Purification of the Drosophila RNA polymerase II general transcription factors.

We describe a fractionation and purification scheme for the Drosophila RNA polymerase II general transcription factors. Drosophila TFIIE, TFIIF, TFIIH, and RNA polymerase II have been purified to greater than 50% homogeneity from Drosophila embryo nuclear extracts. TFIID has been purified 80-fold and is not significantly contaminated with any of the other general factors. This is the first reported identification and purification of Drosophila TFIIH and TFIIE. Further analysis shows that, similar to their mammalian counterparts, Drosophila TFIIH is composed of eight polypeptides sized between 30 and 100 kDa, and Drosophila TFIIE is composed of two polypeptides sized at 34 and 60 kDa. When all of these fractions are combined with recombinant Drosophila TFlIB, a highly purified in vitro transcription system is generated that has not previously been available in Drosophila. The TFIID fraction can be replaced with recombinant Drosophila TBP to give a transcription system that is nearly free of contaminating proteins.

Crystal structure of a human TATA box-binding protein/TATA element complex.

The TATA box-binding protein (TBP) is required by all three eukaryotic RNA polymerases for correct initiation of transcription of ribosomal, messenger, small nuclear, and transfer RNAs. The cocrystal structure of the C-terminal/core region of human TBP complexed with the TATA element of the adenovirus major late promoter has been determined at 1.9 angstroms resolution. Structural and functional analyses of the protein-DNA complex are presented, with a detailed comparison to our 1.9-angstroms resolution structure of Arabidopsis thaliana TBP2 bound to the same TATA box.

The SNAP45 subunit of the small nuclear RNA (snRNA) activating protein complex is required for RNA polymerase II and III snRNA gene transcription and interacts with the TATA box binding protein.

The RNA polymerase II and III small nuclear RNA (snRNA) promoters contain a common basal promoter element, the proximal sequence element (PSE). The PSE binds a multisubunit complex we refer to as the snRNA activating protein complex (SNAPc). At least four polypeptides are visible in purified SNAPc preparations, which migrate with apparent molecular masses of 43, 45, 50, and 190 kDa on SDS/polyacrylamide gels. In addition, purified preparations of SNAPc contain variable amounts of TATA box binding protein (TBP). An important question is whether the PSEs of RNA polymerase II and III snRNA promoters recruit the exact same SNAP complex or slightly different versions of SNAPc, differing, for example, by the presence or absence of a subunit. To address this question, we are isolating cDNAs encoding different subunits of SNAPc. We have previously isolated the cDNA encoding the 43-kDa subunit SNAP43. We now report the isolation of the cDNA that encodes the p45 polypeptide. Antibodies directed against p45 retard the mobility of the SNAPc-PSE complex in an electrophoretic mobility shift assay, indicating that p45 is indeed part of SNAPc. We therefore refer to this protein as SNAP45. SNAP45 is exceptionally proline-rich, interacts strongly with TBP, and, like SNAP43, is required for both RNA polymerase II and III transcription of snRNA genes.

A role for the Msx-1 homeodomain in transcriptional regulation: residues in the N-terminal arm mediate TATA binding protein interaction and transcriptional repression.

In a previous study we showed that the murine homeodomain protein Msx-1 is a potent transcriptional repressor and that this activity is independent of its DNA binding function. The implication of these findings is that repression by Msx-1 is mediated through its association with certain protein factors rather than through its interaction with DNA recognition sites, which prompted investigation of the relevant protein factors. Here we show that Msx-1 interacts directly with the TATA binding protein (TBP) but not with several other general transcription factors. This interaction is mediated by the Msx-1 homeodomain, specifically through residues in the N-terminal arm. These same N-terminal arm residues are required for repression by Msx-1, suggesting a functional relationship between TBP association and transcriptional repression. This is further supported by the observation that addition of excess TBP blocks the repressor action of Msx-1 in in vitro transcription assays. Finally, DNA binding activity is separable from both TBP interaction and repression, which further shows that these other activities of the Msx-1 homeodomain are distinct. Therefore, these findings define a role for the Msx-1 homeodomain, particularly the N-terminal arm residues in protein-protein interaction and transcriptional repression, and implicate a more complex role overall for homeodomains in transcriptional regulation.

