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Haplotype-resolved 3D genome maps reveal RNAPII-mediated allelic regulation in hybrid rice.

To understand how the two parental genomes coordinate transcription in hybrids, chromatin architecture must be resolved at the haplotype level. Here, using phased Bridge-Linker Hi-C, we reconstructed a haplotype-resolved three-dimensional (3D) genome of the elite hybrid rice (Oryza sativa) line Shanyou 63 (SY63). We identified extensive allele-specific chromatin conformations. Furthermore, we generated allele-resolved RNAPII ChIA-PET maps and phased transcriptomes to explore how chromatin interactions contribute to allelic regulation. Although maternal and paternal homologs share broadly similar chromatin features, we detected widespread haplotype-biased RNAPII binding and chromatin looping at high resolution. These allele-specific RNAPII-mediated contacts were significantly associated with biased expression. Stronger RNAPII binding on one haplotype promoted the formation of long-range regulatory loops with distal genes, thereby contributing to allele-biased transcription at a subset of loci, even when promoter-proximal RNAPII occupancy was comparable between alleles. These results demonstrate that subtle differences in RNAPII engagement and 3D regulatory wiring between parental haplotypes can reshape transcriptional output in hybrids, providing new insights into the mechanisms underlying the allelic regulation of gene expression.

Allele-specific chromatin interactions

The ARK2N-CK2 complex initiates transcription-coupled repair through enhancing the interaction of CSB with lesion-stalled RNAPII.

Transcription is extremely important for cellular processes but can be hindered by RNA polymerase II (RNAPII) pausing and stalling. Cockayne syndrome protein B (CSB) promotes the progression of paused RNAPII or initiates transcription-coupled nucleotide excision repair (TC-NER) to remove stalled RNAPII. However, the specific mechanism by which CSB initiates TC-NER upon damage remains unclear. In this study, we identified the indispensable role of the ARK2N-CK2 complex in the CSB-mediated initiation of TC-NER. The ARK2N-CK2 complex is recruited to damage sites through CSB and then phosphorylates CSB. Phosphorylation of CSB enhances its binding to stalled RNAPII, prolonging the association of CSB with chromatin and promoting CSA-mediated ubiquitination of stalled RNAPII. Consistent with this finding, Ark2n-/- mice exhibit a phenotype resembling Cockayne syndrome. These findings shed light on the pivotal role of the ARK2N-CK2 complex in governing the fate of RNAPII through CSB, bridging a critical gap necessary for initiating TC-NER.

DNA Repair Enzymes

Genetic interaction between transcription elongation factor TFIIS and RNA polymerase II.

Little is known about the regions of RNA polymerase II (RNAPII) that are involved in the process of transcript elongation and interaction with elongation factors. One elongation factor, TFIIS, stimulates transcript elongation by binding to RNAPII and facilitating its passage through intrinsic pausing sites in vitro. In Saccharomyces cerevisiae, TFIIS is encoded by the PPR2 gene. Deletion of PPR2 from the yeast genome is not lethal but renders cells sensitive to the uracil analog 6-azauracil (6AU). Here, we show that mutations conferring 6AU sensitivity can also be isolated in the gene encoding the largest subunit of S. cerevisiae RNAPII (RPO21). A screen for mutations in RPO21 that confer 6AU sensitivity identified seven mutations that had been generated by either linker-insertion or random chemical mutagenesis. All seven mutational alterations are clustered within one region of the largest subunit that is conserved among eukaryotic RNAPII. The finding that six of the seven rpo21 mutants failed to grow at elevated temperature underscores the importance of this region for the functional and/or structural integrity of RNAPII. We found that the 6AU sensitivity of the rpo21 mutants can be suppressed by increasing the dosage of the wild-type PPR2 gene, presumably as a result of overexpression of TFIIS. These results are consistent with the proposal that in the rpo21 mutants, the formation of the RNAPII-TFIIS complex is rate limiting for the passage of the mutant enzyme through pausing sites. In addition to implicating a region of the largest subunit of RNAPII in the process of transcript elongation, our observations provide in vivo evidence that TFIIS is involved in transcription by RNAPII.

