PIC. In the yeast PIC, EM and photo-cross-linking studies showed different results for the mutual location of TFIIE and TFIIF along DNA. We have examined the direct interaction between human TFIIF and TFIIE by ESI-MS, SEC, and chemical cross-linking; however, no direct interaction was observed, at least in solution. This is consistent with the previous photo-cross-linking observation that TFIIF and TFIIE flank DNA separately on both sides of the Pol II central cleft in the yeast PIC.

TFIIB and the regulation of transcription by RNA polymerase II

Accurate transcription of a gene by RNA polymerase II requires the assembly of a group of general transcription factors at the promoter. The general transcription factor TFIIB plays a central role in preinitiation complex assembly, providing a bridge between promoter-bound TFIID and RNA polymerase II. TFIIB makes extensive contact with the core promoter via two independent DNA-recognition modules. In addition to interacting with other general transcription factors, TFIIB directly modulates the catalytic center of RNA polymerase II in the transcription complex. Moreover, TFIIB has been proposed as a target of transcriptional activator proteins that act to stimulate preinitiation complex assembly. In this review, we will discuss our current understanding of these activities of TFIIB.

Full and partial genome-wide assembly and disassembly of the yeast transcription machinery in response to heat shock

Eukaryotic genes are controlled by sequence-specific DNA-binding proteins, chromatin regulators, general transcription factors, and elongation factors. Here we examine the genome-wide location of representative members of these groups and their redistribution when the Saccharomyces cerevisiae genome is reprogrammed by heat shock. As expected, assembly of active transcription complexes is coupled to eviction of H2A.Z nucleosomes, and disassembly is coupled to the return of nucleosomes. Remarkably, a large number of promoters assemble into partial preinitiation complexes (partial PICs), containing TFIIA, TFIID (and/or SAGA), TFIIB, TFIIE, and TFIIF. However, RNA polymerase II and TFIIH are generally not recruited, and nucleosomes are not displaced. These promoters may be preparing for additional stress that naturally accompany heat stress. For example, we find that oxidative stress, which often occurs with prolonged exposure of cells to high temperature, converts partial PICs into full PICs. Partial PICs therefore represent novel regulated intermediates that assemble at promoters in the midst of chromatin.

The general transcription machinery and general cofactors

In eukaryotes, the core promoter serves as a platform for the assembly of transcription preinitiation complex (PIC) that includes TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and RNA polymerase II (pol II), which function collectively to specify the transcription start site. PIC formation usually begins with TFIID binding to the TATA box, initiator, and/or downstream promoter element (DPE) found in most core promoters, followed by the entry of other general transcription factors (GTFs) and pol II through either a sequential assembly or a preassembled pol II holoenzyme pathway. Formation of this promoter-bound complex is sufficient for a basal level of transcription. However, for activator-dependent (or regulated) transcription, general cofactors are often required to transmit regulatory signals between gene-specific activators and the general transcription machinery. Three classes of general cofactors, including TBP-associated factors (TAFs), Mediator, and upstream stimulatory activity (USA)-derived positive cofactors (PC1/PARP-1, PC2, PC3/DNA topoisomerase I, and PC4) and negative cofactor 1 (NC1/HMGB1), normally function independently or in combination to fine-tune the promoter activity in a gene-specific or cell-type-specific manner. In addition, other cofactors, such as TAF1, BTAF1, and negative cofactor 2 (NC2), can also modulate TBP or TFIID binding to the core promoter. In general, these cofactors are capable of repressing basal transcription when activators are absent and stimulating transcription in the presence of activators. Here we review the roles of these cofactors and GTFs, as well as TBP-related factors (TRFs), TAF-containing complexes (TFTC, SAGA, SLIK/SALSA, STAGA, and PRC1) and TAF variants, in pol II-mediated transcription, with emphasis on the events occurring after the chromatin has been remodeled but prior to the formation of the first phosphodiester bond.

Assembly of transcription factor IIB at a promoter in vivo requires contact with RNA polymerase II

The general transcription factor TFIIB has a central role in the assembly of the preinitiation complex at the promoter, providing a platform for the entry of RNA polymerase II/TFIIF. We used an RNA interference (RNAi)-based system in which TFIIB expression is ablated in vivo and replaced with a TFIIB derivative that contains a silent mutation and is refractory to the RNAi. Using this approach, we found that transcriptionally defective TFIIB amino-terminal mutants showed distinct effects on the basis of their ability to compete with wild-type TFIIB in vivo. Moreover, analysis of the TFIIB mutant derivatives by chromatin immunoprecipitation showed that promoter occupancy by TFIIB is dependent on the association with RNA polymerase II. Together, our results support a mode of preinitiation complex assembly in which TFIIB/RNA polymerase II recruitment to the promoter occurs in vivo.

