Biology subjects

Sekine, S.-i.

Publications and source records attributed to Sekine, S.-i..

9 recordsLinked to original sources

bioRxiv2026

Structural basis for site-specific histone H3 acetylation-dependent regulation of RNAPII transcription through nucleosomes

Acetylation of histone H3 regulates chromatin dynamics and transcription, but how specific acetylation sites affect RNA polymerase II (RNAPII) transcription through nucleosomes remains unclear. Here, we found that H3 acetylation at Lys56 and Lys122 markedly enhances RNAPII transcription through nucleosomes, whereas acetylation at Lys64 has little effect. To elucidate the structural basis for these functional differences, we determined cryo-electron microscopy (cryo-EM) structures of nucleosomes bearing site-specific acetylation at H3K56, H3K64, or H3K122. The cryo-EM structures revealed that H3K56ac and H3K122ac locally weaken histone-DNA interactions at the DNA entry/exit region and near the dyad, respectively, while H3K64ac induces no detectable structural changes. These structural differences correlate with the observed transcriptional outcomes, indicating that acetylation at H3K56 and H3K122, but not H3K64, alleviates the nucleosomal barrier to RNAPII progression. Our findings provide direct structural evidence that specific acetylations within the histone fold domain of H3 finetune nucleosome dynamics to facilitate RNAPII transcription.

molecular biology↗

bioRxiv2025

Structural basis of asymmetric transcription through a composite nucleosome formed by a hexasome and an octasome

The overlapping dinucleosome (OLDN), a composite chromatin particle consisting of a hexasome and a canonical octasome, forms immediately downstream of transcription start sites, likely through chromatin remodeling activity, and has been proposed to act as a transient regulatory intermediate during transcription. However, how RNA polymerase II (RNAPII) engages with and transcribes through this unusual structure remains unclear. Here, we reconstituted OLDNs in vitro and performed transcription assays with RNAPII. We found that transcription efficiency was markedly higher when RNAPII initiated from the hexasome side than from the octasome side. Cryo-electron microscopy further revealed that transcription from the hexasome side induced pronounced conformational rearrangements, in which RNAPII progression dramatically opened the hexasome-octasome interface. These results uncover the mechanism by which RNAPII senses the intrinsic transcriptional polarity of OLDNs and suggest that OLDNs function as dynamic, directionally sensitive regulators of transcription elongation.

molecular biology↗

bioRxiv2025

Structural basis of RNA polymerase II transcription on the H3-H4 octasome

The H3-H4 octasome is a nucleosome-like particle in which two DNA gyres are wrapped around each H3-H4 tetramer disk, forming a clamshell-like configuration. In the present study, we performed in vitro RNAPII transcription assays with the H3-H4 octasome and found that RNAPII transcribed the H3-H4 octasome more efficiently than the nucleosome. RNAPII paused at only one position, superhelical location (SHL) -4 in the H3-H4 octasome, in contrast to pausing at the SHL(-5), SHL(-2), and SHL(-1) positions in the nucleosome. Cryo-electron microscopy analysis revealed that two H3-H4 tetramer disks are retained when the RNAPII paused at the SHL(-4) position of the H3-H4 octasome. However, when RNAPII reached the SHL(-0.5) position, five base pairs before the dyad position of the H3-H4 octasome, the proximal H3-H4 tetramer was disassembled but the distal H3-H4 tetramer still remained on the DNA. Therefore, RNAPII efficiently transcribes the H3-H4 octasome by stepwise H3-H4 tetramer disassembly.

molecular biology↗

bioRxiv2024

Structural basis of transcription-coupled H3K36 trimethylation by Set2 and RNAPII elongation complex in the nucleosome

