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

Evolution of the Toxb Gene in Pyrenophora tritici-repentis and Related Species

Pyrenophora tritici-repentis (tan spot) is a destructive foliar pathogen of wheat with global impact. This ascomycete fungus possesses a highly plastic open pangenome shaped by the gain and loss of effector genes. This study investigated the allelic variations in the chlorosis-encoding gene ToxB across 422 isolates representing all identified pathotypes and worldwide origins. To gain better insights into ToxB evolution, we examined its presence and variability in other Pyrenophora spp. A ToxB haplotype network was constructed, revealing the evolutionary relationships of this gene (20 haplotypes) across four Pyrenophora species. Notably, toxb, the homolog of ToxB, was detected for the first time in the barley pathogen Pyrenophora teres. The ToxB/toxb genes display evidence of selection that is characterized by loss of function, duplication, and diverse mutations. Within the ToxB/toxb open reading frame, 72 mutations were identified, including 14 synonymous, 55 nonsynonymous, and 3 indel mutations. Remarkably, a, ∼5.6-kb Copia-like retrotransposon, named Copia-1_Ptr, was found inserted in the toxb gene of a race 3 isolate. This insert disrupted the ToxB gene's function, a first case of effector gene disruption by a transposable element in P. tritici-repentis. Additionally, a microsatellite with 25 nucleotide repeats (0 to 10) in the upstream region of ToxB suggested a potential mechanism influencing ToxB expression and regulation. Exploring ToxB-like protein distribution in other ascomycetes revealed the presence of ToxB-like proteins in 19 additional species, including the Leotiomycetes class for the first time. The presence/absence pattern of ToxB-like proteins defied species relatedness compared with a phylogenetic tree, suggesting a past horizontal gene transfer event during the evolution of the ToxB gene. [Formula: see text] Copyright © 2024 His Majesty the King in Right of Canada, as represented by the Minister of Agriculture and Agri-Food. This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Mechanical codes of chemical-scale specificity in DNA motifs

In gene transcription, certain sequences of double-stranded (ds)DNA play a vital role in nucleosome positioning and expression initiation. That dsDNA is deformed to various extents in these processes leads us to ask: Could the genomic DNA also have sequence specificity in its chemical-scale mechanical properties? We approach this question using statistical machine learning to determine the rigidity between DNA chemical moieties. What emerges for the polyA, polyG, TpA, and CpG sequences studied here is a unique trigram that contains the quantitative mechanical strengths between bases and along the backbone. In a way, such a sequence-dependent trigram could be viewed as a DNA mechanical code. Interestingly, we discover a compensatory competition between the axial base-stacking interaction and the transverse base-pairing interaction, and such a reciprocal relationship constitutes the most discriminating feature of the mechanical code. Our results also provide chemical-scale understanding for experimental observables. For example, the long polyA persistence length is shown to have strong base stacking while its complement (polyAc) exhibits high backbone rigidity. The mechanical code concept enables a direct reading of the physical interactions encoded in the sequence which, with further development, is expected to shed new light on DNA allostery and DNA-binding drugs.

Strand Asymmetries Across Genomic Processes

Across biological systems, a number of genomic processes, including transcription, replication, DNA repair, and transcription factor binding, display intrinsic directionalities. These directionalities are reflected in the asymmetric distribution of nucleotides, motifs, genes, transposon integration sites, and other functional elements across the two complementary strands. Strand asymmetries, including GC skews and mutational biases, have shaped the nucleotide composition of diverse organisms. The investigation of strand asymmetries often serves as a method to understand underlying biological mechanisms, including protein binding preferences, transcription factor interactions, retrotransposition, DNA damage and repair preferences, transcription-replication collisions, and mutagenesis mechanisms. Research into this subject also enables the identification of functional genomic sites, such as replication origins and transcription start sites. Improvements in our ability to detect and quantify DNA strand asymmetries will provide insights into diverse functionalities of the genome, the contribution of different mutational mechanisms in germline and somatic mutagenesis, and our knowledge of genome instability and evolution, which all have significant clinical implications in human disease, including cancer. In this review, we describe key developments that have been made across the field of genomic strand asymmetries, as well as the discovery of associated mechanisms.

Genetic characteristics and growth patterns of the hybrid grouper derived from the hybridization of Epinephelus fuscoguttatus (female) × Epinephelus polyphekadion (male)

Hybridization is one of the primary methods used to cultivate farmed grouper species. The hybrid grouper derived from crossing Epinephelus fuscoguttatus (♀) and E. polyphekadion (♂) exhibits growth superiority over its parents. The genetic characteristics and growth patterns of the hybrid grouper have not yet been defined. This study confirms the ploidy level of the hybrid grouper (2n = 48) using chromosome count analysis and flow cytometry. The 5S rDNA family was used to evaluate genetic diversity. Only one 5S class (~400 bp) was detected in the hybrid grouper, which could be used to distinguish between two different types based on nucleotide sequences, likely representing homologous unit classes from the female and male parental species. Growth patterns of 5-8-month-old hybrid groupers were also monitored. In this phase, a positive allometric growth pattern in body mass with total length was found. Body height and body mass were significantly correlated based on correlation and path coefficient, suggesting that body height could serve as an excellent index to increase body mass. These results aid our understanding of the genetic evolution of the hybrid grouper and inform the development of improved rearing techniques.