Amino Acid Sequence

Evidence that mouse promotion-sensitivity gene pro1 is transcribed by RNA polymerase III.

The murine gene pro1 confers susceptibility to tumor promoters upon transfection into an insensitive host cell. Nucleotide analysis over a minimally active domain of 1049 bp reveals signals expected for a gene transcribed by RNA polymerase II (RNAPII). Similar analysis of the complementary strand shows intragenic signals characteristic of genes transcribed by RNA polymerase III (RNAPIII). We have previously characterized a small, pro1-homologous transcript that is constitutively expressed at lower levels in promotion-insensitive JB6 epidermal cells as compared to promotion-sensitive and transformed clonal variants. To identify whether the pro1 RNAPII or RNAPIII transcription unit encodes the pro1-homologous RNA, RNA probes specific for each of the predicted transcripts were generated. The RNA probe specific for the pro1 RNAPIII transcription unit was found to detect the pro1-hybridizing RNA. Ligating the pro1 RNAPII 5'-flanking region to an interferon gamma reporter sequence failed to induce synthesis of the reporter protein. In addition, pro1 transcripts generated from the predicted RNAPII and RNAPIII transcription units were untranslatable in rabbit reticulocyte lysates. These data are consistent with pro1 associated tumor promotion occurring not through an RNAPII intermediate, but through an RNAPIII intermediate.

Animals

Nuclear class 3 PI3K co-activates fasting-specific chromatin remodelling.

Transcriptional remodelling during fasting ensures metabolic adaptation and provides health benefits across species. Although several regulators of fasting-induced transcription and chromatin are known, how nutrient levels directly influence RNA polymerase II (RNAPII) and epigenetic writers remains unclear. Here we show that lipid kinase class 3 phosphatidylinositol 3-kinase (PI3K-3), a master regulator of autophagy, also functions on chromatin as a co-activator of epigenetic writers to promote RNAPII transcription. PI3K-3 overlaps with transcriptionally engaged RNAPII phosphorylated at Ser5 and with Setd1a/COMPASS, the complex that deposits the activating H3K4me3 mark. Nuclear PI3K-3 interacts with RNAPII and Setd1a/COMPASS and promotes their chromatin binding. PI3K-3 loss reduces RNAPII-S5p and H3K4me3 at selected genes, whereas PI3K-3 overexpression co-activates p300/CBP and chromatin-targeted PI3K-3 increases H3K4me3. During starvation, PI3K-3 induces autophagy genes and drives fasted liver towards ketogenesis and lipid degradation. These findings link nutrient stress to chromatin-mediated transcriptional activation.

Chromatin Assembly and Disassembly

A suppressor of an RNA polymerase II mutation of Saccharomyces cerevisiae encodes a subunit common to RNA polymerases I, II, and III.

RNA polymerase II (RNAPII) is a complex multisubunit enzyme responsible for the synthesis of pre-mRNA in eucaryotes. The enzyme is made of two large subunits associated with at least eight smaller polypeptides, some of which are common to all three RNA polymerase species. We have initiated a genetic analysis of RNAPII by introducing mutations in RPO21, the gene encoding the largest subunit of RNAPII in Saccharomyces cerevisiae. We have used a yeast genomic library to isolate plasmids that can suppress a temperature-sensitive mutation in RPO21 (rpo21-4), with the goal of identifying gene products that interact with the largest subunit of RNAPII. We found that increased expression of wild-type RPO26, a single-copy, essential gene encoding a 155-amino-acid subunit common to RNAPI, RNAPII, and RNAPIII, suppressed the rpo21-4 temperature-sensitive mutation. Mutations were constructed in vitro that resulted in single amino acid changes in the carboxy-terminal portion of the RPO26 gene product. One temperature-sensitive mutation, as well as some mutations that did not by themselves generate a phenotype, were lethal in combination with rpo21-4. These results support the idea that the RPO26 and RPO21 gene products interact.

Base Sequence

Integrator promotes the association of TFIID and RNA polymerase II to maintain pluripotency during development.

