The positions of TFIIF and TFIIE in the RNA polymerase II transcription preinitiation complex

Architecture of the RNA polymerase II-Mediator core initiation complex

The conserved co-activator complex Mediator enables regulated transcription initiation by RNA polymerase (Pol) II. Here we reconstitute an active 15-subunit core Mediator (cMed) comprising all essential Mediator subunits from Saccharomyces cerevisiae. The cryo-electron microscopic structure of cMed bound to a core initiation complex was determined at 9.7 Å resolution. cMed binds Pol II around the Rpb4-Rpb7 stalk near the carboxy-terminal domain (CTD). The Mediator head module binds the Pol II dock and the TFIIB ribbon and stabilizes the initiation complex. The Mediator middle module extends to the Pol II foot with a 'plank' that may influence polymerase conformation. The Mediator subunit Med14 forms a 'beam' between the head and middle modules and connects to the tail module that is predicted to bind transcription activators located on upstream DNA. The Mediator 'arm' and 'hook' domains contribute to a 'cradle' that may position the CTD and TFIIH kinase to stimulate Pol II phosphorylation.

Complete architecture of the archaeal RNA polymerase open complex from single-molecule FRET and NPS

The molecular architecture of RNAP II-like transcription initiation complexes remains opaque due to its conformational flexibility and size. Here we report the three-dimensional architecture of the complete open complex (OC) composed of the promoter DNA, TATA box-binding protein (TBP), transcription factor B (TFB), transcription factor E (TFE) and the 12-subunit RNA polymerase (RNAP) from Methanocaldococcus jannaschii. By combining single-molecule Förster resonance energy transfer and the Bayesian parameter estimation-based Nano-Positioning System analysis, we model the entire archaeal OC, which elucidates the path of the non-template DNA (ntDNA) strand and interaction sites of the transcription factors with the RNAP. Compared with models of the eukaryotic OC, the TATA DNA region with TBP and TFB is positioned closer to the surface of the RNAP, likely providing the mechanism by which DNA melting can occur in a minimal factor configuration, without the dedicated translocase/helicase encoding factor TFIIH.

Association of the winged helix motif of the TFIIEα subunit of TFIIE with either the TFIIEβ subunit or TFIIB distinguishes its functions in transcription

In eukaryotes, the general transcription factor TFIIE consists of two subunits, α and β, and plays essential roles in transcription. Structure-function studies indicate that TFIIE has three-winged helix (WH) motifs, with one in TFIIEα and two in TFIIEβ. Recent studies suggested that, by binding to the clamp region of RNA polymerase II, TFIIEα-WH promotes the conformational change that transforms the promoter-bound inactive preinitiation complex to the active complex. Here, to elucidate its roles in transcription, functional analyses of point-mutated human TFIIEα-WH proteins were carried out. In vitro transcription analyses identified two classes of mutants. One class was defective in transcription initiation, and the other was defective in the transition from initiation to elongation. Analyses of the binding of this motif to other general transcription factors showed that the former class was defective in binding to the basic helix-loop-helix motif of TFIIEβ and the latter class was defective in binding to the N-terminal cyclin homology region of TFIIB. Furthermore, TFIIEα-WH bound to the TFIIH XPB subunit at a third distinct region. Therefore, these results provide further insights into the mechanisms underlying RNA polymerase II activation at the initial stages of transcription.

Closing the cohesin ring: structure and function of its Smc3-kleisin interface

Through their association with a kleisin subunit (Scc1), cohesin's Smc1 and Smc3 subunits are thought to form tripartite rings that mediate sister chromatid cohesion. Unlike the structure of Smc1/Smc3 and Smc1/Scc1 interfaces, that of Smc3/Scc1 is not known. Disconnection of this interface is thought to release cohesin from chromosomes in a process regulated by acetylation. We show here that the N-terminal domain of yeast Scc1 contains two α helices, forming a four-helix bundle with the coiled coil emerging from Smc3's adenosine triphosphatase head. Mutations affecting this interaction compromise cohesin's association with chromosomes. The interface is far from Smc3 residues, whose acetylation prevents cohesin's dissociation from chromosomes. Cohesin complexes holding chromatids together in vivo do indeed have the configuration of hetero-trimeric rings, and sister DNAs are entrapped within these.

Relationships of RNA polymerase II genetic interactors to transcription start site usage defects and growth in Saccharomyces cerevisiae

Transcription initiation by RNA Polymerase II (Pol II) is an essential step in gene expression and regulation in all organisms. Initiation requires a great number of factors, and defects in this process can be apparent in the form of altered transcription start site (TSS) selection in Saccharomyces cerevisiae (Baker's yeast). It has been shown previously that TSS selection in S. cerevisiae is altered in Pol II catalytic mutants defective in a conserved active site feature known as the trigger loop. Pol II trigger loop mutants show growth phenotypes in vivo that correlate with biochemical defects in vitro and exhibit wide-ranging genetic interactions. We assessed how Pol II mutant growth phenotypes and TSS selection in vivo are modified by Pol II genetic interactors to estimate the relationship between altered TSS selection in vivo and organismal fitness of Pol II mutants. We examined whether the magnitude of TSS selection defects could be correlated with Pol II mutant-transcription factor double mutant phenotypes. We observed broad genetic interactions among Pol II trigger loop mutants and General Transcription Factor (GTF) alleles, with reduced-activity Pol II mutants especially sensitive to defects in TFIIB. However, Pol II mutant growth defects could be uncoupled from TSS selection defects in some Pol II allele-GTF allele double mutants, whereas a number of other Pol II genetic interactors did not influence ADH1 start site selection alone or in combination with Pol II mutants. Initiation defects are likely only partially responsible for Pol II allele growth phenotypes, with some Pol II genetic interactors able to exacerbate Pol II mutant growth defects while leaving initiation at a model TSS selection promoter unaffected.

