RNA polymerase III and class III transcription factors from Saccharomyces cerevisiae

Functional characterization of Polr3a hypomyelinating leukodystrophy mutations in the S. cerevisiae homolog, RPC160

Mutations in RNA polymerase III (Pol III) cause hypomeylinating leukodystrophy (HLD) and neurodegeneration in humans. POLR3A and POLR3B, the two largest Pol III subunits, together form the catalytic center and carry the majority of disease alleles. Disease-causing mutations include invariant and highly conserved residues that are predicted to negatively affect Pol III activity and decrease transcriptional output. A subset of HLD missense mutations in POLR3A cluster in the pore region that provides nucleotide access to the Pol III active site. These mutations were engineered at the corresponding positions in the Saccharomyces cerevisiae homolog, Rpc160, to evaluate their functional deficits. None of the mutations caused a growth or transcription phenotype in yeast. Each mutation was combined with a frequently occurring pore mutation, POLR3A G672E, which was also wild-type for growth and transcription. The double mutants showed a spectrum of phenotypes from wild-type to lethal, with only the least fit combinations showing an effect on Pol III transcription. In one slow-growing temperature-sensitive mutant the steady-state level of tRNAs was unaffected, however global tRNA synthesis was compromised, as was the synthesis of RPR1 and SNR52 RNAs. Affinity-purified mutant Pol III was broadly defective in both factor-independent and factor-dependent transcription in vitro across genes that represent the yeast Pol III transcriptome. Thus, the robustness of yeast Rpc160 to single Pol III leukodystrophy mutations in the pore domain can be overcome by a second mutation in the domain.

Investigating transcription reinitiation through in vitro approaches

By influencing the number of RNA molecules repeatedly synthesized from the same gene, the control of transcription reinitiation has the potential to shape the transcriptome. Transcription reinitiation mechanisms have been mainly addressed in vitro, through approaches based on both crude and reconstituted systems. These studies support the notion that transcription reinitiation and its regulation rely on dedicated networks of molecular interactions within transcription machineries. At the same time, comparison with in vivo transcription rates suggests that additional mechanisms, factors and conditions must exist in the nucleus, whose biochemical elucidation is a fascinating challenge for future in vitro transcription studies.

The poly(A)-binding protein Nab2 functions in RNA polymerase III transcription

Reuter et al. show that Nab2, a poly(A)-binding protein important for correct poly(A) tail length and nuclear mRNA export, is present at all RNA polymerase III (RNAPIII) transcribed genes. Nab2 is required for the occupancy of RNAPIII and TFIIIB at target genes.

RNA polymerase III (RNAPIII) synthesizes most small RNAs, the most prominent being tRNAs. Although the basic mechanism of RNAPIII transcription is well understood, recent evidence suggests that additional proteins play a role in RNAPIII transcription. Here, we discovered by a genome-wide approach that Nab2, a poly(A)-binding protein important for correct poly(A) tail length and nuclear mRNA export, is present at all RNAPIII transcribed genes. The occupancy of Nab2 at RNAPIII transcribed genes is dependent on transcription. Using a novel temperature-sensitive allele of NAB2, nab2-34, we show that Nab2 is required for the occupancy of RNAPIII and TFIIIB at target genes. Furthermore, Nab2 interacts with RNAPIII, TFIIIB, and RNAPIII transcripts. Importantly, impairment of Nab2 function causes an RNAPIII transcription defect in vivo and in vitro. Taken together, we establish Nab2, an important mRNA biogenesis factor, as a novel player required for RNAPIII transcription by stabilizing TFIIIB and RNAPIII at promoters.

Human RNase P ribonucleoprotein is required for formation of initiation complexes of RNA polymerase III

Human RNase P is implicated in transcription of small non-coding RNA genes by RNA polymerase III (Pol III), but the precise role of this ribonucleoprotein therein remains unknown. We here show that targeted destruction of HeLa nuclear RNase P inhibits transcription of 5S rRNA genes in whole cell extracts, if this precedes the stage of initiation complex formation. Biochemical purification analyses further reveal that this ribonucleoprotein is recruited to 5S rRNA genes as a part of proficient initiation complexes and the activity persists at reinitiation. Knockdown of RNase P abolishes the assembly of initiation complexes by preventing the formation of the initiation sub-complex of Pol III. Our results demonstrate that the structural intactness, but not the endoribonucleolytic activity per se, of RNase P is critical for the function of Pol III in cells and in extracts.

Mass spectrometry reveals stable modules in holo and apo RNA polymerases I and III

RNA polymerases are essential enzymes which transcribe DNA into RNA. Here, we obtain mass spectra of the cellular forms of apo and holo eukaryotic RNA polymerase I and III, defining their composition under different solution conditions. By recombinant expression of subunits within the initiation heterotrimer of Pol III, we derive an interaction network and couple this data with ion mobility data to define topological restraints. Our data agree with available structural information and homology modeling and are generally consistent with yeast two hybrid data. Unexpectedly, elongation complexes of both Pol I and III destabilize the assemblies compared with their apo counterparts. Increasing the pH and ionic strength of apo and holo forms of Pol I and Pol III leads to formation of at least ten stable subcomplexes for both enzymes. Uniquely for Pol III many subcomplexes contain only one of the two largest catalytic subunits. We speculate that these stable subcomplexes represent putative intermediates in assembly pathways.

Conformational flexibility of RNA polymerase III during transcriptional elongation

RNA polymerase (Pol) III is responsible for the transcription of genes encoding small RNAs, including tRNA, 5S rRNA and U6 RNA. Here, we report the electron cryomicroscopy structures of yeast Pol III at 9.9 Å resolution and its elongation complex at 16.5 Å resolution. Particle sub-classification reveals prominent EM densities for the two Pol III-specific subcomplexes, C31/C82/C34 and C37/C53, that can be interpreted using homology models. While the winged-helix-containing C31/C82/C34 subcomplex initiates transcription from one side of the DNA-binding cleft, the C37/C53 subcomplex accesses the transcription bubble from the opposite side of this cleft. The transcribing Pol III enzyme structure not only shows the complete incoming DNA duplex, but also reveals the exit path of newly synthesized RNA. During transcriptional elongation, the Pol III-specific subcomplexes tightly enclose the incoming DNA duplex, which likely increases processivity and provides structural insights into the conformational switch between Pol III-mediated initiation and elongation.