The mechanisms by which the expression of pluripotency and Polycomb networks are harmonized to allow the transition from pluripotency to a differentiated state have not been fully elucidated. Integrator complex regulates transcription pause release and RNA processing in metazoans. We show that Integrator is required for stemness and plays a critical role as early as day 2 in embryonic development. While the catalytic endonuclease activity enhances cellular reprogramming, Integrator recruits RNA polymerase II (RNAPII) to promoters and super enhancers of pluripotency and Polycomb genes. Integrator coordinates expression of pluripotency and Polycomb networks by fostering the association of RNAPII and basal transcription factors. We pinpoint a critical role for TATA-binding protein-associated factors (TAFs) in Integrator entry into the preinitiation complex. Taken together, beyond its role in RNAPII pause release, Integrator recruitment of RNAPII ensures an orderly cellular differentiation during development.

RNA Polymerase II

A single cluster of RNA Polymerase II molecules is stably associated with active genes.

In eukaryotic nuclei, transcription is associated with the clustering of RNA Polymerase II (RNAPII) molecules. The mechanisms underlying cluster formation, their interactions with genes, and their impact on transcriptional activity remain heavily debated. Here we take advantage of the naturally occurring increase in transcriptional activity during Zygotic Genome Activation (ZGA) in Drosophila melanogaster embryos to characterize the functional roles of RNAPII clusters in a developmental context. Using single-molecule tracking and lattice light-sheet microscopy, we find that RNAPII cluster formation depends on transcription initiation, and that cluster lifetimes depend on transcriptional activity when not constrained by interphase duration. We show that single clusters are stably associated with active gene loci during transcription and that cluster intensities are strongly correlated with transcriptional output. Collectively our data and simulations on cluster formation kinetics show that RNAPII clusters reflect local accumulations of transcriptionally engaged polymerases and do not form through higher-order mechanisms such as phase separation.

Journal Article

Improved spike-in normalization clarifies the relationship between active histone modifications and transcription.

Spike-in normalization enables quantitative analysis of chromatin immunoprecipitation sequencing (ChIP-seq) signal. Here we introduce a robust dual spike-in normalization approach for ChIP-seq (ChIP-wrangler), optimize parameters and verify its accuracy in quantifying changes in ChIP-seq signal and detecting technical artifacts. We use ChIP-wrangler to revisit recent claims that active histone marks depend on transcription. We show that acute depletion of RNA polymerase II (RNAPII) has a modest impact on H3K27ac levels, with only 6% of peaks significantly changing after RNAPII depletion, indicating that histone acetylation maintenance is not entirely dependent on ongoing transcription. Promoters and enhancers are differentially affected, with 82% of decreasing acetylation peaks located at promoter-distal elements with enhancer-related motifs. ChIP-wrangler provides increased rigor and 'guardrails' for successful spike-in normalization and, as applied here, refines the understanding of crosstalk between RNAPII activity and transcription-associated histone marks.

Histones

Novel control elements in the alpha-1 tubulin gene promoter from Chlamydomonas reinhardii.

Alpha-1 tubulin is the principal alpha-tubulin isotype found in the flagella of the unicellular green alga, Chlamydomonas reinhardii. Although the pattern of tubulin mRNA accumulation and utilization has been examined in some detail in Chlamydomonas (Lefebvre and Rosenbaum 1986), the transcriptional mechanisms establishing tubulin mRNA levels are not understood. To begin an analysis of the alpha-1 tubulin gene transcriptional control elements, we studied a number of promoter mutants of this gene from Chlamydomonas. These mutants, assayed by injection into Xenopus oocyte nuclei, delimit the promoter to 36 bp of DNA upstream of the cap site and 73 bp of the untranslated mRNA leader. A major rate-controlling element lies in a short GC-rich sequence positioned between the TATA homology and the mRNA cap site (position + 1). A similar sequence motif has been found in the same position upstream of all four tubulin genes of Chlamydomonas (Brunke et al. 1984). A 10 bp linker insertion within this sequence abolishes transcription. A far upstream sequence, located in a fragment between -400 and -800, is an efficiency element, whose deletion inhibits transcription in vivo by about 30%. The upstream element (ue) also has the unique ability to drive RNA polymerase II (RNAPII) transcription in vivo when isolated from all downstream promoter elements, unlike any control element described to date. These results suggest that a sequence within the upstream element is an entry site for RNAPII into the tubulin transcription unit.