RNA polymerase II conserved protein domains as platforms for protein-protein interactions

RNA polymerase II establishes many protein-protein interactions with transcriptional regulators to coordinate gene expression, but little is known about protein domains involved in the contact with them. We use a new approach to look for conserved regions of the RNA pol II of S. cerevisiae located at the surface of the structure of the complex, hypothesizing that they might be involved in the interaction with transcriptional regulators. We defined five different conserved domains and demonstrate that all of them make contact with transcriptional regulators.

The X-ray crystal structure of the euryarchaeal RNA polymerase in an open-clamp configuration

The archaeal transcription apparatus is closely related to the eukaryotic RNA polymerase II (Pol II) system. Archaeal RNA polymerase (RNAP) and Pol II evolved from a common ancestral structure and the euryarchaeal RNAP is the simplest member of the extant archaeal/eukaryotic RNAP family. Here we report the first crystal structure of euryarchaeal RNAP from Thermococcus kodakarensis (Tko). This structure reveals that the clamp domain is able to swing away from the main body of RNAP in the presence of the Rpo4/Rpo7 stalk by coordinated movements of these domains. More detailed structure-function analysis of yeast Pol II and Tko RNAP identifies structural additions to Pol II that correspond to the binding sites of Pol II-specific general transcription factors including TFIIF, TFIIH and Mediator. Such comparisons provide a framework for dissecting interactions between RNAP and these factors during formation of the pre-initiation complex.

Conserved architecture of the core RNA polymerase II initiation complex

During transcription initiation at promoters of protein-coding genes, RNA polymerase (Pol) II assembles with TBP, TFIIB and TFIIF into a conserved core initiation complex that recruits additional factors. The core complex stabilizes open DNA and initiates RNA synthesis, and it is conserved in the Pol I and Pol III transcription systems. Here, we derive the domain architecture of the yeast core pol II initiation complex during transcription initiation. The yeast complex resembles the human initiation complex and reveals that the TFIIF Tfg2 winged helix domain swings over promoter DNA. An 'arm' and a 'charged helix' in TFIIF function in transcription start site selection and initial RNA synthesis, respectively, and apparently extend into the active centre cleft. Our model provides the basis for further structure-function analysis of the entire transcription initiation complex.

Genetic interaction analysis of point mutations enables interrogation of gene function at a residue-level resolution: exploring the applications of high-resolution genetic interaction mapping of point mutations

We have achieved a residue-level resolution of genetic interaction mapping - a technique that measures how the function of one gene is affected by the alteration of a second gene - by analyzing point mutations. Here, we describe how to interpret point mutant genetic interactions, and outline key applications for the approach, including interrogation of protein interaction interfaces and active sites, and examination of post-translational modifications. Genetic interaction analysis has proven effective for characterizing cellular processes; however, to date, systematic high-throughput genetic interaction screens have relied on gene deletions or knockdowns, which limits the resolution of gene function analysis and poses problems for multifunctional genes. Our point mutant approach addresses these issues, and further provides a tool for in vivo structure-function analysis that complements traditional biophysical methods. We also discuss the potential for genetic interaction mapping of point mutations in human cells and its application to personalized medicine.

Insights into how Spt5 functions in transcription elongation and repressing transcription coupled DNA repair

Spt5, a transcription elongation factor, and Rpb4, a subunit of RNA polymerase II (RNAP II) that forms a subcomplex with Rpb7, play important roles in transcription elongation and repression of transcription coupled DNA repair (TCR) in eukaryotic cells. How Spt5 physically interacts with RNAP II, and if and/or how Spt5 and Rpb4/7 coordinate to achieve the distinctive functions have been enigmatic. By site-specific incorporation of the unnatural amino acid p-benzoyl-L-phenylalanine, a photoreactive cross-linker, we mapped interactions between Spt5 and RNAP II in Saccharomyces cerevisiae. Through its KOW4-5 domains, Spt5 extensively interacts with Rpb4/7. Spt5 also interacts with Rpb1 and Rpb2, two largest subunits of RNAP II, at the clamp, protrusion and wall domains. These interactions may lock the clamp to the closed conformation and enclose the DNA being transcribed in the central cleft of RNAP II. Deletion of Spt5 KOW4-5 domains decreases transcription elongation and derepresses TCR. Our findings suggest that Spt5 is a key coordinator for holding the RNAP II complex in a closed conformation that is highly competent for transcription elongation but repressive to TCR.

An in vivo photo-cross-linking approach reveals a homodimerization domain of Aha1 in S. cerevisiae

Protein-protein interactions play an essential role in almost any biological processes. Therefore, there is a particular need for methods which describe the interactions of a defined target protein in its physiological context. Here we report a method to photo-cross-link interacting proteins in S. cerevisiae by using the non-canonical amino acid p-azido-L-phenylalanine (pAzpa). Based on the expanded genetic code the photoreactive non-canonical amino acid pAzpa was site-specifically incorporated at eight positions into a domain of Aha1 that was previously described to bind Hsp90 in vitro to function as a cochaperone of Hsp90 and activates its ATPase activity. In vivo photo-cross-linking to the cognate binding partner of Aha1 was carried out by irradiation of mutant strains with UV light (365 nm) to induce covalent intermolecular bonds. Surprisingly, an interaction between Aha1 and Hsp90 was not detected, although, we could confirm binding of suppressed pAzpa containing Aha1 to Hsp90 by native co-immunoprecipitation. However, a homodimer consisting of two covalently crosslinked Aha1 monomers was identified by mass spectrometry. This homodimer could also be confirmed using p-benzoyl-L-phenylalanine, another photoreactive non-canonical amino acid. Crosslinking was highly specific as it was dependent on irradiation using UV light, the exact position of the non-canonical amino acid in the protein sequence as well as on the addition of the non-canonical amino acid to the growth medium. Therefore it seems possible that an interaction of Aha1 with Hsp90 takes place at different positions than previously described in vitro highlighting the importance of in vivo techniques to study protein-protein interactions. Accordingly, the expanded genetic code can easily be applied to other S. cerevisiae proteins to study their interaction under physiological relevant conditions in vivo.