The role of the general transcription factor IIF in androgen receptor-dependent transcription

The androgen receptor (AR) is a member of the steroid receptor subfamily of nuclear receptors and is important for normal male sexual differentiation and fertility. The major transactivation function of the AR, termed activation function 1 (AF1), is modular in structure and has been mapped to the N terminus of the protein. To understand better the mechanisms whereby the AR activates transcription, we have established a novel cell-free transcription assay. This is based on the use of a dual reporter gene template, containing promoter proximal and distal G-less cassettes, which result in different size transcripts that can be easily detected and quantified. The promoter proximal transcript gives an indication of transcription initiation and promoter escape, whereas the relative levels of the distal transcript indicate elongation efficiency. The AR-AF1-Lex protein enhanced production of both transcripts whereas, in the absence of DNA binding, the AF1 domain squelched both initiation and elongation. Mutations in the transactivation domain that impaired transactivation and/or binding of the general transcription factor IIF (TFIIF) were found to reduce the ability of AR-AF1 to squelch transcription. Addition of recombinant TFIIF reversed squelching of the promoter-proximal but not the -distal G-less transcript, whereas addition of TATA-binding protein failed to reverse squelching of either transcript. Taken together, these results demonstrate that the AR N-terminal transactivation function, AF1, has the potential to regulate transcription at both the level of initiation and elongation, and that interactions with TFIIF are important during preinitiation complex assembly/open complex formation and/or promoter escape.

Prediction of the general transcription factors associated with RNA polymerase II in Plasmodium falciparum: conserved features and differences relative to other eukaryotes

BACKGROUND

To date, only a few transcription factors have been identified in the genome of the parasite Plasmodium falciparum, the causative agent of malaria. Moreover, no detailed molecular analysis of its basal transcription machinery, which is otherwise well-conserved in the crown group of eukaryotes, has yet been reported. In this study, we have used a combination of sensitive sequence analysis methods to predict the existence of several parasite encoded general transcription factors associated with RNA polymerase II.

RESULTS

Several orthologs of general transcription factors associated with RNA polymerase II can be predicted among the hypothetical proteins of the P. falciparum genome using the two-dimensional Hydrophobic Cluster Analysis (HCA) together with profile-based search methods (PSI-BLAST). These predicted orthologous genes encoding putative transcription factors include the large subunit of TFIIA and two candidates for its small subunit, the TFIIE beta-subunit, which would associate with the previously known TFIIE alpha-subunit, the TFIIF beta-subunit, as well as the p62/TFB1 subunit of the TFIIH core. Within TFIID, the putative orthologs of TAF1, TAF2, TAF7 and TAF10 were also predicted. However, no candidates for TAFs with classical histone fold domain (HFD) were found, suggesting an unusual architecture of TFIID complex of RNA polymerase II in the parasite.

CONCLUSION

Taken together, these results suggest that more general transcription factors may be present in the P. falciparum proteome than initially thought. The prediction of these orthologous general transcription factors opens the way for further studies dealing with transcriptional regulation in P. falciparum. These alternative and sensitive sequence analysis methods can help to identify candidates for other transcriptional regulatory factors in P. falciparum. They will also facilitate the prediction of biological functions for several orphan proteins from other apicomplexan parasites such as Toxoplasma gondii, Cryptosporidium parvum and Eimeria.

Amino acid substitutions in yeast TFIIF confer upstream shifts in transcription initiation and altered interaction with RNA polymerase II

Transcription factor IIF (TFIIF) is required for transcription of protein-encoding genes by eukaryotic RNA polymerase II. In contrast to numerous studies establishing a role for higher eukaryotic TFIIF in multiple steps of the transcription cycle, relatively little has been reported regarding the functions of TFIIF in the yeast Saccharomyces cerevisiae. In this study, site-directed mutagenesis, plasmid shuffle complementation assays, and primer extension analyses were employed to probe the functional domains of the S. cerevisiae TFIIF subunits Tfg1 and Tfg2. Analyses of 35 Tfg1 alanine substitution mutants and 19 Tfg2 substitution mutants identified 5 mutants exhibiting altered properties in vivo. Primer extension analyses revealed that the conditional growth properties exhibited by the tfg1-E346A, tfg1-W350A, and tfg2-L59K mutants were associated with pronounced upstream shifts in transcription initiation in vivo. Analyses of double mutant strains demonstrated functional interactions between the Tfg1 mutations and mutations in Tfg2, TFIIB, and RNA polymerase II. Importantly, biochemical results demonstrated an altered interaction between mutant TFIIF protein and RNA polymerase II. These results provide direct evidence for the involvement of S. cerevisiae TFIIF in the mechanism of transcription start site utilization and support the view that a TFIIF-RNA polymerase II interaction is a determinant in this process.

Tfg3, a subunit of the general transcription factor TFIIF in Schizosaccharomyces pombe, functions under stress conditions

TFIIF is a general transcription factor (GTF) that binds to RNA polymerase II (pol II) for subsequent recruitment of pol II to a promoter. TFIIF of Saccharomyces cerevisiae contains a small subunit, designated Tfg3, in addition to two conserved subunits, TFIIFalpha (Tfg1) and TFIIFbeta (Tfg2). In this study, we characterized Tfg3 of Schizosaccharomyces pombe. Using Tfg3 fused to green fluorescent protein (GFP), we found that Tfg3 is located in nuclei, and it is assembled into the C-terminal domain phosphatase (Fcp1)/TFIIF/pol II complex via interactions with TFIIFalpha and TFIIFbeta. As in the case of S.cerevisiae, Tfg3 in S.pombe forms part of another GTF, namely TFIID. The TFIID complex isolated from S.pombe that had been cultured at elevated temperatures included increased levels of Tfg3. The interaction of recombinant Tfg3 with TATA-binding protein (TBP), the central subunit of TFIID, was temperature-dependent. Moreover, a mutant of S.pombe that lacked the gene for Tfg3 was sensitive to a battery of stresses including temperature up-shift. Starting from a mutant with tfg3- mutation, we isolated five species of multicopy suppressors. Expression levels of the suppressor genes were lower in the mutant cell than in wild-type cell at an elevated temperature. Taken together, we propose that Tfg3 is involved in transcriptional regulation under stress conditions, in particular, at high temperatures.

