Transcription factor B contacts promoter DNA near the transcription start site of the archaeal transcription initiation complex

Early Evolution of Transcription Systems and Divergence of Archaea and Bacteria

DNA template-dependent multi-subunit RNA polymerases (RNAPs) found in all three domains of life and some viruses are of the two-double-Ψ-β-barrel (DPBB) type. The 2-DPBB protein format is also found in some RNA template-dependent RNAPs and a major replicative DNA template-dependent DNA polymerase (DNAP) from Archaea (PolD). The 2-DPBB family of RNAPs and DNAPs probably evolved prior to the last universal common cellular ancestor (LUCA). Archaeal Transcription Factor B (TFB) and bacterial σ factors include homologous strings of helix-turn-helix units. The consequences of TFB-σ homology are discussed in terms of the evolution of archaeal and bacterial core promoters. Domain-specific DPBB loop inserts functionally connect general transcription factors to the RNAP active site. Archaea appear to be more similar to LUCA than Bacteria. Evolution of bacterial σ factors from TFB appears to have driven divergence of Bacteria from Archaea, splitting the prokaryotic domains.

RNA extension drives a stepwise displacement of an initiation-factor structural module in initial transcription

All organisms-bacteria, archaea, and eukaryotes-have a transcription initiation factor that contains a structural module that binds within the RNA polymerase (RNAP) active-center cleft and interacts with template-strand single-stranded DNA (ssDNA) in the immediate vicinity of the RNAP active center. This transcription initiation-factor structural module preorganizes template-strand ssDNA to engage the RNAP active center, thereby facilitating binding of initiating nucleotides and enabling transcription initiation from initiating mononucleotides. However, this transcription initiation-factor structural module occupies the path of nascent RNA and thus presumably must be displaced before or during initial transcription. Here, we report four sets of crystal structures of bacterial initially transcribing complexes that demonstrate and define details of stepwise, RNA-extension-driven displacement of the "σ-finger" of the bacterial transcription initiation factor σ. The structures reveal that-for both the primary σ-factor and extracytoplasmic (ECF) σ-factors, and for both 5'-triphosphate RNA and 5'-hydroxy RNA-the "σ-finger" is displaced in stepwise fashion, progressively folding back upon itself, driven by collision with the RNA 5'-end, upon extension of nascent RNA from ∼5 nt to ∼10 nt.

Structural basis of ECF-σ-factor-dependent transcription initiation

Extracytoplasmic (ECF) σ factors, the largest class of alternative σ factors, are related to primary σ factors, but have simpler structures, comprising only two of six conserved functional modules in primary σ factors: region 2 (σR2) and region 4 (σR4). Here, we report crystal structures of transcription initiation complexes containing Mycobacterium tuberculosis RNA polymerase (RNAP), M. tuberculosis ECF σ factor σ^L, and promoter DNA. The structures show that σR2 and σR4 of the ECF σ factor occupy the same sites on RNAP as in primary σ factors, show that the connector between σR2 and σR4 of the ECF σ factor–although shorter and unrelated in sequence–follows the same path through RNAP as in primary σ factors, and show that the ECF σ factor uses the same strategy to bind and unwind promoter DNA as primary σ factors. The results define protein-protein and protein-DNA interactions involved in ECF-σ-factor-dependent transcription initiation.

No structural data have been available for RNA polymerase holoenzymes or transcription initiation complexes that contain extracytoplasmic σ factors. Here the authors report the crystal structures of transcription initiation complexes comprising Mycobacterium tuberculosis RNA polymerase, extracytoplasmic σ factor σ^L and promoter DNA.

Transcriptional Regulation in Archaea: From Individual Genes to Global Regulatory Networks

Archaea are major contributors to biogeochemical cycles, possess unique metabolic capabilities, and resist extreme stress. To regulate the expression of genes encoding these unique programs, archaeal cells use gene regulatory networks (GRNs) composed of transcription factor proteins and their target genes. Recent developments in genetics, genomics, and computational methods used with archaeal model organisms have enabled the mapping and prediction of global GRN structures. Experimental tests of these predictions have revealed the dynamical function of GRNs in response to environmental variation. Here, we review recent progress made in this area, from investigating the mechanisms of transcriptional regulation of individual genes to small-scale subnetworks and genome-wide global networks. At each level, archaeal GRNs consist of a hybrid of bacterial, eukaryotic, and uniquely archaeal mechanisms. We discuss this theme from the perspective of the role of individual transcription factors in genome-wide regulation, how these proteins interact to compile GRN topological structures, and how these topologies lead to emergent, high-level GRN functions. We conclude by discussing how systems biology approaches are a fruitful avenue for addressing remaining challenges, such as discovering gene function and the evolution of GRNs.