Genetic code expansion as a tool to study regulatory processes of transcription

The expansion of the genetic code with non-canonical amino acids (ncAA) enables the chemical and biophysical properties of proteins to be tailored, inside cells, with a previously unattainable level of precision. A wide range of ncAA with functions not found in canonical amino acids have been genetically encoded in recent years and have delivered insights into biological processes that would be difficult to access with traditional approaches of molecular biology. A major field for the development and application of novel ncAA-functions has been transcription and its regulation. This is particularly attractive, since advanced DNA sequencing- and proteomics-techniques continue to deliver vast information on these processes on a global level, but complementing methodologies to study them on a detailed, molecular level and in living cells have been comparably scarce. In a growing number of studies, genetic code expansion has now been applied to precisely control the chemical properties of transcription factors, RNA polymerases and histones, and this has enabled new insights into their interactions, conformational changes, cellular localizations and the functional roles of posttranslational modifications.

Expanding and reprogramming the genetic code of cells and animals

Genetic code expansion and reprogramming enable the site-specific incorporation of diverse designer amino acids into proteins produced in cells and animals. Recent advances are enhancing the efficiency of unnatural amino acid incorporation by creating and evolving orthogonal ribosomes and manipulating the genome. Increasing the number of distinct amino acids that can be site-specifically encoded has been facilitated by the evolution of orthogonal quadruplet decoding ribosomes and the discovery of mutually orthogonal synthetase/tRNA pairs. Rapid progress in moving genetic code expansion from bacteria to eukaryotic cells and animals (C. elegans and D. melanogaster) and the incorporation of useful unnatural amino acids has been aided by the development and application of the pyrrolysyl-transfer RNA (tRNA) synthetase/tRNA pair for unnatural amino acid incorporation. Combining chemoselective reactions with encoded amino acids has facilitated the installation of posttranslational modifications, as well as rapid derivatization with diverse fluorophores for imaging.

Mapping the protein interaction network for TFIIB-related factor Brf1 in the RNA polymerase III preinitiation complex

TFIIB-related factor Brf1 is essential for RNA polymerase (Pol) III recruitment and open-promoter formation in transcription initiation. We site specifically incorporated a nonnatural amino acid cross-linker into Brf1 to map its protein interaction targets in the preinitiation complex (PIC). Our cross-linking analysis in the N-terminal domain of Brf1 indicated a pattern of multiple protein interactions reminiscent of TFIIB in the Pol active-site cleft. In addition to the TFIIB-like protein interactions, the Brf1 cyclin repeat subdomain is in contact with the Pol III-specific C34 subunit. With site-directed hydroxyl radical probing, we further revealed the binding between Brf1 cyclin repeats and the highly conserved region connecting C34 winged-helix domains 2 and 3. In contrast to the N-terminal domain of Brf1, the C-terminal domain contains extensive binding sites for TBP and Bdp1 to hold together the TFIIIB complex on the promoter. Overall, the domain architecture of the PIC derived from our cross-linking data explains how individual structural subdomains of Brf1 integrate the protein network from the Pol III active center to the promoter for transcription initiation.

Structural insights into transcription initiation by RNA polymerase II

Transcriptional regulation is one of the most important steps in control of cell identity, growth, differentiation, and development. Many signaling pathways controlling these processes ultimately target the core transcription machinery that, for protein coding genes, consists of RNA polymerase II (Pol II) and the general transcription factors (GTFs). New studies on the structure and mechanism of the core assembly and how it interfaces with promoter DNA and coactivator complexes have given tremendous insight into early steps in the initiation process, genome-wide binding, and mechanisms conserved for all nuclear and archaeal Pols. Here, we review recent developments in dissecting the architecture of the Pol II core machinery with a focus on early and regulated steps in transcription initiation.

From structure to systems: high-resolution, quantitative genetic analysis of RNA polymerase II

RNA polymerase II (RNAPII) lies at the core of dynamic control of gene expression. Using 53 RNAPII point mutants, we generated a point mutant epistatic miniarray profile (pE-MAP) comprising ∼60,000 quantitative genetic interactions in Saccharomyces cerevisiae. This analysis enabled functional assignment of RNAPII subdomains and uncovered connections between individual regions and other protein complexes. Using splicing microarrays and mutants that alter elongation rates in vitro, we found an inverse relationship between RNAPII speed and in vivo splicing efficiency. Furthermore, the pE-MAP classified fast and slow mutants that favor upstream and downstream start site selection, respectively. The striking coordination of polymerization rate with transcription initiation and splicing suggests that transcription rate is tuned to regulate multiple gene expression steps. The pE-MAP approach provides a powerful strategy to understand other multifunctional machines at amino acid resolution.

Pds5 promotes and protects cohesin acetylation

Cohesin's Smc1 and Smc3 subunits form V-shaped heterodimers, the nucleotide binding domains (NBDs) of which bind the C- and N-terminal domains, respectively, of the α-kleisin subunit, forming a large tripartite ring within in which sister DNAs are entrapped, and thereby held together (sister chromatid cohesion). During replication, establishment of stable cohesion is dependent on Eco1-mediated acetylation of Smc3's NBD, which is thought to prevent dissociation of α-kleisin from Smc3, thereby locking shut a "DNA exit gate." How Scc3 and Pds5, regulatory subunits bound to α-kleisin, regulate cohesion establishment and maintenance is poorly understood. We show here that by binding to α-kleisin adjacent to its Smc3 nucleotide binding N-terminal domain, Pds5 not only promotes cohesin's release from chromatin but also mediates de novo acetylation of Smc3 by Eco1 during S phase and subsequently prevents de-acetylation by the deacetylase Hos1/HDAC8. By first promoting cohesin's release from chromosomes and subsequently creating and guarding the chemical modification responsible for blocking release, Pds5 enables chromosomal cohesin to switch during S phase from a state of high turnover to one capable of tenaciously holding sister chromatids together for extended periods of time, a duality that has hitherto complicated analysis of this versatile cohesin subunit.