Protein-protein interactions in eukaryotic transcription initiation: structure of the preinitiation complex.

We have used alanine scanning to analyze protein-protein interactions by human TATA-element binding protein (TBP) within the transcription preinitiation complex. The results indicate that TBP interacts with RNA polymerase II and general transcription factors IIA, IIB, and IIF within the functional transcription preinitiation complex and define the determinants of TBP for each of these interactions. The results permit construction of a model for the structure of the preinitiation complex.

Core promoter-specific function of a mutant transcription factor TFIID defective in TATA-box binding.

In conjunction with other general initiation factors, the TATA box-binding protein (TBP) can direct basal transcription by RNA polymerase II from TATA-containing promoters, but its stable interaction with TBP-associated factors (TAFs) in the TFIID complex is required both for activator-dependent transcription and for basal transcription directed by an initiator element. We have generated a TATA-binding-defective TFIID complex containing an amino acid substitution in the DNA-binding surface of its TBP subunit. This mutated TFIID is defective in both basal and activated transcription from core promoters containing only a TATA box but supports transcription from initiator-containing promoters independently of the presence or absence of a TATA sequence. Our results show that a functional initiator element is needed to bypass the requirement for an active TATA DNA-binding surface in TFIID and imply that gene-specific transcription can be achieved by modulating distinct core promoter-specific TFIID functions--e.g., TBP-TATA versus TAF-initiator interactions.

The ability to associate with activation domains in vitro is not required for the TATA box-binding protein to support activated transcription in vivo.

The TATA box-binding protein (TBP) interacts in vitro with the activation domains of many viral and cellular transcription factors and has been proposed to be a direct target for transcriptional activators. We have examined the functional relevance of activator-TBP association in vitro to transcriptional activation in vivo. We show that alanine substitution mutations in a single loop of TBP can disrupt its association in vitro with the activation domains of the herpes simplex virus activator VP16 and of the human tumor suppressor protein p53; these mutations do not, however, disrupt the transcriptional response of TBP to either activation domain in vivo. Moreover, we show that a region of VP16 distinct from its activation domain can also tightly associate with TBP in vitro, but fails to activate transcription in vivo. These data suggest that the ability of TBP to interact with activation domains in vitro is not directly relevant to its ability to support activated transcription in vivo.

The activation domain of GAL4 protein mediates cooperative promoter binding with general transcription factors in vivo.

Most proteins that activate RNA polymerase II-mediated transcription in eukaryotic cells contain sequence-specific DNA-binding domains and "activation" regions. The latter bind general transcription factors and/or coactivators and are required for high-level transcription. Their function in vivo is unknown. Since several activation domains bind the TATA-binding protein (TBP), TBP-associated factors, or other general factors in vitro, one role of the activation domain may be to facilitate promoter occupancy by supporting cooperative binding of the activator and general transcription factors. Using the GAL4 system of yeast, we have tested this model in vivo. It is demonstrated that the presence of a TATA box (the TBP binding site) facilitates binding of GAL4 protein to low- and moderate-affinity sites and that the activation domain modulates these effects. These results support the cooperative binding model for activation domain function in vivo.

Transcription factor TFIID is a direct functional target of the adenovirus E1A transcription-repression domain.

The 243-amino acid adenovirus E1A oncoprotein both positively and negatively modulates the expression of cellular genes involved in the regulation of cell growth. The E1A transcription repression function appears to be linked with its ability to induce cellular DNA synthesis, cell proliferation, and cell transformation, as well as to inhibit cell differentiation. The mechanism by which E1A represses the transcription of various promoters has proven enigmatic. Here we provide several lines of evidence that the "TATA-box" binding protein (TBP) component of transcription factor TFIID is a cellular target of the E1A repression function encoded within the E1A N-terminal 80 amino acids. (i) The E1A N-terminal 80 amino acids [E1A-(1-80)protein] efficiently represses basal transcription from TATA-containing core promoters in vitro. (ii) TBP reverses completely E1A repression in vitro. (iii) TBP restores transcriptional activity to E1A-(1-80) protein affinity-depleted nuclear extracts. (iv) The N-terminal repression domain of E1A interacts directly and specifically with TBP in vitro. These results may help explain how E1A represses a set of genes that lack common upstream promoter elements.

