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Assembly pathway for mitoribosome The biogenesis of ribosomes is a multistep process facilitated by assembly factors. Saurer et al. provided structural information on the maturation process of the mitochondrial ribosome, or mitoribosome, in the parasitic protozoan Trypanosoma brucei (see the Perspective by Karbstein). Cells evolved a dedicated machinery for maturation of the small subunit of the mitoribosome, including the formation of three distinct and well-structured assembly intermediates. Comparison of intermediates and the mature mitoribosome reveals how assembly factors and ribosomal proteins work together to fold and stabilize ribosomal RNA. Science , this issue p. 1144 ; see also p. 1077

In contrast to the bacterial translation machinery, mitoribosomes and mitochondrial translation factors are highly divergent in terms of composition and architecture. There is increasing evidence that the biogenesis of mitoribosomes is an intricate pathway, involving many assembly factors. To better understand this process, we investigated native assembly intermediates of the mitoribosomal large subunit from the human parasite Trypanosoma brucei using cryo-electron microscopy. We identify 28 assembly factors, 6 of which are homologous to bacterial and eukaryotic ribosome assembly factors. They interact with the partially folded rRNA by specifically recognizing functionally important regions such as the peptidyltransferase center. The architectural and compositional comparison of the assembly intermediates indicates a stepwise modular assembly process, during which the rRNA folds toward its mature state. During the process, several conserved GTPases and a helicase form highly intertwined interaction networks that stabilize distinct assembly intermediates. The presented structures provide general insights into mitoribosomal maturation. © 2020 Elsevier Inc. The structures of two assembly intermediates of the Trypanosoma brucei mitoribosomal large subunit in combination with biochemical analysis provide insights into the stepwise mitoribosomal biogenesis process that involves numerous assembly factors functioning as enzymes or scaffold components. © 2020 Elsevier Inc. ISSN:1097-2765 ISSN:1097-4164

Structure of the largest, most complex ribosome Ribosomes are two-subunit ribonucleoprotein assemblies that catalyze the translation of messenger RNA into protein. Ribosomal RNAs (rRNAs) play key structural and functional roles. Ramrath et al. report the high-resolution structure of mitochondrial ribosomes from the unicellular parasite Trypanosoma brucei that contain the smallest known rRNAs. The trypanosomal mitoribosome is the most complex ribosomal assembly characterized, with two rRNAs and 126 proteins. The increased protein subunits have substituted for rRNA as an architectural scaffold. The structure also reveals the minimal core needed for ribosome function. Science , this issue p. eaau7735

Science, 348 (6232), ISSN:0036-8075, ISSN:1095-9203

[This corrects the article DOI: 10.1371/journal.pbio.1002557.].

Nature, 505 (7484), ISSN:0028-0836, ISSN:1476-4687

The mitochondrial ribosome, which translates all mitochondrial DNA (mtDNA)-encoded proteins, should be tightly regulated pre- and post-transcriptionally. Recently, we found RNA-DNA differences (RDDs) at human mitochondrial 16S (large) rRNA position 947 that were indicative of post-transcriptional modification. Here, we show that these 16S rRNA RDDs result from a 1-methyladenosine (m1A) modification introduced by TRMT61B, thus being the first vertebrate methyltransferase that modifies both tRNA and rRNAs. m1A947 is conserved in humans and all vertebrates having adenine at the corresponding mtDNA position (90% of vertebrates). However, this mtDNA base is a thymine in 10% of the vertebrates and a guanine in the 23S rRNA of 95% of bacteria, suggesting alternative evolutionary solutions. m1A, uridine, or guanine may stabilize the local structure of mitochondrial and bacterial ribosomes. Experimental assessment of genome-edited Escherichia coli showed that unmodified adenine caused impaired protein synthesis and growth. Our findings revealed a conserved mechanism of rRNA modification that has been selected instead of DNA mutations to enable proper mitochondrial ribosome function., PLoS Biology, 14 (9), ISSN:1544-9173, ISSN:1545-7885

Nature, 515 (7526), ISSN:0028-0836, ISSN:1476-4687