Structure of RNA polymerase II pre-initiation complex at 2.9 Å defines initial DNA opening.

Transcription initiation requires assembly of the RNA polymerase II (Pol II) pre-initiation complex (PIC) and opening of promoter DNA. Here, we present the long-sought high-resolution structure of the yeast PIC and define the mechanism of initial DNA opening. We trap the PIC in an intermediate state that contains half a turn of open DNA located 30-35 base pairs downstream of the TATA box. The initially opened DNA region is flanked and stabilized by the polymerase "clamp head loop" and the TFIIF "charged region" that both contribute to promoter-initiated transcription. TFIIE facilitates initiation by buttressing the clamp head loop and by regulating the TFIIH translocase. The initial DNA bubble is then extended in the upstream direction, leading to the open promoter complex and enabling start-site scanning and RNA synthesis. This unique mechanism of DNA opening may permit more intricate regulation than in the Pol I and Pol III systems.

Structural Basis of Poxvirus Transcription: Transcribing and Capping Vaccinia Complexes.

Poxviruses use virus-encoded multisubunit RNA polymerases (vRNAPs) and RNA-processing factors to generate m7G-capped mRNAs in the host cytoplasm. In the accompanying paper, we report structures of core and complete vRNAP complexes of the prototypic Vaccinia poxvirus (Grimm et al., 2019; in this issue of Cell). Here, we present the cryo-electron microscopy (cryo-EM) structures of Vaccinia vRNAP in the form of a transcribing elongation complex and in the form of a co-transcriptional capping complex that contains the viral capping enzyme (CE). The trifunctional CE forms two mobile modules that bind the polymerase surface around the RNA exit tunnel. RNA extends from the vRNAP active site through this tunnel and into the active site of the CE triphosphatase. Structural comparisons suggest that growing RNA triggers large-scale rearrangements on the surface of the transcription machinery during the transition from transcription initiation to RNA capping and elongation. Our structures unravel the basis for synthesis and co-transcriptional modification of poxvirus RNA.

Structural Basis of Poxvirus Transcription: Vaccinia RNA Polymerase Complexes.

Poxviruses encode a multisubunit DNA-dependent RNA polymerase (vRNAP) that carries out viral gene expression in the host cytoplasm. We report cryo-EM structures of core and complete vRNAP enzymes from Vaccinia virus at 2.8 Å resolution. The vRNAP core enzyme resembles eukaryotic RNA polymerase II (Pol II) but also reveals many virus-specific features, including the transcription factor Rap94. The complete enzyme additionally contains the transcription factor VETF, the mRNA processing factors VTF/CE and NPH-I, the viral core protein E11, and host tRNAGln. This complex can carry out the entire early transcription cycle. The structures show that Rap94 partially resembles the Pol II initiation factor TFIIB, that the vRNAP subunit Rpo30 resembles the Pol II elongation factor TFIIS, and that NPH-I resembles chromatin remodeling enzymes. Together with the accompanying paper (Hillen et al., 2019), these results provide the basis for unraveling the mechanisms of poxvirus transcription and RNA processing.

Three-step mechanism of promoter escape by RNA polymerase II.

The transition from transcription initiation to elongation is highly regulated in human cells but remains incompletely understood at the structural level. In particular, it is unclear how interactions between RNA polymerase II (RNA Pol II) and initiation factors are broken to enable promoter escape. Here, we reconstitute RNA Pol II promoter escape in vitro and determine high-resolution structures of initially transcribing complexes containing 8-, 10-, and 12-nt ordered RNAs and two elongation complexes containing 14-nt RNAs. We suggest that promoter escape occurs in three major steps. First, the growing RNA displaces the B-reader element of the initiation factor TFIIB without evicting TFIIB. Second, the rewinding of the transcription bubble coincides with the eviction of TFIIA, TFIIB, and TBP. Third, the binding of DSIF and NELF facilitates TFIIE and TFIIH dissociation, establishing the paused elongation complex. This three-step model for promoter escape fills a gap in our understanding of the initiation-elongation transition of RNA Pol II transcription.

Resolution of transcription-induced hexasome-nucleosome complexes by Chd1 and FACT.