RNA polymerase III-specific general transcription factor IIIC contains a heterodimer resembling TFIIF Rap30/Rap74

Transcription of tRNA-encoding genes by RNA polymerase (Pol) III requires the six-subunit general transcription factor IIIC that uses subcomplexes τA and τB to recognize two gene-internal promoter elements named A- and B-box. The Schizosaccharomyces pombe τA subcomplex comprises subunits Sfc1, Sfc4 and Sfc7. The crystal structure of the Sfc1/Sfc7 heterodimer reveals similar domains and overall domain architecture to the Pol II-specific general transcription factor TFIIF Rap30/Rap74. The N-terminal Sfc1/Sfc7 dimerization module consists of a triple β-barrel similar to the N-terminal TFIIF Rap30/Rap74 dimerization module, whereas the C-terminal Sfc1 DNA-binding domain contains a winged-helix domain most similar to the TFIIF Rap30 C-terminal winged-helix domain. Sfc1 DNA-binding domain recognizes single and double-stranded DNA by an unknown mechanism. Several features observed for A-box recognition by τA resemble the recognition of promoters by bacterial RNA polymerase, where σ factor unfolds double-stranded DNA and stabilizes the non-coding DNA strand in an open conformation. Such a function has also been proposed for TFIIF, suggesting that the observed structural similarity between Sfc1/Sfc7 and TFIIF Rap30/Rap74 might also reflect similar functions.

Hybrid electron microscopy-FRET imaging localizes the dynamical C-terminus of Tfg2 in RNA polymerase II-TFIIF with nanometer precision

TFIIF-a general transcription factor comprising two conserved subunits can associate with RNA polymerase II (RNAPII) tightly to regulate the synthesis of messenger RNA in eukaryotes. Herein, a hybrid method that combines electron microscopy (EM) and Förster resonance energy transfer (FRET) is described and used to localize the C-terminus of the second TFIIF subunit (Tfg2) in the architecture of RNAPII-TFIIF. In the first stage, a poly-histidine tag appended to the Tfg2 C-terminus was labeled with nickel-NTA nanogold and a seven-step single particle EM protocol was devised to obtain the region accessible by the nanogold in 3D, suggesting the Tfg2 C-terminus is proximal to the clamp of RNAPII. Next, the C-termini of the Rpb2 and the Rpb4 subunits of RNAPII, adjacent to the clamp, were selected for placing FRET satellites to enable the nano-positioning (NP) analysis, by which the localization precision was improved such that the Tfg2 C-terminus was found to dwell on the clamp ridge but could move to the clamp top during transcription. Because the tag receptive to the EM or FRET probes can be readily introduced to any protein subunit, this hybrid approach is generally applicable to complement cryo-EM study of many protein complexes to nanometer precision.

Structural visualization of key steps in human transcription initiation

Eukaryotic transcription initiation requires the assembly of general transcription factors into a pre-initiation complex that ensures the accurate loading of RNA polymerase II (Pol II) at the transcription start site. The molecular mechanism and function of this assembly have remained elusive due to lack of structural information. Here we have used an in vitro reconstituted system to study the stepwise assembly of human TBP, TFIIA, TFIIB, Pol II, TFIIF, TFIIE and TFIIH onto promoter DNA using cryo-electron microscopy. Our structural analyses provide pseudo-atomic models at various stages of transcription initiation that illuminate critical molecular interactions, including how TFIIF engages Pol II and promoter DNA to stabilize both the closed pre-initiation complex and the open-promoter complex, and to regulate start--initiation complexes, combined with the localization of the TFIIH helicases XPD and XPB, support a DNA translocation model of XPB and explain its essential role in promoter opening.

The non-coding B2 RNA binds to the DNA cleft and active-site region of RNA polymerase II

The B2 family of short interspersed elements is transcribed into non-coding RNA by RNA polymerase III. The ~180-nt B2 RNA has been shown to potently repress mRNA transcription by binding tightly to RNA polymerase II (Pol II) and assembling with it into complexes on promoter DNA, where it keeps the polymerase from properly engaging the promoter DNA. Mammalian Pol II is an ~500-kDa complex that contains 12 different protein subunits, providing many possible surfaces for interaction with B2 RNA. We found that the carboxy-terminal domain of the largest Pol II subunit was not required for B2 RNA to bind Pol II and repress transcription in vitro. To identify the surface on Pol II to which the minimal functional region of B2 RNA binds, we coupled multi-step affinity purification, reversible formaldehyde cross-linking, peptide sequencing by mass spectrometry, and analysis of peptide enrichment. The Pol II peptides most highly recovered after cross-linking to B2 RNA mapped to the DNA binding cleft and active-site region of Pol II. These studies determine the location of a defined nucleic acid binding site on a large, native, multi-subunit complex and provide insight into the mechanism of transcriptional repression by B2 RNA.