Dicistronic tRNA-5S rRNA genes in Yarrowia lipolytica: an alternative TFIIIA-independent way for expression of 5S rRNA genes

In eukaryotes, genes transcribed by RNA polymerase III (Pol III) carry their own internal promoters and as such, are transcribed as individual units. Indeed, a very few cases of dicistronic Pol III genes are yet known. In contrast to other hemiascomycetes, 5S rRNA genes of Yarrowia lipolytica are not embedded into the tandemly repeated rDNA units, but appear scattered throughout the genome. We report here an unprecedented genomic organization: 48 over the 108 copies of the 5S rRNA genes are located 3' of tRNA genes. We show that these peculiar tRNA-5S rRNA dicistronic genes are expressed in vitro and in vivo as Pol III transcriptional fusions without the need of the 5S rRNA gene-specific factor TFIIIA, the deletion of which displays a viable phenotype. We also report the existence of a novel putative non-coding Pol III RNA of unknown function about 70 nucleotide-long (RUF70), the 13 genes of which are devoid of internal Pol III promoters and located 3' of the 13 copies of the tDNA-Trp (CCA). All genes embedded in the various dicistronic genes, fused 5S rRNA genes, RUF70 genes and their leader tRNA genes appear to be efficiently transcribed and their products correctly processed in vivo.

The transcription reinitiation properties of RNA polymerase III in the absence of transcription factors

Transcription reinitiation by RNA polymerase (Pol) III proceeds through facilitated recycling, a process by which the terminating Pol III, assisted by the transcription factors TFIIIB and TFIIIC, rapidly reloads onto the same transcription unit. To get further insight into the Pol III transcription mechanism, we analyzed the kinetics of transcription initiation and reinitiation of a simplified in vitro transcription system consisting only of Pol III and template DNA. The data indicates that, in the absence of transcription factors, first-round transcription initiation by Pol III proceeds at a normal rate, while facilitated reinitiation during subsequent cycles is compromised.

Selectivity and proofreading both contribute significantly to the fidelity of RNA polymerase III transcription

We examine here the mechanisms ensuring the fidelity of RNA synthesis by RNA polymerase III (Pol III). Misincorporation could only be observed by using variants of Pol III deficient in the intrinsic RNA cleavage activity. Determination of relative rates of the reactions producing correct and erroneous transcripts at a specific position on a tRNA gene, combined with computational methods, demonstrated that Pol III has a highly efficient proofreading activity increasing its transcriptional fidelity by a factor of 10(3) over the error rate determined solely by selectivity (1.8 x 10(-4)). We show that Pol III slows down synthesis past a misincorporation to achieve efficient proofreading. We discuss our findings in the context of transcriptional fidelity studies performed on RNA Pols, proposing that the fidelity of transcription is more crucial for Pol III than Pol II.

Insights into transcription initiation and termination from the electron microscopy structure of yeast RNA polymerase III

RNA polymerase III (RNAPIII) synthesizes tRNA, 5S RNA, U6 snRNA, and other small RNAs. The structure of yeast RNAPIII, determined at 17 A resolution by cryo-electron microscopy and single-particle analysis, reveals a hand-like shape typical of RNA polymerases. Compared to RNAPII, RNAPIII is characterized by a bulkier stalk and by prominent features extending from the DNA binding cleft. We attribute the latter primarily to five RNAPIII-specific subunits, present as two distinct subcomplexes (C82/C34/C31 and C53/C37). Antibody labeling experiments localize the C82/C34/C31 subcomplex to the clamp side of the DNA binding cleft, consistent with its known role in transcription initiation. The C53/C37 subcomplex appears to be situated across the cleft, near the presumed location of downstream DNA, accounting for its role in transcription termination. Our structure rationalizes available mutagenesis and biochemical data and provides insights into RNAPIII-mediated transcription.

Requirement of Nhp6 proteins for transcription of a subset of tRNA genes and heterochromatin barrier function in Saccharomyces cerevisiae

A key event in tRNA gene (tDNA) transcription by RNA polymerase (Pol) III is the TFIIIC-dependent assembly of TFIIIB upstream of the transcription start site. Different tDNA upstream sequences bind TFIIIB with different affinities, thereby modulating tDNA transcription. We found that in the absence of Nhp6 proteins, the influence of the 5'-flanking region on tRNA gene transcription is dramatically enhanced in Saccharomyces cerevisiae. Expression of a tDNA bearing a suboptimal TFIIIB binding site, but not of a tDNA preceded by a strong TFIIIB binding region, was strongly dependent on Nhp6 in vivo. Upstream sequence-dependent stimulation of tRNA gene transcription by Nhp6 could be reproduced in vitro, and Nhp6 proteins were found associated with tRNA genes in yeast cells. We also show that both transcription and silencing barrier activity of a tDNA(Thr) at the HMR locus are compromised in the absence of Nhp6. Our data suggest that Nhp6 proteins are important components of Pol III chromatin templates that contribute both to the robustness of tRNA gene expression and to positional effects of Pol III transcription complexes.

A Minimal Promoter for TFIIIC-dependent in Vitro Transcription of snoRNA and tRNA Genes by RNA Polymerase III

The Saccharomyces cerevisiae SNR52 gene is unique among the snoRNA coding genes in being transcribed by RNA polymerase III. The primary transcript of SNR52 is a 250-nucleotide precursor RNA from which a long leader sequence is cleaved to generate the mature snR52 RNA. We found that the box A and box B sequence elements in the leader region are both required for the in vivo accumulation of the snoRNA. As expected box B, but not box A, was absolutely required for stable TFIIIC, yet in vitro. Surprisingly, however, the box B was found to be largely dispensable for in vitro transcription of SNR52, whereas the box A-mutated template effectively recruited TFIIIB; yet it was transcriptionally inactive. Even in the complete absence of box B and both upstream TATA-like and T-rich elements, the box A still directed efficient, TFIIIC-dependent transcription. Box B-independent transcription was also observed for two members of the tRNA(Asn)(GTT) gene family, but not for two tRNA(Pro)(AGG) gene copies. Fully recombinant TFIIIC supported box B-independent transcription of both SNR52 and tRNA(Asn) genes, but only in the presence of TFIIIB reconstituted with a crude B'' fraction. Non-TFIIIB component(s) in this fraction were also required for transcription of wild-type SNR52. Transcription of the box B-less tRNA(Asn) genes was strongly influenced by their 5'-flanking regions, and it was stimulated by TBP and Brf1 proteins synergistically. The box A can thus be viewed as a core TFIIIC-interacting element that, assisted by upstream TFIIIB-DNA contacts, is sufficient to promote class III gene transcription.