To maintain the nucleosome organization of transcribed genes, ATP-dependent chromatin remodelers collaborate with histone chaperones. Here, we show that at the 5' ends of yeast genes, RNA polymerase II (RNAPII) generates hexasomes that occur directly adjacent to nucleosomes. The resulting hexasome-nucleosome complexes are then resolved by Chd1. We present two cryoelectron microscopy (cryo-EM) structures of Chd1 bound to a hexasome-nucleosome complex before and after restoration of the missing inner H2A/H2B dimer by FACT. Chd1 uniquely interacts with the complex, positioning its ATPase domain to shift the hexasome away from the nucleosome. In the absence of the inner H2A/H2B dimer, its DNA-binding domain (DBD) packs against the ATPase domain, suggesting an inhibited state. Restoration of the dimer by FACT triggers a rearrangement that displaces the DBD and stimulates Chd1 remodeling. Our results demonstrate how chromatin remodelers interact with a complex nucleosome assembly and suggest how Chd1 and FACT jointly support transcription by RNAPII.

Structural insights into human co-transcriptional capping.

Co-transcriptional capping of the nascent pre-mRNA 5' end prevents degradation of RNA polymerase (Pol) II transcripts and suppresses the innate immune response. Here, we provide mechanistic insights into the three major steps of human co-transcriptional pre-mRNA capping based on six different cryoelectron microscopy (cryo-EM) structures. The human mRNA capping enzyme, RNGTT, first docks to the Pol II stalk to position its triphosphatase domain near the RNA exit site. The capping enzyme then moves onto the Pol II surface, and its guanylyltransferase receives the pre-mRNA 5'-diphosphate end. Addition of a GMP moiety can occur when the RNA is ∼22 nt long, sufficient to reach the active site of the guanylyltransferase. For subsequent cap(1) methylation, the methyltransferase CMTR1 binds the Pol II stalk and can receive RNA after it is grown to ∼29 nt in length. The observed rearrangements of capping factors on the Pol II surface may be triggered by the completion of catalytic reaction steps and are accommodated by domain movements in the elongation factor DRB sensitivity-inducing factor (DSIF).

Structural basis of transcription reduction by a promoter-proximal +1 nucleosome.

At active human genes, the +1 nucleosome is located downstream of the RNA polymerase II (RNA Pol II) pre-initiation complex (PIC). However, at inactive genes, the +1 nucleosome is found further upstream, at a promoter-proximal location. Here, we establish a model system to show that a promoter-proximal +1 nucleosome can reduce RNA synthesis in vivo and in vitro, and we analyze its structural basis. We find that the PIC assembles normally when the edge of the +1 nucleosome is located 18 base pairs (bp) downstream of the transcription start site (TSS). However, when the nucleosome edge is located further upstream, only 10 bp downstream of the TSS, the PIC adopts an inhibited state. The transcription factor IIH (TFIIH) shows a closed conformation and its subunit XPB contacts DNA with only one of its two ATPase lobes, inconsistent with DNA opening. These results provide a mechanism for nucleosome-dependent regulation of transcription initiation.

Structural basis of catalytic activation in human splicing.

Pre-mRNA splicing follows a pathway driven by ATP-dependent RNA helicases. A crucial event of the splicing pathway is the catalytic activation, which takes place at the transition between the activated Bact and the branching-competent B* spliceosomes. Catalytic activation occurs through an ATP-dependent remodelling mediated by the helicase PRP2 (also known as DHX16)1-3. However, because PRP2 is observed only at the periphery of spliceosomes3-5, its function has remained elusive. Here we show that catalytic activation occurs in two ATP-dependent stages driven by two helicases: PRP2 and Aquarius. The role of Aquarius in splicing has been enigmatic6,7. Here the inactivation of Aquarius leads to the stalling of a spliceosome intermediate-the BAQR complex-found halfway through the catalytic activation process. The cryogenic electron microscopy structure of BAQR reveals how PRP2 and Aquarius remodel Bact and BAQR, respectively. Notably, PRP2 translocates along the intron while it strips away the RES complex, opens the SF3B1 clamp and unfastens the branch helix. Translocation terminates six nucleotides downstream of the branch site through an assembly of PPIL4, SKIP and the amino-terminal domain of PRP2. Finally, Aquarius enables the dissociation of PRP2, plus the SF3A and SF3B complexes, which promotes the relocation of the branch duplex for catalysis. This work elucidates catalytic activation in human splicing, reveals how a DEAH helicase operates and provides a paradigm for how helicases can coordinate their activities.

Allosteric transcription stimulation by RNA polymerase II super elongation complex.