RNA Pol III promoters-key players in precisely targeted plant genome editing

The clustered regularly interspaced short palindrome repeat (CRISPR)/CRISPR-associated protein Cas) system is a powerful and highly precise gene-editing tool in basic and applied research for crop improvement programs. CRISPR/Cas tool is being extensively used in plants to improve crop yield, quality, and nutritional value and make them tolerant to environmental stresses. CRISPR/Cas system consists of a Cas protein with DNA endonuclease activity and one CRISPR RNA transcript that is processed to form one or several short guide RNAs that direct Cas9 to the target DNA sequence. The expression levels of Cas proteins and gRNAs significantly influence the editing efficiency of CRISPR/Cas-mediated genome editing. This review focuses on insights into RNA Pol III promoters and their types that govern the expression levels of sgRNA in the CRISPR/Cas system. We discussed Pol III promoters structural and functional characteristics and their comparison with Pol II promoters. Further, the use of synthetic promoters to increase the targeting efficiency and overcome the structural, functional, and expressional limitations of RNA Pol III promoters has been discussed. Our review reports various studies that illustrate the use of endogenous U6/U3 promoters for improving editing efficiency in plants and the applicative approach of species-specific RNA pol III promoters for genome editing in model crops like Arabidopsis and tobacco, cereals, legumes, oilseed, and horticultural crops. We further highlight the significance of optimizing these species-specific promoters' systematic identification and validation for crop improvement and biotic and abiotic stress tolerance through CRISPR/Cas mediated genome editing.

MEPP: more transparent motif enrichment by profiling positional correlations

Score-based motif enrichment analysis (MEA) is typically applied to regulatory DNA to infer transcription factors (TFs) that may modulate transcription and chromatin state in different conditions. Most MEA methods determine motif enrichment independent of motif position within a sequence, even when those sequences harbor anchor points that motifs and their bound TFs may functionally interact with in a distance-dependent fashion, such as other TF binding motifs, transcription start sites (TSS), sequencing assay cleavage sites, or other biologically meaningful features. We developed motif enrichment positional profiling (MEPP), a novel MEA method that outputs a positional enrichment profile of a given TF's binding motif relative to key anchor points (e.g. transcription start sites, or other motifs) within the analyzed sequences while accounting for lower-order nucleotide bias. Using transcription initiation and TF binding as test cases, we demonstrate MEPP's utility in determining the sequence positions where motif presence correlates with measures of biological activity, inferring positional dependencies of binding site function. We demonstrate how MEPP can be applied to interpretation and hypothesis generation from experiments that quantify transcription initiation, chromatin structure, or TF binding measurements. MEPP is available for download from https://github.com/npdeloss/mepp.

A Comparative Study of 5S rDNA Non-Transcribed Spacers in Elaeagnaceae Species

5S rDNA is organized as a cluster of tandemly repeated monomers that consist of the conservative 120 bp coding part and non-transcribed spacers (NTSs) with different lengths and sequences among different species. The polymorphism in the 5S rDNA NTSs of closely related species is interesting for phylogenetic and evolutional investigations, as well as for the development of molecular markers. In this study, the 5S rDNA NTSs were amplified with universal 5S1/5S2 primers in some species of the Elaeagnaceae Adans. family. The polymerase chain reaction (PCR) products of five Elaeagnus species had similar lengths near 310 bp and were different from Shepherdia canadensis (L.) Nutt. and Sh. argentea (Pusch.) Nutt. samples (260 bp and 215 bp, respectively). The PCR products were cloned and sequenced. An analysis of the sequences revealed that intraspecific levels of NTS identity are high (approximately 95–96%) and similar in the Elaeagnus L. species. In Sh. argentea, this level was slightly lower due to the differences in the poly-T region. Moreover, the intergeneric and intervarietal NTS identity levels were studied and compared. Significant differences between species (except E. multiflora Thunb. and E. umbellata Thunb.) and genera were found. Herein, a range of the NTS features is discussed. This study is another step in the investigation of the molecular evolution of Elaeagnaceae and may be useful for the development of species-specific DNA markers in this family.

Isolation and characterization of 5S rDNA sequences in catfishes genome (Heptapteridae and Pseudopimelodidae): perspectives for rDNA studies in fish by C0t method

Sequences of 5S ribosomal RNA (rRNA) are extensively used in fish cytogenomic studies, once they have a flexible organization at the chromosomal level, showing inter- and intra-specific variation in number and position in karyotypes. Sequences from the genome of Imparfinis schubarti (Heptapteridae) were isolated, aiming to understand the organization of 5S rDNA families in the fish genome. The isolation of 5S rDNA from the genome of I. schubarti was carried out by reassociation kinetics (C0t) and PCR amplification. The obtained sequences were cloned for the construction of a micro-library. The obtained clones were sequenced and hybridized in I. schubarti and Microglanis cottoides (Pseudopimelodidae) for chromosome mapping. An analysis of the sequence alignments with other fish groups was accomplished. Both methods were effective when using 5S rDNA for hybridization in I. schubarti genome. However, the C0t method enabled the use of a complete 5S rRNA gene, which was also successful in the hybridization of M. cottoides. Nevertheless, this gene was obtained only partially by PCR. The hybridization results and sequence analyses showed that intact 5S regions are more appropriate for the probe operation, due to conserved structure and motifs. This study contributes to a better understanding of the organization of multigene families in catfish's genomes.

The orientation of transcription factor binding site motifs in gene promoter regions: does it matter?

Background

Gene expression is to large degree regulated by the specific binding of protein transcription factors to cis-regulatory transcription factor binding sites in gene promoter regions. Despite the identification of hundreds of binding site sequence motifs, the question as to whether motif orientation matters with regard to the gene expression regulation of the respective downstream genes appears surprisingly underinvestigated.