The Transcriptional Regulator TFB-RF1 Activates Transcription of a Putative ABC Transporter in Pyrococcus furiosus

Transcription factor B recruiting factor 1 (TFB-RF1; PF1088) is a transcription regulator which activates transcription on archaeal promoters containing weak TFB recognition elements (BRE) by recruiting TFB to the promoter. The mechanism of activation is described in detail, but nothing is known about the biological function of this protein in Pyrococcus furiosus. The protein is located in an operon structure together with the hypothetical gene pf1089 and western blot as well as end-point RT-PCR experiments revealed an extremely low expression rate of both proteins. Furthermore, conditions to induce the expression of the operon are not known. By introducing an additional copy of tfb-RF1 using a Pyrococcus shuttle vector we could circumvent the lacking expression of both proteins under standard growth conditions as indicated by western blot as well as end-point RT-PCR experiments. A ChIP-seq experiment revealed an additional binding site of TFB-RF1 in the upstream region of the pf1011/1012 operon, beside the expected target of the pf1089/tfb-RF1 region. This operon codes for a putative ABC transporter which is most-related to a multidrug export system and in vitro analysis using gel shift assays, DNase I footprinting and in vitro transcription confirmed the activator function of TFB-RF1 on the corresponding promoter. These findings are also in agreement with in vivo data, as RT-qPCR experiments also indicate transcriptional activation of both operons. Taken together, the overexpression strategy of tfb-RF1 enabled the identification of an additional operon of the TFB-RF1 regulon which indicates a transport-related function and provides a promising starting position to decipher the physiological function of the TFB-RF1 gene regulatory network in P. furiosus.

Transcription Regulation in Archaea

The known diversity of metabolic strategies and physiological adaptations of archaeal species to extreme environments is extraordinary. Accurate and responsive mechanisms to ensure that gene expression patterns match the needs of the cell necessitate regulatory strategies that control the activities and output of the archaeal transcription apparatus. Archaea are reliant on a single RNA polymerase for all transcription, and many of the known regulatory mechanisms employed for archaeal transcription mimic strategies also employed for eukaryotic and bacterial species. Novel mechanisms of transcription regulation have become apparent by increasingly sophisticated in vivo and in vitro investigations of archaeal species. This review emphasizes recent progress in understanding archaeal transcription regulatory mechanisms and highlights insights gained from studies of the influence of archaeal chromatin on transcription.

Photoaffinity labeling of transcription factors by DNA-templated crosslinking

Characterization of transcription factor-DNA interaction is of high importance in elucidating the molecular mechanisms of gene transcriptions. DNA-based affinity probes were developed to capture and identify transcription factors by covalent crosslinking; however, the requirement of a crosslinker on the affinity probe remains a disadvantage, as the crosslinker itself often interferes with the protein-DNA interactions. We report a dual-probe method able to capture DNA-binding transcription factors with unmodified protein-binding sites in scenarios where conventional probes have failed. We have also shown the method's converse application in selecting specific transcription factor-binding DNA sequences from a probe library and its extension to studying proteins recognizing epigenetic marks. This study may provide a new tool for exploring DNA-binding proteins in biology.

Super-resolution microscopy reveals decondensed chromatin structure at transcription sites

Remodeling of the local chromatin structure is essential for the regulation of gene expression. While a number of biochemical and bioimaging experiments suggest decondensed chromatin structures are associated with transcription, a direct visualization of DNA and transcriptionally active RNA polymerase II (RNA pol II) at super-resolution is still lacking. Here we investigate the structure of chromatin isolated from HeLa cells using binding activatable localization microscopy (BALM). The sample preparation method preserved the structural integrity of chromatin. Interestingly, BALM imaging of the chromatin spreads revealed the presence of decondensed chromatin as gap structures along the spreads. These gaps were enriched with phosphorylated S5 RNA pol II, and were sensitive to the cellular transcriptional state. Taken together, we could visualize the decondensed chromatin regions together with active RNA pol II for the first time using super-resolution microscopy.

Transcription regulation in the third domain

The ability of organisms to sense and respond to their environment is essential to their survival. This is no different for members of the third domain of life, the Archaea. Archaea are found in diverse and often extreme habitats. However, their ability to sense and respond to their environment at the level of gene expression has been understudied when compared to bacteria and eukaryotes. Over the last decade, the field has expanded, and a variety of unique and interesting regulatory schemes have been unraveled. In this review, the current state of knowledge of archaeal transcription regulation is explored.

The Sulfolobus initiator element is an important contributor to promoter strength

Basal elements in archaeal promoters, except for putative initiator elements encompassing transcription start sites, are well characterized. Here, we employed the Sulfolobus araS promoter as a model to study the function of the initiator element (Inr) in archaea. We have provided evidence for the presence of a third core promoter element, the Sulfolobus Inr, whose action depends on a TATA box and the TFB recognition element (BRE). Substitution mutations in the araS Inr did not alter the location of the transcription start site. Using systematic mutagenesis, the most functional araS Inr was defined as +1 GAGAMK +6 (where M is A/C and K is G/T). Furthermore, WebLogo analysis of a subset of promoters with coding sequences for 5' untranslated regions (UTRs) larger than 4 nucleotides (nt) in Sulfolobus solfataricus P2 identified an Inr consensus that exactly matches the functional araS Inr sequence. Moreover, mutagenesis of 3 randomly selected promoters confirmed the Inr sequences to be important for basal promoter strength in the subgroup. Importantly, the result of the araS Inr being added to the Inr-less promoters indicates that the araS Inr, the core promoter element, is able to enhance the strength of Inr-less promoters. We infer that transcription factor B (TFB) and subunits of RNA polymerase bind the Inr to enhance promoter strength. Taken together, our data suggest that the presence or absence of an Inr on basal promoters is important for global gene regulation in Sulfolobus.