RNA polymerase II/III transcription specificity determined by TATA box orientation.

The TATA box sequence in eukaryotes is located about 25 bp upstream of many genes transcribed by RNA polymerase II (Pol II) and some genes transcribed by RNA polymerase III (Pol III). The TATA box is recognized in a sequence-specific manner by the TATA box-binding protein (TBP), an essential factor involved in the initiation of transcription by all three eukaryotic RNA polymerases. We have investigated the recognition of the TATA box by the Pol II and Pol III basal transcription machinery and its role in establishing the RNA polymerase specificity of the promoter. Artificial templates were constructed that contained a canonical TATA box as the sole promoter element but differed in the orientation of the 8-bp TATA box sequence. As expected, Pol II initiated transcription in unfractionated nuclear extracts downstream of the "forward" TATA box. In distinct contrast, transcription that initiated downstream of the "reverse" TATA box was carried out specifically by Pol III. Importantly, this effect was observed regardless of the source of the DNA either upstream or downstream of the TATA sequence. These findings suggest that TBP may bind in opposite orientations on Pol II and Pol III promoters and that opposite, yet homologous, surfaces of TBP may be utilized by the Pol II and Pol III basal machinery for the initiation of transcription.

Evolutionary conservation of human TATA-binding-polypeptide-associated factors TAFII31 and TAFII80 and interactions of TAFII80 with other TAFs and with general transcription factors.

Human transcription initiation factor TFIID is composed of the TATA-binding polypeptide (TBP) and at least 13 TBP-associated factors (TAFs) that collectively or individually are involved in activator-dependent transcription. To investigate protein-protein interactions involved in TFIID assembly and in TAF-mediated activator functions, we have cloned and expressed cDNAs encoding human TAFII80 and TAFII31. Coimmunoprecipitation assays showed that TAFII80 interacted with TAFII250, TAFII31, TAFII20, and TBP, but not with TAFII55. Similar assays showed that TAFII80 interacted with TFIIE alpha and with TFIIF alpha (RAP74) but not with TFIIB, TFIIE beta, or TFIIF beta (RAP30). Further studies with TAFII80 mutations revealed three distinct interaction domains which fall within regions conserved in human TAFII80, Drosophila TAFII60, and yeast TAFII60. The N terminus of TAFII80 (residues 1-100) interacts with both TAFII31 and TAFII20, while two C-terminal regions are involved, respectively, in interactions with TAFII250 and TFIIF alpha (RAP74) (residues 203-276) and with TBP and TFIIE alpha (residues 377-505). The interactions between TAFII80 and general factors TFIIE alpha and TFIIF alpha (RAP74) could be important for recruitment of GTFs during activator-dependent transcription. Because TAFs 80, 31, and 20 show sequence similarities to histones H4, H3, and H2B, as well as some parallel interactions, this subset of TAFs may form a related core structure within TFIID.

Identification and characterization of a TFIID-like multiprotein complex from Saccharomyces cerevisiae.

Although the mechanisms of transcriptional regulation by RNA polymerase II are apparently highly conserved from yeast to man, the identification of a yeast TATA-binding protein (TBP)-TBP-associated factor (TAFII) complex comparable to the metazoan TFIID component of the basal transcriptional machinery has remained elusive. Here, we report the isolation of a yeast TBP-TAFII complex which can mediate transcriptional activation by GAL4-VP16 in a highly purified yeast in vitro transcription system. We have cloned and sequenced the genes encoding four of the multiple yeast TAFII proteins comprising the TBP-TAFII multisubunit complex and find that they are similar at the amino acid level to both human and Drosophila TFIID subunits. Using epitope-tagging and immunoprecipitation experiments, we demonstrate that these genes encode bona fide TAF proteins and show that the yeast TBP-TAFII complex is minimally composed of TBP and seven distinct yTAFII proteins ranging in size from M(r) = 150,000 to M(r) = 25,000. In addition, by constructing null alleles of the cloned TAF-encoding genes, we show that normal function of the TAF-encoding genes is essential for yeast cell viability.