The super elongation complex (SEC) contains the positive transcription elongation factor b (P-TEFb) and the subcomplex ELL2-EAF1, which stimulates RNA polymerase II (RNA Pol II) elongation. Here, we report the cryoelectron microscopy (cryo-EM) structure of ELL2-EAF1 bound to a RNA Pol II elongation complex at 2.8 Å resolution. The ELL2-EAF1 dimerization module directly binds the RNA Pol II lobe domain, explaining how SEC delivers P-TEFb to RNA Pol II. The same site on the lobe also binds the initiation factor TFIIF, consistent with SEC binding only after the transition from transcription initiation to elongation. Structure-guided functional analysis shows that the stimulation of RNA elongation requires the dimerization module and the ELL2 linker that tethers the module to the RNA Pol II protrusion. Our results show that SEC stimulates elongation allosterically and indicate that this stimulation involves stabilization of a closed conformation of the RNA Pol II active center cleft.

Structure of the human Mediator-RNA polymerase II pre-initiation complex.

Mediator is a conserved coactivator complex that enables the regulated initiation of transcription at eukaryotic genes1-3. Mediator is recruited by transcriptional activators and binds the pre-initiation complex (PIC) to stimulate the phosphorylation of RNA polymerase II (Pol II) and promoter escape1-6. Here we prepare a recombinant version of human Mediator, reconstitute a 50-subunit Mediator-PIC complex and determine the structure of the complex by cryo-electron microscopy. The head module of Mediator contacts the stalk of Pol II and the general transcription factors TFIIB and TFIIE, resembling the Mediator-PIC interactions observed in the corresponding complex in yeast7-9. The metazoan subunits MED27-MED30 associate with exposed regions in MED14 and MED17 to form the proximal part of the Mediator tail module that binds activators. Mediator positions the flexibly linked cyclin-dependent kinase (CDK)-activating kinase of the general transcription factor TFIIH near the linker to the C-terminal repeat domain of Pol II. The Mediator shoulder domain holds the CDK-activating kinase subunit CDK7, whereas the hook domain contacts a CDK7 element that flanks the kinase active site. The shoulder and hook domains reside in the Mediator head and middle modules, respectively, which can move relative to each other and may induce an active conformation of the CDK7 kinase to allosterically stimulate phosphorylation of the C-terminal domain.

Promoter Distortion and Opening in the RNA Polymerase II Cleft.

Transcription initiation requires opening of promoter DNA in the RNA polymerase II (Pol II) pre-initiation complex (PIC), but it remains unclear how this is achieved. Here we report the cryo-electron microscopic (cryo-EM) structure of a yeast PIC that contains underwound, distorted promoter DNA in the closed Pol II cleft. The DNA duplex axis is offset at the upstream edge of the initially melted DNA region (IMR) where DNA opening begins. Unstable IMRs are found in a subset of yeast promoters that we show can still initiate transcription after depletion of the transcription factor (TF) IIH (TFIIH) translocase Ssl2 (XPB in human) from the nucleus in vivo. PIC-induced DNA distortions may thus prime the IMR for melting and may explain how unstable IMRs that are predicted in promoters of Pol I and Pol III can open spontaneously. These results suggest that DNA distortion in the polymerase cleft is a general mechanism that contributes to promoter opening.

Nucleosome-bound SOX2 and SOX11 structures elucidate pioneer factor function.

'Pioneer' transcription factors are required for stem-cell pluripotency, cell differentiation and cell reprogramming1,2. Pioneer factors can bind nucleosomal DNA to enable gene expression from regions of the genome with closed chromatin. SOX2 is a prominent pioneer factor that is essential for pluripotency and self-renewal of embryonic stem cells3. Here we report cryo-electron microscopy structures of the DNA-binding domains of SOX2 and its close homologue SOX11 bound to nucleosomes. The structures show that SOX factors can bind and locally distort DNA at superhelical location 2. The factors also facilitate detachment of terminal nucleosomal DNA from the histone octamer, which increases DNA accessibility. SOX-factor binding to the nucleosome can also lead to a repositioning of the N-terminal tail of histone H4 that includes residue lysine 16. We speculate that this repositioning is incompatible with higher-order nucleosome stacking, which involves contacts of the H4 tail with a neighbouring nucleosome. Our results indicate that pioneer transcription factors can use binding energy to initiate chromatin opening, and thereby facilitate nucleosome remodelling and subsequent transcription.

Structure of SWI/SNF chromatin remodeller RSC bound to a nucleosome.