A nonconserved surface of the TFIIB zinc ribbon domain plays a direct role in RNA polymerase II recruitment

The general transcription factor TFIIB is a highly conserved and essential component of the eukaryotic RNA polymerase II (pol II) transcription initiation machinery. It consists of a single polypeptide with two conserved structural domains: an amino-terminal zinc ribbon structure (TFIIB(ZR)) and a carboxy-terminal core (TFIIB(CORE)). We have analyzed the role of the amino-terminal region of human TFIIB in transcription in vivo and in vitro. We identified a small nonconserved surface of the TFIIB(ZR) that is required for pol II transcription in vivo and for different types of basal pol II transcription in vitro. Consistent with a general role in transcription, this TFIIB(ZR) surface is directly involved in the recruitment of pol II to a TATA box-containing promoter. Curiously, although the amino-terminal human TFIIB(ZR) domain can recruit both human pol II and yeast (Saccharomyces cerevisiae) pol II, the yeast TFIIB amino-terminal region recruits yeast pol II but not human pol II. Thus, a critical process in transcription from many different promoters-pol II recruitment-has changed in sequence specificity during eukaryotic evolution.

RNA Polymerase II/TFIIF Structure and Conserved Organization of the Initiation Complex

The structure of an RNA polymerase II/general transcription factor TFIIF complex was determined by cryo-electron microscopy and single particle analysis. Density due to TFIIF was not concentrated in one area but rather was widely distributed across the surface of the polymerase. The largest subunit of TFIIF interacted with the dissociable Rpb4/Rpb7 polymerase subunit complex and with the mobile "clamp." The distribution of the second largest subunit of TFIIF was very similar to that previously reported for the sigma subunit in the bacterial RNA polymerase holoenzyme, consisting of a series of globular domains extending along the polymerase active site cleft. This result indicates that the second TFIIF subunit is a true structural homolog of the bacterial sigma factor and reveals an important similarity of the transcription initiation mechanism between bacteria and eukaryotes. The structure of the RNAPII/TFIIF complex suggests a model for the organization of a minimal transcription initiation complex.

Transcription factor IIB acetylates itself to regulate transcription

Acetylation is a well-known regulatory post-translational modification, but a biological function for acetylation in regulating basal transcription factors has not been reported. Here we show that the general transcription factor TFIIB, which is required for the initiation of eukaryotic polymerase II transcription, is acetylated. TFIIB is also an autoacetyltransferase, although it shares no sequence homology with any known acetyltransferases. In the absence of other enzymes, it binds acetyl-coenzyme A (acetyl-CoA), and catalyses the transfer of the acetyl group onto a specific lysine residue (K238). Both recombinant and cellular TFIIB can autoacetylate, markedly stabilizing the interaction between TFIIB and transcription factor TFIIF and activating transcription in vitro and in cells. A K238A mutant, which cannot be autoacetylated, does not show this activation of transcription. Our findings suggest that there is a regulatory pathway controlling acetylation of TFIIB, and they link acetyl-CoA with basal gene transcription.

CTD phosphatase: role in RNA polymerase II cycling and the regulation of transcript elongation

The repetitive C-terminal domain (CTD) of the largest RNA polymerase II subunit plays a critical role in the regulation of gene expression. The activity of the CTD is dependent on its state of phosphorylation. A variety of CTD kinases act on RNA polymerase II at specific steps in the transcription cycle and preferentially phosphorylate distinct positions within the CTD consensus repeat. A single CTD phosphatase has been identified and characterized that in concert with CTD kinases establishes the level of CTD phosphorylation. The involvement of CTD phosphatase in controlling the progression of RNAP II around the transcription cycle, the mobilization of stored RNAP IIO, and the regulation of transcript elongation and RNA processing is discussed.

BRFU, a TFIIB-like factor, is directly recruited to the TATA-box of polymerase III small nuclear RNA gene promoters through its interaction with TATA-binding protein

The human snRNA genes transcribed by RNA polymerase II (pol II) and III (pol III) have different core promoter elements. Both gene types contain similar proximal sequence elements (PSEs) but differ in the absence (pol II) or presence (pol III) of a TATA-box, which, together with the PSE, determines the assembly of a pol III-specific pre-initiation complex. BRFU is a factor exclusively required for transcription of the pol III-type snRNA genes. We report that recruitment of BRFU to the TATA-box of these promoters is TATA-binding protein (TBP)-dependent. BRFU in turn stabilizes TBP on TATA-containing template and extends the TBP footprint both upstream and downstream of the TATA element. The core domain of TBP is sufficient for BRFU.TBP.DNA complex formation and for interaction with BRFU off the template. We have mapped amino acid residues within TBP and domains of BRFU that mediate this interaction. BRFU has no specificity for sequences flanking the TATA-box and also forms a stable complex on the TATA-box of the pol II-specific adenovirus major late promoter (AdMLP). Furthermore, pol III-type transcription can initiate from an snRNA gene promoter containing an AdMLP TATA-box and flanking sequences. Therefore, the polymerase recruitment is not simply determined by the sequence of the TATA-box and immediate flanking sequences.