The RNA polymerase trigger loop functions in all three phases of the transcription cycle

The trigger loop (TL) forms a conserved element in the RNA polymerase active centre that functions in the elongation phase of transcription. Here, we show that the TL also functions in transcription initiation and termination. Using recombinant variants of RNA polymerase from Pyrococcus furiosus and a reconstituted transcription system, we demonstrate that the TL is essential for initial RNA synthesis until a complete DNA–RNA hybrid is formed. The archaeal TL is further important for transcription fidelity during nucleotide incorporation, but not for RNA cleavage during proofreading. A conserved glutamine residue in the TL binds the 2’-OH group of the nucleoside triphosphate (NTP) to discriminate NTPs from dNTPs. The TL also prevents aberrant transcription termination at non-terminator sites.

A Monte Carlo-based framework enhances the discovery and interpretation of regulatory sequence motifs

BACKGROUND

Discovery of functionally significant short, statistically overrepresented subsequence patterns (motifs) in a set of sequences is a challenging problem in bioinformatics. Oftentimes, not all sequences in the set contain a motif. These non-motif-containing sequences complicate the algorithmic discovery of motifs. Filtering the non-motif-containing sequences from the larger set of sequences while simultaneously determining the identity of the motif is, therefore, desirable and a non-trivial problem in motif discovery research.

RESULTS

We describe MotifCatcher, a framework that extends the sensitivity of existing motif-finding tools by employing random sampling to effectively remove non-motif-containing sequences from the motif search. We developed two implementations of our algorithm; each built around a commonly used motif-finding tool, and applied our algorithm to three diverse chromatin immunoprecipitation (ChIP) data sets. In each case, the motif finder with the MotifCatcher extension demonstrated improved sensitivity over the motif finder alone. Our approach organizes candidate functionally significant discovered motifs into a tree, which allowed us to make additional insights. In all cases, we were able to support our findings with experimental work from the literature.

CONCLUSIONS

Our framework demonstrates that additional processing at the sequence entry level can significantly improve the performance of existing motif-finding tools. For each biological data set tested, we were able to propose novel biological hypotheses supported by experimental work from the literature. Specifically, in Escherichia coli, we suggested binding site motifs for 6 non-traditional LexA protein binding sites; in Saccharomyces cerevisiae, we hypothesize 2 disparate mechanisms for novel binding sites of the Cse4p protein; and in Halobacterium sp. NRC-1, we discoverd subtle differences in a general transcription factor (GTF) binding site motif across several data sets. We suggest that small differences in our discovered motif could confer specificity for one or more homologous GTF proteins. We offer a free implementation of the MotifCatcher software package at http://www.bme.ucdavis.edu/facciotti/resources_data/software/.

A dual promoter region with overlapping activator sequences drives the expression of gas vesicle protein genes in haloarchaea

Gas vesicle formation in haloarchaea involves 14 gas vesicle protein (gvp) genes. The strong promoter P(A) drives the expression of gvpACNO, which encodes the major gas vesicle structural proteins GvpA and GvpC, whereas the oppositely oriented promoter P(D) initiates the synthesis of the two regulator proteins, GvpD and GvpE. GvpE activates P(A) and P(D), and requires a 20 nt upstream activator sequence (UAS). UAS(A) and UAS(D) partially overlap in the centre of the 35 bp intergenic region. The basal and GvpE-induced activities of P(A) and P(D) were investigated in Haloferax volcanii transformants. Each UAS consists of two 8 nt portions (P(A), 1A+2A; P(D), 1D+2D), and mutations in the overlapping 1A and 1D portions affected the GvpE induction of both promoters. Substitution of one of the UAS portions by a nonsense sequence showed that a complete UAS is required for activation. The activation of P(A) was more efficient compared with P(D). Promoter P(A) with UAS(A) in configuration 1A+1A was still activated by GvpE, but P(D) was not inducible with UAS(D) in configuration 1D+1D. The TATA box and/or transcription factor B recognition element (BRE) were exchanged between P(A) and P(D). All elements of P(A) functioned well in the environment of 'P(D)' and transferred the stronger P(A) activity to 'P(D)'. In contrast, the respective 'P(A)' chimeras were less active, and BRE(D) was not functional in the environment of 'P(A)'. The relative strengths of the two promoters were substantially determined by the BRE. A 4 nt scanning mutagenesis uncovered an additional regulatory element in the region between TATA(D) and the transcriptional start site of gvpD.

Activation of archaeal transcription mediated by recruitment of transcription factor B

Archaeal promoters consist of a TATA box and a purine-rich adjacent upstream sequence (transcription factor B (TFB)-responsive element (BRE)), which are bound by the transcription factors TATA box-binding protein (TBP) and TFB. Currently, only a few activators of archaeal transcription have been experimentally characterized. The best studied activator, Ptr2, mediates activation by recruitment of TBP. Here, we present a detailed biochemical analysis of an archaeal transcriptional activator, PF1088, which was identified in Pyrococcus furiosus by a bioinformatic approach. Operon predictions suggested that an upstream gene, pf1089, is polycistronically transcribed with pf1088. We demonstrate that PF1088 stimulates in vitro transcription by up to 7-fold when the pf1089 promoter is used as a template. By DNase I and hydroxyl radical footprinting experiments, we show that the binding site of PF1088 is located directly upstream of the BRE of pf1089. Mutational analysis indicated that activation requires the presence of the binding site for PF1088. Furthermore, we show that activation of transcription by PF1088 is dependent upon the presence of an imperfect BRE and is abolished when the pf1089 BRE is replaced with a BRE from a strong archaeal promoter. Gel shift experiments showed that TFB recruitment to the pf1089 operon is stimulated by PF1088, and TFB seems to stabilize PF1088 operator binding even in the absence of TBP. Taken together, these results represent the first biochemical evidence for a transcriptional activator working as a TFB recruitment factor in Archaea, for which the designation TFB-RF1 is suggested.