Chromatin-remodelling complexes of the SWI/SNF family function in the formation of nucleosome-depleted, transcriptionally active promoter regions (NDRs)1,2. In the yeast Saccharomyces cerevisiae, the essential SWI/SNF complex RSC3 contains 16 subunits, including the ATP-dependent DNA translocase Sth14,5. RSC removes nucleosomes from promoter regions6,7 and positions the specialized +1 and -1 nucleosomes that flank NDRs8,9. Here we present the cryo-electron microscopy structure of RSC in complex with a nucleosome substrate. The structure reveals that RSC forms five protein modules and suggests key features of the remodelling mechanism. The body module serves as a scaffold for the four flexible modules that we call DNA-interacting, ATPase, arm and actin-related protein (ARP) modules. The DNA-interacting module binds extra-nucleosomal DNA and is involved in the recognition of promoter DNA elements8,10,11 that influence RSC functionality12. The ATPase and arm modules sandwich the nucleosome disc with the Snf2 ATP-coupling (SnAC) domain and the finger helix, respectively. The translocase motor of the ATPase module engages with the edge of the nucleosome at superhelical location +2. The mobile ARP module may modulate translocase-nucleosome interactions to regulate RSC activity5. The RSC-nucleosome structure provides a basis for understanding NDR formation and the structure and function of human SWI/SNF complexes that are frequently mutated in cancer13.

Structure of replicating SARS-CoV-2 polymerase.

The new coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) uses an RNA-dependent RNA polymerase (RdRp) for the replication of its genome and the transcription of its genes1-3. Here we present a cryo-electron microscopy structure of the SARS-CoV-2 RdRp in an active form that mimics the replicating enzyme. The structure comprises the viral proteins non-structural protein 12 (nsp12), nsp8 and nsp7, and more than two turns of RNA template-product duplex. The active-site cleft of nsp12 binds to the first turn of RNA and mediates RdRp activity with conserved residues. Two copies of nsp8 bind to opposite sides of the cleft and position the second turn of RNA. Long helical extensions in nsp8 protrude along exiting RNA, forming positively charged 'sliding poles'. These sliding poles can account for the known processivity of RdRp that is required for replicating the long genome of coronaviruses3. Our results enable a detailed analysis of the inhibitory mechanisms that underlie the antiviral activity of substances such as remdesivir, a drug for the treatment of coronavirus disease 2019 (COVID-19)4.

Structure of the transcription coactivator SAGA.

Gene transcription by RNA polymerase II is regulated by activator proteins that recruit the coactivator complexes SAGA (Spt-Ada-Gcn5-acetyltransferase)1,2 and transcription factor IID (TFIID)2-4. SAGA is required for all regulated transcription5 and is conserved among eukaryotes6. SAGA contains four modules7-9: the activator-binding Tra1 module, the core module, the histone acetyltransferase (HAT) module and the histone deubiquitination (DUB) module. Previous studies provided partial structures10-14, but the structure of the central core module is unknown. Here we present the cryo-electron microscopy structure of SAGA from the yeast Saccharomyces cerevisiae and resolve the core module at 3.3 Å resolution. The core module consists of subunits Taf5, Sgf73 and Spt20, and a histone octamer-like fold. The octamer-like fold comprises the heterodimers Taf6-Taf9, Taf10-Spt7 and Taf12-Ada1, and two histone-fold domains in Spt3. Spt3 and the adjacent subunit Spt8 interact with the TATA box-binding protein (TBP)2,7,15-17. The octamer-like fold and its TBP-interacting region are similar in TFIID, whereas Taf5 and the Taf6 HEAT domain adopt distinct conformations. Taf12 and Spt20 form flexible connections to the Tra1 module, whereas Sgf73 tethers the DUB module. Binding of a nucleosome to SAGA displaces the HAT and DUB modules from the core-module surface, allowing the DUB module to bind one face of an ubiquitinated nucleosome.

Yeast PIC-Mediator structure with RNA polymerase II C-terminal domain.

