Structures of transcription preinitiation complex engaged with the +1 nucleosome

The choreography of chromatin in RNA polymerase III regulation

Regulation of eukaryotic gene expression involves a dynamic interplay between the core transcriptional machinery, transcription factors, and chromatin organization and modification. While this applies to transcription by all RNA polymerase complexes, RNA polymerase III (RNAPIII) seems to be atypical with respect to its mechanisms of regulation. One distinctive feature of most RNAPIII transcribed genes is that they are devoid of nucleosomes, which relates to the high levels of transcription. Moreover, most of the regulatory sequences are not outside but within the transcribed open chromatin regions. Yet, several lines of evidence suggest that chromatin factors affect RNAPIII dynamics and activity and that gene sequence alone does not explain the observed regulation of RNAPIII. Here we discuss the role of chromatin modification and organization of RNAPIII transcribed genes and how they interact with the core transcriptional RNAPIII machinery and regulatory DNA elements in and around the transcribed genes.

Nucleosomal DNA has topological memory

One elusive aspect of the chromosome architecture is how it constrains the DNA topology. Nucleosomes stabilise negative DNA supercoils by restraining a DNA linking number difference (∆Lk) of about -1.26. However, whether this capacity is uniform across the genome is unknown. Here, we calculate the ∆Lk restrained by over 4000 nucleosomes in yeast cells. To achieve this, we insert each nucleosome in a circular minichromosome and perform Topo-seq, a high-throughput procedure to inspect the topology of circular DNA libraries in one gel electrophoresis. We show that nucleosomes inherently restrain distinct ∆Lk values depending on their genomic origin. Nucleosome DNA topologies differ at gene bodies (∆Lk = -1.29), intergenic regions (∆Lk = -1.23), rDNA genes (∆Lk = -1.24) and telomeric regions (∆Lk = -1.07). Nucleosomes near the transcription start and termination sites also exhibit singular DNA topologies. Our findings demonstrate that nucleosome DNA topology is imprinted by its native chromatin context and persists when the nucleosome is relocated.

Does TFIIH move nucleosomes?

Transcription factor (TF) IIH is a factor involved in transcription, DNA repair, mitosis, and telomere stability. These functions stem from its helicase/ATPase and kinase activities. Recent reports on the structure and function of the transcription machinery, as well as chromosome compaction during mitosis, suggest that TFIIH also influences nucleosome movement, are explored here.

A conserved function of corepressors is to nucleate assembly of the transcriptional preinitiation complex

The plant corepressor TPL is recruited to diverse chromatin contexts, yet its mechanism of repression remains unclear. Previously, we have leveraged the fact that TPL retains its function in a synthetic transcriptional circuit in the yeast model Saccharomyces cerevisiae to localize repressive function to two distinct domains. Here, we employed two unbiased whole genome approaches to map the physical and genetic interactions of TPL at a repressed locus. We identified SPT4, SPT5 and SPT6 as necessary for repression with the SPT4 subunit acting as a bridge connecting TPL to SPT5 and SPT6. We also discovered the association of multiple additional constituents of the transcriptional preinitiation complex at TPL-repressed promoters, specifically those involved in early transcription initiation events. These findings were validated in yeast and plants through multiple assays, including a novel method to analyze conditional loss of function of essential genes in plants. Our findings support a model where TPL nucleates preassembly of the transcription activation machinery to facilitate rapid onset of transcription once repression is relieved.

Structural perspectives on transcription in chromatin

In eukaryotes, all genetic processes take place in the cell nucleus, where DNA is packaged as chromatin in 'beads-on-a-string' nucleosome arrays. RNA polymerase II (RNAPII) transcribes protein-coding and many non-coding genes in this chromatin environment. RNAPII elongates RNA while passing through multiple nucleosomes and maintaining the integrity of the chromatin structure. Recent structural studies have shed light on the detailed mechanisms of this process, including how transcribing RNAPII progresses through a nucleosome and reassembles it afterwards, and how transcription elongation factors, chromatin remodelers, and histone chaperones participate in these processes. Other studies have also illuminated the crucial role of nucleosomes in preinitiation complex assembly and transcription initiation. In this review we outline these advances and discuss future perspectives.