RNA polymerase II mutations conferring defects in poly(A) site cleavage and termination in Saccharomyces cerevisiae

Transcription termination by RNA polymerase (Pol) II is an essential but poorly understood process. In eukaryotic nuclei, the 3′ ends of mRNAs are generated by cleavage and polyadenylation, and the same sequence elements that specify that process are required for downstream release of the polymerase from the DNA. Although Pol II is known to bind proteins required for both events, few studies have focused on Pol II mutations as a means to uncover the mechanisms that couple polyadenylation and termination. We performed a genetic screen in the yeast Saccharomyces cerevisiae to isolate mutations in the N-terminal half of Rpb2, the second largest Pol II subunit, that conferred either a decreased or increased response to a well-characterized poly(A) site. Most of the mutant alleles encoded substitutions affecting either surface residues or conserved active site amino acids at positions important for termination by other RNA polymerases. Reverse transcription polymerase chain reaction experiments revealed that transcript cleavage at the poly(A) site was impaired in both classes of increased readthrough mutants. Transcription into downstream sequences beyond where termination normally occurs was also probed. Although most of the tested readthrough mutants showed a reduction in termination concomitant with the reduced poly(A) usage, these processes were uncoupled in at least one mutant strain. Several rpb2 alleles were found to be similar or identical to published mutants associated with defective TFIIF function. Tests of these and additional mutations known to impair Rpb2−TFIIF interactions revealed similar decreased readthrough phenotypes, suggesting that TFIIF may have a role in 3′ end formation and termination.

The C terminus of the histone chaperone Asf1 cross-links to histone H3 in yeast and promotes interaction with histones H3 and H4

The central histone H3/H4 chaperone Asf1 comprises a highly conserved globular core and a divergent C-terminal tail. While the function and structure of the Asf1 core are well known, the function of the tail is less well understood. Here, we have explored the role of the yeast (yAsf1) and human (hAsf1a and hAsf1b) Asf1 tails in Saccharomyces cerevisiae. We show, using a photoreactive, unnatural amino acid, that Asf1 tail residue 210 cross-links to histone H3 in vivo and, further, that loss of C-terminal tail residues 211 to 279 weakens yAsf1-histone binding affinity in vitro nearly 200-fold. Via several yAsf1 C-terminal truncations and yeast-human chimeric proteins, we found that truncations at residue 210 increase transcriptional silencing and that the hAsf1a tail partially substitutes for full-length yAsf1 with respect to silencing but that full-length hAsf1b is a better overall substitute for full-length yAsf1. In addition, we show that the C-terminal tail of Asf1 is phosphorylated at T270 in yeast. Loss of this phosphorylation site does not prevent coimmunoprecipitation of yAsf1 and Rad53 from yeast extracts, whereas amino acid residue substitutions at the Asf1-histone H3/H4 interface do. Finally, we show that residue substitutions in yAsf1 near the CAF-1/HIRA interface also influence yAsf1's function in silencing.

Photocrosslinking approaches to interactome mapping

Photocrosslinking approaches can be used to map interactome networks within the context of living cells. Photocrosslinking methods rely on use of metabolic engineering or genetic code expansion to incorporate photocrosslinking analogs of amino acids or sugars into cellular biomolecules. Immunological and mass spectrometry techniques are used to analyze crosslinked complexes, thereby defining specific interactomes. Because photocrosslinking can be conducted in native, cellular settings, it can be used to define context-dependent interactions. Photocrosslinking methods are also ideally suited for determining interactome dynamics, mapping interaction interfaces, and identifying transient interactions in which intrinsically disordered proteins and glycoproteins engage. Here we discuss the application of cell-based photocrosslinking to the study of specific problems in immune cell signaling, transcription, membrane protein dynamics, nucleocytoplasmic transport, and chaperone-assisted protein folding.

RNA polymerase III subunit architecture and implications for open promoter complex formation

Transcription initiation by eukaryotic RNA polymerase (Pol) III relies on the TFIIE-related subcomplex C82/34/31. Here we combine cross-linking and hydroxyl radical probing to position the C82/34/31 subcomplex around the Pol III active center cleft. The extended winged helix (WH) domains 1 and 4 of C82 localize to the polymerase domains clamp head and clamp core, respectively, and the two WH domains of C34 span the polymerase cleft from the coiled-coil region of the clamp to the protrusion. The WH domains of C82 and C34 apparently cooperate with other mobile regions flanking the cleft during promoter DNA binding, opening, and loading. Together with published data, our results complete the subunit architecture of Pol III and indicate that all TFIIE-related components of eukaryotic and archaeal transcription systems adopt an evolutionarily conserved location in the upper part of the cleft that supports their functions in open promoter complex formation and stabilization.

Yeast RNA polymerase III transcription factors and effectors

Recent data indicate that the well-defined transcription machinery of RNA polymerase III (Pol III) is probably more complex than commonly thought. In this review, we describe the yeast basal transcription factors of Pol III and their involvements in the transcription cycle. We also present a list of proteins detected on genes transcribed by Pol III (class III genes) that might participate in the transcription process. Surprisingly, several of these proteins are involved in RNA polymerase II transcription. Defining the role of these potential new effectors in Pol III transcription in vivo will be the challenge of the next few years. This article is part of a Special Issue entitled: Transcription by Odd Pols.

Basic mechanisms of RNA polymerase II activity and alteration of gene expression in Saccharomyces cerevisiae

Transcription by RNA polymerase II (Pol II), and all RNA polymerases for that matter, may be understood as comprising two cycles. The first cycle relates to the basic mechanism of the transcription process wherein Pol II must select the appropriate nucleoside triphosphate (NTP) substrate complementary to the DNA template, catalyze phosphodiester bond formation, and translocate to the next position on the DNA template. Performing this cycle in an iterative fashion allows the synthesis of RNA chains that can be over one million nucleotides in length in some larger eukaryotes. Overlaid upon this enzymatic cycle, transcription may be divided into another cycle of three phases: initiation, elongation, and termination. Each of these phases has a large number of associated transcription factors that function to promote or regulate the gene expression process. Complicating matters, each phase of the latter transcription cycle are coincident with cotranscriptional RNA processing events. Additionally, transcription takes place within a highly dynamic and regulated chromatin environment. This chromatin environment is radically impacted by active transcription and associated chromatin modifications and remodeling, while also functioning as a major platform for Pol II regulation. This review will focus on our basic knowledge of the Pol II transcription mechanism, and how altered Pol II activity impacts gene expression in vivo in the model eukaryote Saccharomyces cerevisiae. This article is part of a Special Issue entitled: RNA Polymerase II Transcript Elongation.

