Poster Presentation The 42nd Lorne Conference on Protein Structure and Function 2017

Structure of RNA polymerase I transcribing ribosomal DNA genes (#221)

Simon Neyer 1 , Michael Kunz 2 , Christian Geiss 2 , Merle Hantsche 1 , Victor-Valentin Hodirnau 2 , Anja Seybert 2 , Christoph Engel 1 , Margot P. Scheffer 2 , Patrick Cramer 1 , Achilleas S. Frangakis 2
  1. Max Planck Institute for Biophysical Chemistry, Goettingen, LOWER SAXONY, Germany
  2. Buchmann Institute for Molecular Life Sciences and Institute for Biopyhsics, Goethe University Frankfurt, Frankfurt, Hessen, Germany

RNA polymerase I (Pol I) is a highly processive enzyme that transcribes ribosomal DNA (rDNA) and regulates growth of eukaryotic cells1,2. Crystal structures of free Pol I from the yeast Saccharomyces cerevisiae revealed dimers of the enzyme stabilized by a ‘connector’ element and an expanded cleft containing the active centre in an inactive conformation3,4. The central bridge helix was unfolded and a Pol-I-specific ‘expander’ element occupied the DNA-template-binding site. The structure of Pol I in its active transcribing conformation is yet to be determined whereas structures of Pol II and Pol III have been solved with bound DNA template and RNA transcript5,6. Here we report structures of active transcribing Pol I from yeast solved by two different cryo-electron microscopy approaches. A single-particle structure at 3.8 Å resolution reveals a contracted active centre cleft with bound DNA and RNA, and a narrowed pore beneath the active site that no longer holds the RNA cleavage-stimulating domain of subunit A12.2. A structure at 29 Å resolution, that was determined from cryo-electron tomograms of Pol I enzymes transcribing cellular rDNA, confirms contraction of the cleft and reveals that incoming and exiting rDNA enclose an angle of ~150°. The structures suggest a model for the regulation of transcription elongation in which contracted and expanded polymerase conformations are associated with active and inactive states, respectively.

 

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