Construction and characterization of a gradually inducible expression vector for Halobacterium salinarum, based on the kdp promoter

Gradually inducible expression vectors which are governed by variations of growth conditions are powerful tools for gene expression of conditionally lethal mutants. Furthermore, controlled expression allows monitoring of overproduction of proteins at various stages in their expressing hosts. For Halobacterium salinarum, which is often used as a paradigm for halophilic archaea, such an inducible expression system is not available to date. Here we show that the kdp promoter (Pkdp), which facilitates gene expression upon K(+) limitation, can be used to establish such a system for molecular applications. Pkdp features a rather high expression rate, with an approximately 50-fold increase that can be easily varied by K(+) concentrations in the growth medium. Besides the construction of an expression vector, our work describes the characterization of expression patterns and, thus, offers a gradually inducible expression system to the scientific community.

Archaeal RNA polymerase and transcription regulation

To elucidate the mechanism of transcription by cellular RNA polymerases (RNAPs), high-resolution X-ray crystal structures together with structure-guided biochemical, biophysical, and genetics studies are essential. The recently solved X-ray crystal structures of archaeal RNAP allow a structural comparison of the transcription machinery among all three domains of life. The archaea were once thought of closely related to bacteria, but they are now considered to be more closely related to the eukaryote at the molecular level than bacteria. According to these structures, the archaeal transcription apparatus, which includes RNAP and general transcription factors (GTFs), is similar to the eukaryotic transcription machinery. Yet, the transcription regulators, activators and repressors, encoded by archaeal genomes are closely related to bacterial factors. Therefore, archaeal transcription appears to possess an intriguing hybrid of eukaryotic-type transcription apparatus and bacterial-like regulatory mechanisms. Elucidating the transcription mechanism in archaea, which possesses a combination of bacterial and eukaryotic transcription mechanisms that are commonly regarded as separate and mutually exclusive, can provide data that will bring basic transcription mechanisms across all life forms.

Archaeal promoter architecture and mechanism of gene activation

Sulfolobus solfataricus and Sulfolobus islandicus contain several genes exhibiting D-arabinose-inducible expression and these systems are ideal for studying mechanisms of archaeal gene expression. At sequence level, only two highly conserved cis elements are present on the promoters: a regulatory element named ara box directing arabinose-inducible expression and the basal promoter element TATA, serving as the binding site for the TATA-binding protein. Strikingly, these promoters possess a modular structure that allows an essentially inactive basal promoter to be strongly activated. The invoked mechanisms include TFB (transcription factor B) recruitment by the ara-box-binding factor to activate gene expression and modulation of TFB recruitment efficiency to yield differential gene expression.

Mitochondrial transcription factor Mtf1 traps the unwound non-template strand to facilitate open complex formation

The catalytic subunit of the mitochondrial (mt) RNA polymerase (RNAP) is highly homologous to the bacteriophage T7/T3 RNAP. Unlike the phage RNAP, however, the mtRNAP relies on accessory proteins to initiate promoter-specific transcription. Rpo41, the catalytic subunit of the Saccharomyces cerevisiae mtRNAP, requires Mtf1 for opening the duplex promoter. To elucidate the role of Mtf1 in promoter-specific DNA opening, we have mapped the structural organization of the mtRNAP using site-specific protein-DNA photo-cross-linking studies. Both Mtf1 and Rpo41 cross-linked to distinct sites on the promoter DNA, but the dominant cross-links were those of the Mtf1, which indicates a direct role of Mtf1 in promoter-specific binding and initiation. Strikingly, Mtf1 cross-linked with a high efficiency to the melted region of the promoter DNA, based on which we suggest that Mtf1 facilitates DNA melting by trapping the non-template strand in the unwound conformation. Additional strong cross-links of the Mtf1 were observed with the -8 to -10 base-paired region of the promoter. The cross-linking results were incorporated into a structural model of the mtRNAP-DNA, created from a homology model of the C-terminal domain of Rpo41 and the available structure of Mtf1. The promoter DNA is sandwiched between Mtf1 and Rpo41 in the structural model, and Mtf1 closely associates mainly with one face of the promoter across the entire nona-nucleotide consensus sequence. Overall, the studies reveal that in many ways the role of Mtf1 is analogous to the transcription factors of the multisubunit RNAPs, which provides an intriguing link between single- and multisubunit RNAPs.

RNA polymerase II-TFIIB structure and mechanism of transcription initiation

To initiate gene transcription, RNA polymerase II (Pol II) requires the transcription factor IIB (B). Here we present the crystal structure of the complete Pol II-B complex at 4.3 A resolution, and complementary functional data. The results indicate the mechanism of transcription initiation, including the transition to RNA elongation. Promoter DNA is positioned over the Pol II active centre cleft with the 'B-core' domain that binds the wall at the end of the cleft. DNA is then opened with the help of the 'B-linker' that binds the Pol II rudder and clamp coiled-coil at the edge of the cleft. The DNA template strand slips into the cleft and is scanned for the transcription start site with the help of the 'B-reader' that approaches the active site. Synthesis of the RNA chain and rewinding of upstream DNA displace the B-reader and B-linker, respectively, to trigger B release and elongation complex formation.

