Mitochondria teach ribosome assembly

Ribosome assembly factors regulate structure formation to prevent misfolding Mitochondrial ribosomes translate messenger RNAs (mRNAs) encoded in mitochondrial DNA. Many of the resulting proteins are components of the electron transport chain, an essential mitochondrial pathway that produces the cellular energy source adenosine triphosphate (ATP). These mitoribosomes are assembled from mitochondrially encoded ribosomal RNAs (rRNAs) and from ribosomal proteins (RPs) mostly imported from the cytosol. In keeping with the evolutionary origin of mitochondria, mitoribosomes are more closely related to bacterial ribosomes than to their cytoplasmic counterparts, although often their rRNAs are smaller and additional RPs are recruited. An extreme example of this is found in trypanosomes, single-celled parasites that cause sleeping sickness. On page 1144 of this issue, Saurer et al. (1) report the identification and structure of three assembly intermediates of the Trypanosoma brucei mitoribosomal small subunit (SSU). Comparison to assembly intermediates from cytoplasmic ribosomes and to those from bacteria suggests universal and specific assembly problems and their solutions.