90 S pre-ribosome transformation into the primordial 40 S subunit

Science. 2020 Sep 18;369(6510):1470-1476. doi: 10.1126/science.abb4119.


Production of small ribosomal subunits initially requires the formation of a 90S precursor followed by an enigmatic process of restructuring into the primordial pre-40S subunit. We elucidate this process by biochemical and cryo-electron microscopy analysis of intermediates along this pathway in yeast. First, the remodeling RNA helicase Dhr1 engages the 90S pre-ribosome, followed by Utp24 endonuclease-driven RNA cleavage at site A1, thereby separating the 5'-external transcribed spacer (ETS) from 18S ribosomal RNA. Next, the 5'-ETS and 90S assembly factors become dislodged, but this occurs sequentially, not en bloc. Eventually, the primordial pre-40S emerges, still retaining some 90S factors including Dhr1, now ready to unwind the final small nucleolar U3-18S RNA hybrid. Our data shed light on the elusive 90S to pre-40S transition and clarify the principles of assembly and remodeling of large ribonucleoproteins.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cryoelectron Microscopy
  • DEAD-box RNA Helicases / chemistry
  • DEAD-box RNA Helicases / metabolism*
  • Nuclear Proteins / chemistry
  • Nuclear Proteins / metabolism*
  • Protein Conformation
  • RNA Cleavage
  • RNA, Ribosomal, 18S / chemistry
  • RNA, Ribosomal, 18S / metabolism
  • Ribosomal Proteins / metabolism
  • Ribosome Subunits, Large, Eukaryotic / chemistry
  • Ribosome Subunits, Large, Eukaryotic / metabolism*
  • Ribosome Subunits, Small, Eukaryotic / chemistry
  • Ribosome Subunits, Small, Eukaryotic / metabolism*
  • Saccharomyces cerevisiae / metabolism*
  • Saccharomyces cerevisiae Proteins / chemistry
  • Saccharomyces cerevisiae Proteins / metabolism*


  • Nuclear Proteins
  • PNO1 protein, S cerevisiae
  • RNA, Ribosomal, 18S
  • Ribosomal Proteins
  • Saccharomyces cerevisiae Proteins
  • Utp24 protein, S cerevisiae
  • ECM16 protein, S cerevisiae
  • DEAD-box RNA Helicases