Interplay between the transcription preinitiation complex and the +1 nucleosome

Eukaryotic transcription starts with the assembly of a preinitiation complex (PIC) on core promoters. Flanking this region is the +1 nucleosome, the first nucleosome downstream of the core promoter. While this nucleosome is rich in epigenetic marks and plays a key role in transcription regulation, how the +1 nucleosome interacts with the transcription machinery has been a long-standing question. Here, we summarize recent structural and functional studies of the +1 nucleosome in complex with the PIC. We specifically focus on how differently organized promoter-nucleosome templates affect the assembly of the PIC and PIC-Mediator on chromatin and result in distinct transcription initiation.

Collaborators or competitors: the communication between RNA polymerase II and the nucleosome during eukaryotic transcription

Decades of scientific research have been devoted to unraveling the intricacies of eukaryotic transcription since the groundbreaking discovery of eukaryotic RNA polymerases in the late 1960s. RNA polymerase II, the polymerase responsible for mRNA synthesis, has always attracted the most attention. Despite its structural resemblance to its bacterial counterpart, eukaryotic RNA polymerase II faces a unique challenge in progressing transcription due to the presence of nucleosomes that package DNA in the nuclei. In this review, we delve into the impact of RNA polymerase II and histone signaling on the progression of eukaryotic transcription. We explore the pivotal points of interactions that bridge the RNA polymerase II and histone signaling systems. Finally, we present an analysis of recent cryo-electron microscopy structures, which captured RNA polymerase II-nucleosome complexes at different stages of the transcription cycle. The combination of the signaling crosstalk and the direct visualization of RNA polymerase II-nucleosome complexes provides a deeper understanding of the communication between these two major players in eukaryotic transcription.

Physical Peculiarity of Two Sites in Human Promoters: Universality and Diverse Usage in Gene Function

Since the discovery of physical peculiarities around transcription start sites (TSSs) and a site corresponding to the TATA box, research has revealed only the average features of these sites. Unsettled enigmas include the individual genes with these features and whether they relate to gene function. Herein, using 10 physical properties of DNA, including duplex DNA free energy, base stacking energy, protein-induced deformability, and stabilizing energy of Z-DNA, we clarified for the first time that approximately 97% of the promoters of 21,056 human protein-coding genes have distinctive physical properties around the TSS and/or position -27; of these, nearly 65% exhibited such properties at both sites. Furthermore, about 55% of the 21,056 genes had a minimum value of regional duplex DNA free energy within TSS-centered ±300 bp regions. Notably, distinctive physical properties within the promoters and free energies of the surrounding regions separated human protein-coding genes into five groups; each contained specific gene ontology (GO) terms. The group represented by immune response genes differed distinctly from the other four regarding the parameter of the free energies of the surrounding regions. A vital suggestion from this study is that physical-feature-based analyses of genomes may reveal new aspects of the organization and regulation of genes.

Quantitative analysis of transcription start site selection reveals control by DNA sequence, RNA polymerase II activity and NTP levels

Transcription start site (TSS) selection is a key step in gene expression and occurs at many promoter positions over a wide range of efficiencies. Here we develop a massively parallel reporter assay to quantitatively dissect contributions of promoter sequence, nucleoside triphosphate substrate levels and RNA polymerase II (Pol II) activity to TSS selection by 'promoter scanning' in Saccharomyces cerevisiae (Pol II MAssively Systematic Transcript End Readout, 'Pol II MASTER'). Using Pol II MASTER, we measure the efficiency of Pol II initiation at 1,000,000 individual TSS sequences in a defined promoter context. Pol II MASTER confirms proposed critical qualities of S. cerevisiae TSS -8, -1 and +1 positions, quantitatively, in a controlled promoter context. Pol II MASTER extends quantitative analysis to surrounding sequences and determines that they tune initiation over a wide range of efficiencies. These results enabled the development of a predictive model for initiation efficiency based on sequence. We show that genetic perturbation of Pol II catalytic activity alters initiation efficiency mostly independently of TSS sequence, but selectively modulates preference for the initiating nucleotide. Intriguingly, we find that Pol II initiation efficiency is directly sensitive to guanosine-5'-triphosphate levels at the first five transcript positions and to cytosine-5'-triphosphate and uridine-5'-triphosphate levels at the second position genome wide. These results suggest individual nucleoside triphosphate levels can have transcript-specific effects on initiation, representing a cryptic layer of potential regulation at the level of Pol II biochemical properties. The results establish Pol II MASTER as a method for quantitative dissection of transcription initiation in eukaryotes.