Nucleosome depletion activates poised RNA polymerase III at unconventional transcription sites in Saccharomyces cerevisiae

RNA polymerase (pol) III, assisted by the transcription factors TFIIIC and TFIIIB, transcribes small untranslated RNAs, such as tRNAs. In addition to known pol III-transcribed genes, the Saccharomyces cerevisiae genome contains loci (ZOD1, ETC1-8) associated to incomplete pol III transcription complexes (Moqtaderi, Z., and Struhl, K. (2004) Mol. Cell. Biol. 24, 4118-4127). We show that a short segment of the ZOD1 locus, containing box A and box B promoter elements and a termination signal between them, directs the pol III-dependent production of a small RNA both in vitro and in vivo. In yeast cells, the levels of both ZOD1- and ETC5-specific transcripts were dramatically enhanced upon nucleosome depletion. Remarkably, transcription factor and pol III occupancy at the corresponding loci did not change significantly upon derepression, thus suggesting that chromatin opening activates poised pol III to transcription. Comparative genomic analysis revealed that the ZOD1 promoter is the only surviving portion of a tDNA(Ile) ancestor, whose transcription capacity has been preserved throughout evolution independently from the encoded RNA product. Similarly, another TFIIIC/TFIIIB-associated locus, close to the YGR033c open reading frame, was found to be the strictly conserved remnant of an ancient tDNA(Arg). The maintenance, by eukaryotic genomes, of chromatin-repressed, non-coding transcription units has implications for both genome expression and organization.

Distinct modes of TATA box utilization by the RNA polymerase III transcription machineries from budding yeast and higher plants

The TATA box is a key upstream control element for basal tRNA gene transcription by RNA polymerase III in some eukaryotes, such as the fission yeast (Schizosaccharomyces pombe) and higher plants, but not in others such as the budding yeast (Saccharomyces cerevisiae). To gain information on this differential TATA box requirement, we examined side-by-side the in vitro transcription properties of TATA-containing and TATA-mutated plant and S. cerevisiae tDNAs in homologous in vitro transcription systems from both organisms and in a hybrid system in which yeast TBP was replaced by its plant homologue. The data support the general conclusion that specific features of the plant transcription machinery, rather than upstream region architecture per se, are responsible for the much stronger TATA box dependence of the plant system. In both systems, however, a strong influence of the TATA box on transcription start site selection was observed. This was particularly striking in the case of plant tDNAs, where TATA-rich upstream regions were found to favour the use of alternative initiation sites. Replacement of yeast TBP with its plant counterpart did not confer any general TATA box responsiveness to the yeast transcription machinery. Interactions involving components other than TBP are thus responsible for the strong TATA box requirement of plant tDNA transcription.

Nhp6 is a transcriptional initiation fidelity factor for RNA polymerase III transcription in vitro and in vivo

The binding of the RNA polymerase III (pol III) transcription factor TFIIIC to the box A intragenic promoter element of tRNA genes specifies the placement of TFIIIB on upstream-lying DNA. In turn, TFIIIB recruits pol III to the promoter and specifies transcription initiating 17-19 base pairs upstream of box A. The resolution of the pol III transcription apparatus into recombinant TFIIIB, highly purified TFIIIC, and pol III is accompanied by a loss of precision in specifying where transcription initiation occurs due to heterogeneous placement of TFIIIB. In this paper we show that Nhp6a, an abundant high mobility group B (HMGB) family, non-sequence-specific DNA-binding protein in Saccharomyces cerevisiae restores transcriptional initiation fidelity to this highly purified in vitro system. Restoration of initiation fidelity requires the presence of Nhp6a prior to TFIIIB-DNA complex formation. Chemical nuclease footprinting of TFIIIC- and TFIIIB-TFIIIC-DNA complexes reveals that Nhp6a markedly alters the TFIIIC footprint over box A and reduces the size of the TFIIIB footprint on upstream DNA sequence. Analyses of unprocessed tRNAs from yeast lacking Nhp6a and its closely related paralogue Nhp6b demonstrate that Nhp6 is required for transcriptional initiation fidelity of some but not all tRNA genes, in vivo.

A subcomplex of RNA polymerase III subunits involved in transcription termination and reinitiation

While initiation of transcription by RNA polymerase III (Pol III) has been thoroughly investigated, molecular mechanisms driving transcription termination remain poorly understood. Here we describe how the characterization of the in vitro transcriptional properties of a Pol III variant (Pol IIIdelta), lacking the C11, C37, and C53 subunits, revealed crucial information about the mechanisms of Pol III termination and reinitiation. The specific requirement for the C37-C53 complex in terminator recognition was determined. This complex was demonstrated to slow down elongation by the enzyme, adding to the evidence implicating the elongation rate as a critical determinant of correct terminator recognition. In addition, the presence of the C37-C53 complex required the simultaneous addition of C11 to Pol IIIdelta for the enzyme to reinitiate after the first round of transcription, thus uncovering a role for polymerase subunits in the facilitated recycling process. Interestingly, we demonstrated that the role of C11 in recycling was independent of its role in RNA cleavage. The data presented allowed us to propose a model of Pol III termination and its links to reinitiation.

Rpc25, a conserved RNA polymerase III subunit, is critical for transcription initiation

Rpc25 is a strongly conserved subunit of RNA polymerase III with homology to Rpa43 in RNA polymerase I, Rpb7 in RNA polymerase II and the archaeal RpoE subunit. A central domain of Rpc25 can replaced the corresponding region of Rpb7 with little or no growth defect, underscoring the functional relatedness of these proteins. Rpc25 forms a heterodimer with Rpc17, another conserved component of RNA polymerase III. A conditional mutant (rpc25-S100P) impairs this interaction. rpc25-S100P and another conditional mutant obtained by complementation with the Schizosaccharomyces pombe subunit (rpc25-Sp) were investigated for the properties of their purified RNA polymerase III. The mutant enzymes were defective in the specific synthesis of pre-tRNA transcripts but acted at a wild-type level on poly[d(A-T)] templates. They were also indistinguishable from wild type in transcript elongation, cleavage and termination. These data indicate that Rpc25 is needed for transcription initiation but is not critical for the elongating properties of RNA polymerase III.