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.

Sub1 associates with Spt5 and influences RNA polymerase II transcription elongation rate

The transcriptional coactivator Sub1 has been implicated in several steps of mRNA metabolism in yeast, such as the activation of transcription, termination, and 3'-end formation. In addition, Sub1 globally regulates RNA polymerase II phosphorylation, and most recently it has been shown that it is a functional component of the preinitiation complex. Here we present evidence that Sub1 plays a significant role in transcription elongation by RNA polymerase II (RNAPII). We show that SUB1 genetically interacts with the gene encoding the elongation factor Spt5, that Sub1 influences Spt5 phosphorylation of the carboxy-terminal domain of RNAPII largest subunit by the kinase Bur1, and that both Sub1 and Spt5 copurify in the same complex, likely during early transcription elongation. Indeed, our data indicate that Sub1 influences Spt5-Rpb1 interaction. In addition, biochemical and molecular data show that Sub1 influences transcription elongation of constitutive and inducible genes and associates with coding regions in a transcription-dependent manner. Taken together, our results indicate that Sub1 associates with Spt5 and influences Spt5-Rpb1 complex levels and consequently transcription elongation rate.

Interaction of the mediator head module with RNA polymerase II

Mediator, a large (21 polypeptides, MW ∼1 MDa) complex conserved throughout eukaryotes, plays an essential role in control of gene expression by conveying regulatory signals that influence the activity of the preinitiation complex. However, the precise mode of interaction between Mediator and RNA polymerase II (RNAPII), and the mechanism of regulation by Mediator remain elusive. We used cryo-electron microscopy and reconstituted in vitro transcription assays to characterize a transcriptionally-active complex including the Mediator Head module and components of a minimum preinitiation complex (RNAPII, TFIIF, TFIIB, TBP, and promoter DNA). Our results reveal how the Head interacts with RNAPII, affecting its conformation and function.

Promoter clearance by RNA polymerase II

Many changes must occur to the RNA polymerase II (pol II) transcription complex as it makes the transition from initiation into transcript elongation. During this intermediate phase of transcription, contact with initiation factors is lost and stable association with the nascent transcript is established. These changes collectively comprise promoter clearance. Once the transcript elongation complex has reached a point where its properties are indistinguishable from those of complexes with much longer transcripts, promoter clearance is complete. The clearance process for pol II consists of a number of steps and it extends for a surprisingly long distance downstream of transcription start. This article is part of a Special Issue entitled: RNA polymerase II Transcript Elongation.

The Spt4-Spt5 complex: a multi-faceted regulator of transcription elongation

In all domains of life, elongating RNA polymerases require the assistance of accessory factors to maintain their processivity and regulate their rate. Among these elongation factors, the Spt5/NusG factors stand out. Members of this protein family appear to be the only transcription accessory proteins that are universally conserved across all domains of life. In archaea and eukaryotes, Spt5 associates with a second protein, Spt4. In addition to regulating elongation, the eukaryotic Spt4-Spt5 complex appears to couple chromatin modification states and RNA processing to transcription elongation. This review discusses the experimental bases for our current understanding of Spt4-Spt5 function and recent studies that are beginning to elucidate the structure of Spt4-Spt5/RNA polymerase complexes and mechanism of Spt4-Spt5 action. This article is part of a Special Issue entitled: RNA polymerase II Transcript Elongation.

Regulation of mammalian transcription by Gdown1 through a novel steric crosstalk revealed by cryo-EM

In mammals, a distinct RNA polymerase II form, RNAPII(G) contains a novel subunit Gdown1 (encoded by POLR2M), which represses gene activation, only to be reversed by the multisubunit Mediator co-activator. Here, we employed single-particle cryo-electron microscopy (cryo-EM) to disclose the architectures of RNAPII(G), RNAPII and RNAPII in complex with the transcription initiation factor TFIIF, all to ~19 Å. Difference analysis mapped Gdown1 mostly to the RNAPII Rpb5 shelf-Rpb1 jaw, supported by antibody labelling experiments. These structural features correlate with the moderate increase in the efficiency of RNA chain elongation by RNAP II(G). In addition, our updated RNAPII-TFIIF map showed that TFIIF tethers multiple regions surrounding the DNA-binding cleft, in agreement with cross-linking and biochemical mapping. Gdown1's binding sites overlap extensively with those of TFIIF, with Gdown1 sterically excluding TFIIF from RNAPII, herein demonstrated by competition assays using size exclusion chromatography. In summary, our work establishes a structural basis for Gdown1 impeding initiation at promoters, by obstruction of TFIIF, accounting for an additional dependent role of Mediator in activated transcription.

Architecture of the RNA polymerase II preinitiation complex and mechanism of ATP-dependent promoter opening

Yeast RNA polymerase (Pol) II general factor TFIIE and the TFIIH subunit Ssl2/XPB function in transition of the preinitiation complex (PIC) to the open complex. We find that the three TFIIE winged helix (WH) domains form a heterodimer, with the Tfa1/TFIIEα WH binding the Pol II clamp and the Tfa2/TFIIEβ tandem WH domain encircling promoter DNA that becomes single stranded in the open complex. Ssl2 lies adjacent to TFIIE, enclosing downstream promoter DNA. In contrast to previous proposals, comparison of the PIC and open complex models strongly suggests that Ssl2 promotes DNA opening by functioning as a double stranded DNA translocase, feeding 15 bp of double stranded DNA into the Pol II cleft. Right-handed threading of DNA through the Ssl2 binding groove, combined with the fixed position of upstream promoter DNA, will lead to DNA unwinding and the open state.

