The late stages of yeast mitoribosome large subunit biogenesis

The Saccharomyces cerevisiae mitoribosome synthesizes eight mitochondrial DNA-encoded proteins essential for oxidative phosphorylation. Mitoribosome large subunit (mtLSU) biogenesis involves the conserved DEAD-box helicase Mrh4 and the GTPases Mtg1/GTPBP7 and Mtg2/GTPBP5. Here, we have employed genetic, biochemical, in vitro reconstitution, and cryo-EM approaches to elucidate their hierarchical action during the late stages of mtLSU assembly. We show that Mrh4-mediated bL33m incorporation precedes Mtg1 recruitment to the 21S rRNA. Cryo-EM structures of mitoribosome assembly intermediates accumulating in the absence of Mtg1 or uL16m reveal that Mtg1 restructures the 21S rRNA H73-75 and H93 domains to their mature fold. This subsequently allows the structuring of neighboring peptidyl transfer center region helices and the incorporation of uL6m, uL16m, bL35m, and bL36m during late mtLSU maturation. Unexpectedly, monosomes containing immature mtLSU assemble in Mrh4-, bL33m-, uL16m-, Mtg1-, and Mtg2-depleted mitochondria, at levels that increase with the maturation state of the mtLSU particle. Our data have shed light on the rRNA folding events and the structuring of the MRPs that occur during the late stages of assembly. They have provided insight into the roles of assembly factors Mrh4, Mtg1, and Mtg2 during the process and revealed evolutionarily conserved mechanisms underlying mitochondrial ribosome assembly.