Sequence context effects on oligo(dT) termination signal recognition by Saccharomyces cerevisiae RNA polymerase III

Eukaryotic RNA polymerase (Pol) III terminates transcription at short runs of T residues in the coding DNA strand. By genomic analysis, we found that T(5) and T(4) are the shortest Pol III termination signals in yeasts and mammals, respectively, and that, at variance with yeast, oligo(dT) terminators longer than T(5) are very rare in mammals. In Saccharomyces cerevisiae, the strength of T(5) as a terminator was found to be largely influenced by both the upstream and the downstream sequence context. In particular, the CT sequence, which is naturally present downstream of T(5) in the 3'-flank of some tDNAs, was found to act as a terminator-weakening element that facilitates translocation by reducing Pol III pausing at T(5). In contrast, tDNA transcription termination was highly efficient when T(5) was followed by an A or G residue. Surprisingly, however, when a termination-proficient T(5) signal was taken out from the tDNA context and placed downstream of a fragment of the SCR1 gene, its termination activity was compromised, both in vitro and in vivo. Even the T(6) sequence, acting as a strong terminator in tRNA gene contexts, was unexpectedly weak within the SNR52 transcription unit, where it naturally occurs. The observed sequence context effects reflect intrinsic recognition properties of Pol III, because they were still observed in a simplified in vitro transcription system only consisting of purified RNA polymerase and template DNA. Our findings strengthen the notion that termination signal recognition by Pol III is influenced in a complex way by the region surrounding the T cluster and suggest that read-through transcription beyond T clusters might play a significant role in the biogenesis of class III gene products.

Two steps in Maf1-dependent repression of transcription by RNA polymerase III

In Saccharomyces cerevisiae, Maf1 is essential for mediating the repression of transcription by RNA polymerase (pol) III in response to diverse cellular conditions. These conditions activate distinct signaling pathways that converge at or above Maf1. Thus, Maf1-dependent repression is thought to involve a common set of downstream inhibitory effects on the pol III machinery. Here we provide support for this view and define two steps in Maf1-dependent transcriptional repression. We show that chlorpromazine (CPZ)-induced repression of pol III transcription is achieved by inhibiting de novo assembly of transcription factor (TF) IIIB onto DNA as well as the recruitment of pol III to preassembled TFIIIB.DNA complexes. Additionally Brf1 was identified as a target of repression in extracts of CPZ-treated cells. Maf1-Brf1 and Maf1-pol III interactions were implicated in the inhibition of TFIIIB.DNA complex assembly and polymerase recruitment by recombinant Maf1. Co-immunoprecipitation experiments confirmed these interactions in yeast extracts and demonstrated that Maf1 does not differentially sequester Brf1 or pol III under repressing conditions. The results suggest that Maf1 functions by a non-stoichiometric mechanism to repress pol III transcription.

Distinct roles of transcription factors TFIIIB and TFIIIC in RNA polymerase III transcription reinitiation

Eukaryotic RNA polymerase (Pol) III is recruited to target promoters by a stable preinitiation complex containing transcription factors TFIIIC and TFIIIB. After the first transcription cycle, reinitiation proceeds through facilitated recycling, a process by which the terminating Pol III rapidly reloads onto the same transcription unit. Here, we show that Pol III is repeatedly recaptured in vitro by the first transcribed gene, even in the presence of a juxtaposed competitor promoter complex, thus suggesting that facilitated recycling is not merely due to a stochastic reassociation process favored by the small size of class III genes. The transcription factor requirements for facilitated reinitiation were investigated by taking advantage of Pol III templates that support both TFIIIC-dependent and TFIIIC-independent transcription. A TFIIIC-less transcription system, in which TFIIIB was reconstituted from recombinant TATA box-binding protein and Brf1 proteins and a crude fraction containing the Bdp1 component, was sufficient to direct efficient Pol III recycling on short ( approximately 100 bp) class III genes. Unexpectedly, however, on longer (>300 bp) transcription units, reinitiation in the presence of TFIIIB alone was compromised, and TFIIIC was further required to reestablish a high reinitiation rate. Transcription reinitiation was also severely impaired when recombinant Bdp1 protein replaced the corresponding crude fraction in reconstituted TFIIIB. The data reveal an unexpected complexity in the Pol III reinitiation mechanism and suggest the existence of a handing-back network between Pol III, TFIIIC, and TFIIIB on actively transcribed class III genes.

The RNA polymerase III transcriptome revealed by genome-wide localization and activity-occupancy relationships

RNA polymerase III (Pol III) transcribes small untranslated RNAs, such as tRNAs. To define the Pol III transcriptome in Saccharomyces cerevisiae, we performed genome-wide chromatin immunoprecipitation using subunits of Pol III, TFIIIB and TFIIIC. Virtually all of the predicted targets of Pol III, as well as several novel candidates, were occupied by Pol III machinery. Interestingly, TATA box-binding protein occupancy was greater at Pol III targets than virtually all Pol II targets, and the highly occupied Pol II targets are generally strongly transcribed. The temporal relationships between factor occupancy and gene activity were then investigated at selected targets. Nutrient deprivation rapidly reduced both Pol III transcription and Pol III occupancy of both a tRNA gene and RPR1. In contrast, TFIIIB remained bound, suggesting that TFIIIB release is not a critical aspect of the onset of repression. Remarkably, TFIIIC occupancy increased dramatically during repression. Nutrient addition generally reestablished transcription and initial occupancy levels. Our results are consistent with active Pol III displacing TFIIIC, and with inactivation/release of Pol III enabling TFIIIC to bind, marking targets for later activation. These studies reveal new aspects of the kinetics, dynamics, and targets of the Pol III system.

The tau95 subunit of yeast TFIIIC influences upstream and downstream functions of TFIIIC.DNA complexes

The yeast transcription factor IIIC (TFIIIC) is organized in two distinct multisubunit domains, tauA and tauB, that are respectively responsible for TFIIIB assembly and stable anchoring of TFIIIC on the B block of tRNA genes. Surprisingly, we found that the removal of tauA by mild proteolysis stabilizes the residual tauB.DNA complexes at high temperatures. Focusing on the well conserved tau95 subunit that belongs to the tauA domain, we found that the tau95-E447K mutation has long distance effects on the stability of TFIIIC.DNA complexes and start site selection. Mutant TFIIIC.DNA complexes presented a shift in their 5' border, generated slow-migrating TFIIIB.DNA complexes upon stripping TFIIIC by heparin or heat treatment, and allowed initiation at downstream sites. In addition, mutant TFIIIC.DNA complexes were highly unstable at high temperatures. Coimmunoprecipitation experiments indicated that tau95 participates in the interconnection of tauA with tauB via its contacts with tau138 and tau91 polypeptides. The results suggest that tau95 serves as a scaffold critical for tauA.DNA spatial configuration and tauB.DNA stability.