Promoter-specific shifts in transcription initiation conferred by yeast TFIIB mutations are determined by the sequence in the immediate vicinity of the start sites

The general transcription factor IIB (TFIIB) is required for transcription of class II genes by RNA polymerase II. Previous studies demonstrated that mutations in the Saccharomyces cerevisiae SUA7 gene, which encodes TFIIB, can alter transcription initiation patterns in vivo. To further delineate the functional domain and residues of TFIIB involved in transcription start site utilization, a genetic selection was used to isolate S. cerevisiae TFIIB mutants exhibiting downstream shifts in transcription initiation in vivo. Both dominant and recessive mutations conferring downstream shifts were identified at multiple positions within a highly conserved homology block in the N-terminal region of the protein. The TFIIB mutations conferred downstream shifts in transcription initiation at the ADH1 and CYC1 promoters, whereas no significant shifts were observed at the HIS3 promoter. Analysis of a series of ADH1-HIS3 hybrid promoters and variant ADH1 and HIS3 promoters containing insertions, deletions, or site-directed base substitutions revealed that the feature that renders a promoter sensitive to TFIIB mutations is the sequence in the immediate vicinity of the normal start sites. We discuss these results in light of possible models for the mechanism of start site utilization by S. cerevisiae RNA polymerase II and the role played by TFIIB.

Direct interaction between the subunit RAP30 of transcription factor IIF (TFIIF) and RNA polymerase subunit 5, which contributes to the association between TFIIF and RNA polymerase II

The general transcription factor IIF (TFIIF) assembled in the initiation complex, and RAP30 of TFIIF, have been shown to associate with RNA polymerase II (pol II), although it remains unclear which pol II subunit is responsible for the interaction. We examined whether TFIIF interacts with RNA polymerase II subunit 5 (RPB5), the exposed domain of which binds transcriptional regulatory factors such as hepatitis B virus X protein and a novel regulatory protein, RPB5-mediating protein. The results demonstrated that RPB5 directly binds RAP30 in vitro using purified recombinant proteins and in vivo in COS1 cells transiently expressing recombinant RAP30 and RPB5. The RAP30-binding region was mapped to the central region (amino acids (aa) 47-120) of RPB5, which partly overlaps the hepatitis B virus X protein-binding region. Although the middle part (aa 101-170) and the N-terminus (aa 1-100) of RAP30 independently bound RPB5, the latter was not involved in the RPB5 binding when RAP30 was present in TFIIF complex. Scanning of the middle part of RAP30 by clustered alanine substitutions and then point alanine substitutions pinpointed two residues critical for the RPB5 binding in in vitro and in vivo assays. Wild type but not mutants Y124A and Q131A of RAP30 coexpressed with FLAG-RAP74 efficiently recovered endogenous RPB5 to the FLAG-RAP74-bound anti-FLAG M2 resin. The recovered endogenous RPB5 is assembled in pol II as demonstrated immunologically. Interestingly, coexpression of the central region of RPB5 and wild type RAP30 inhibited recovery of endogenous pol II to the FLAG-RAP74-bound M2 resin, strongly suggesting that the RAP30-binding region of RPB5 inhibited the association of TFIIF and pol II. The exposed domain of RPB5 interacts with RAP30 of TFIIF and is important for the association between pol II and TFIIF.

Crystal structure of the C-terminal domain of the RAP74 subunit of human transcription factor IIF

The x-ray structure of a C-terminal fragment of the RAP74 subunit of human transcription factor (TF) IIF has been determined at 1.02-A resolution. The alpha/beta structure is strikingly similar to the globular domain of linker histone H5 and the DNA-binding domain of hepatocyte nuclear factor 3gamma (HNF-3gamma), making it a winged-helix protein. The surface electrostatic properties of this compact domain differ significantly from those of bona fide winged-helix transcription factors (HNF-3gamma and RFX1) and from the winged-helix domains found within the RAP30 subunit of TFIIF and the beta subunit of TFIIE. RAP74 has been shown to interact with the TFIIF-associated C-terminal domain phosphatase FCP1, and a putative phosphatase binding site has been identified within the RAP74 winged-helix domain.

Novel dimerization fold of RAP30/RAP74 in human TFIIF at 1.7 A resolution

General transcription factor IIF (TFIIF) is required for transcription by RNA polymerase II; it consists minimally of a heterodimer of RNA polymerase-associated proteins RAP30 and RAP74. According to solution and mutagenesis studies, the multiple domains of RAP30 and RAP74 bind PolII, TFIIB, TAF250 and DNA in interactions that are essential for transcription initiation and elongation. The X-ray structure of the RAP30/RAP74 interaction domains at 1.7 A resolution reveals a novel "triple barrel" dimerization fold and suggests with mutant data that interactions with the transcription apparatus are mediated not only by this tripartite beta-barrel, but also via flexible loops and alpha and beta-structures extending from it.