Results

We pursued a statistical approach by probing 293 reported non-palindromic transcription factor binding site and ten core promoter motifs in Arabidopsis thaliana for evidence of any relevance of motif orientation based on mapping statistics and effects on the co-regulation of gene expression of the respective downstream genes. Although positional intervals closer to the transcription start site (TSS) were found with increased frequencies of motifs exhibiting orientation preference, a corresponding effect with regard to gene expression regulation as evidenced by increased co-expression of genes harboring the favored orientation in their upstream sequence could not be established. Furthermore, we identified an intrinsic orientational asymmetry of sequence regions close to the TSS as the likely source of the identified motif orientation preferences. By contrast, motif presence irrespective of orientation was found associated with pronounced effects on gene expression co-regulation validating the pursued approach. Inspecting motif pairs revealed statistically preferred orientational arrangements, but no consistent effect with regard to arrangement-dependent gene expression regulation was evident.

Conclusions

Our results suggest that for the motifs considered here, either no specific orientation rendering them functional across all their instances exists with orientational requirements instead depending on gene-locus specific additional factors, or that the binding orientation of transcription factors may generally not be relevant, but rather the event of binding itself.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2549-x) contains supplementary material, which is available to authorized users.

Role of indirect readout mechanism in TATA box binding protein-DNA interaction

Gene expression generally initiates from recognition of TATA-box binding protein (TBP) to the minor groove of DNA of TATA box sequence where the DNA structure is significantly different from B-DNA. We have carried out molecular dynamics simulation studies of TBP-DNA system to understand how the DNA structure alters for efficient binding. We observed rigid nature of the protein while the DNA of TATA box sequence has an inherent flexibility in terms of bending and minor groove widening. The bending analysis of the free DNA and the TBP bound DNA systems indicate presence of some similar structures. Principal coordinate ordination analysis also indicates some structural features of the protein bound and free DNA are similar. Thus we suggest that the DNA of TATA box sequence regularly oscillates between several alternate structures and the one suitable for TBP binding is induced further by the protein for proper complex formation.

RNA polymerase III accurately initiates transcription from RNA polymerase II promoters in vitro

In eukaryotes, there are three major RNA polymerases (Pol) in the nucleus, which are commonly described as transcribing non-overlapping subsets of genes. Structural studies have highlighted a conserved core shared among all three transcription systems. Initiation of human Pol III from TATA box-containing Pol II promoters under conditions with impaired Pol II transcription activity have been described previously. RNA polymerase III and Pol II were found to co-localize at the promoters of the c-myc gene and the RPPH1 sRNA in vivo. Here, I report that Pol III can, like Pol II, initiate transcription from most tested Pol II core promoters when assayed with crude human nuclear extracts (HSK, SNF, or Dignam). Both polymerases often initiate from the same transcription start site, and depend on a TATA box or AT-rich region but not the downstream promoter element (DPE) or the motif ten element (MTE). Moderate (∼2-fold) changes in the ratio of DNA template to nuclear extract were sufficient to change Pol II-mediated transcription to a mixture of Pol II- and Pol III-, or to a solely Pol III-dependent initiation of transcription from Pol II promoters. Polymerase specificity is thus not fixed but a variable that depends on the properties of the promoter and the transcription conditions. These findings provide functional evidence for a close similarity between the Pol II and Pol III transcription complexes, and additionally explain previous controversies in the literature.

Coadaptation of isoacceptor tRNA genes and codon usage bias for translation efficiency in Aedes aegypti and Anopheles gambiae

The transfer RNAs (tRNAs) are essential components of translational machinery. We determined that tRNA isoacceptors (tRNAs with different anticodons but incorporating the same amino acid in protein synthesis) show differential copy number abundance, genomic distribution patterns and sequence evolution between Aedes aegypti and Anopheles gambiae mosquitoes. The tRNA-Ala genes are present in unusually high copy number in the Ae. aegypti genome but not in An. gambiae. Many of the tRNA-Ala genes of Ae. aegypti are flanked by a highly conserved sequence that is not observed in An. gambiae. The relative abundance of tRNA isoacceptor genes is correlated with preferred (or optimal) and nonpreferred (or rare) codons for ∼2-4% of the predicted protein coding genes in both species. The majority (∼74-85%) of these genes are related to pathways involved with translation, energy metabolism and carbohydrate metabolism. Our results suggest that these genes and the related pathways may be under translational selection in these mosquitoes.

Biochemical characterization of Sf9 Sp-family-like protein factors reveals interesting features

We earlier documented the involvement of novel Sp-family-like protein factors in transcription from the Autographa californica nucleopolyhedrovirus (AcNPV) polyhedrin (polh) gene promoter [Ramachandran et al. (2001) J. Biol. Chem. 276: 23440-23449]. These zinc-dependent Sp-like factors bind to two putative Sp-factor-binding motifs, present within the AcSp sequence upstream of the polh promoter, with very high affinity (K(d) = 2.1 x 10(-12) M). Like other polh-promoter-associated host transcription factors, these Sp-like factors display tolerance to high ion concentrations up to even 3 M NaCl. An electrophoretic mobility shift assay demonstrated a probable cross-talk between the Spodoptera frugiperda (Sf9) Sp-family-like proteins and the TFIID complex. In complementary experiments, specific replacements of the Sp-factor-binding motifs with TATA-like elements resulted in expression of a luciferase reporter gene to almost the same level as that obtained with a wild-type native construct. Our results point to the possibility of the involvement of TFIID and Sf9 Sp protein interaction in transcription from the baculovirus polyhedrin promoter.

