ジャーナル: Nature / 年: 2025 タイトル: Structural insights into human Pol III transcription initiation in action. 著者: Qianmin Wang / Yulei Ren / Qianwei Jin / Xizi Chen / Yanhui Xu / 要旨: RNA polymerase III (Pol III) transcribes highly demanded RNAs grouped into three types of classical promoters, including type 1 (5S rRNA), type 2 (tRNA) and type 3 (short non-coding RNAs, such as U6, ...RNA polymerase III (Pol III) transcribes highly demanded RNAs grouped into three types of classical promoters, including type 1 (5S rRNA), type 2 (tRNA) and type 3 (short non-coding RNAs, such as U6, 7SK and RNase H1) promoters. While structures of the Pol III preinitiation complex (PIC) and elongation complex (EC) have been determined, the mechanism underlying the transition from initiation to elongation remains unclear. Here we reconstituted seven human Pol III transcribing complexes (TC4, TC5, TC6, TC8, TC10, TC12 and TC13) halted on U6 promoters with nascent RNAs of 4-13 nucleotides. Cryo-electron microscopy structures captured initially transcribing complexes (ITCs; TC4 and TC5) and ECs (TC6-13). Together with KMnO footprinting, the data reveal extensive modular rearrangements: the transcription bubble expands from PIC to TC5, followed by general transcription factor (GTF) dissociation and abrupt bubble collapse from TC5 to TC6, marking the ITC-EC transition. In TC5, SNAPc and TFIIIB remain bound to the promoter and Pol III, while the RNA-DNA hybrid adopts a tilted conformation with template DNA blocked by BRF2, a TFIIIB subunit. Hybrid forward translocation during ITC-EC transition triggers BRF2-finger retraction, GTF release and transcription-bubble collapse. Pol III then escapes the promoter while GTFs stay bound upstream, potentially enabling reinitiation. These findings reveal molecular insights into Pol III dynamics and reinitiation mechanisms on type 3 promoters of highly demanded small RNAs, with the earliest documented initiation-elongation transition for an RNA polymerase.
全体 : Human RNA Polymerase III de novo transcribing complex 6 on 7SK pr...
全体
名称: Human RNA Polymerase III de novo transcribing complex 6 on 7SK promoter (7SK-TC6)
要素
複合体: Human RNA Polymerase III de novo transcribing complex 6 on 7SK promoter (7SK-TC6)
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超分子 #1: Human RNA Polymerase III de novo transcribing complex 6 on 7SK pr...
超分子
名称: Human RNA Polymerase III de novo transcribing complex 6 on 7SK promoter (7SK-TC6) タイプ: complex / ID: 1 / 親要素: 0 詳細: RNA polymerase III transcribing complex 6 RNA polymerase III in transcription elongation stage
由来(天然)
生物種: Homo sapiens (ヒト)
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実験情報
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構造解析
手法
クライオ電子顕微鏡法
解析
単粒子再構成法
試料の集合状態
particle
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試料調製
緩衝液
pH: 8
凍結
凍結剤: ETHANE-PROPANE
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電子顕微鏡法
顕微鏡
TFS KRIOS
撮影
フィルム・検出器のモデル: GATAN K3 (6k x 4k) / 平均電子線量: 50.0 e/Å2
電子線
加速電圧: 300 kV / 電子線源: FIELD EMISSION GUN
電子光学系
照射モード: FLOOD BEAM / 撮影モード: DARK FIELD / 最大 デフォーカス(公称値): 2.5 µm / 最小 デフォーカス(公称値): 1.5 µm