Structural insights into distinct mechanisms of RNA polymerase II and III recruitment to snRNA promoters

Nat Commun. 2025 Jan 2;16(1):141. doi: 10.1038/s41467-024-55553-8.

Abstract

RNA polymerase III (Pol III) transcribes short, essential RNAs, including the U6 small nuclear RNA (snRNA). At U6 snRNA genes, Pol III is recruited by the snRNA Activating Protein Complex (SNAPc) and a Brf2-containing TFIIIB complex, forming a pre-initiation complex (PIC). Uniquely, SNAPc also recruits Pol II at the remaining splicesosomal snRNA genes (U1, 2, 4 and 5). The mechanism of SNAPc cross-polymerase engagement and the role of the SNAPC2 and SNAPC5 subunits remain poorly defined. Here, we present cryo-EM structures of the full-length SNAPc-containing Pol III PIC assembled on the U6 snRNA promoter in the open and melting states at 3.2-4.2 Å resolution. The structural comparison revealed differences with the Saccharomyces cerevisiae Pol III PIC and the basis of selective SNAPc engagement within Pol III and Pol II PICs. Additionally, crosslinking mass spectrometry localizes SNAPC2 and SNAPC5 near the promoter DNA, expanding upon existing descriptions of snRNA Pol III PIC structure.

MeSH terms

  • Cryoelectron Microscopy*
  • Humans
  • Models, Molecular
  • Promoter Regions, Genetic*
  • RNA Polymerase II* / chemistry
  • RNA Polymerase II* / genetics
  • RNA Polymerase II* / metabolism
  • RNA Polymerase III* / chemistry
  • RNA Polymerase III* / genetics
  • RNA Polymerase III* / metabolism
  • RNA, Small Nuclear* / chemistry
  • RNA, Small Nuclear* / genetics
  • RNA, Small Nuclear* / metabolism
  • Saccharomyces cerevisiae Proteins* / chemistry
  • Saccharomyces cerevisiae Proteins* / genetics
  • Saccharomyces cerevisiae Proteins* / metabolism
  • Saccharomyces cerevisiae* / genetics
  • Saccharomyces cerevisiae* / metabolism
  • Transcription Factors

Substances

  • RNA, Small Nuclear
  • RNA Polymerase III
  • RNA Polymerase II
  • Saccharomyces cerevisiae Proteins
  • U6 small nuclear RNA
  • SNAPC protein complex, human
  • Transcription Factors