The Saccharomyces cerevisiae RNA polymerase III recruitment factor subunits Brf1 and Bdp1 impose a strict sequence preference for the downstream half of the TATA box

Association of the TATA-binding protein (TBP) with its cognate site within eukaryotic promoters is key to accurate and efficient transcriptional initiation. To achieve recruitment of Saccharomyces cerevisiae RNA polymerase III, TBP is associated with two additional factors, Brf1 and Bdp1, to form the initiation factor TFIIIB. Previous data have suggested that the structure or dynamics of the TBP–DNA complex may be altered upon entry of Brf1 and Bdp1 into the complex. We show here, using the altered specificity TBP mutant TBPm3 and an iterative in vitro selection assay, that entry of Brf1 and Bdp1 into the complex imposes a strict sequence preference for the downstream half of the TATA box. Notably, the selected sequence (TGTAAATA) is a perfect match to the TATA box of the RNA polymerase III-transcribed U6 small nuclear RNA (SNR6) gene. We suggest that the selected T•A base pair step at the downstream end of the 8 bp TBP site may provide a DNA flexure that promotes TFIIIB-DNA complex formation.

Analysis of the 5S rRNA gene promoter from Acanthamoeba castellanii

The promoter region of the Acanthamoeba 5S rRNA gene was analysed by in vitro transcription of several 5' and 3' deletion and substitution mutants, as well as a series of linker scanning mutants. The promoter consists of three sequence regions contained entirely within the gene; two of these correspond to the A and C boxes that bind TFIIIA, found in the genes from other genera. In addition, a region immediately 3' to the transcription start site has a strong effect on initiation efficiency. No strict requirement was found for specific sequences 5' to the transcription start site. Substitution of a consensus TATA box at -29 had only a modest effect on transcription, and deletion or substitution of sequences between -15 and -10 as well as -34 and -21 was only modestly more active than the wild-type template. Analysis of 3' deletions sets the 3' end-point of the promoter between +79 and +97, and demonstrates the importance of a T-rich region in transcription termination. Taken together, these results suggest that promoter elements within the Acanthamoeba 5S RNA gene are somewhat redundant, with the exception of a sequence between +50 and +60, which functions in binding TFIIIA. Remarkably, polymerase chain reaction product templates containing only non-specific 5' ends between -6 and +1 relative to the transcription start site are fully functional, demonstrating that no external DNA scaffold is needed for TFIIIB and RNA polymerase III binding, and that productive initiation can be mediated solely by protein-DNA interactions within the coding region of the 5S gene.

Mechanism of cyst specific protein 21 mRNA induction during Acanthamoeba differentiation

The Acanthamoeba cyst specific protein 21 (CSP21) gene is tightly repressed in growing cells and highly induced early during differentiation into a dormant cyst. This increase is mediated by the rate of transcription of the CSP21 gene as determined by nuclear run-on assays. The promoter region of the CSP21 gene was analyzed by transcript start site mapping and in vitro transcription of wild-type or mutant templates, using extracts from growing cells. A sequence located 3' to a modified TATA box completely inhibits transcription and removal of this region permits robust transcription utilizing a start site approximately 35 base pairs downstream of the TATA box. Sequences 5' to the TATA box had no effect on transcription, suggesting that anti-repression is the only mechanism required for CSP21 induction. Fractionation of nuclear extracts yielded a fraction capable of transcription from the CSP21 promoter, and a fraction containing a promoter-specific repressing activity. Anti-repression may thus be a major mechanism regulating differentiation or maintenance of the proliferative cycle in Acanthamoeba.

A TATA binding protein mutant with increased affinity for DNA directs transcription from a reversed TATA sequence in vivo

The TATA-binding protein (TBP) nucleates the assembly and determines the position of the preinitiation complex at RNA polymerase II-transcribed genes. We investigated the importance of two conserved residues on the DNA binding surface of Saccharomyces cerevisiae TBP to DNA binding and sequence discrimination. Because they define a significant break in the twofold symmetry of the TBP-TATA interface, Ala100 and Pro191 have been proposed to be key determinants of TBP binding orientation and transcription directionality. In contrast to previous predictions, we found that substitution of an alanine for Pro191 did not allow recognition of a reversed TATA box in vivo; however, the reciprocal change, Ala100 to proline, resulted in efficient utilization of this and other variant TATA sequences. In vitro assays demonstrated that TBP mutants with the A100P and P191A substitutions have increased and decreased affinity for DNA, respectively. The TATA binding defect of TBP with the P191A mutation could be intragenically suppressed by the A100P substitution. Our results suggest that Ala100 and Pro191 are important for DNA binding and sequence recognition by TBP, that the naturally occurring asymmetry of Ala100 and Pro191 is not essential for function, and that a single amino acid change in TBP can lead to elevated DNA binding affinity and recognition of a reversed TATA sequence.

A noncoding RNA regulates human protease-activated receptor-1 gene during embryogenesis

Activation of the human protease-activated receptor-1 (PAR-1) by thrombin leads to myriad functions essential for maintaining vascular integrity. Upregulation of PAR-1 expression is considered important in atherosclerosis, angiogenesis and tumor metastasis. In vitro analysis of the human PAR-1 promoter function revealed a positive regulatory element between -4.2 and -3.2 kb of the transcription start site. This element was examined in transgenic mice containing either 4.1 or 2.9 kb of the 5' flanking sequence driving a LacZ reporter gene. Only the 4.1 kb PAR-1 transgene was expressed in vivo and only during embryonic development. The transgene expression was observed only in developing arteries and not in veins. Further examination of this putative regulatory sequence identified a novel noncoding RNA (ncR-uPAR:noncoding RNA upstream of the PAR-1) gene at -3.4 kb. The ncR-uPAR upregulated PAR-1-core promoter-driven luciferase activity and mRNA expression in vitro in a Pol II-dependent manner. This noncoding RNA appears to act in trans, albeit locally at the adjacent PAR-1 promoter. These data suggest that an untranslated RNA plays a role in PAR-1 gene expression during embryonic growth.