Visualizing RNA extrusion and DNA wrapping in transcription elongation complexes of bacterial and eukaryotic RNA polymerases

Transcription ternary complexes of Escherichia coli RNA polymerase and yeast RNA polymerase III have been analyzed by atomic force microscopy. Using the method of nucleotide omission and different DNA templates, E.coli RNAP has been stalled at position +24, +70 and +379 and RNAP III at position +377 from the starting site. Conformational analysis of E.coli RNAP elongation complexes reveals an average DNA compaction of 22nm and a DNA deformation compatible with approximately 180 degrees DNA wrapping against the enzyme. The extent of protein-DNA interaction attributed to wrapping, however, is less than that of corresponding open promoter complexes. DNA wrapping was also observed for RNAP III elongation complexes, which showed a DNA compaction of 30nm. When the RNA polymerases were stalled far from the promoter (+379 and +377), the growing RNA transcript was often visible and it was prevalently seen exiting from the enzyme on the opposite side relative to the smallest angle subtended by the upstream and downstream DNA arms. Surprisingly, we found that many complexes had a second RNAP, not involved in transcription, bound to the growing RNA of a ternary complex. DNA wrapping in the elongation complex suggests a possible mechanism by which the polymerase may overcome the physical barrier to transcription imposed by the nucleosomes.

An Rpb4/Rpb7-like complex in yeast RNA polymerase III contains the orthologue of mammalian CGRP-RCP

The essential C17 subunit of yeast RNA polymerase (Pol) III interacts with Brf1, a component of TFIIIB, suggesting a role for C17 in the initiation step of transcription. The protein sequence of C17 (encoded by RPC17) is conserved from yeasts to humans. However, mammalian homologues of C17 (named CGRP-RCP) are known to be involved in a signal transduction pathway related to G protein-coupled receptors, not in transcription. In the present work, we first establish that human CGRP-RCP is the genuine orthologue of C17. CGRP-RCP was found to functionally replace C17 in Deltarpc17 yeast cells; the purified mutant Pol III contained CGRP-RCP and had a decreased specific activity but initiated faithfully. Furthermore, CGRP-RCP was identified by mass spectrometry in a highly purified human Pol III preparation. These results suggest that CGRP-RCP has a dual function in mammals. Next, we demonstrate by genetic and biochemical approaches that C17 forms with C25 (encoded by RPC25) a heterodimer akin to Rpb4/Rpb7 in Pol II. C17 and C25 were found to interact genetically in suppression screens and physically in coimmunopurification and two-hybrid experiments. Sequence analysis and molecular modeling indicated that the C17/C25 heterodimer likely adopts a structure similar to that of the archaeal RpoE/RpoF counterpart of the Rpb4/Rpb7 complex. These RNA polymerase subunits appear to have evolved to meet the distinct requirements of the multiple forms of RNA polymerases.

HIV-1 Tat interaction with RNA polymerase II C-terminal domain (CTD) and a dynamic association with CDK2 induce CTD phosphorylation and transcription from HIV-1 promoter

Human immunodeficiency virus, type 1 (HIV-1), Tat protein activates viral gene expression through promoting transcriptional elongation by RNA polymerase II (RNAPII). In this process Tat enhances phosphorylation of the C-terminal domain (CTD) of RNAPII by activating cell cycle-dependent kinases (CDKs) associated with general transcription factors of the promoter complex, specifically CDK7 and CDK9. We reported a Tat-associated T-cell-derived kinase, which contained CDK2. Here, we provide further evidence that CDK2 is involved in Tat-mediated CTD phosphorylation and in HIV-1 transcription in vitro. Tat-mediated CTD phosphorylation by CDK2 required cysteine 22 in the activation domain of Tat and amino acids 42-72 of Tat. CDK2 phosphorylated Tat itself, apparently by forming dynamic contacts with amino acids 15-24 and 36-49 of Tat. Also, amino acids 24-36 and 45-72 of Tat interacted with CTD. CDK2 associated with RNAPII and was found in elongation complexes assembled on HIV-1 long-terminal repeat template. Recombinant CDK2/cyclin E stimulated Tat-dependent HIV-1 transcription in reconstituted transcription assay. Immunodepletion of CDK2/cyclin E in HeLa nuclear extract blocked Tat-dependent transcription. We suggest that CDK2 is part of a transcription complex that is required for Tat-dependent transcription and that interaction of Tat with CTD and a dynamic association of Tat with CDK2/cyclin E stimulated CTD phosphorylation by CDK2.

Intragenic promoter adaptation and facilitated RNA polymerase III recycling in the transcription of SCR1, the 7SL RNA gene of Saccharomyces cerevisiae

The SCR1 gene, coding for the 7SL RNA of the signal recognition particle, is the last known class III gene of Saccharomyces cerevisiae that remains to be characterized with respect to its mode of transcription and promoter organization. We show here that SCR1 represents a unique case of a non-tRNA class III gene in which intragenic promoter elements (the TFIIIC-binding A- and B-blocks), corresponding to the D and TpsiC arms of mature tRNAs, have been adapted to a structurally different small RNA without losing their transcriptional function. In fact, despite the presence of an upstream canonical TATA box, SCR1 transcription strictly depends on the presence of functional, albeit quite unusual, A- and B-blocks and requires all the basal components of the RNA polymerase III transcription apparatus, including TFIIIC. Accordingly, TFIIIC was found to protect from DNase I digestion an 80-bp region comprising the A- and B-blocks. B-block inactivation completely compromised TFIIIC binding and transcription capacity in vitro and in vivo. An inactivating mutation in the A-block selectively affected TFIIIC binding to this promoter element but resulted in much more dramatic impairment of in vivo than in vitro transcription. Transcriptional competition and nucleosome disruption experiments showed that this stronger in vivo defect is due to a reduced ability of A-block-mutated SCR1 to compete with other genes for TFIIIC binding and to counteract the assembly of repressive chromatin structures through TFIIIC recruitment. A kinetic analysis further revealed that facilitated RNA polymerase III recycling, far from being restricted to typical small sized class III templates, also takes place on the 522-bp-long SCR1 gene, the longest known class III transcriptional unit.