Molecular basis of transcription initiation in Archaea

Compared with eukaryotes, the archaeal transcription initiation machinery-commonly known as the Pre-Initiation Complex-is relatively simple. The archaeal PIC consists of the TFIIB ortholog TFB, TBP, and an 11-subunit RNA polymerase (RNAP). The relatively small size of the entire archaeal PIC makes it amenable to structural analysis. Using purified RNAP, TFB, and TBP from the thermophile Pyrococcus furiosus, we assembled the biochemically active PIC at 65ºC. The intact archaeal PIC was isolated by implementing a cross-linking technique followed by size-exclusion chromatography, and the structure of this 440 kDa assembly was determined using electron microscopy and single-particle reconstruction techniques. Combining difference maps with crystal structure docking of various sub-domains, TBP and TFB were localized within the macromolecular PIC. TBP/TFB assemble near the large RpoB subunit and the RpoD/L "foot" domain behind the RNAP central cleft. This location mimics that of yeast TBP and TFIIB in complex with yeast RNAP II. Collectively, these results define the structural organization of the archaeal transcription machinery and suggest a conserved core PIC architecture.

Conservation between the RNA polymerase I, II, and III transcription initiation machineries

Recent studies of the three eukaryotic transcription machineries revealed that all initiation complexes share a conserved core. This core consists of the RNA polymerase (I, II, or III), the TATA box-binding protein (TBP), and transcription factors TFIIB, TFIIE, and TFIIF (for Pol II) or proteins structurally and functionally related to parts of these factors (for Pol I and Pol III). The conserved core initiation complex stabilizes the open DNA promoter complex and directs initial RNA synthesis. The periphery of the core initiation complex is decorated by additional polymerase-specific factors that account for functional differences in promoter recognition and opening, and gene class-specific regulation. This review outlines the similarities and differences between these important molecular machines.

Evidence that RNA polymerase II and not TFIIB is responsible for the difference in transcription initiation patterns between Saccharomyces cerevisiae and Schizosaccharomyces pombe

The basal eukaryotic transcription machinery for protein coding genes is highly conserved from unicellular yeast to higher eukaryotes. Whereas TATA-containing promoters in human cells usually contain a single transcription start site (TSS) located ∼30 bp downstream of the TATA element, transcription in the yeast Schizosaccharomyces pombe and Saccharomyces cerevisiae typically initiates at multiple sites within a window ranging from 30 to 70 bp or 40 to 200 bp downstream of a TATA element, respectively. By exchanging highly purified factors between reconstituted S. pombe and S. cerevisiae transcription systems, we confirmed previous observations that the dual exchange of RNA polymerase II (RNAPII) and transcription factor IIB (TFIIB) confer the distinct initiation patterns between these yeast species. Surprisingly, however, further genetic and biochemical assays of TFIIB chimeras revealed that TFIIB and the proposed B-finger/reader domain do not play a role in determining the distinct initiation patterns between S. pombe and S. cerevisiae, but rather, these patterns are solely due to differences in RNAPII. These results are discussed within the context of a proposed model for the mechanistic coupling of the efficiency of early phosphodiester bond formation during productive TSS utilization and intrinsic elongation proficiency.

Performance analysis of orthogonal pairs designed for an expanded eukaryotic genetic code

Background

The suppression of amber stop codons with non-canonical amino acids (ncAAs) is used for the site-specific introduction of many unusual functions into proteins. Specific orthogonal aminoacyl-tRNA synthetase (o-aaRS)/amber suppressor tRNA_CUA pairs (o-pairs) for the incorporation of ncAAs in S. cerevisiae were previously selected from an E. coli tyrosyl-tRNA synthetase/tRNA_CUA mutant library. Incorporation fidelity relies on the specificity of the o-aaRSs for their ncAAs and the ability to effectively discriminate against their natural substrate Tyr or any other canonical amino acid.

Methodology/Principal Findings

We used o-pairs previously developed for ncAAs carrying reactive alkyne-, azido-, or photocrosslinker side chains to suppress an amber mutant of human superoxide dismutase 1 in S. cerevisiae. We found worse incorporation efficiencies of the alkyne- and the photocrosslinker ncAAs than reported earlier. In our hands, amber suppression with the ncAA containing the azido group did not occur at all. In addition to the incorporation experiments in S. cerevisiae, we analyzed the catalytic properties of the o-aaRSs in vitro. Surprisingly, all o-aaRSs showed much higher preference for their natural substrate Tyr than for any of the tested ncAAs. While it is unclear why efficiently recognized Tyr is not inserted at amber codons, we speculate that metabolically inert ncAAs accumulate in the cell, and for this reason they are incorporated despite being weak substrates for the o-aaRSs.

Conclusions/Significance

O-pairs have been developed for a whole plethora of ncAAs. However, a systematic and detailed analysis of their catalytic properties is still missing. Our study provides a comprehensive scrutiny of o-pairs developed for the site-specific incorporation of reactive ncAAs in S. cerevisiae. It suggests that future development of o-pairs as efficient biotechnological tools will greatly benefit from sound characterization in vivo and in vitro in parallel to monitoring intracellular ncAA levels.