Dynamic simulations of 13 TATA variants refine kinetic hypotheses of sequence/activity relationships 1 1Edited by B. Honig

The fundamental relationship between DNA sequence/deformability and biological function has attracted numerous experimental and theoretical studies. A classic prototype system used for such studies in eukaryotes is the complex between the TATA element transcriptional regulator and the TATA-box binding protein (TBP). The recent crystallographic study by Burley and co-workers demonstrated the remarkable structural similarity contrasted to different transcriptional activity of 11 TBP/DNA complexes in which the DNAs differed by single base-pairs. By simulating these TATA variants and two other single base-pair variants that were not crystallizable, we uncover sequence-dependent structural, energetic, and flexibility properties that tailor TATA elements to TBP interactions, complementing many previous studies by refining kinetic hypotheses on sequence/activity correlations. The factors that combine to produce favorable elements for TBP activity include overall flexibility; minor groove widening, as well as roll, rise, and shift increases at the ends of the TATA element; untwisting within the TATA element accompanied by large roll at the TATA element ends; and relatively low maximal water densities around the DNA. These features accompany the severe deformation induced by the minor-groove binding protein, which kinks the TATA element at the ends and displaces local water molecules to form stabilizing hydrophobic contacts. Interestingly, the preferred bending direction itself is not a significant predictor of activity disposition, although certain variants (such as wild-type AdMLP, 5'-TATA4G-3', and inactive A29, 5'-TA6G-3') exhibit large preferred bends in directions consistent with their activity or inactivity (major groove and minor groove bends, respectively). These structural, flexibility, and hydration preferences, identified here and connected to a new crystallographic study of a larger group of DNA variants than reported to date, highlight the profound influence of single base-pair DNA variations on DNA motion. Our refined kinetic hypothesis suggests the functional implications of these motions in a kinetic model of TATA/TBP recognition, inviting further theoretical and experimental research.

Development of an inducible pol III transcription system essentially requiring a mutated form of the TATA-binding protein

We attempted to devise a transcription system in which a particular DNA sequence of interest could be inducibly expressed under the control of a modified polymerase III (pol III) promoter. Its activation requires a mutated transcription factor not contained endogenously in human cells. We constructed such a promoter by fusing elements of the beta-lactamase gene of Escherichia coli, containing a modified TATA-box and a pol III terminator, to the initiation region of the human U6 gene. This construct functionally resembles a 5'-regulated pol III gene and its transcribed segment can be exchanged for an arbitrary sequence. Its transcription in vitro by pol III requires the same factors as the U6 gene with the major exception that the modified TATA-box of this construct only interacts with a TATA-binding protein (TBP) mutant (TBP-DR2) but not with TBP wild-type (TBPwt). Its transcription therefore requires TBP-DR2 exclusively instead of TBPWT: In order to render the system inducible, we fused the gene coding for TBP-DR2 to a tetracycline control element and stably transfected this new construct into HeLa cells. Induction of such a stable and viable clone with tetracycline resulted in the expression of functional TBP-DR2. This system may conceptually be used in the future to inducibly express an arbitrary DNA sequence in vivo under the control of the above mentioned promoter.

Similarities and differences in the conformation of protein-DNA complexes at the U1 and U6 snRNA gene promoters

Most small nuclear RNAs (snRNAs) are synthesized by RNA polymerase II, but U6 snRNA is synthesized by RNA polymerase III. In the fruit fly Drosophila melanogaster the RNA polymerase specificity of the snRNA genes is determined by a few nucleotide differences within the proximal sequence element (PSE), a conserved sequence located approximately 40-65 bp upstream of the transcription start site. The PSE is essential for transcription of both RNA polymerase II-transcribed and RNA polymerase III-transcribed snRNA genes and is recognized in Drosophila by a multi-subunit protein factor termed DM:PBP. Previous studies that employed site-specific protein-DNA photocrosslinking indicated that the conformation of the DNA-protein complex is different depending upon whether DM:PBP is bound to a U1 or U6 PSE sequence. These conformational differences of the complex probably represent an early step in determining the selection of the correct RNA polymerase. We have now obtained evidence that DM:PBP modestly bends the DNA upon interacting with the PSE and that the direction of DNA bending is similar for both the U1 and U6 PSEs. Under the assumption that DM:PBP does not significantly twist the DNA, the direction of the bend in both cases is toward the face of the DNA helix contacted by the 45 kDa subunit of DM:PBP. Together with data from partial proteolysis assays, these results indicate that the conformational differences in the complexes of DM:PBP with the U1 and U6 PSEs more likely occur at the protein level rather than at the DNA level.

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.

