Mechanism for the initiation of co-transcriptional pre-60 S assembly

Eukaryotic ribosomal large subunit (60 S ) assembly requires an internal transcribed spacer 2 (ITS2) to license both nucleolar and nuclear pre-60 S assembly intermediates. The underlying molecular mechanisms responsible for nucleation of pre-60 S assembly, quality control, and installation of ITS2 during co-transcriptional stages remain unknown. Here we report the earliest co-transcriptional assembly intermediates of the eukaryotic 60 S subunits. Together with biochemical assays, our data reveal the architecture of co-transcriptional pre-60 S assembly initiation and progression, as well as the molecular logic of an assembly checkpoint. This study highlights an evolutionary solution by which complex RNA folding processes can be parallelized and integrated via biological AND-gating.