Structure of a (Cys3His) zinc ribbon, a ubiquitous motif in archaeal and eucaryal transcription

Transcription factor IIB (TFIIB) is an essential component in the formation of the transcription initiation complex in eucaryal and archaeal transcription. TFIIB interacts with a promoter complex containing the TATA-binding protein (TBP) to facilitate interaction with RNA polymerase II (RNA pol II) and the associated transcription factor IIF (TFIIF). TFIIB contains a zinc-binding motif near the N-terminus that is directly involved in the interaction with RNA pol II/TFIIF and plays a crucial role in selecting the transcription initiation site. The solution structure of the N-terminal residues 2-59 of human TFIIB was determined by multidimensional NMR spectroscopy. The structure consists of a nearly tetrahedral Zn(Cys)3(His)1 site confined by type I and "rubredoxin" turns, three antiparallel beta-strands, and disordered loops. The structure is similar to the reported zinc-ribbon motifs in several transcription-related proteins from archaea and eucarya, including Pyrococcus furiosus transcription factor B (PfTFB), human and yeast transcription factor IIS (TFIIS), and Thermococcus celer RNA polymerase II subunit M (TcRPOM). The zinc-ribbon structure of TFIIB, in conjunction with the biochemical analyses, suggests that residues on the beta-sheet are involved in the interaction with RNA pol II/TFIIF, while the zinc-binding site may increase the stability of the beta-sheet.

Intramolecular interaction of yeast TFIIB in transcription control

The general transcription factor TFIIB is a key component in the eukaryotic RNA polymerase II (RNAPII) transcriptional machinery. We have previously shown that a yeast TFIIB mutant (called YR1m4) with four amino acid residues in a species-specific region changed to corresponding human residues affects the expression of genes activated by different activators in vivo. We report here that YR1m4 can interact with several affected activators in vitro. In addition, YR1m4 and other mutants with amino acid alterations within the same region can interact with TATA-binding protein (TBP) and RNAPII normally. However, YR1m4 is defective in supporting activator-independent transcription in assays con-ducted both in vitro and in vivo. We further demonstrate that the interaction between the C-terminal core domain and the N-terminal region is weakened in YR1m4 and other related TFIIB mutants. These results suggest that the intramolecular interaction property of yeast TFIIB plays an important role in transcription regulation in cells.

The conformation of the transcription factor TFIIB modulates the response to transcriptional activators in vivo

The general transcription factor TFIIB plays a crucial role in the assembly of the transcriptional preinitiation complex and has also been proposed as a target of transcriptional activator proteins (reviewed in [1]). TFIIB is composed of two domains which are engaged in an intramolecular interaction that is disrupted upon interaction with the activation domain of the Herpesvirus VP16 protein in vitro [2] [3]. The significance of this event for transcriptional activation is not known, however. The amino-terminal intramolecular interaction domain is the most conserved region of TFIIB and plays a role in transcription start-site selection [4] [5] [6]. In addition, we have shown previously that the integrity of this region is required for transcriptional activation in vivo [4]. Here, we have defined a charge cluster at the amino terminus of TFIIB that is required for transcriptional activation in vivo. We found that this domain determines the affinity of the TFIIB intramolecular interaction and the ability of TFIIB to interact with a transcriptional activation domain, but not with components of the holoenzyme. Our results suggest that the intramolecular interaction in TFIIB regulates transcriptional activation in vivo.

Immunoaffinity purification of the RAP30 subunit of human transcription factor IIF

RAP30, an RNA polymerase-associated protein (RAP) of approximately 30 kDa, is a component of the eukaryotic general transcription factor IIF (TFIIF). We have isolated a monoclonal antibody (MAb) that can be used to purify RAP30 under nondenaturing conditions. This MAb (designated 1RAP1) is a unique type of MAb that we have designated "polyol-responsive MAb." Polyol-responsive MAbs are high-affinity antibodies that release antigen in a buffer containing a low-molecular-weight polyhydroxylated compound (polyol) and a nonchaotropic salt. RAP30, contained on pET11d, was expressed in Escherichia coli by culturing and inducing protein expression at 26 degrees C. Under these conditions, approximately 50% of the RAP30 remains soluble. Inclusion bodies were removed from the cell lysate by centrifugation, the supernatant was treated with polyethyleneimine at 0.5 M NaCl to remove nucleic acids, and the soluble protein was applied directly to MAb-conjugated Sepharose. After extensive washing, RAP30 was eluted with buffer containing 0. 75 M ammonium sulfate and 40% propylene glycol. RAP30 produced by this procedure stimulates transcription from a minimal promoter. This is a rapid method for purifying unmodified RAP30 without renaturing the protein from inclusion bodies.

An interaction between the N-terminal region and the core domain of yeast TFIIB promotes the formation of TATA-binding protein-TFIIB-DNA complexes

The general transcription factor IIB (TFIIB) plays an essential role in transcription of protein-coding genes by eukaryotic RNA polymerase II. We previously identified a yeast TFIIB mutant (R64E) that exhibited increased activity in the formation of stable TATA-binding protein-TFIIB-DNA (DB) complexes in vitro. We report here that the homologous human TFIIB mutant (R53E) also displayed increased activity in DB complex formation in vitro. Biochemical analyses revealed that the increased activity of the R64E mutant in DB complex formation was associated with an altered protease sensitivity of the protein and an enhanced interaction between the N-terminal region and the C-terminal core domain. These results suggest that the intramolecular interaction in yeast TFIIB stabilizes a productive conformation of the protein for the association with promoter-bound TATA-binding protein.