The TATA motif, the CAA motif and the poly(T) transcription termination motif are all important for transcription re-initiation on plant tRNA genes

The effect of alteration of 5' and 3' flanking sequences on the transcription of plant tRNA genes was analysed using an RNA polymerase III-dependent in vitro transcription system derived from nuclei of cultured tobacco cells. A TATA-like sequence and the CAA motif frequently observed upstream of plant tRNA genes, and the poly(T) stretch usually present downstream, were shown to be necessary for efficient re-initiation of transcription. The CAA motif was shown to be a transcription initiation site. Introduction of the CAA and TATA-like motifs into a gene naturally lacking them greatly enhanced transcription by promoting efficient re-initiation.

A TRF1:BRF complex directs Drosophila RNA polymerase III transcription

It has been generally accepted that the TATA binding protein (TBP) is a universal mediator of transcription by RNA polymerase I, II, and III. Here we report that the TBP-related factor TRF1 rather than TBP is responsible for RNA polymerase III transcription in Drosophila. Immunoprecipitation and in vitro transcription assays using immunodepleted extracts supplemented with recombinant proteins reveals that a TRF1:BRF complex is required to reconstitute transcription of tRNA, 5S and U6 RNA genes. In vivo, the majority of TRF1 is complexed with BRF and these two proteins colocalize at many polytene chromosome sites containing RNA pol III genes. These data suggest that in Drosophila, TRF1 rather than TBP forms a complex with BRF that plays a major role in RNA pol III transcription.

Minor groove DNA-protein contacts upstream of a tRNA gene detected with a synthetic DNA binding ligand

Transcription factor IIIB (TFIIIB) is composed of the TATA box binding protein (TBP) and class III gene-specific TBP-associated factors (TAFs). TFIIIB is brought to a site centered approximately 35 bp upstream from the transcription start site of tRNA genes via protein-protein interactions with the intragenic promoter-recognition factor TFIIIC. Since TBP interacts with TATA elements through the minor groove of DNA, we asked whether TFIIIB interacts with DNA in the minor groove. Polyamides containing pyrrole (Py) and imidazole (Im) amino acids are synthetic DNA ligands that bind to predetermined sequences in the minor groove of double helical DNA. These small molecules have been shown to interfere with protein-DNA interactions in the minor groove. A series of DNA constructs was generated in which the binding site for a Py-Im polyamide was placed at various distances upstream from a tRNA gene transcription start site. We find that a match polyamide will effectively inhibit tRNA gene transcription when its binding site is located within 33 bp of the transcription start site of the Xenopus TyrD tRNA gene. Moreover, in the presence of polyamide, RNA polymerase III is redirected to a new transcription initiation site located approximately one DNA helical turn downstream from the native start site. Our results suggest that a subunit of TFIIIB, possibly TBP, makes an essential minor groove DNA contact centered approximately 30 bp upstream from the tRNA gene.

hTFIIIB-beta stably binds to pol II promoters and recruits RNA polymerase III in a hTFIIIC1 dependent way

It has been shown that under specific conditions, transcription of protein coding genes can be efficiently initiated by RNA polymerase (pol) III in vitro. We examined the formation and composition of such pol III transcription complexes on the duck histone H5 and alphaA-globin promoters and found that the essential step for the formation of pol III transcription complexes on these pol II promoters was the stable binding of transcription factor (TF) IIIB-beta. For this process, the intact TFIIIB-beta complex, consisting of TBP and associated factors (TAFs) was needed and the prior association of pol III assembly factors was not necessary. We demonstrate for the first time that hTFIIIB-beta alone is able to bind to pol II promoter DNA. This resulted in a very stable complex which was resistant to high concentrations of heparin. Although immunodepletion revealed that TBP is essentially required for complex formation, other components of hTFIIIB-beta must also be involved, since TBP itself is unable to form heparin-resistant complexes and does not mediate pol III commitment per se. pol III is recruited to these pol II promoters in a strictly TFIIIC1 dependent way. After binding of TFIIIB-beta, the addition of TFIIIC1 and pol III were sufficient to yield productive pol III transcription complexes, which utilized the correct pol II initiation site. From these findings, we postulate that TFIIIC1 is involved in the recruitment of pol III and may thus form a bridge between TFIIIB-beta and the enzyme. This finding provides the first evidence for functional contacts between TFIIIC1 and pol III, which could be of general importance for the assembly of pol III transcription complexes.

Autoregulation of eukaryotic transcription factors

The structures of several promoters regulating the expression of eukaryotic transcription factors have in recent years been examined. In many cases there is good evidence for autoregulation, in which a given factor binds to its own promoter and either activates or represses transcription. Autoregulation occurs in all eukaryotes and is an important component in controlling expression of basal, cell cycle specific, inducible response and cell type-specific factors. The basal factors are autoregulatory, being strictly necessary for their own expression, and as such must be epigenetically inherited. Autoregulation of stimulus response factors typically serves to amplify cellular signals transiently and also to attenuate the response whether or not a given inducer remains. Cell cycle-specific transcription factors are positively and negatively autoregulatory, but this frequently depends on interlocking circuits among family members. Autoregulation of cell type-specific factors results in a form of cellular memory that can contribute, or define, a determined state. Autoregulation of transcription factors provides a simple circuitry, useful in many cellular circumstances, that does not require the involvement of additional factors, which, in turn, would need to be subject to another hierarchy of regulation. Autoregulation additionally can provide a direct means to sense and control the cellular conce]ntration of a given factor. However, autoregulatory loops are often dependent on cellular pathways that create the circumstances under which autoregulation occurs.

