The linker domain of basal transcription factor TFIIB controls distinct recruitment and transcription stimulation functions

Structure and Function of RNA Polymerases and the Transcription Machineries

In all living organisms, the flow of genetic information is a two-step process: first DNA is transcribed into RNA, which is subsequently used as template for protein synthesis during translation. In bacteria, archaea and eukaryotes, transcription is carried out by multi-subunit RNA polymerases (RNAPs) sharing a conserved architecture of the RNAP core. RNAPs catalyse the highly accurate polymerisation of RNA from NTP building blocks, utilising DNA as template, being assisted by transcription factors during the initiation, elongation and termination phase of transcription. The complexity of this highly dynamic process is reflected in the intricate network of protein-protein and protein-nucleic acid interactions in transcription complexes and the substantial conformational changes of the RNAP as it progresses through the transcription cycle.In this chapter, we will first briefly describe the early work that led to the discovery of multisubunit RNAPs. We will then discuss the three-dimensional organisation of RNAPs from the bacterial, archaeal and eukaryotic domains of life, highlighting the conserved nature, but also the domain-specific features of the transcriptional apparatus. Another section will focus on transcription factors and their role in regulating the RNA polymerase throughout the different phases of the transcription cycle. This includes a discussion of the molecular mechanisms and dynamic events that govern transcription initiation, elongation and termination.

A Single-Molecule View of Archaeal Transcription

The discovery of the archaeal domain of life is tightly connected to an in-depth analysis of the prokaryotic RNA world. In addition to Carl Woese's approach to use the sequence of the 16S rRNA gene as phylogenetic marker, the finding of Karl Stetter and Wolfram Zillig that archaeal RNA polymerases (RNAPs) were nothing like the bacterial RNAP but are more complex enzymes that resemble the eukaryotic RNAPII was one of the key findings supporting the idea that archaea constitute the third major branch on the tree of life. This breakthrough in transcriptional research 40years ago paved the way for in-depth studies of the transcription machinery in archaea. However, although the archaeal RNAP and the basal transcription factors that fine-tune the activity of the RNAP during the transcription cycle are long known, we still lack information concerning the architecture and dynamics of archaeal transcription complexes. In this context, single-molecule measurements were instrumental as they provided crucial insights into the process of transcription initiation, the architecture of the initiation complex and the dynamics of mobile elements of the RNAP. In this review, we discuss single-molecule approaches suitable to examine molecular mechanisms of transcription and highlight findings that shaped our understanding of the archaeal transcription apparatus. We furthermore explore the possibilities and challenges of next-generation single-molecule techniques, for example, super-resolution microscopy and single-molecule tracking, and ask whether these approaches will ultimately allow us to investigate archaeal transcription in vivo.

The cutting edge of archaeal transcription

The archaeal RNA polymerase (RNAP) is a double-psi β-barrel enzyme 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. Understanding archaeal transcription is, therefore, key to delineate the universally conserved fundamental mechanisms of transcription as well as the evolution of the archaeo-eukaryotic transcription machineries. The dynamic interplay between RNAP subunits, transcription factors and nucleic acids dictates the activity of RNAP and ultimately gene expression. This review focusses on recent progress in our understanding of (i) the structure, function and molecular mechanisms of known and less characterized factors including Elf1 (Elongation factor 1), NusA (N-utilization substance A), TFS4, RIP and Eta, and (ii) their evolution and phylogenetic distribution across the expanding tree of Archaea.

Displacement of the transcription factor B reader domain during transcription initiation

Transcription initiation by archaeal RNA polymerase (RNAP) and eukaryotic RNAP II requires the general transcription factor (TF) B/ IIB. Structural analyses of eukaryotic transcription initiation complexes locate the B-reader domain of TFIIB in close proximity to the active site of RNAP II. Here, we present the first crosslinking mapping data that describe the dynamic transitions of an archaeal TFB to provide evidence for structural rearrangements within the transcription complex during transition from initiation to early elongation phase of transcription. Using a highly specific UV-inducible crosslinking system based on the unnatural amino acid para-benzoyl-phenylalanine allowed us to analyze contacts of the Pyrococcus furiosus TFB B-reader domain with site-specific radiolabeled DNA templates in preinitiation and initially transcribing complexes. Crosslink reactions at different initiation steps demonstrate interactions of TFB with DNA at registers +6 to +14, and reduced contacts at +15, with structural transitions of the B-reader domain detected at register +10. Our data suggest that the B-reader domain of TFB interacts with nascent RNA at register +6 and +8 and it is displaced from the transcribed-strand during the transition from +9 to +10, followed by the collapse of the transcription bubble and release of TFB from register +15 onwards.