An upstream activation element exerting differential transcriptional activation on an archaeal promoter

Microorganisms can utilize different sugars as energy and carbon sources and the genes involved in sugar metabolism often exhibit highly regulated expression. To study cis-acting elements controlling arabinose-responsive expression in archaea, the promoter of the Sulfolobus solfataricus araS gene encoding an arabinose binding protein was characterized using an Sulfolobus islandicus reporter gene system. The minimal active araS promoter (P(araS)) was found to be 59 nucleotides long and harboured four promoter elements: an ara-box, an upstream transcription factor B-responsive element (BRE), a TATA-box and a proximal promoter element, each of which contained important nucleotides that either greatly decreased or completely abolished promoter activity upon mutagenesis. The basal araS promoter was virtually inactive due to intrinsically weak BRE element, and the upstream activating sequence (UAS) ara-box activated the basal promoter by recruiting transcription factor B to its BRE. While this UAS ensured a general expression from an inactive or weak basal promoter in the presence of other tested carbon resources, it exhibited a strong arabinose-responsive transcriptional activation. To our knowledge, this represents the first example of an archaeal UAS that exhibits differential activation to the expression on the same promoter in the presence of different carbon sources.

Probing differential Ag+-nucleobase interactions with isothermal titration calorimetry (ITC): Towards patterned DNA metallization

DNA has been successfully used as a scaffold for the fabrication of metallic nanowires, primarily based on the electrostatic complexation and reduction of the metal cations on the negatively charged sugar-phosphate backbone. Here, we probe the differential binding affinities of nucleobases for silver ions using sensitive isothermal titration calorimetry (ITC) measurements of the reaction enthalpies, which go in order: C > G > A > or = T. Using the disparity between the interaction of cytosine (strong binding) and thymine (weak binding) with silver ions, we have successfully generated silver nanoparticle doublets and triplets on custom-made oligonucleotides, C(30)-T(40)-C(30) and C(20)-T(20)-C(20)-T(20)-C(20), respectively. Thus, a new and simple method of generating metallized DNA wires is presented, based entirely on the nucleotide sequence of DNA. The concept could be extended to other cations and complex DNA sequences in order to achieve intricately patterned DNA constructs.

Evolution of complex RNA polymerases: the complete archaeal RNA polymerase structure

The archaeal RNA polymerase (RNAP) shares structural similarities with eukaryotic RNAP II but requires a reduced subset of general transcription factors for promoter-dependent initiation. To deepen our knowledge of cellular transcription, we have determined the structure of the 13-subunit DNA-directed RNAP from Sulfolobus shibatae at 3.35 Å resolution. The structure contains the full complement of subunits, including RpoG/Rpb8 and the equivalent of the clamp-head and jaw domains of the eukaryotic Rpb1. Furthermore, we have identified subunit Rpo13, an RNAP component in the order Sulfolobales, which contains a helix-turn-helix motif that interacts with the RpoH/Rpb5 and RpoA'/Rpb1 subunits. Its location and topology suggest a role in the formation of the transcription bubble.

Protein-coding gene promoters in Methanocaldococcus (Methanococcus) jannaschii

Although Methanocaldococcus (Methanococcus) jannaschii was the first archaeon to have its genome sequenced, little is known about the promoters of its protein-coding genes. To expand our knowledge, we have experimentally identified 131 promoters for 107 protein-coding genes in this genome by mapping their transcription start sites. Compared to previously identified promoters, more than half of which are from genes for stable RNAs, the protein-coding gene promoters are qualitatively similar in overall sequence pattern, but statistically different at several positions due to greater variation among their sequences. Relative binding affinity for general transcription factors was measured for 12 of these promoters by competition electrophoretic mobility shift assays. These promoters bind the factors less tightly than do most tRNA gene promoters. When a position weight matrix (PWM) was constructed from the protein gene promoters, factor binding affinities correlated with corresponding promoter PWM scores. We show that the PWM based on our data more accurately predicts promoters in the genome and transcription start sites than could be done with the previously available data. We also introduce a PWM logo, which visually displays the implications of observing a given base at a position in a sequence.

Static and kinetic site-specific protein-DNA photocrosslinking: analysis of bacterial transcription initiation complexes

Static site-specific protein-DNA photocrosslinking permits identification of protein-DNA interactions within multiprotein-DNA complexes. Kinetic site-specific protein-DNA photocrosslinking - involving rapid-quench-flow mixing and pulsed-laser irradiation - permits elucidation of pathways and kinetics of formation of protein-DNA interactions within multiprotein-DNA complexes. We present detailed protocols for application of static and kinetic site-specific protein-DNA photocrosslinking to bacterial transcription initiation complexes.