Structure and function of a human transcription factor TFIIIB subunit that is evolutionarily conserved and contains both TFIIB- and high-mobility-group protein 2-related domains.

Transcription factor TFIIIB plays a central role in transcription initiation by RNA polymerase III on genes encoding tRNA, 5S rRNA, and other small structural RNAs. We report the purification of a human TFIIIB-derived complex containing only the TATA-binding polypeptide (TBP) and a 90-kDa subunit (TFIIIB90) and the isolation of a cDNA clone encoding the 90-kDa subunit. The N-terminal half of TFIIIB90 exhibits sequence similarity to the yeast TFIIIB70 (BRF) and the class II transcription factor TFIIB and interacts weakly with TBP. The C-terminal half of TFIIIB90 contains a high-mobility-group protein 2 (HMG2)-related domain and interacts strongly with TBP. Recombinant TFIIIB90 plus recombinant human TBP substitute for human TFIIIB in a complementation assay for transcription of 5S, tRNA, and VA1 RNA genes, and both the TFIIB-related domain and the HMG2-related domain are required for this activity. TFIIIB90 is also required for transcription of human 7SK and U6 RNA genes by RNA polymerase III, but apparently within a complex distinct from the TBP/TFIIIB90 complex.

Transcription in archaea: similarity to that in eucarya.

We present homologies between archaeal and eucaryal DNA-dependent RNA polymerase (RNAP) subunits and transcription factors. The sequences of the Sulfolobus acidocaldarius subunits D, E, and N and alignments with eucaryal homologs are presented here. The similarities between archaeal transcription factors and their eucaryal homologs TFIIB and TBP have been established in other laboratories. The archaeal RNAP subunits H, K, and N, respectively, show high sequence similarity to ABC27, ABC23, and ABC10 beta (found in all three eucaryal RNAPs); subunit D, to AC40 (common to polymerase II and polymerase III) and B44 (polymerase II); and subunit L, to AC19 and B12.5. The similarity of subunit D and its eucaryal homologs to bacterial alpha is limited to the "alpha-motif," which is also present in subunit L and its eucaryal homologs. Genes encoding homologs of the related eucaryal RNAP subunits A12.2/B12.6 and also homologs of eucaryal transcription elongation factors of the TFIIS family have been detected in Sulfolobus acidocaldarius and Thermococcus celer. In archaea, the protein is not an RNAP subunit. Together with the sequence similarities between archaeal box A-containing and eucaryal TATA box-containing promoters, this shows that the archaeal and eucaryal transcription systems are truly homologous and that they differ structurally and functionally from the bacterial transcription machinery. In contrast, however, a number of genes for the archaeal transcription apparatus are organized in clusters resembling the clusters of transcription-associated genes in Bacteria.

Molecular cloning and expression of the 32-kDa subunit of human TFIID reveals interactions with VP16 and TFIIB that mediate transcriptional activation.

Transcription factor TFIID consists of TATA binding protein (TBP) and at least eight TBP-associated factors (TAFs). As TAFs are required for activated but not basal transcription, we have proposed that TAFs act as coactivators to mediate signals between activators and the basal transcription machinery. Here we report the cloning, expression, and biochemical characterization of the 32-kDa subunit of human (h) TFIID, termed hTAFII32. We find that hTAFII32 is the human homologue of Drosophila TAFII40. In vitro protein-protein interaction assays reveal that as observed with Drosophila TAFII40, hTAFII32 interacts with the C-terminal 39-amino acid activation domain of the acidic transactivator viral protein 16 (VP16) as well as with the general transcription factor TFIIB. Moreover, a partial recombinant TFIID complex containing hTAFII32 was capable of mediating in vitro transcriptional activation by the VP16 activation domain. These findings indicate that specific activator-coactivator interactions have been conserved between human and Drosophila and provide additional support for the function of these interactions in mediating transcriptional activation.