Molecular Mechanisms of Transcription Initiation-Structure, Function, and Evolution of TFE/TFIIE-Like Factors and Open Complex Formation

Transcription initiation requires that the promoter DNA is melted and the template strand is loaded into the active site of the RNA polymerase (RNAP), forming the open complex (OC). The archaeal initiation factor TFE and its eukaryotic counterpart TFIIE facilitate this process. Recent structural and biophysical studies have revealed the position of TFE/TFIIE within the pre-initiation complex (PIC) and illuminated its role in OC formation. TFE operates via allosteric and direct mechanisms. Firstly, it interacts with the RNAP and induces the opening of the flexible RNAP clamp domain, concomitant with DNA melting and template loading. Secondly, TFE binds physically to single-stranded DNA in the transcription bubble of the OC and increases its stability. The identification of the β-subunit of archaeal TFE enabled us to reconstruct the evolutionary history of TFE/TFIIE-like factors, which is characterised by winged helix (WH) domain expansion in eukaryotes and loss of metal centres including iron-sulfur clusters and Zinc ribbons. OC formation is an important target for the regulation of transcription in all domains of life. We propose that TFE and the bacterial general transcription factor CarD, although structurally and evolutionary unrelated, show interesting parallels in their mechanism to enhance OC formation. We argue that OC formation is used as a way to regulate transcription in all domains of life, and these regulatory mechanisms coevolved with the basal transcription machinery.

Robotic high-throughput purification of affinity-tagged recombinant proteins

Affinity purification of recombinant proteins has become the method of choice to obtain good quantities and qualities of proteins for a variety of downstream biochemical applications. While manual or FPLC-assisted purification techniques are generally time-consuming and labor-intensive, the advent of high-throughput technologies and liquid handling robotics has simplified and accelerated this process significantly. Additionally, without the human factor as a potential source of error, automated purification protocols allow for the generation of large numbers of proteins simultaneously and under directly comparable conditions. The delivered material is ideal for activity comparisons of different variants of the same protein. Here, we present our strategy for the simultaneous purification of up to 24 affinity-tagged proteins for activity measurements in biochemical assays. The protocol described is suitable for the scale typically required in individual research laboratories.

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.

Emergence and expansion of TFIIB-like factors in the plant kingdom

Many gene families in higher plants have expanded in number, giving rise to diverse protein paralogs with specialized biochemical functions. For instance, plant general transcription factors such as TFIIB have expanded in number and in some cases perform specialized transcriptional functions in the plant cell. To date, no comprehensive genome-wide identification of the TFIIB gene family has been conducted in the plant kingdom. To determine the extent of TFIIB expansion in plants, I used the remote homology program HHPred to search for TFIIB homologs in the plant kingdom and performed a comprehensive analysis of eukaryotic TFIIB gene families. I discovered that higher plants encode more than 10 different TFIIB-like proteins. In particular, Arabidopsis thaliana encodes 14 different TFIIB-like proteins and predicted domain architectures of the newly identified TFIIB-like proteins revealed that they have unique modular domain structures that are divergent in sequence and size. Phylogenetic analysis of selected eukaryotic organisms showed that most life forms encode three major TFIIB subfamilies that include TFIIB, Brf, Rrn7/TAF1B/MEE12 subfamilies, while all plants and some algae species encode one or two additional TFIIB-related protein subfamilies. A subset of A. thaliana GTFs have also expanded in number, indicating that GTF diversification and expansion is a general phenomenon in higher plants. Together, these findings were used to generate a model for the evolutionary history of TFIIB-like proteins in eukaryotes.

Promoter independent abortive transcription assays unravel functional interactions between TFIIB and RNA polymerase

TFIIB-like general transcription factors are required for transcription initiation by all eukaryotic and archaeal RNA polymerases (RNAPs). TFIIB facilitates both recruitment and post-recruitment steps of initiation; in particular, TFIIB stimulates abortive initiation. X-ray crystallography of TFIIB-RNAP II complexes shows that the TFIIB linker region penetrates the RNAP active center, yet the impact of this arrangement on RNAP activity and underlying mechanisms remains elusive. Promoter-independent abortive initiation assays exploit the intrinsic ability of RNAP enzymes to initiate transcription from nicked DNA templates and record the formation of the first phosphodiester bonds. These assays can be used to measure the effect of transcription factors such as TFIIB and RNAP mutations on abortive transcription.

RNA polymerase II transcription: structure and mechanism

A minimal RNA polymerase II (pol II) transcription system comprises the polymerase and five general transcription factors (GTFs) TFIIB, -D, -E, -F, and -H. The addition of Mediator enables a response to regulatory factors. The GTFs are required for promoter recognition and the initiation of transcription. Following initiation, pol II alone is capable of RNA transcript elongation and of proofreading. Structural studies reviewed here reveal roles of GTFs in the initiation process and shed light on the transcription elongation mechanism. This article is part of a Special Issue entitled: RNA Polymerase II Transcript Elongation.

