90<i>S</i> pre-ribosome transformation into the primordial 40<i>S</i> subunit.

Abstract

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