Kinase activity and phosphorylation of the largest subunit of TFIIF transcription factor

The largest subunit of the human basal transcription factor TFIIFalpha (also called RAP74) was reported previously to be the target of some phospho/dephosphorylation process. We show that TFIIFalpha possesses a serine/threonine kinase activity, allowing an autophosphorylation of the two residues at position serine 385 and threonine 389. Mutation analysis strongly suggests that autophosphorylation of both sites regulates the transcription elongation process. Moreover we also evidence three additional phosphorylation sites located at positions 207-230, 271-283, and 335-344. These sites are phosphorylated by casein kinase II-like kinases and TAF(II)250, a component of TFIID.

The role of human TFIIB in transcription start site selection in vitro and in vivo

The general transcription factor TFIIB plays a crucial role in selecting the transcription initiation site in yeast. We have analyzed the human homologs of TFIIB mutants that have previously been shown to affect transcription start site selection in the yeast Saccharomyces cerevisiae. Despite the distinct mechanisms of transcription start site selection observed in S. cerevisiae and humans, the role of TFIIB in this process is similar. However, unlike their yeast counterparts, the human mutants do not show a severe defect in supporting either basal transcription or transcription stimulated by an acidic activator in vitro. Transient transfection analysis revealed that, in addition to a role in transcription start site selection, human TFIIB residue Arg-66 performs a critical function in vivo that is bypassed in vitro. Furthermore, although correct transcription start site selection is dependent upon an arginine residue at position 66 in human TFIIB, innate function in vivo is determined by the charge of the residue alone. Our observations raise questions as to the evolutionary conservation of TFIIB and uncover an additional function for TFIIB that is required in vivo but can be bypassed in vitro.

FCP1, the RAP74-interacting subunit of a human protein phosphatase that dephosphorylates the carboxyl-terminal domain of RNA polymerase IIO

TFIIF (RAP30/74) is a general initiation factor that also increases the rate of elongation by RNA polymerase II. A two-hybrid screen for RAP74-interacting proteins produced cDNAs encoding FCP1a, a novel, ubiquitously expressed human protein that interacts with the carboxyl-terminal evolutionarily conserved domain of RAP74. Related cDNAs encoding FCP1b lack a carboxyl-terminal RAP74-binding domain of FCP1a. FCP1 is an essential subunit of a RAP74-stimulated phosphatase that processively dephosphorylates the carboxyl-terminal domain of the largest RNA polymerase II subunit. FCP1 is also a stoichiometric component of a human RNA polymerase II holoenzyme complex.

Structure of the human transcription factor TFIIF revealed by limited proteolysis with trypsin

In this study, the human general transcription factor IIF (TFIIF), a heteromeric complex of RAP74 and RAP30 subunits, was subjected to limited proteolysis with trypsin. The central region of RAP74 was demonstrated to be highly sensitive to trypsin while both the N- and C-terminal regions contained trypsin-resistant structures. In contrast, RAP30 digestion occurred after proteolysis of RAP74. The digestion pattern of RAP74 recruited into the preinitiation complex showed no marked difference from that of IIF, while RAP30 in the complex was protected from trypsin. These results indicate that RAP74 apparently contains three structural domains, the central one of which is externally surfaced and unstructured, but RAP30 is internally wrapped by RAP74. Furthermore, the accessibility of the central region of RAP74 is unaltered in the minimal preinitiation complex, while RAP30 is involved in promoter recognition through its DNA binding activity.

Wrapping of promoter DNA around the RNA polymerase II initiation complex induced by TFIIF

The formation of the RNA polymerase II (Pol II) initiation complex was analyzed using site-specific protein-DNA photo-cross-linking. We show that the RAP74 subunit of transcription factor (TF) IIF, through its RAP30-binding domain and an adjacent region necessary for the formation of homomeric interactions in vitro, dramatically alters the distribution of RAP30, TFIIE, and Pol II along promoter DNA between positions -40 and +26. This isomerization of the complex, which requires both TFIIF and TFIIE, is accompanied by tight wrapping of the promoter DNA for almost a full turn around Pol II. Addition of TFIIH enhances photo-cross-linking of Pol II to a number of promoter positions, suggesting that TFIIH tightens the DNA wrap around the enzyme. We present a general model to describe transcription initiation.

Structural homology between the Rap30 DNA-binding domain and linker histone H5: implications for preinitiation complex assembly

The three-dimensional structure of the human Rap30 DNA-binding domain has been solved by multinuclear NMR spectroscopy. The structure of the globular domain is strikingly similar to that of linker histone H5 and its fold places Rap30 into the "winged" helix-turn-helix family of eukaryotic transcription factors. Although the domain interacts weakly with DNA, the binding surface was identified and shown to be consistent with the structure of the HNF-3/fork head-DNA complex. The architecture of the Rap30 DNA-binding domain has important implications for the function of Rap30 in the assembly of the preinitiation complex. In analogy to the function of linker histones in chromatin formation, the fold of the Rap30 DNA-binding domain suggests that its role in transcription initiation may be that of a condensation factor for preinitiation complex assembly. Functional similarity to linker histones may explain the dependence of Rap30 binding on the bent DNA environment induced by the TATA box-binding protein. Cryptic sequence identity and functional homology between the Rap30 DNA-binding domain and region 4 of Escherichia coli sigma70 may indicate that the sigma factors also possess a linker histone-like activity in the formation of a prokaryotic closed complex.

