9K2G

Human RNA Polymerase III de novo transcribing complex 13 (TC13)

Summary for 9K2G

• Entry DOI: 10.2210/pdb9k2g/pdb
• EMDB information: 61994
• Descriptor: DNA-directed RNA polymerase III subunit RPC1, DNA-directed RNA polymerases I, II, and III subunit RPABC5, DNA-directed RNA polymerases I and III subunit RPAC2, ... (25 entities in total)
• Functional Keywords: elongation complex, transcription/dna/rna, transcription-dna-rna complex
• Biological source: Homo sapiens (human); More
• Total number of polymer chains: 21
• Total formula weight: 823441.62

Authors

Wang, Q.,Ren, Y.,Jin, Q.,Chen, X.,Xu, Y. (deposition date: 2024-10-17, release date: 2025-04-16, Last modification date: 2025-06-18)

Primary citation

Wang, Q.,Ren, Y.,Jin, Q.,Chen, X.,Xu, Y.
Structural insights into human Pol III transcription initiation in action.
Nature, 2025

PubMed Abstract: 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.

PubMed: 40468065
DOI: 10.1038/s41586-025-09093-w

Experimental method

ELECTRON MICROSCOPY (3 Å)