A whole-genome approach to identifying protein binding sites: promoters in Methanocaldococcus (Methanococcus) jannaschii

We have adapted an electrophoretic mobility shift assay (EMSA) to isolate genomic DNA fragments that bind the archaeal transcription initiation factors TATA-binding protein (TBP) and transcription factor B (TFB) to perform a genome-wide search for promoters. Mobility-shifted fragments were cloned, tested for their ability to compete with known promoter-containing fragments for a limited concentration of transcription factors, and sequenced. We applied the method to search for promoters in the genome of Methanocaldococcus jannaschii. Selection was most efficient for promoters of tRNA genes and genes for several presumed small non-coding RNAs (ncRNA). Protein-coding gene promoters were dramatically underrepresented relative to their frequency in the genome. The repeated isolation of these genomic regions was partially rectified by including a hybridization-based screening. Sequence alignment of the affinity-selected promoters revealed previously identified TATA box, BRE, and the putative initiator element. In addition, the conserved bases immediately upstream and downstream of the BRE and TATA box suggest that the composition and structure of archaeal natural promoters are more complicated.

Activation of transcription initiation by Spx: formation of transcription complex and identification of a Cis-acting element required for transcriptional activation

The Spx protein of Bacillus subtilis interacts with RNA polymerase (RNAP) to activate transcription initiation in response to thiol-oxidative stress. Protein-DNA cross-linking analysis of reactions containing RNAP, Spx and trxA (thioredoxin) or trxB (thioredoxin reductase) promoter DNA was undertaken to uncover the organization of the Spx-activated transcription initiation complex. Spx induced contact between the RNAP sigma(A) subunit and the -10 promoter sequence of trxA and B, and contact of the betabeta' subunits with core promoter DNA. No Spx-DNA contact was detected. Spx mutants, Spx(C10A) and Spx(G52R.), or RNAP alpha C-terminal domain mutants that impair productive Spx-RNAP interaction did not induce heightened sigma and betabeta' contact with the core promoter. Deletion analysis and the activity of hybrid promoter constructs having upstream trxB DNA fused at positions -31, -36 and -41 of the srf (surfactin synthetase) promoter indicated that a cis-acting site between -50 and -36 was required for Spx activity. Mutations at -43 and -44 of trxB abolished Spx-dependent transcription and Spx-induced cross-linking between the sigma subunit and the -10 region. These data are consistent with a model that Spx activation requires contact between the Spx/RNAP complex and upstream promoter DNA, which allows Spx-induced engagement of the sigma and large subunits with the core promoter.

TrpY regulation of trpB2 transcription in Methanothermobacter thermautotrophicus

TrpY binds specifically to TRP box sequences upstream of trpB2, but the repression of trpB2 transcription requires additional TrpY assembly that is stimulated by but not dependent on the presence of tryptophan. Inhibitory complex formation is prevented by insertions within the regulatory region and by a G149R substitution in TrpY, even though TrpY(G149R) retains both TRP box DNA- and tryptophan-binding abilities.

Archaeal transcription: function of an alternative transcription factor B from Pyrococcus furiosus

The genome of the hyperthermophile archaeon Pyrococcus furiosus encodes two transcription factor B (TFB) paralogs, one of which (TFB1) was previously characterized in transcription initiation. The second TFB (TFB2) is unusual in that it lacks recognizable homology to the archaeal TFB/eukaryotic TFIIB B-finger motif. TFB2 functions poorly in promoter-dependent transcription initiation, but photochemical cross-linking experiments indicated that the orientation and occupancy of transcription complexes formed with TFB2 at the strong gdh promoter are similar to the orientation and occupancy of transcription complexes formed with TFB1. Initiation complexes formed by TFB2 display a promoter opening defect that can be bypassed with a preformed transcription bubble, suggesting a mechanism to explain the low TFB2 transcription activity. Domain swaps between TFB1 and TFB2 showed that the low activity of TFB2 is determined mainly by its N terminus. The low activity of TFB2 in promoter opening and transcription can be partially relieved by transcription factor E (TFE). The results indicate that the TFB N-terminal region, containing conserved Zn ribbon and B-finger motifs, is important in promoter opening and that TFE can compensate for defects in the N terminus through enhancement of promoter opening.

Structure and function of archaeal RNA polymerases

RNA polymerases (RNAPs) are essential to all life forms and therefore deserve our special attention. The archaeal RNAP is closely related to eukaryotic RNAPII in terms of subunit composition and architecture, promoter elements and basal transcription factors required for the initiation and elongation phase of transcription. RNAPs of this class are large and sophisticated enzymes that interact in a complex manner with DNA/RNA scaffolds, substrates NTPs and a plethora of transcription factors - interactions that often result in an allosteric regulation of RNAP activity. The 12 subunits of RNAP play distinct roles including RNAP assembly and stability, catalysis and functional contacts with exogenous factors. Due to the availability of structural information of RNAPs at high-resolution and wholly recombinant archaeal transcription systems, we are beginning to understand the molecular mechanisms of archaeal RNAPs and transcription in great detail.

TFIIB and the regulation of transcription by RNA polymerase II

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

Archaeal minichromosome maintenance (MCM) helicase can unwind DNA bound by archaeal histones and transcription factors

Protein-DNA complexes must be disassembled to facilitate DNA replication. Replication forks contain a helicase that unwinds the duplex DNA at the front of the fork. The minichromosome maintenance helicase from the archaeon Methanothermobacter thermautotrophicus required only ATP to unwind DNA bound into complexes by the M. thermautotrophicus archaeal histone HMtA2, transcription repressor TrpY, or into a transcription pre-initiation complex by M. thermautotrophicus TATA-box-binding protein, transcription factor B, and RNA polymerase. In contrast, the minichromosome maintenance helicase was unable to unwind DNA bound by this archaeal RNA polymerase in a stalled transcript-elongating complex.