The N-terminal region of yeast TFIIB contains two adjacent functional domains involved in stable RNA polymerase II binding and transcription start site selection

The general transcription factor IIB (TFIIB) is required for accurate and efficient transcription of protein-coding genes by RNA polymerase II (RNAPII). To define functional domains in the highly conserved N-terminal region of TFIIB, we have analyzed 14 site-directed substitution mutants of yeast TFIIB for their ability to support cell viability, transcription in vitro, accurate start site selection in vitro and in vivo, and to form stable complexes with purified RNAPII in vitro. Mutations impairing the formation of stable TFIIB.RNAPII complexes mapped to the zinc ribbon fold, whereas mutations conferring downstream shifts in transcription start site selection were identified at multiple positions within a highly conserved homology block adjacent and C-terminal to the zinc ribbon. These results demonstrate that the N-terminal region of yeast TFIIB contains two separable and adjacent functional domains involved in stable RNAPII binding and transcription start site selection, suggesting that downstream shifts in transcription start site selection do not result from impairment of stable TFIIB.RNAPII binding. We discuss models for yeast start site selection in which TFIIB may affect the ability of preinitiation complexes to interact with downstream DNA or to affect start site recognition by a scanning polymerase.

Molecular genetics of the RNA polymerase II general transcriptional machinery

Transcription initiation by RNA polymerase II (RNA pol II) requires interaction between cis-acting promoter elements and trans-acting factors. The eukaryotic promoter consists of core elements, which include the TATA box and other DNA sequences that define transcription start sites, and regulatory elements, which either enhance or repress transcription in a gene-specific manner. The core promoter is the site for assembly of the transcription preinitiation complex, which includes RNA pol II and the general transcription fctors TBP, TFIIB, TFIIE, TFIIF, and TFIIH. Regulatory elements bind gene-specific factors, which affect the rate of transcription by interacting, either directly or indirectly, with components of the general transcriptional machinery. A third class of transcription factors, termed coactivators, is not required for basal transcription in vitro but often mediates activation by a broad spectrum of activators. Accordingly, coactivators are neither gene-specific nor general transcription factors, although gene-specific coactivators have been described in metazoan systems. Transcriptional repressors include both gene-specific and general factors. Similar to coactivators, general transcriptional repressors affect the expression of a broad spectrum of genes yet do not repress all genes. General repressors either act through the core transcriptional machinery or are histone related and presumably affect chromatin function. This review focuses on the global effectors of RNA polymerase II transcription in yeast, including the general transcription factors, the coactivators, and the general repressors. Emphasis is placed on the role that yeast genetics has played in identifying these factors and their associated functions.

Properties of PC4 and an RNA polymerase II complex in directing activated and basal transcription in vitro

A human RNA polymerase II (pol II) complex was isolated from a HeLa-derived cell line that conditionally expresses an epitope-tagged RPB9 subunit of human pol II. The isolated FLAG-tagged pol II complex (f:pol II) contains a subset of general transcription factors but is devoid of TFIID and TFIIA. In conjunction with TATA-binding protein (TBP) or TFIID, f:pol II is able to mediate both basal and activated transcription by Gal4-VP16 when a transcriptional coactivator PC4 is also provided. Interestingly, PC4, in the absence of a transcriptional activator, actually functions as a repressor to inhibit basal transcription. Remarkably, TBP is able to mediate activator function in this transcription system. The presence of TBP-associated factors, however, helps overcome PC4 repression and further enhance the level of activation mediated by TBP. Alleviation of PC4 repression can also be achieved by preincubation of the transcriptional components with the DNA template. Sarkosyl disruption of preinitiation complex formation further illustrates that PC4 can only inhibit transcription prior to the assembly of a functional preinitiation complex. These results suggest that PC4 represses basal transcription by preventing the assembly of a functional preinitiation complex, but it has no effect on the later steps of the transcriptional process.

Functions of the N- and C-terminal domains of human RAP74 in transcriptional initiation, elongation, and recycling of RNA polymerase II

Transcription factor IIF (TFIIF) cooperates with RNA polymerase II (pol II) during multiple stages of the transcription cycle including preinitiation complex assembly, initiation, elongation, and possibly termination and recycling. Human TFIIF appears to be an alpha2beta2 heterotetramer of RNA polymerase II-associating protein 74- and 30-kDa subunits (RAP74 and RAP30). From inspection of its 517-amino-acid (aa) sequence, the RAP74 subunit appears to comprise separate N- and C-terminal domains connected by a flexible loop. In this study, we present functional data that strongly support this model for RAP74 architecture and further show that the N- and C-terminal domains and the central loop of RAP74 have distinct roles during separate phases of the transcription cycle. The N-terminal domain of RAP74 (minimally aa 1 to 172) is sufficient to deliver pol II into a complex formed on the adenovirus major late promoter with the TATA-binding protein, TFIIB, and RAP30. A more complete N-terminal domain fragment (aa 1 to 217) strongly stimulates both accurate initiation and elongation by pol II. The region of RAP74 between aa 172 and 205 and a subregion between aa 170 and 178 are critical for both accurate initiation and elongation, and mutations in these regions have similar effects on initiation and elongation. Based on these observations, RAP74 appears to have similar functions in initiation and elongation. The central region and the C-terminal domain of RAP74 do not contribute strongly to single-round accurate initiation or elongation stimulation but do stimulate multiple-round transcription in an extract system.