Base Sequence

ZC3H4/Restrictor Exerts a Stranglehold on Pervasive Transcription.

The regulation of transcription by RNA polymerase II (RNAPII) underpins all cellular processes and is perturbed in thousands of diseases. In humans, RNAPII transcribes ∼20000 protein-coding genes and engages in apparently futile non-coding transcription at thousands of other sites. Despite being so ubiquitous, this transcription is usually attenuated soon after initiation and the resulting products are immediately degraded by the nuclear exosome. We and others have recently described a new complex, "Restrictor", which appears to control such unproductive transcription. Underpinned by the RNA binding protein, ZC3H4, Restrictor curtails unproductive/pervasive transcription genome-wide. Here, we discuss these recent discoveries and speculate on some of the many unknowns regarding Restrictor function and mechanism.

Humans

The CGG triplet repeat binding protein 1 counteracts R-loop induced transcription-replication stress.

The CGG triplet repeat binding protein 1 (CGGBP1) binds to CGG repeats and has several important cellular functions, but how this DNA sequence-specific binding factor affects transcription and replication processes is an open question. Here, we show that CGGBP1 binds human gene promoters containing short (<&#x2009;5) CGG-repeat tracts prone to R-loop formation. Loss of CGGBP1 leads to deregulated transcription, transcription-replication-conflicts (TRCs) and accumulation of Serine-5 phosphorylated RNA polymerase II (RNAPII), indicative of promoter-proximal stalling and a defect in transcription elongation. Consistently, an episomal CGG-repeat-containing model locus as well as endogenous genes show deregulated transcription, R-loop accumulation and increased RNAPII chromatin occupancy in CGGBP1-depleted cells. We identify the DEAD-box RNA:DNA helicases DDX41 and DHX15 as interaction partners specifically recruited by CGGBP1. Co-depletion experiments show that DDX41 and CGGBP1 work in the same pathway to unwind R-loops and avoid TRCs. Together, our work shows that short trinucleotide repeats are a source of genome-destabilizing secondary structures, and cells rely on specific DNA-binding factors to maintain proper transcription and replication coordination at short CGG repeats.

Humans

Transcription factor IID mutants defective for interaction with transcription factor IIA.

Transcription factor IID (TFIID) recognizes the TATA element of promoters transcribed by RNA polymerase II (RNAPII) and serves as the base for subsequent association by other general transcription factors and RNAPII. The carboxyl-terminal domain of TFIID is highly conserved and contains an imperfect repetition of a 60-amino acid sequence. These repeats are separated by a region rich in basic amino acids. Mutagenesis of the lysines in this region resulted in a conditioned phenotype in vivo, and the mutant proteins were defective for interactions with transcription factor IIA in vitro. Binding of TFIID to DNA was unaffected. These results suggest that the basic domain of TFIID is important for protein-protein interactions.

Amino Acid Sequence

DNA topoisomerase II promotes N6-adenosine mRNA methylation.

DNA topoisomerase II (TOP2) is an enzyme that regulates DNA topology, primarily by removing DNA supercoiling. This function is crucial during transcription, as the movement of RNA polymerase II (RNAPII) generates torsional stress. However, the specific role of TOP2 in the regulation of gene expression remains to be fully elucidated, as both TOP2 inhibitors and poisons have been shown to upregulate specific genes. In this study, we show that TOP2 poisoning negatively affects transcription elongation of genes repressed at the level of promoter-proximal pausing. Importantly, this effect is counteracted by defective mRNA N6-adenosine methylation (m6A), which results in altered RNA turnover and pre-mRNA splicing. We propose that TOP2 serves a dual function, supporting the maintenance of basal transcription elongation while simultaneously promoting m6A modification in pre-mRNAs to reduce the overall gene expression output.

RNA Methylation

Crosstalk between chromatin state and ATM signalling in DNA damage-induced transcription stress.