Protein-protein interactions in the archaeal transcriptional machinery: binding studies of isolated RNA polymerase subunits and transcription factors

Transcription in Archaea is directed by a pol II-like RNA polymerase and homologues of TBP and TFIIB (TFB) but the crystal structure of the archaeal enzyme and the subunits involved in recruitment of RNA polymerase to the promoter-TBP-TFB-complex are unknown. We described here the cloning expression and purification of 11 bacterially expressed subunits of the Pyrococcus furiosus RNAP. Protein interactions of subunits with each other and of archaeal transcription factors TFB and TFB with RNAP subunits were studied by Far-Western blotting and reconstitution of subcomplexes from single subunits in solution. In silico comparison of a consensus sequence of archaeal RNAP subunits with the sequence of yeast pol II subunits revealed a high degree of conservation of domains of the enzymes forming the cleft and catalytic center of the enzyme. Interaction studies with the large subunits were complicated by the low solubility of isolated subunits B, A', and A'', but an interaction network of the smaller subunits of the enzyme was established. Far-Western analyses identified subunit D as structurally important key polypeptide of RNAP involved in interactions with subunits B, L, N, and P and revealed also a strong interaction of subunits E' and F. Stable complexes consisting of subunits E' and F, of D and L and a BDLNP-subcomplex were reconstituted and purified. Gel shift analyses revealed an association of the BDLNP subcomplex with promoter-bound TBP-TFB. These results suggest a major role of subunit B (Rpb2) in RNAP recruitment to the TBP-TFB promoter complex.

Quantitative analysis of in vivo initiator selection by yeast RNA polymerase II supports a scanning model

Initiation of transcription by RNA polymerase II (RNAP II) on Saccharomyces cerevisiae messenger RNA (mRNA) genes typically occurs at multiple sites 40-120 bp downstream of the TATA box. The mechanism that accommodates this extended and variable promoter architecture is unknown, but one model suggests that RNAP II forms an open promoter complex near the TATA box and then scans the template DNA strand for start sites. Unlike most protein-coding genes, small nuclear RNA gene transcription starts predominantly at a single position. We identify a highly efficient initiator element as the primary start site determinant for the yeast U4 small nuclear RNA gene, SNR14. Consistent with the scanning model, transcription of an SNR14 allele with tandemly duplicated start sites initiates primarily from the upstream site, yet the downstream site is recognized with equivalent efficiency by the diminished population of RNAP II molecules that encounter it. A quantitative in vivo assay revealed that SNR14 initiator efficiency is nearly perfect (approximately 90%), which explains the precision of U4 RNA 5' end formation. Initiator efficiency was reduced by cis-acting mutations at -8, -7, -1, and +1 and trans-acting substitutions in the TFIIB B-finger. These results expand our understanding of RNAP II initiation preferences and provide new support for the scanning model.

Determinants of transcription initiation by archaeal RNA polymerase

Transcription in Archaea is catalyzed by an RNA polymerase that is most similar to eukaryotic RNA polymerases both in subunit composition and in transcription initiation factor requirements. Recent studies on archaeal transcription in diverse members of this domain have contributed new details concerning the functions of promoters and transcription factors in guiding initiation by RNA polymerase, and phylogenetic arguments have allowed modeling of archaeal transcription initiation complexes by comparison with recently described models of eukaryotic and bacterial transcription initiation complexes. Important new advances in reconstitution of archaeal transcription complexes from fully recombinant components is permitting testing of hypotheses derived from and informed by these structural models, and will help bring the study of archaeal transcription to the levels of understanding currently enjoyed by bacterial and eukaryotic RNA polymerase II transcription.

Regulation of tryptophan operon expression in the archaeon Methanothermobacter thermautotrophicus

Conserved trp genes encode enzymes that catalyze tryptophan biosynthesis in all three biological domains, and studies of their expression in Bacteria and eukaryotes have revealed a variety of different regulatory mechanisms. The results reported here provide the first detailed description of an archaeal trp gene regulatory system. We have established that the trpEGCFBAD operon in Methanothermobacter thermautotrophicus is transcribed divergently from a gene (designated trpY) that encodes a tryptophan-sensitive transcription regulator. TrpY binds to TRP box sequences (consensus, TGTACA) located in the overlapping promoter regions between trpY and trpE, inhibiting trpY transcription in the absence of tryptophan and both trpY and trpEGCFBAD transcription in the presence of tryptophan. TrpY apparently inhibits trpY transcription by blocking RNA polymerase access to the site of trpY transcription initiation and represses trpEGCFBAD transcription by preventing TATA box binding protein (TBP) binding to the TATA box sequence. Given that residue 2 (W2) is the only tryptophan in TrpY and in TrpY homologues in other Euryarchaea and that there is only one tryptophan codon in the entire trpEGCFBAD operon (trpB encodes W175), expression of the trp operon may also be regulated in vivo by the supply of charged tRNA(Trp) available to translate the second codon of the trpY mRNA.