RNA polymerases reshape chromatin and coordinate transcription on individual fibers

During eukaryotic transcription, RNA polymerases must initiate and pause within a crowded, complex environment, surrounded by nucleosomes and other transcriptional activity. This environment creates a spatial arrangement along individual chromatin fibers ripe for both competition and coordination, yet these relationships remain largely unknown owing to the inherent limitations of traditional structural and sequencing methodologies. To address these limitations, we employed long-read chromatin fiber sequencing (Fiber-seq) to visualize RNA polymerases within their native chromatin context at single-molecule and near single-nucleotide resolution along up to 30 kb fibers. We demonstrate that Fiber-seq enables the identification of single-molecule RNA Polymerase (Pol) II and III transcription associated footprints, which, in aggregate, mirror bulk short-read sequencing-based measurements of transcription. We show that Pol II pausing destabilizes downstream nucleosomes, with frequently paused genes maintaining a short-term memory of these destabilized nucleosomes. Furthermore, we demonstrate pervasive direct coordination and anti-coordination between nearby Pol II genes, Pol III genes, transcribed enhancers, and insulator elements. This coordination is largely limited to spatially organized elements within 5 kb of each other, implicating short-range chromatin environments as a predominant determinant of coordinated polymerase initiation. Overall, transcription initiation reshapes surrounding nucleosome architecture and coordinates nearby transcriptional machinery along individual chromatin fibers.

Unveiling the noncanonical activation mechanism of CDKs: insights from recent structural studies

The Cyclin-dependent kinases (CDKs) play crucial roles in a range of essential cellular processes. While the classical two-step activation mechanism is generally applicable to cell cycle-related CDKs, both CDK7 and CDK8, involved in transcriptional regulation, adopt distinct mechanisms for kinase activation. In both cases, binding to their respective cyclin partners results in only partial activity, while their full activation requires the presence of an additional subunit. Recent structural studies of these two noncanonical kinases have provided unprecedented insights into their activation mechanisms, enabling us to understand how the third subunit coordinates the T-loop stabilization and enhances kinase activity. In this review, we summarize the structure and function of CDK7 and CDK8 within their respective functional complexes, while also describing their noncanonical activation mechanisms. These insights open new avenues for targeted drug discovery and potential therapeutic interventions in various diseases related to CDK7 and CDK8.

Nucleosomes unwrapped: Structural perspectives on transcription through chromatin

Transcription of most protein-coding genes requires the passage of RNA polymerase II through chromatin. Chromatin with its fundamental unit, the nucleosome, represents a barrier to transcription. How RNA polymerase II and associated factors traverse through nucleosomes and how chromatin architecture is maintained have remained largely enigmatic. Only recently, cryo-EM structures have visualized the transcription process through chromatin. These structures have elucidated how transcription initiation and transcription elongation influence and are influenced by a chromatinized DNA substrate. This review provides a summary of our current structural understanding of transcription through chromatin, highlighting common mechanisms during nucleosomal traversal and novel regulatory mechanisms that have emerged in the last five years.

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.

Opposing Roles of FACT for Euchromatin and Heterochromatin in Yeast

DNA is stored in the nucleus of a cell in a folded state; however, only the necessary genetic information is extracted from the required group of genes. The key to extracting genetic information is chromatin ambivalence. Depending on the chromosomal region, chromatin is characterized into low-density "euchromatin" and high-density "heterochromatin", with various factors being involved in its regulation. Here, we focus on chromatin regulation and gene expression by the yeast FACT complex, which functions in both euchromatin and heterochromatin. FACT is known as a histone H2A/H2B chaperone and was initially reported as an elongation factor associated with RNA polymerase II. In budding yeast, FACT activates promoter chromatin by interacting with the transcriptional activators SBF/MBF via the regulation of G1/S cell cycle genes. In fission yeast, FACT plays an important role in the formation of higher-order chromatin structures and transcriptional repression by binding to Swi6, an HP1 family protein, at heterochromatin. This FACT property, which refers to the alternate chromatin-regulation depending on the binding partner, is an interesting phenomenon. Further analysis of nucleosome regulation within heterochromatin is expected in future studies.