Multiple roles of the tau131 subunit of yeast transcription factor IIIC (TFIIIC) in TFIIIB assembly

Yeast transcription factor IIIC (TFIIIC) plays a key role in assembling the transcription initiation factor TFIIIB on class III genes after TFIIIC-DNA binding. The second largest subunit of TFIIIC, tau131, is thought to initiate TFIIIB assembly by interacting with Brf1/TFIIIB70. In this work, we have analyzed a TFIIIC mutant (tau131-DeltaTPR2) harboring a deletion in tau131 removing the second of its 11 tetratricopeptide repeats. Remarkably, this thermosensitive mutation was selectively suppressed in vivo by overexpression of B"/TFIIIB90, but not Brf1 or TATA-binding protein. In vitro, the mutant factor preincubated at restrictive temperature bound DNA efficiently but lost transcription factor activity. The in vitro transcription defect was abolished at high concentrations of B" but not Brf1. Copurification experiments of baculovirus-expressed proteins confirmed a direct physical interaction between tau131 and B". tau131, therefore, appears to be involved in the recruitment of both Brf1 and B".

Maf1p, a negative effector of RNA polymerase III in Saccharomyces cerevisiae

Although yeast RNA polymerase III (Pol III) and the auxiliary factors TFIIIC and TFIIIB are well characterized, the mechanisms of class III gene regulation are poorly understood. Previous studies identified MAF1, a gene that affects tRNA suppressor efficiency and interacts genetically with Pol III. We show here that tRNA levels are elevated in maf1 mutant cells. In keeping with the higher levels of tRNA observed in vivo, the in vitro rate of Pol III RNA synthesis is significantly increased in maf1 cell extracts. Mutations in the RPC160 gene encoding the largest subunit of Pol III which reduce tRNA levels were identified as suppressors of the maf1 growth defect. Interestingly, Maf1p is located in the nucleus and coimmunopurifies with epitope-tagged RNA Pol III. These results indicate that Maf1p acts as a negative effector of Pol III synthesis. This potential regulator of Pol III transcription is likely conserved since orthologs of Maf1p are present in other eukaryotes, including humans.

Transcription of the Schizosaccharomyces pombe U2 gene in vivo and in vitro is directed by two essential promoter elements

As compared to the metazoan small nuclear RNAs (snRNAs), relatively little is known about snRNA synthesis in unicellular organisms. We have analyzed the transcription of the Schizosaccharomyces pombe U2 snRNA gene in vivo and in the homologous in vitro system. Deletion and linker-scanning analyses show that the S.pombe U2 promoter contains at least two elements: the spUSE centered at -55, which functions as an activator, and a TATA box at -26, which is essential for basal transcription. These data point to a similar architecture among S.pombe, plant and invertebrate snRNA promoters. Factors recognizing the spUSE can be detected in whole cell extracts by DNase I footprinting and competition studies show that the binding of these factors correlates with transcriptional activity. Electrophoretic mobility shift assays and gel-filtration chromatography revealed a native molecular mass of approximately 200 kDa for the spUSE binding activity. Two polypeptides of molecular masses 25 and 65 kDa were purified by virtue of their ability to specifically bind the spUSE.

High-mobility-group proteins NHP6A and NHP6B participate in activation of the RNA polymerase III SNR6 gene

Transcription of yeast class III genes involves the formation of a transcription initiation complex that comprises RNA polymerase III (Pol III) and the general transcription factors TFIIIB and TFIIIC. Using a genetic screen for positive regulators able to compensate for a deficiency in a promoter element of the SNR6 gene, we isolated the NHP6A and NHP6B genes. Here we show that the high-mobility-group proteins NHP6A and NHP6B are required for the efficient transcription of the SNR6 gene both in vivo and in vitro. The transcripts of wild-type and promoter-defective SNR6 genes decreased or became undetectable in an nhp6ADelta nhp6BDelta double-mutant strain, and the protection over the TATA box of the wild-type SNR6 gene was lost in nhp6ADelta nhp6BDelta cells at 37 degrees C. In vitro, NHP6B specifically stimulated the transcription of SNR6 templates up to fivefold in transcription assays using either cell nuclear extracts from nhp6ADelta nhp6BDelta cells or reconstituted transcription systems. Finally, NHP6B activated SNR6 transcription in a TFIIIC-independent assay. These results indicate that besides the general transcription factors TFIIIB and TFIIIC, additional auxillary factors are required for the optimal transcription of at least some specific Pol III genes.

RNA polymerase III transcription complexes on chromosomal 5S rRNA genes in vivo: TFIIIB occupancy and promoter opening

Quantitative analysis of multiple-hit potassium permanganate (KMnO(4)) footprinting has been carried out in vivo on Saccharomyces cerevisiae 5S rRNA genes. The results fix the number of open complexes at steady state in exponentially growing cells at between 8 and 17% of the 150 to 200 chromosomal copies. UV and dimethyl sulfate footprinting set the transcription factor TFIIIB occupancy at 23 to 47%. The comparison between the two values suggests that RNA polymerase III binding or promoter opening is the rate-limiting step in 5S rRNA transcription in vivo. Inhibition of RNA elongation in vivo by cordycepin confirms this result. An experimental system that is capable of providing information on the mechanistic steps involved in regulatory events in S. cerevisiae cells has been established.

Isolation and cloning of four subunits of a fission yeast TFIIIC complex that includes an ortholog of the human regulatory protein TFIIICbeta

Eukaryotic tRNA genes are controlled by proximal and downstream elements that direct transcription by RNA polymerase (pol) III. Transcription factors (TFs) that reside near the initiation site are related in Saccharomyces cerevisiae and humans, while those that reside at or downstream of the B box share no recognizable sequence relatedness. Human TFIIICbeta is a transcriptional regulator that exhibits no homology to S. cerevisiae sequences on its own. We cloned an essential Schizosaccharomyces pombe gene that encodes a protein, Sfc6p, with homology to the S. cerevisiae TFIIIC subunit, TFC6p, that extends to human TFIIICbeta. We also isolated and cloned S. pombe homologs of three other TFIIIC subunits, Sfc3p, Sfc4p, and Sfc1p, the latter two of which are conserved from S. cerevisiae to humans, while the former shares homology with the S. cerevisiae B box-binding homolog only. Sfc6p is a component of a sequence-specific DNA-binding complex that also contains the B box-binding homolog, Sfc3p. Immunoprecipitation of Sfc3p further revealed that Sfc1p, Sfc3p, Sfc4p, and Sfc6p are associated in vivo and that the isolated Sfc3p complex is active for pol III-mediated transcription of a S. pombe tRNA gene in vitro. These results establish a link between the downstream pol III TFs in yeast and humans.