Protein kinase C inhibits transcription from the RNA polymerase III promoter of the human c-myc gene

The c-myc promoter has a unique characteristic showing both RNA polymerase II (pol II) and RNA polymerase III (pol III) activities. Previous studies demonstrated that activating PKC results in upregulation of c-myc expression from its pol II promoter. However, how PKC activation affects expression from the pol III promoter of the c-myc gene is not well understood. This study examines the effect of PKC on the pol III transcription from the c-myc gene by using an in vitro system. We report the inhibition of the c-myc pol III transcript by activating PKC. Further, either a phosphocellulose fraction of HeLa whole cell extract (WCE) enriched for transcription factor TF IIIB, or recombinant TATA-box binding protein could restore the inhibited c-myc pol III transcription under conditions that activate PKC. A role has been proposed for the c-myc pol III transcript in the regulation of c-myc gene expression. Therefore, this report discusses the significance of the downregulation of c-myc expression from its pol III promoter and the possible interplay between the pol II and pol III promoters of this gene.

The proximal sequence element (PSE) plays a major role in establishing the RNA polymerase specificity of Drosophila U-snRNA genes

Most small nuclear RNA (snRNA) genes are transcribed by RNA polymerase II, but some (e.g., U6) are transcribed by RNA polymerase III. In vertebrates a TATA box at a fixed distance downstream of the proximal sequence element (PSE) acts as a dominant determinant for recruiting RNA polymerase III to U6 gene promoters. In contrast, vertebrate snRNA genes that contain a PSE but lack a TATA box are transcribed by RNA polymerase II. In plants, transcription of both classes of snRNA genes requires a TATA box in addition to an upstream sequence element (USE), and polymerase specificity is determined by the spacing between these two core promoter elements. In these examples, the PSE (or USE) is interchangeable between the two classes of snRNA genes. Here we report the surprising finding that the Drosophila U1 and U6 PSEs cannot functionally substitute for each other; rather, determination of RNA polymerase specificity is an intrinsic property of the PSE sequence itself. The alteration of two or three base pairs near the 3'-end of the U1 and U6 PSEs was sufficient to switch the RNA polymerase specificity of Drosophila snRNA promoters in vitro. These findings reveal a novel mechanism for achieving RNA polymerase specificity at insect snRNA promoters.

Implication of 5'-flanking sequence elements in expression of a plant tRNA(Leu) gene

A comparison of 5'-flanking sequences from 68 different nuclear plant tRNA genes was analyzed to find consensus sequences. Three conserved features stood out, all of which are present in the tRNA(Leu) gene used in this study: (1) a high proportion of A and T residues upstream of all tRNA genes; (2) a region of low duplex stability about 30-35 bp before the coding sequence, often containing a TATA-box like motif; (3) a CAA triplet in the region of the presumed transcription start. The effect of replacement of the AT-rich upstream sequences with GC-rich sequences or unrelated AT-rich sequences was tested by progressive deletions and by inserting randomly cloned sequences upstream of the tRNA gene. GC-rich 5'-flanking sequences were found to be generally incompatible with high levels of expression. The TATA-box like motifs and the CAA triplet were removed or altered by deletion or directed mutagenesis. Mutation of the CAA triplet significantly decreased expression of the tRNA(Leu) gene, suggesting that this CAA triplet is important for transcription efficiency, but mutation or elimination of the TATA-box like motifs generally had little effect. The presence or absence of each of these features in tRNA genes from other organisms is discussed; there are clear and interesting differences between plant tRNA genes and those of yeast and mammals.

Characterization of a Drosophila proximal-sequence-element-binding protein involved in transcription of small nuclear RNA genes

In a wide variety of eukaryotic organisms, transcription of small nuclear RNA (snRNA) genes is dependent upon a proximal sequence element (PSE) located upstream of position -40 relative to the transcription start site. There is little or no existent knowledge concerning the PSE-binding proteins of organisms other than human. Here, we report the purification of a fraction enriched in the Drosophila melanogaster PSE-binding protein (DmPBP). DmPBP forms a highly specific complex with the PSE. The protein stimulates transcription from the U1 gene promoter by RNA polymerase II and from the U6 gene promoter by RNA polymerase III in Drosophila nuclear extracts, and activation is dependent upon the presence of a PSE. The molecular mass of native DmPBP as measured by gel-filtration chromatography is 375 kDa. Two polypeptides (apparent molecular masses 59 kDa and 61 kDa) appear to be in close contact with the DNA in that they can be very efficiently and specifically crosslinked to the PSE sequence by ultraviolet irradiation.

Expression and purification of the RNA polymerase III transcription specificity factor IIIB70 from Saccharomyces cerevisiae and its cooperative binding with TATA-binding protein

Transcription by RNA polymerase III (pol III) in yeast requires the assembly of an initiation complex comprising the TATA-binding protein (TBP), a 90-kDa polypeptide (TFIIIB90), and a 70-kDa polypeptide (TFIIIB70). TFIIIB70 interacts with TBP, a unique pol III subunit, C34, and the 131-kDa subunit of the pol III-specific complex, TFIIIC. TFIIIB70 was expressed in Escherichia coli and purified to homogeneity. The specific transcription activity of rTFIIIB70 is 22-58% that of the native yeast and in vitro synthesized factor. However, only a small fraction (0.07-0.32%) of the TFIIIB70 from these sources results in the synthesis of full-length RNA. The data suggest that TFIIIB70 function may be limited by an unfavorable recruitment equilibrium into the preinitiation complex. Quantitative DNase I "footprint" titrations of yeast TBP to the adenovirus major late promoter were conducted at a series of constant TFIIIB70 concentrations. A value of -0.7 +/- 0.2 kcal/mol was determined for the cooperative free energy of formation of the TBP.TFIIIB70.DNA complex at concentrations of TFIIIB70 sufficient to partition all of the binding cooperativity to the TBP binding isotherm. A Kd of 44 +/- 23 nM characterizes the TFIIIB70 concentration dependence of the TBP.TFIIIB70 cooperativity. The relationship deltalog K/deltalog (TFIIIB70) is consistent with the linkage of a single molecule of TFIIIB70 with the TBP-promoter binding reaction.