Direct modulation of RNA polymerase core functions by basal transcription factors

Archaeal RNA polymerases (RNAPs) are recruited to promoters through the joint action of three basal transcription factors: TATA-binding protein, TFB (archaeal homolog of TFIIB), and TFE (archaeal homolog of TFIIE). Our results demonstrate several new insights into the mechanisms of TFB and TFE during the transcription cycle. (i) The N-terminal Zn ribbon of TFB displays a surprising degree of redundancy for the recruitment of RNAP during transcription initiation in the archaeal system. (ii) The B-finger domain of TFB participates in transcription initiation events by stimulating abortive and productive transcription in a recruitment-independent function. TFB thus combines physical recruitment of the RNAP with an active role in influencing the catalytic properties of RNAP during transcription initiation. (iii) TFB mutations are complemented by TFE, thereby demonstrating that both factors act synergistically during transcription initiation. (iv) An additional function of TFE is to dynamically alter the nucleic acid-binding properties of RNAP by stabilizing the initiation complex and destabilizing elongation complexes.

Archaeal transcription and its regulators

The relatively complex archaeal RNA polymerases are constructed along eukaryotic lines, and require two initiation factors for promoter recognition and specific transcription that are homologues of the RNA polymerase II TATA-binding protein and TFIIB. Many archaea also produce histones. In contrast, the transcriptional regulators encoded by archaeal genomes are primarily of bacterial rather than eukaryotic type. It is this combination of elements commonly regarded as separate and mutually exclusive that promises unifying insights into basic transcription mechanisms across all three domains of life.

High-resolution protein-DNA contacts for the yeast RNA polymerase II general transcription machinery

We used site-specific protein-DNA photo-cross-linking to define contact points between Saccharomyces cerevisiae RNA polymerase II (RNAP II) and the general transcription factors TBP, TFIIB, and TFIIF on promoter DNA. We present three key findings: (i) the overall pattern of cross-link sites is remarkably similar between the yeast and the previously described human system, even though transcription initiates downstream of the DNA-TBP-TFIIB-RNAP II-TFIIF complex in the S. cerevisiae system; (ii) the yeast Rpb7 subunit of RNAP II forms strong and reproducible cross-links to both strands of promoter DNA; and (iii) a TFIIB arginine-78 to cysteine replacement (R78C), which shifts start site selection downstream of normal, does not affect TFIIB-DNA cross-links prior to promoter melting but instead affects downstream TFIIF-DNA interactions. These results support the premise that the overall structure of the RNAP II preinitiation complex is similar in all eukaryotes and imply that yeast RNAP II is able to scan template DNA downstream of the preinitiation complex for acceptable start sites. The novel Rpb7-DNA contact sites imply that either promoter DNA does not follow a straight path from TATA to the initiation site or the topology of Rpb7 within the DNA-TBP-TFIIB-RNAP II-TFIIF complex is different from that defined in the 12-subunit RNAP II X-ray structure. We discuss the implications of these results for the mechanism of preinitiation complex assembly and promoter melting.

RNA polymerase II structure: from core to functional complexes

New structural studies of RNA polymerase II (Pol II) complexes mark the beginning of a detailed mechanistic analysis of the eukaryotic mRNA transcription cycle. Crystallographic models of the complete Pol II, together with new biochemical and electron microscopic data, give insights into transcription initiation. The first X-ray analysis of a Pol II complex with a transcription factor, the elongation factor TFIIS, supports the idea that the polymerase has a 'tunable' active site that switches between mRNA synthesis and cleavage. The new studies also show that domains of transcription factors can enter polymerase openings, to modulate function during transcription.

Mapping the Location of TFIIB within the RNA Polymerase II Transcription Preinitiation Complex

Biochemical probes positioned on the surface of the general transcription factor TFIIB were used to probe the architecture of the RNA polymerase II (Pol II) transcription preinitiation complex (PIC). In PICs, the TFIIB linker and core domains are positioned over the central cleft and wall of Pol II. This positioning is not observed in the smaller Pol II-TFIIB complex. These results lead to a new model for the structure of the PIC, which agrees with most previously documented protein-DNA interactions within Pol II and archaea PICs. Specific interaction of the TFIIB core domain with Pol II positions and orients the promoter DNA over the Pol II central cleft, and TBP-DNA bending leads to bending of the promoter around the surface of Pol II. The TFIIF subunit Tfg1 was found in close proximity to the TFIIB B finger, linker, and core domains, suggesting that these two factors closely cooperate during initiation.

Transcription by Methanothermobacter thermautotrophicus RNA polymerase in vitro releases archaeal transcription factor B but not TATA-box binding protein from the template DNA

Transcription initiation in Archaea requires the assembly of a preinitiation complex containing the TATA- box binding protein (TBP), transcription factor B (TFB), and RNA polymerase (RNAP). The results reported establish the fate of Methanothermobacter thermautotrophicus TBP and TFB following transcription initiation by M. thermautotrophicus RNAP in vitro. TFB is released after initiation, during extension of the transcript from 4 to 24 nucleotides, but TBP remains bound to the template DNA. Regulation of archaeal transcription initiation by a repressor competition with TBP for TATA-box region binding must accommodate this observation.

Structure and mechanism of the RNA polymerase II transcription machinery

Advances in structure determination of the bacterial and eukaryotic transcription machinery have led to a marked increase in the understanding of the mechanism of transcription. Models for the specific assembly of the RNA polymerase II transcription machinery at a promoter, conformational changes that occur during initiation of transcription, and the mechanism of initiation are discussed in light of recent developments.