A single 2'-O-methylation of ribosomal RNA gates assembly of a functionalribosome

Abstract

RNA modifications are widespread in biology, and particularly abundant in ribosomal RNA. However, the significance of these modifications is not well understood. We show that methylation of a single universally conserved nucleotide, in the catalytic center of the large subunit, gates ribosome assembly. Massively parallel mutational scanning of the essential nuclear GTPase Nog2 identified important interactions with ribosomal RNA helix 92, particularly with the methylated A-site base Gm2922. We found that 2-O-methylation of G2922 is needed for efficient nuclear export of the large subunit. Critically, we identified single amino acid changes in Nog2 that completely bypass its dependence on G2922 methylation. By solving the cryo-EM structure of the unmodified nascent subunit, we reveal how methylation flips Gm2922 into the active site channel of Nog2. This work demonstrates that a single RNA modification is a critical checkpoint in ribosome biogenesis, and suggests that RNA modifications can play an important role in regulation and assembly of macromolecular machines.