TATA-box DNA binding activity and subunit composition for RNA polymerase III transcription factor IIIB from Xenopus laevis

The RNA polymerase III transcription initiation factor TFIIIB contains the TATA-box-binding protein (TBP) and polymerase III-specific TBP-associated factors (TAFs). Previous studies have shown that DNA oligonucleotides containing the consensus TATA-box sequence inhibit polymerase III transcription, implying that the DNA binding domain of TBP is exposed in TFIIIB. We have investigated the TATA-box DNA binding activity of Xenopus TFIIIB, using transcription inhibition assays and a gel mobility shift assay. Gel shift competition assays with mutant and nonspecific DNAs demonstrate the specificity of the TFIIIB-TATA box DNA complex. The apparent dissociation constant for this protein-DNA interaction is approximately 0.4 nM, similar to the affinity of yeast TBP for the same sequence. TFIIIB transcriptional activity and TATA-box binding activity cofractionate during a series of four ion-exchange chromatographic steps, and reconstituted transcription reactions demonstrate that the TATA-box DNA-protein complex contains TFIIIB TAF activity. Polypeptides with apparent molecular masses of 75 and 92 kDa are associated with TBP in this complex. These polypeptides were renatured after elution from sodium dodecyl sulfate-gels and tested individually and in combination for TFIIIB TAF activity. Recombinant TBP along with protein fractions containing the 75- and 92-kDa polypeptides were sufficient to reconstitute TFIIIB transcriptional activity and DNA binding activity, suggesting that Xenopus TFIIIB is composed of TBP along with these polypeptides.

Role of TATA box sequence and orientation in determining RNA polymerase II/III transcription specificity

Work from a number of laboratories has indicated that the TATA box sequence can act as a basal promoter element not only for RNA polymerase II (RNAP II) transcription, but also for transcription by RNA polymerase III (RNAP III). We previously reported that, in the absence of other cis-acting elements, the canonical TATA sequence TATAAAAA specifically supported transcription by RNAP II in an unfractionated Drosophila nuclear extract, whereas the sequence TTTTTATA (the same sequence in reverse orientation) directed RNAP III transcription. We have now examined a variety of other TATA box sequences with regard to RNA polymerase selectivity and their ability to support RNAP III transcription. The results have allowed us to rank these TATA box sequences with respect to their relative strengths as RNAP III promoter elements in unfractionated Drosophila extracts. Further, the data indicate that T residues at positions 2 and 4 of the TATA box appear to be important determinants of RNAP III selectivity in this system, whereas A residues at these positions favor RNAP II transcription. Finally, the data suggest that transcription factors TFIID and TFIIIB, although both capable of binding a variety of TATA elements, have distinct sequence preferences for recognizing the TATA box and possibly the surrounding DNA.

TATA elements direct bi-directional transcription by RNA polymerases II and III

Eukaryotic promoter elements specify the direction and efficiency of transcription, as well as the type of RNA polymerase to be used. One such element, the TATA box, is thought to participate in determining the direction of transcription and can function within promoters for RNA polymerase II or III, depending on the sequence context. In this report the ability of four different TATA boxes to support transcription in vitro was determined. It was found that TATA elements are not directional. However, they support transcription by RNA polymerases II and III. An upstream activating sequence was found to stimulate downstream transcription by RNA polymerase II and to inhibit upstream transcription by RNA polymerases II and III. Thus a promoter necessarily consists of a TATA element and upstream sequences in order to specify the direction of transcription and the type of polymerase to be used.

The symmetry of the yeast U6 RNA gene's TATA box and the orientation of the TATA-binding protein in yeast TFIIIB

The central RNA polymerase III (Pol III) transcription factor TFIIIB is composed of the TATA-binding protein (TBP), Brf, a protein related to TFIIB, and the product of the newly cloned TFC5 gene. TFIIIB assembles autonomously on the upstream promoter of the yeast U6 snRNA (SNR6) gene in vitro, through the interaction of its TBP subunit with a consensus TATA box located at base pair -30. As both the DNA-binding domain of TBP and the U6 TATA box are nearly twofold symmetrical, we have examined how the binding polarity of TFIIIB is determined. We find that TFIIIB can bind to the U6 promoter in both directions, that TBP is unable to discern the natural polarity of the TATA element and that, as a consequence, the U6 TATA box is functionally symmetrical. A modest preference for TFIIIB binding in the natural direction of the U6 promoter is instead dictated by flanking DNA. Because the assembly of TFIIIB on the yeast U6 gene in vivo occurs via a TFIIIC-dependent mechanism, we investigated the influence of TFIIIC on the binding polarity of TFIIIB. TFIIIC places TFIIIB on the promoter in one direction only; thus, it is TFIIIC that primarily specifies the direction of transcription. Experiments using TFIIIB reconstituted with the altered DNA specificity mutant TBPm3 demonstrate that in the TFIIIB-U6 promoter complex, the carboxy-terminal repeat of TBP contacts the upstream half of the TATA box. This orientation of yeast TBP in Pol III promoter-bound TFIIIB is the same as in Pol II promoter-bound TFIID and in TBP-DNA complexes that have been analyzed by X-ray crystallography.