The DNA Damage Response (DDR) is a highly regulated process that safeguards genomic integrity against DNA lesions. Increasing evidence supports a reciprocal relationship between damaged chromatin architecture and the signalling pathways that coordinate the DDR. However, the mechanisms underlying this interplay in response to transcription-blocking DNA lesions remain largely unexplored. Here, we show that stalling of RNA polymerase II (RNAPII) at such lesions induces local chromatin acetylation, mediated primarily by the histone acetyltransferase p300. The resulting chromatin relaxation stimulates the dissociation of mature co-transcriptional spliceosomes from nascent RNA and promotes RNA:DNA hybrid (R-loop) formation, leading to ATM activation. In turn, activated ATM modulates chromatin conformation by phosphorylating histone H2A.X and triggering p38MAPK/MSK1-dependent histone H3S10 phosphorylation. Our findings highlight the cross-regulation between chromatin state and ATM signalling as a key component of the cellular response to transcription stress.

Ataxia Telangiectasia Mutated Proteins

Should I stay or should I go: TFIIIC as assembly factor and barrier in RNA polymerase III transcription.

Critical for the regulation of eukaryotic gene transcription is the assembly and interplay of general transcription factors (GTFs) with RNA polymerases (RNAPs), leading to the formation of pre-initiation complexes (PICs) as a rate-limiting step in transcription activation. Compared with RNAPII PIC assembly involving many GTFs, activators, and co-activators, RNAPIII PIC assembly is less complex, involving mainly the four GTFs TFIIIA, TFIIIB, TFIIIC, and snRNA activating protein complex with only a few additional factors. The RNAPIII-specific GTF TFIIIC is present in type I and II promoters. One prominent area of investigation has been the dynamic interaction between TFIIIC and its promoter elements, the varying affinities of TFIIIC toward these elements, and the flexible linker within TFIIIC. Additionally, evidence suggests that TFIIIC may play a dual role, acting as an assembly factor that positions TFIIIB during PIC formation and as a barrier during RNAPIII-mediated transcription. By summarizing recent structural, biochemical, and genomic data, this review explores the mechanisms by which RNAPIII-specific GTFs, with a focus on TFIIIC, dynamically regulate RNAPIII transcription.

RNA Polymerase III

Machine learning on multiple epigenetic features reveals H3K27Ac as a driver of gene expression prediction across patients with glioblastoma.

Epigenetic mechanisms play a crucial role in driving transcript expression and shaping the phenotypic plasticity of glioblastoma stem cells (GSCs), contributing to tumor heterogeneity and therapeutic resistance. These mechanisms dynamically regulate the expression of key oncogenic and stemness-associated genes, enabling GSCs to adapt to environmental cues and evade targeted therapies. Importantly, epigenetic reprogramming allows GSCs to transition between cellular states, including therapy-resistant mesenchymal-like phenotypes, underscoring the need for epigenetic-targeting strategies to disrupt these adaptive processes. Understanding these epigenetic drivers of gene expression provides a foundation for novel therapeutic interventions aimed at eradicating GSCs and improving glioblastoma outcomes. Using machine learning (ML), we employ cross-patient prediction of transcript expression in GSCs by combining epigenetic features from various sources, including ATAC-seq, CTCF ChIP-seq, RNAPII ChIP-seq, H3K27Ac ChIP-seq, and RNA-seq. We investigate different ML and deep learning (DL) models for this task and ultimately build our final pipeline using XGBoost. The model trained on one patient generalizes to other 11 patients with high performance. Notably, H3K27Ac alone from a single patient is sufficient to predict gene expression in all 11 patients. Furthermore, the distribution of H3K27Ac peaks across the genomes of all patients is remarkably similar. These findings suggest that GSCs share a common distributional pattern of enhancer activity characterized by H3K27Ac, which can be utilized to predict gene expression in GSCs across patients. In summary, while GSCs are known for their transcriptomic and phenotypic heterogeneity, we propose that they share a common epigenetic pattern of enhancer activation that defines their underlying transcriptomic expression pattern. This pattern can predict gene expression across patient samples, providing valuable insights into the biology of GSCs.

Glioblastoma