Trimethylation of the histone H3K36 residue (H3K36me3) plays an indispensable role in ensuring transcription fidelity by suppressing undesired cryptic transcription in chromatin. The H3K36me3 modification is accomplished by Set2/SETD2 during transcription elongation by the RNA polymerase II elongation complex (EC). Here we found that the Set2-mediated H3K36me3 deposition occurs primarily on the nucleosome reassembling behind the EC. Cryo-electron microscopy structures of the transcribing EC complexed with Set2 and the reassembled nucleosome revealed that Set2 is anchored by the Spt6 subunit of the EC and captures an H3 N-terminal tail of the nucleosome. Abrogation of the Set2-Spt6 interaction leads to defective transcription-coupled H3K36me3 deposition. These insights elucidate the structure-based mechanism of transcription-coupled H3K36me3 deposition in chromatin. One-Sentence SummaryCryo-EM structures of the EC-Set2-nucleosome complex reveal the mechanism of H3 Lys36 trimethylation in chromatin.

biochemistry↗

bioRxiv2024

Structural basis of eukaryotic transcription termination by the Rat1 exonuclease complex

The 5-3 exoribonuclease Rat1 (or Xrn2) plays a pivotal role in termination of mRNA transcription by eukaryotic RNA polymerase II (RNAPII). Rat1 forms a complex with its partner proteins, Rai1 and Rtt103, and acts as a "torpedo" to bind transcribing RNAPII and dissociate DNA/RNA from it. Here we report the cryo-electron microscopy structures of a Rat1-Rai1-Rtt103 complex and three Rat1-Rai1-associated RNAPII complexes (type-1, type-1b, and type-2) from the yeast Komagataella phaffii. The Rat1-Rai1-Rtt103 complex structure revealed that Rat1 and Rai1 form a heterotetramer, with a single Rtt103 bound between two Rai1 molecules. In the type-1 complex, Rat1-Rai1 forms a heterodimer and directly binds to the RNA exit site of transcribing RNAPII to extract RNA into the Rat1 nuclease active site. This interaction changes the RNA path in favor of termination (the "pre-termination" state). The type-1b and type-2 complexes have no bound DNA/RNA, likely representing the "post-termination" states. These structures shed light on the mechanism of eukaryotic mRNA transcription termination.

biochemistry↗

bioRxiv2024

Structural insight into bacterial co-transcriptional translation initiation

In bacteria, transcription and translation are tightly coupled, forming a transcription-translation complex (TTC) between RNA polymerase (RNAP) and the ribosome. As nascent mRNA emerging from RNAP is susceptible to ribonuclease digestion, undesired RNA folding, and R-loop formation, immediate TTC formation is important. Here, we report the cryo-electron microscopy structures that capture the translation initiation complex assembly on transcribing RNAP. As a short mRNA emerging from RNAP, the 30S ribosomal subunit binds the RNAP on its inter-subunit side, interacting with the mRNA 5-region and the initiator tRNA. The RNAP could relocate to the canonical mRNA-entry site of 30S, threading the mRNA into a path formed between RNAP and 30S. The subsequent 50S joining establishes a TTC. These structures illustrate the transcription-coupled translation initiation, while protecting mRNA. One-Sentence SummaryCryo-EM captures the ribosome assembly on transcribing RNA polymerase in the presence of translation initiation factors.

biochemistry↗

bioRxiv2022

Structural basis of the transcription termination factor Rho engagement with transcribing RNA polymerase

Transcription termination is an essential step in transcription by RNA polymerase (RNAP) and crucial for gene regulation. For many bacterial genes, transcription termination is mediated by the ATP-dependent RNA translocase/helicase Rho, which causes the dissociation of RNA/DNA from RNAP elongation complex (EC). However, structural basis of the interplay between Rho and RNAP remains obscure. Here we report the cryo-electron microscopy structure of the Rho-engaged EC. The Rho hexamer binds RNAP through the C-terminal domains, which surround the RNA-exit site of RNAP, directing the nascent RNA seamlessly from the RNA exit to the Rho central channel. The {beta}-flap tip at the RNA exit is critical to the Rho-dependent RNA release, and its deletion causes an alternative Rho-RNAP binding mode, which is irrelevant to termination. The Rho-binding site overlaps with the binding sites of other macromolecules, such as ribosomes, providing a general basis of gene regulation. TeaserCryo-EM captures the structure of an RNA polymerase elongation complex engaged with the termination factor Rho.

biochemistry↗

bioRxiv2022

ERK2-topoisomerase II regulatory axis is important for gene activation in immediate early genes