Mutational analysis of the D1/E1 core helices and the conserved N-terminal region of yeast transcription factor IIB (TFIIB): identification of an N-terminal mutant that stabilizes TATA-binding protein-TFIIB-DNA complexes

The general transcription factor IIB (TFIIB) plays an essential role in transcription of protein-coding genes by RNA polymerase II. We have used site-directed mutagenesis to assess the role of conserved amino acids in several important regions of yeast TFIIB. These include residues in the highly conserved amino-terminal region and basic residues in the D1 and E1 core domain alpha-helices. Acidic substitutions of residues K190 (D1) and K201 (E1) resulted in growth impairments in vivo, reduced basal transcriptional activity in vitro, and an inability to form stable TFIIB-TATA-binding protein-DNA (DB) complexes. Significantly, these mutants retained the ability to respond to acidic activators in vivo and to the Gal4-VP16 activator in vitro, supporting the view that these basic residues play a role in basal transcription. In addition, 14 single-amino-acid substitutions were introduced in the conserved amino-terminal region. Three of these mutants, the L50D, R64E, and R78L mutants, displayed altered growth properties in vivo and were compromised for supporting transcription in vitro. The L50D mutant was impaired for RNA polymerase II interaction, while the R64E mutant exhibited altered transcription start site selection both in vitro and in vivo and, surprisingly, was more active than the wild type in the formation of stable DB complexes. These results support the view that the amino-terminal domain is involved in the direct interaction between yeast TFIIB and RNA polymerase II and suggest that this domain may interact with DNA and/or modulate the formation of a DB complex.

Transcriptional activation by TFIIB mutants that are severely impaired in interaction with promoter DNA and acidic activation domains

Biochemical experiments indicate that the general transcription factor IIB (TFIIB) can interact directly with acidic activation domains and that activators can stimulate transcription by increasing recruitment of TFIIB to promoters. For promoters at which recruitment of TFIIB to promoters is limiting in vivo, one would predict that transcriptional activity should be particularly sensitive to TFIIB mutations that decrease the association of TFIIB with promoter DNA and/or with activation domains; i.e., such TFIIB mutations should exacerbate a limiting step that occurs in wild-type cells. Here, we describe mutations on the DNA-binding surface of TFIIB that severely affect both TATA-binding protein (TBP)-TFIIB-TATA complex formation and interaction with the VP16 activation domain in vitro. These TFIIB mutations affect the stability of the TBP-TFIIB-TATA complex in vivo because they are synthetically lethal in combination with TBP mutants impaired for TFIIB binding. Interestingly, these TFIIB derivatives support viability, and they efficiently respond to Gal4-VP16 and natural acidic activators in different promoter contexts. These results suggest that in vivo, recruitment of TFIIB is not generally a limiting step for acidic activators. However, one TFIIB derivative shows reduced transcription of GAL4, suggesting that TFIIB may be limiting at a subset of promoters in vivo.

A cryptic DNA binding domain at the COOH terminus of TFIIIB70 affects formation, stability, and function of preinitiation complexes

TFIIIC-dependent assembly of yeast TFIIIB on class III genes unmasks a high avidity of TFIIIB for DNA. TFIIIB contains TATA-binding protein (TBP), TFIIIB90/B", and TFIIIB70/Brf1, which is homologous to TFIIB. Using limited proteolysis, we have found that the COOH terminus of TFIIIB70 (residues 510-596) forms a protease-resistant domain that binds DNA tightly as seen by Southwestern, DNase I footprinting, and gel shift assays. Consistent with a role for this DNA binding activity, preinitiation complexes were formed less efficiently with truncated TFIIIB70 lacking the COOH-terminal domain and displayed an increased sensitivity to heparin. B' (TFIIIB70 + TBP).TFIIIC.DNA complexes were also particularly unstable. In addition, TFIIIB.TFIIIC.DNA complexes containing truncated TFIIIB70 were impaired in promoting transcription initiation.

RAP74 induces promoter contacts by RNA polymerase II upstream and downstream of a DNA bend centered on the TATA box

RAP74, the large subunit of transcription factor IIF, associates with a preinitiation complex containing RNA polymerase II (pol II) and other general initiation factors. We have mapped the location of RAP74 in close proximity to promoter DNA at similar distances both upstream and downstream of a DNA bend centered on the TATA box. Binding of RAP74 induces a conformational change that affects the position of pol II relative to that of the DNA. This reorganization of the preinitiation complex minimally requires the N-terminal region of RAP74 containing both its RAP30-binding domain and another region necessary for accurate transcription in vitro. We propose a role for RAP74 in controlling the topological organization of the pol II preinitiation complex.