TFIIIC-independent in vitro transcription of yeast tRNA genes

The most peculiar transcriptional property of eukaryotic tRNA genes, as well as of other genes served by RNA polymerase III, is their complete dependence on the intragenic interaction platform provided by transcription factor IIIC (TFIIIC) for the productive assembly of the TBP-containing initiation factor TFIIIB. The sole exception, in yeast, is the U6 RNA gene, which is able to exploit a TATAAATA element, 30 bp upstream of the transcription start site, for the TFIIIC-independent assembly of TFIIIB. To find out whether this extragenic core promoter organization and autonomous TFIIIB assembly capacity are unique features of the U6 gene or also apply to other genes transcribed by RNA polymerase III, we scanned the 5'-flanking regions (up to position -100) of the entire tRNA gene set of Saccharomyces cerevisiae searching for U6-like TATA motifs. Four tRNA genes harboring such a sequence motif around position -30 were identified and found to be transcribed in vitro by a minimal system only composed of TFIIIB and RNA polymerase III. In this system, start site selection is not at all affected by the absence of TFIIIC, which, when added, significantly stimulates transcription by determining an increase in the number, rather than in the efficiency of utilization, of productive initiation complexes. A specific TBP-TATA element interaction is absolutely required for TFIIIC-independent transcription, but the nearby sequence context also contributes to the efficiency of autonomous TFIIIB assembly. The existence of a TFIIIB assembly pathway leading to the faithful transcription of natural eukaryotic tRNA genes in the absence of TFIIIC provides novel insights into the functional flexibility of the eukaryotic tRNA gene transcription machinery and on its evolution from an ancestral RNA polymerase III system relying on upstream, TATA- centered control elements.

A novel subunit of yeast RNA polymerase III interacts with the TFIIB-related domain of TFIIIB70

There is limited information on how eukaryotic RNA polymerases (Pol) recognize their cognate preinitiation complex. We have characterized a polypeptide copurifying with yeast Pol III. This protein, C17, was found to be homologous to a mammalian protein described as a hormone receptor. Deletion of the corresponding gene, RPC17, was lethal and its regulated extinction caused a selective defect in transcription of class III genes in vivo. Two-hybrid and coimmunoprecipitation experiments indicated that C17 interacts with two Pol III subunits, one of which, C31, is important for the initiation reaction. C17 also interacted with TFIIIB70, the TFIIB-related component of TFIIIB. The interaction domain was found to be in the N-terminal, TFIIB-like half of TFIIIB70, downstream of the zinc ribbon and first imperfect repeat. Although Pol II similarly interacts with TFIIB, it is notable that C17 has no similarity to any Pol II subunit. The data indicate that C17 is a novel specific subunit of Pol III which participates together with C34 in the recruitment of Pol III by the preinitiation complex.

A subunit of yeast TFIIIC participates in the recruitment of TATA-binding protein

TFIIIC plays a key role in nucleating the assembly of the initiation factor TFIIIB on class III genes. We have characterized an essential gene, TFC8, encoding the 60-kDa polypeptide, tau60, present in affinity-purified TFIIIC. Hemagglutinin-tagged variants of tau60 were found to be part of TFIIIC-tDNA complexes and to reside at least in part in the downstream DNA-binding domain tauB. Unexpectedly, the thermosensitive phenotype of N-terminally tagged tau60 was suppressed by overexpression of tau95, which belongs to the tauA domain, and by two TFIIIB components, TATA-binding protein (TBP) and B"/TFIIIB90 (but not by TFIIIB70). Mutant TFIIIC was deficient in the activation of certain tRNA genes in vitro, and the transcription defect was selectively alleviated by increasing TBP concentration. Coimmunoprecipitation experiments support a direct interaction between TBP and tau60. It is suggested that tau60 links tauA and tauB domains and participates in TFIIIB assembly via its interaction with TBP.

Functional characterization of ABC10alpha, an essential polypeptide shared by all three forms of eukaryotic DNA-dependent RNA polymerases

ABC10alpha is a small polypeptide shared by the three yeast RNA polymerases. Homologous polypeptides in higher eukaryotes have a zinc-binding CX(2)C. CX(2)C motif and a conserved basic C-terminal end. These features are also found in archaeal gene products that may encode an RNA polymerase subunit. The CX(2)C. CX(2)C motif is partly dispensable, since only its first cysteine is essential for growth, whereas the basic C-terminal end is critical in vivo. A mutant in the latter domain has an RNA polymerase III-specific defect and, in vitro, impairs RNA polymerase III assembly. Polymerase activity was, however, not affected in various faithful transcription assays. The mutant is suppressed by a high gene dosage of the second largest subunit of RNA polymerase III, whereas the homologous subunits of RNA polymerase I and II have aggravating effects. In a two-hybrid assay, ABC10alpha binds to the C-terminal half of the second largest subunit of RNA polymerase I, in a way that requires the integrity of the CX(2)C. CX(2)C motif. Thus, ABC10alpha appears to interact directly with the second largest subunit during polymerase assembly. This interaction is presumably a major rate-limiting step in assembly, since diploid cells containing only one functional gene copy for ABC10alpha have a partial growth defect.

Domain organization and functional properties of yeast transcription factor IIIA species with different zinc stoichiometries

Transcription factor IIIA (TFIIIA) binds to the 5 S rRNA gene through its zinc finger domain and directs the assembly of a multiprotein complex that promotes transcription initiation by RNA polymerase III. Limited proteolysis of TFIIIA forms with different zinc stoichiometries, in combination with DNA binding and in vitro transcription analyses, have been used herein to investigate the domain organization and zinc requirements of Saccharomyces cerevisiae TFIIIA. Species containing either nine, six, or three zinc equivalents were produced by reductive resaturation and controlled metal depletion of recombinant TFIIIA. Partial digestion of the metal-saturated, 9 Zn2+-liganded factor yields a stable intermediate comprising the eight N-terminal zinc fingers, and a less stable fragment corresponding to a C-terminal portion including the ninth finger. Proteolyzed TFIIIA has the same 5 S DNA binding ability of the intact protein yet no longer supports in vitro 5 S rRNA synthesis. Both the structural compactness and the 5 S DNA binding ability of the TFIIIA form only containing 3 zinc ions are severely compromised. In contrast, the 6 Zn2+-liganded species was found to be indistinguishable from metal-saturated TFIIIA. By demonstrating the existence of three classes of zinc-binding sites contributing differently to yeast TFIIIA structure and function, the present study provides new evidence for the remarkable flexibility built into this complex transcription factor.

