Production of small ribosomal subunits initially requires the formation of a 90 S precursor followed by an enigmatic process of restructuring into the primordial pre-40 S 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 90 S pre-ribosome, followed by Utp24 endonuclease-driven RNA cleavage at site A 1 , thereby separating the 5'-external transcribed spacer (ETS) from 18 S ribosomal RNA. Next, the 5'-ETS and 90 S assembly factors become dislodged, but this occurs sequentially, not en bloc. Eventually, the primordial pre-40 S emerges, still retaining some 90 S factors including Dhr1, now ready to unwind the final small nucleolar U3-18 S RNA hybrid. Our data shed light on the elusive 90 S to pre-40 S transition and clarify the principles of assembly and remodeling of large ribonucleoproteins.
Organizational Affiliation:
Gene Center, Department of Biochemistry, University of Munich, 81377 Munich, Germany.
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