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

Mitochondrial protein import is essential for all eukaryotes and mediated by hetero-oligomeric protein translocases thought to be conserved within all eukaryotes. We have identified and analysed the function and architecture of the non-conventional outer membrane (OM) protein translocase in the early diverging eukaryote Trypanosoma brucei. It consists of six subunits that show no obvious homology to translocase components of other species. Two subunits are import receptors that have a unique topology and unique protein domains and thus evolved independently of the prototype receptors Tom20 and Tom70. Our study suggests that protein import receptors were recruited to the core of the OM translocase after the divergence of the major eukaryotic supergroups. Moreover, it links the evolutionary history of mitochondrial protein import receptors to the origin of the eukaryotic supergroups.

Mitochondria are essential for eukaryotic life and more than 95% of their proteins are imported as precursors from the cytosol. The targeting signals for this posttranslational import are conserved in all eukaryotes. However, this conservation does not hold true for the protein translocase of the mitochondrial outer membrane that serves as entry gate for essentially all precursor proteins. Only two of its subunits, Tom40 and Tom22, are conserved and thus likely were present in the last eukaryotic common ancestor. Tom7 is found in representatives of all supergroups except the Excavates. This suggests that it was added to the core of the translocase after the Excavates segregated from all other eukaryotes. A comparative analysis of the biochemically and functionally characterized outer membrane translocases of yeast, plants, and trypanosomes, which represent three eukaryotic supergroups, shows that the receptors that recognize the conserved import signals differ strongly between the different systems. They present a remarkable example of convergent evolution at the molecular level. The structural diversity of the functionally conserved import receptors therefore provides insight into the early evolutionary history of mitochondria.

Unlike most other model eukaryotes, Trypanosoma brucei and its relatives have a single mitochondrion with a single-unit mitochondrial genome that is termed kinetoplast DNA (kDNA). Replication of the kDNA is coordinated with the cell cycle. During binary mitochondrial fission and prior to cytokinesis, the replicated kDNA has to be faithfully segregated to the daughter organelles. This process depends on the tripartite attachment complex (TAC) that physically links the kDNA across the two mitochondrial membranes with the basal body of the flagellum. Thus, the TAC couples segregation of the replicated kDNA with segregation of the basal bodies of the old and the new flagellum. In this Review, we provide an overview of the role of the TAC in kDNA inheritance in T. brucei. We focus on recent advances regarding the molecular composition of the TAC, and discuss how the TAC is assembled and how its subunits are targeted to their respective TAC subdomains. Finally, we will contrast the segregation of the single-unit kDNA in trypanosomes to mitochondrial genome inheritance in yeast and mammals, both of which have numerous mitochondria that each contain multiple genomes.

To test the significance of cyclic Notch activity for somite formation in mice, we analyzed embryos expressing activated Notch (NICD) throughout the presomitic mesoderm (PSM). Embryos expressing NICD formed up to 18 somites. Expression in the PSM of Hes7, Lfng, and Spry2 was no longer cyclic, whereas Axin2 was expressed dynamically. NICD expression led to caudalization of somites, and loss of Notch activity to their rostralization. Thus, segmentation and anterior–posterior somite patterning can be uncoupled, differential Notch signaling is not required to form segment borders, and Notch is unlikely to be the pacemaker of the segmentation clock.