High-throughput purification of affinity-tagged recombinant proteins

X-ray crystallography is the method of choice for obtaining a detailed view of the structure of proteins. Such studies need to be complemented by further biochemical analyses to obtain detailed insights into structure/function relationships. Advances in oligonucleotide- and gene synthesis technology make large-scale mutagenesis strategies increasingly feasible, including the substitution of target residues by all 19 other amino acids. Gain- or loss-of-function phenotypes then allow systematic conclusions to be drawn, such as the contribution of particular residues to catalytic activity, protein stability and/or protein-protein interaction specificity. In order to attribute the different phenotypes to the nature of the mutation--rather than to fluctuating experimental conditions--it is vital to purify and analyse the proteins in a controlled and reproducible manner. High-throughput strategies and the automation of manual protocols on robotic liquid-handling platforms have created opportunities to perform such complex molecular biological procedures with little human intervention and minimal error rates. Here, we present a general method for the purification of His-tagged recombinant proteins in a high-throughput manner. In a recent study, we applied this method to a detailed structure-function investigation of TFIIB, a component of the basal transcription machinery. TFIIB is indispensable for promoter-directed transcription in vitro and is essential for the recruitment of RNA polymerase into a preinitiation complex. TFIIB contains a flexible linker domain that penetrates the active site cleft of RNA polymerase. This linker domain confers two biochemically quantifiable activities on TFIIB, namely (i) the stimulation of the catalytic activity during the 'abortive' stage of transcript initiation, and (ii) an additional contribution to the specific recruitment of RNA polymerase into the preinitiation complex. We exploited the high-throughput purification method to generate single, double and triple substitution and deletions mutations within the TFIIB linker and to subsequently analyse them in functional assays for their stimulation effect on the catalytic activity of RNA polymerase. Altogether, we generated, purified and analysed 381 mutants--a task which would have been time-consuming and laborious to perform manually. We produced and assayed the proteins in multiplicates which allowed us to appreciate any experimental variations and gave us a clear idea of the reproducibility of our results. This method serves as a generic protocol for the purification of His-tagged proteins and has been successfully used to purify other recombinant proteins. It is currently optimised for the purification of 24 proteins but can be adapted to purify up to 96 proteins.

Rethinking the role of TFIIF in transcript initiation by RNA polymerase II

TFIIF is considered to be a general transcription factor, based on the fact that it is essential for assembly of RNA polymerase II preinitiation complexes on fully double-stranded templates in vitro. Existing models assign various tasks to TFIIF during preinitiation complex formation and transcript initiation. Recent results do not support all aspects of those models but they do emphasize the significance of the interaction of TFIIF and TFIIB. Other recent findings raise the possibility that a fraction of RNA polymerase II transcription complex assembly proceeds through a pathway that is independent of TFIIF.

The Bridge Helix of RNA polymerase acts as a central nanomechanical switchboard for coordinating catalysis and substrate movement

The availability of in vitro assembly systems to produce recombinant archaeal RNA polymerases (RNAPs) offers one of the most powerful experimental tools for investigating the still relatively poorly understood molecular mechanisms underlying RNAP function. Over the last few years, we pioneered new robot-based high-throughput mutagenesis approaches to study structure/function relationships within various domains surrounding the catalytic center. The Bridge Helix domain, which appears in numerous X-ray structures as a 35-amino-acid-long alpha helix, coordinates the concerted movement of several other domains during catalysis through kinking of two discrete molecular hinges. Mutations affecting these kinking mechanisms have a direct effect on the specific catalytic activity of RNAP and can in some instances more than double it. Molecular dynamics simulations have established themselves as exceptionally useful for providing additional insights and detailed models to explain the underlying structural motions.

Revealing the functions of TFIIB

The TFIIB linker domain stimulates the catalytic activity of archaeal RNAP. By characterising a range of super-stimulating mutants we identified a novel rate-limiting step in transcription initiation. Our results help to interpret structural findings and pave the way towards higher-resolution structures of the RNAP-TFIIB linker interface.

The nucleotide addition cycle of RNA polymerase is controlled by two molecular hinges in the Bridge Helix domain

Background

Cellular RNA polymerases (RNAPs) are complex molecular machines that combine catalysis with concerted conformational changes in the active center. Previous work showed that kinking of a hinge region near the C-terminus of the Bridge Helix (BH-H_C) plays a critical role in controlling the catalytic rate.

Results

Here, new evidence for the existence of an additional hinge region in the amino-terminal portion of the Bridge Helix domain (BH-H_N) is presented. The nanomechanical properties of BH-H_N emerge as a direct consequence of the highly conserved primary amino acid sequence. Mutations that are predicted to influence its flexibility cause corresponding changes in the rate of the nucleotide addition cycle (NAC). BH-H_N displays functional properties that are distinct from BH-H_C, suggesting that conformational changes in the Bridge Helix control the NAC via two independent mechanisms.

Conclusions

The properties of two distinct molecular hinges in the Bridge Helix of RNAP determine the functional contribution of this domain to key stages of the NAC by coordinating conformational changes in surrounding domains.