The RNA cleavage activity of RNA polymerase III is mediated by an essential TFIIS-like subunit and is important for transcription termination

Budding yeast RNA polymerase III (Pol III) contains a small, essential subunit, named C11, that is conserved in humans and shows a strong homology to TFIIS. A mutant Pol III, heterocomplemented with Schizosaccharomyces pombe C11, was affected in transcription termination in vivo. A purified form of the enzyme (Pol III Delta), deprived of C11 subunit, initiated properly but ignored pause sites and was defective in termination. Remarkably, Pol III Delta lacked the intrinsic RNA cleavage activity of complete Pol III. In vitro reconstitution experiments demonstrated that Pol III RNA cleavage activity is mediated by C11. Mutagenesis in C11 of two conserved residues, which are critical for the TFIIS-dependent cleavage activity of Pol II, is lethal. Immunoelectron microscopy data suggested that C11 is localized on the mobile thumb-like stalk of the polymerase. We propose that C11 allows the enzyme to switch between an RNA elongation and RNA cleavage mode and that the essential role of the Pol III RNA cleavage activity is to remove the kinetic barriers to the termination process. The integration of TFIIS function into a specific Pol III subunit may stem from the opposite requirements of Pol III and Pol II in terms of transcript length and termination efficiency.

A chimeric subunit of yeast transcription factor IIIC forms a subcomplex with tau95

The multisubunit yeast transcription factor IIIC (TFIIIC) is a multifunctional protein required for promoter recognition, transcription factor IIIB recruitment, and chromatin antirepression. We report the isolation and characterization of TFC7, an essential gene encoding the 55-kDa polypeptide, tau55, present in affinity-purified TFIIIC. tau55 is a chimeric protein generated by an ancient chromosomal rearrangement. Its C-terminal half is essential for cell viability and sufficient to ensure TFIIIC function in DNA binding and transcription assays. The N-terminal half is nonessential and highly similar to a putative yeast protein encoded on another chromosome and to a cyanobacterial protein of unknown function. Partial deletions of the N-terminal domain impaired tau55 function at a high temperature or in media containing glycerol or ethanol, suggesting a link between PolIII transcription and metabolic pathways. Interestingly, tau55 was found, together with TFIIIC subunit tau95, in a protein complex which was distinct from TFIIIC and which may play a role in the regulation of PolIII transcription, possibly in relation to cell metabolism.

Efficient synthesis, termination and release of RNA polymerase III transcripts in Xenopus extracts depleted of La protein

La proteins are conserved, abundant and predominantly nuclear phosphoproteins which bind to the 3'-U termini of newly synthesized RNA polymerase III transcripts. The human La protein has been implicated in the synthesis, termination and release of such transcripts. Here we examine the potential transcriptional properties of La in Xenopus laevis, using a homologous tRNA gene as template. Immunodepletion of La from cell-free extracts leads to the formation of tRNA precursors lacking 3'-U residues. This shortening can be uncoupled from RNA polymerase III transcription, indicating that it results from nuclease degradation rather than incomplete synthesis. Extracts containing <1% of the normal La protein content synthesize tRNA precursors just as well as complete extracts, with no change in termination efficiency, and the vast majority of these full-length transcripts are not associated with the template or with residual La protein. Hence, Xenopus La seems not to function as an initiation, termination or release factor for RNA polymerase III. Consistent with the recently discovered role of La in yeast tRNA maturation in vivo, recombinant Xenopus La prevents 3'-exonucleolytic degradation of tRNA precursors in vitro. A conserved RNA chaperone function may best explain the abundance of La in eukaryotic nuclei.

Differential expression of individual suppressor tRNA(Trp) gene gene family members in vitro and in vivo in the nematode Caenorhabditis elegans

Eight different amber suppressor tRNA (suptRNA) mutations in the nematode Caenorhabditis elegans have been isolated; all are derived from members of the tRNA(Trp) gene family (K. Kondo, B. Makovec, R. H. Waterston, and J. Hodgkin, J. Mol. Biol. 215:7-19, 1990). Genetic assays of suppressor activity suggested that individual tRNA genes were differentially expressed, probably in a tissue- or developmental stage-specific manner. We have now examined the expression of representative members of this gene family both in vitro, using transcription in embryonic cell extracts, and in vivo, by assaying suppression of an amber-mutated lacZ reporter gene in animals carrying different suptRNA mutations. Individual wild-type tRNA(Trp) genes and their amber-suppressing counterparts appear to be transcribed and processed identically in vitro, suggesting that the behavior of suptRNAs should reflect wild-type tRNA expression. The levels of transcription of different suptRNA genes closely parallel the extent of genetic suppression in vivo. The results suggest that differential expression of tRNA genes is most likely at the transcriptional rather than the posttranscriptional level and that 5' flanking sequences play a role in vitro, and probably in vivo as well. Using suppression of a lacZ(Am) reporter gene as a more direct assay of suptRNA activity in individual cell types, we have again observed differential expression which correlates with genetic and in vitro transcription results. This provides a model system to more extensively study the basis for differential expression of this tRNA gene family.

Tau91, an essential subunit of yeast transcription factor IIIC, cooperates with tau138 in DNA binding

Transcription factor IIIC (TFIIIC) (or tau) is a large multisubunit and multifunctional factor required for transcription of all class III genes in Saccharomyces cerevisiae. It is responsible for promoter recognition and TFIIIB assembly. We report here the cloning and characterization of TFC6, an essential gene encoding the 91-kDa polypeptide, tau91, present in affinity-purified TFIIIC. Tau91 has a predicted molecular mass of 74 kDa. It harbors a central cluster of His and Cys residues and has basic and acidic amino acid regions, but it shows no specific similarity to known proteins or predicted open reading frames. The TFIIIC subunit status of tau91 was established by the following biochemical and genetic evidence. Antibodies to tau91 bound TFIIIC-DNA complexes in gel shift assays; in vivo, a B block-deficient U6 RNA gene (SNR6) harboring GAL4 binding sites was reactivated by fusing the GAL4 DNA binding domain to tau91; and a point mutation in TFC6 (tau91-E330K) was found to suppress the thermosensitive phenotype of a tfc3-G349E mutant affected in the B block binding subunit (tau138). The suppressor mutation alleviated the DNA binding and transcription defects of mutant TFIIIC in vitro. These results indicated that tau91 cooperates with tau138 for DNA binding. Recombinant tau91 by itself did not interact with a tRNA gene, although it showed a strong affinity for single-stranded DNA.