The transcription of stress-inducible genes requires synchronized and robust activation, which is critical for organismal survival and homeostasis. The function of the mitogen-activated protein kinase (MAPK) signaling pathway is required for the activation of immediate early genes (IEGs), including EGR1 and FOS, for cell growth and proliferation1-3. In addition, recent studies have identified topoisomerase II (TOP2) as one of the important regulators of the transcriptional activation of IEGs4-6. However, the mechanism underlying transcriptional regulation involving TOP2 in IEG activation has remained unknown. Here, we demonstrate that ERK2, but not ERK1, is important for IEG transcriptional activation and report a critical ELK1 binding sequence for ERK2 function at the EGR1 gene. Our data indicate that both ERK1 and ERK2 extensively phosphorylate the C-terminal domain of TOP2B at mutual and distinctive residues. Although both ERK1 and ERK2 enhance the catalytic rate of TOP2B required to relax positive DNA supercoiling, ERK1 can relax the DNA by itself and produces a semi-relaxed DNA, which is apparently resistant to TOP2B catalysis. Inhibition of ERK2 kinase activity or ERK2 knock-down interferes with transcription and deregulates TOP2B in IEGs. Furthermore, we obtained the first cryo-EM structure of the human cell-purified TOP2B and etoposide together with the EGR1 transcriptional start site (50 nt; -30 to +20) that has the strongest affinity to TOP2B within -423 to +332. The structure elucidated in our studies showed TOP2B-mediated breakage and dramatic bending of the double-stranded DNA, comparable to previously reported structures of TOP2. Our cell-based analyses showed transcriptional activation by etoposide and transcriptional inhibition by ICRF193 at EGR1 and FOS, suggesting that TOP2B-mediated DNA break to favor transcriptional activation. Taken together, this study suggests that activated ERK2 phosphorylates TOP2B to regulate TOP2-DNA interactions and favor transcriptional activation in IEGs. We propose that TOP2B association, catalysis, and dissociation on its substrate DNA are important processes for regulating transcription and that ERK2-mediated TOP2B phosphorylation may be key for the catalysis and dissociation steps.

molecular biology↗

bioRxiv2021

Unidirectional ion transport mechanism of a light-driven chloride pump revealed using X-ray free electron lasers

Light-driven chloride-pumping rhodopsins actively transport anions, including various halide ions, across cell membranes. Recent studies using time-resolved serial femtosecond crystallography (TR-SFX) have uncovered the structural changes and ion transfer mechanisms in light-driven cation-pumping rhodopsins. However, the mechanism by which the conformational changes pump an anion to achieve unidirectional ion transport, from the extracellular side to the cytoplasmic side, in anion-pumping rhodopsins remains enigmatic. We have collected TR-SFX data of Nonlabens marinus rhodopsin-3 (NM-R3), derived from a marine flavobacterium, at 10 s and 1 ms time-points after photoexcitation. Our structural analysis reveals the conformational alterations during ion transfer and after ion release. Movements of the retinal chromophore initially displace a conserved tryptophan to the cytoplasmic side of NM-R3, accompanied with a slight shift of the halide ion bound to the retinal. After ion release, the inward movements of helix C and helix G and the lateral displacements of the retinal block access to the extracellular side of NM-R3. Anomalous signal data have also been obtained from NM-R3 crystals containing iodide ions. The anomalous density maps provide insight into the halide binding site for ion transfer in NM-R3. SignificanceLight-driven chloride pumps have been identified in various species, including archaea and marine flavobacteria. The function of ion transportation controllable by light is utilized for optogenetics tools in neuroscience. Chloride pumps differ among species, in terms of amino acid homology and structural similarity. Our time-resolved crystallographic studies using X-ray free electron lasers reveal the molecular mechanism of halide ion transfer in a light-driven chloride pump from a marine flavobacterium. Our data indicate a common mechanism in chloride pumping rhodopsins, as compared to previous low temperature trapping studies of chloride pumps. These findings are significant not only for further improvements of optogenetic tools but also for a general understanding of the ion pumping mechanisms of microbial rhodopsins.

molecular biology↗