For transcription initiation, RNA polymerase II (Pol II) forms a preinitiation complex (PIC) that associates with the general coactivator Mediator. Whereas atomic models of the human PIC-Mediator structure have been reported, structures for its yeast counterpart remain incomplete. Here, we present an atomic model for the yeast PIC with core Mediator, including the Mediator middle module that was previously poorly resolved and including subunit Med1 that was previously lacking. We observe three peptide regions containing eleven of the 26 heptapeptide repeats of the flexible C-terminal repeat domain (CTD) of Pol II. Two of these CTD regions bind between the Mediator head and middle modules and form defined CTD-Mediator interactions. CTD peptide 1 binds between the Med6 shoulder and Med31 knob domains, whereas CTD peptide 2 forms additional contacts with Med4. The third CTD region (peptide 3) binds in the Mediator cradle and associates with the Mediator hook. Comparisons with the human PIC-Mediator structure show that the central region in peptide 1 is similar and forms conserved contacts with Mediator, whereas peptides 2 and 3 exhibit distinct structures and Mediator interactions.

Neutralization of SARS-CoV-2 by highly potent, hyperthermostable, and mutation-tolerant nanobodies.

Monoclonal anti-SARS-CoV-2 immunoglobulins represent a treatment option for COVID-19. However, their production in mammalian cells is not scalable to meet the global demand. Single-domain (VHH) antibodies (also called nanobodies) provide an alternative suitable for microbial production. Using alpaca immune libraries against the receptor-binding domain (RBD) of the SARS-CoV-2 Spike protein, we isolated 45 infection-blocking VHH antibodies. These include nanobodies that can withstand 95°C. The most effective VHH antibody neutralizes SARS-CoV-2 at 17-50 pM concentration (0.2-0.7 µg per liter), binds the open and closed states of the Spike, and shows a tight RBD interaction in the X-ray and cryo-EM structures. The best VHH trimers neutralize even at 40 ng per liter. We constructed nanobody tandems and identified nanobody monomers that tolerate the K417N/T, E484K, N501Y, and L452R immune-escape mutations found in the Alpha, Beta, Gamma, Epsilon, Iota, and Delta/Kappa lineages. We also demonstrate neutralization of the Beta strain at low-picomolar VHH concentrations. We further discovered VHH antibodies that enforce native folding of the RBD in the E. coli cytosol, where its folding normally fails. Such "fold-promoting" nanobodies may allow for simplified production of vaccines and their adaptation to viral escape-mutations.

Multi-particle cryo-EM refinement with M visualizes ribosome-antibiotic complex at 3.5 Å in cells.

Cryo-electron microscopy (cryo-EM) enables macromolecular structure determination in vitro and inside cells. In addition to aligning individual particles, accurate registration of sample motion and three-dimensional deformation during exposures are crucial for achieving high-resolution reconstructions. Here we describe M, a software tool that establishes a reference-based, multi-particle refinement framework for cryo-EM data and couples a comprehensive spatial deformation model to in silico correction of electron-optical aberrations. M provides a unified optimization framework for both frame-series and tomographic tilt-series data. We show that tilt-series data can provide the same resolution as frame-series data on a purified protein specimen, indicating that the alignment step no longer limits the resolution obtainable from tomographic data. In combination with Warp and RELION, M resolves to residue level a 70S ribosome bound to an antibiotic inside intact bacterial cells. Our work provides a computational tool that facilitates structural biology in cells.

Structure of an inactive RNA polymerase II dimer.

Eukaryotic gene transcription is carried out by three RNA polymerases: Pol I, Pol II and Pol III. Although it has long been known that Pol I can form homodimers, it is unclear whether and how the two other RNA polymerases dimerize. Here we present the cryo-electron microscopy (cryo-EM) structure of a mammalian Pol II dimer at 3.5 Å resolution. The structure differs from the Pol I dimer and reveals that one Pol II copy uses its RPB4-RPB7 stalk to penetrate the active centre cleft of the other copy, and vice versa, giving rise to a molecular handshake. The polymerase clamp domain is displaced and mobile, and the RPB7 oligonucleotide-binding fold mimics the DNA-RNA hybrid that occupies the cleft during active transcription. The Pol II dimer is incompatible with nucleic acid binding as required for transcription and may represent an inactive storage form of the polymerase.

Coronavirus-Replikation: Mechanismus und Inhibition durch Remdesivir.

Coronaviruses use an RNA-dependent RNA polymerase to replicate and transcribe their RNA genome. The structure of the SARS-CoV-2 polymerase was determined by cryo-electron microscopy within a short time in spring 2020. The structure explains how the viral enzyme synthesizes RNA and how it replicates the exceptionally large genome in a processive manner. The most recent structure-function studies further reveal the mechanism of polymerase inhibition by remdesivir, an approved drug for the treatment of COVID-19.