Dynamic architecture of a minimal RNA polymerase II open promoter complex

The open promoter complex (OC) is a central intermediate during transcription initiation that contains a DNA bubble. Here, we employ single-molecule Förster resonance energy transfer experiments and Nano-Positioning System analysis to determine the three-dimensional architecture of a minimal OC consisting of promoter DNA, including a TATA box and an 11-nucleotide mismatched region around the transcription start site, TATA box-binding protein (TBP), RNA polymerase (Pol) II, and general transcription factor (TF)IIB and TFIIF. In this minimal OC, TATA-DNA and TBP reside above the Pol II cleft between clamp and protrusion domains. Downstream DNA is dynamically loaded into and unloaded from the Pol II cleft at a timescale of seconds. The TFIIB core domain is displaced from the Pol II wall, where it is located in the closed promoter complex. These results reveal large overall structural changes during the initiation-elongation transition, which are apparently accommodated by the intrinsic flexibility of TFIIB.

Crosslinking-MS analysis reveals RNA polymerase I domain architecture and basis of rRNA cleavage

RNA polymerase (Pol) I contains a 10-subunit catalytic core that is related to the core of Pol II and includes subunit A12.2. In addition, Pol I contains the heterodimeric subcomplexes A14/43 and A49/34.5, which are related to the Pol II subcomplex Rpb4/7 and the Pol II initiation factor TFIIF, respectively. Here we used lysine-lysine crosslinking, mass spectrometry (MS) and modeling based on five crystal structures, to extend the previous homology model of the Pol I core, to confirm the location of A14/43 and to position A12.2 and A49/34.5 on the core. In the resulting model of Pol I, the C-terminal ribbon (C-ribbon) domain of A12.2 reaches the active site via the polymerase pore, like the C-ribbon of the Pol II cleavage factor TFIIS, explaining why the intrinsic RNA cleavage activity of Pol I is strong, in contrast to the weak cleavage activity of Pol II. The A49/34.5 dimerization module resides on the polymerase lobe, like TFIIF, whereas the A49 tWH domain resides above the cleft, resembling parts of TFIIE. This indicates that Pol I and also Pol III are distantly related to a Pol II-TFIIS-TFIIF-TFIIE complex.

Transcriptional regulation in Saccharomyces cerevisiae: transcription factor regulation and function, mechanisms of initiation, and roles of activators and coactivators

Here we review recent advances in understanding the regulation of mRNA synthesis in Saccharomyces cerevisiae. Many fundamental gene regulatory mechanisms have been conserved in all eukaryotes, and budding yeast has been at the forefront in the discovery and dissection of these conserved mechanisms. Topics covered include upstream activation sequence and promoter structure, transcription factor classification, and examples of regulated transcription factor activity. We also examine advances in understanding the RNA polymerase II transcription machinery, conserved coactivator complexes, transcription activation domains, and the cooperation of these factors in gene regulatory mechanisms.

Activator-mediator binding stabilizes RNA polymerase II orientation within the human mediator-RNA polymerase II-TFIIF assembly

The human Mediator complex controls RNA polymerase II (pol II) function in ways that remain incompletely understood. Activator-Mediator binding alters Mediator structure, and these activator-induced structural shifts appear to play key roles in regulating transcription. A recent cryo-electron microscopy (EM) analysis revealed that pol II adopted a stable orientation within a Mediator-pol II-TFIIF assembly in which Mediator was bound to the activation domain of viral protein 16 (VP16). Whereas TFIIF was shown to be important for orienting pol II within this assembly, the potential role of the activator was not assessed. To determine how activator binding might affect pol II orientation, we isolated human Mediator-pol II-TFIIF complexes in which Mediator was not bound to an activator. Cryo-EM analysis of this assembly, coupled with pol II crystal structure docking, revealed that pol II binds Mediator at the same general location; however, in contrast to VP16-bound Mediator, pol II does not appear to stably orient in the absence of an activator. Variability in pol II orientation might be important mechanistically, perhaps to enable sense and antisense transcription at human promoters. Because Mediator interacts extensively with pol II, these results suggest that Mediator structural shifts induced by activator binding help stably orient pol II prior to transcription initiation.

Subunit architecture of general transcription factor TFIIH

Structures of complete 10-subunit yeast TFIIH and of a nested set of subcomplexes, containing 5, 6, and 7 subunits, have been determined by electron microscopy (EM) and 3D reconstruction. Consistency among all the structures establishes the location of the "minimal core" subunits (Ssl1, Tfb1, Tfb2, Tfb4, and Tfb5), and additional densities can be specifically attributed to Rad3, Ssl2, and the TFIIK trimer. These results can be further interpreted by placement of previous X-ray structures into the additional densities to give a preliminary picture of the RNA polymerase II preinitiation complex. In this picture, the key catalytic components of TFIIH, the Ssl2 ATPase/helicase and the Kin28 protein kinase are in proximity to their targets, downstream promoter DNA and the RNA polymerase C-terminal domain.

The initiation factor TFE and the elongation factor Spt4/5 compete for the RNAP clamp during transcription initiation and elongation

TFIIE and the archaeal homolog TFE enhance DNA strand separation of eukaryotic RNAPII and the archaeal RNAP during transcription initiation by an unknown mechanism. We have developed a fluorescently labeled recombinant M. jannaschii RNAP system to probe the archaeal transcription initiation complex, consisting of promoter DNA, TBP, TFB, TFE, and RNAP. We have localized the position of the TFE winged helix (WH) and Zinc ribbon (ZR) domains on the RNAP using single-molecule FRET. The interaction sites of the TFE WH domain and the transcription elongation factor Spt4/5 overlap, and both factors compete for RNAP binding. Binding of Spt4/5 to RNAP represses promoter-directed transcription in the absence of TFE, which alleviates this effect by displacing Spt4/5 from RNAP. During elongation, Spt4/5 can displace TFE from the RNAP elongation complex and stimulate processivity. Our results identify the RNAP “clamp” region as a regulatory hot spot for both transcription initiation and transcription elongation.