Emergence of the primordial pre-60S from the 90S pre-ribosome

Synthesis of ribosomes begins in the nucleolus with formation of the 90S pre-ribosome, during which the pre 40S and pre-60S pathways diverge by pre-rRNA cleavage. However, it remains unclear how, after this uncoupling, the earliest pre-60S subunit continues to develop. Here, we reveal a large-subunit intermediate at the beginning of its construction when still linked to the 90S, the precursor to the 40S subunit. This primordial pre 60S is characterized by the SPOUT domain methyltransferase Upa1-Upa2, large a-solenoid scaffolds, Mak5, one of several RNA helicases, and two small nucleolar RNA (snoRNAs), C/D box snR190 and H/ACA box snR37. The emerging pre-60S does not efficiently disconnect from the 90S pre-ribosome in a dominant mak5 helicase mutant, allowing a 70-nm 90S-pre-60S bipartite particle to be visualized by electron microscopy. Our study provides insight into the assembly pathway when the still-connected nascent 40S and 60S subunits are beginning to separate.

Automated summary

The synthesis of ribosomes in the nucleolus involves the formation of pre-ribosomes, from which the 40S and 60S pathways diverge. However, it is unknown how the earliest pre-60S subunit continues to develop after this separation. This study reveals a large-subunit intermediate, connected to the 90S pre-ribosome, that contains specific proteins and RNAs. The inability to disconnect from the 90S in a mutant allows for the visualization of a 70nm 90S-pre-60S particle. This research provides insights into the assembly pathway of the still-connected nascent 40S and 60S subunits.