A Translation-Like Cycle Is a Quality Control Checkpoint for Maturing 40S Ribosome Subunits

Assembly factors (AFs) prevent premature translation initiation on small (40S) ribosomal subunit assembly intermediates by blocking ligand binding. However, it is unclear how AFs are displaced from maturing 40S ribosomes, if or how maturing subunits are assessed for fidelity, and what prevents premature translation initiation once AFs dissociate. Here we show that maturation involves a translation-like cycle whereby the translation factor eIF5B, a GTPase, promotes joining of large (60S) subunits with pre-40S subunits to give 80S-like complexes, which are subsequently disassembled by the termination factor Rli1, an ATPase. The AFs Tsr1 and Rio2 block the mRNA channel and initiator tRNA binding site, and therefore 80S-like ribosomes lack mRNA or initiator tRNA. After Tsr1 and Rio2 dissociate from 80S-like complexes Rli1-directed displacement of 60S subunits allows for translation initiation. This cycle thus provides a functional test of 60S subunit binding and the GTPase site before ribosomes enter the translating pool. [Display omitted] ► 60S ribosomes join maturing pre-40S subunits in a translation-like cycle ► This cycle is conserved, eIF5B-dependent, and provides a fidelity checkpoint ► The resulting 80S-like ribosomes are not translation initiation intermediates ► Rli1•Dom34 disassemble 80S-like ribosomes for translation initiation Discovery of a translation-like cycle that involves the assembly and disassembly of inactive 80S-like ribosomes lacking mRNA and tRNA identifies a quality control step where cells test the functionality of immature 40S subunits before the final stages of ribosome maturation are allowed to proceed.