Project description:The archaeal ribosome is of the eukaryotic type. Genomic and phylogenetic studies have indicated that TACK and Asgard, the closest relatives of eukaryotes, have ribosomes containing eukaryotic ribosomal proteins not found in other archaeal branches, eS25, eS26 and eS30. In our study, we investigated the case of Saccharolobus solfataricus, a crenarchaea belonging to the TACK branch, which mainly uses leaderless mRNAs. We characterized the small ribosomal subunit of S. Solfataricus bound to SD-leadered or leaderless mRNAs. Cryo-EM structures show for the first time archaeal versions of the eS25, eS26 and eS30 proteins bound to the small subunit. In addition, we identify two novel ribosomal proteins named aS33 and aS34 as well as a domain of eS6, that highlight the diversity of archaeal ribosomes. Leaderless mRNAs are bound to the small ribosomal subunit and the 5'-triphosphate group contributes to their binding. Archaeal eS26 is in the mRNA exit channel wrapped around the 3' end of ribosomal RNA as it is in eukaryotes. Its position is not compatible with an SD:antiSD duplex in the mRNA exit channel. Overall, our results suggest a role of eS26 in translation regulation and possible evolutionary routes from archaeal to eukaryotic translation.
Project description:Early eukaryotic ribosome biogenesis involves large multi-protein complexes, which co-transcriptionally associate with pre-ribosomal RNA to form the small subunit processome. The precise mechanisms by which two of the largest multi-protein complexes – UtpA and UtpB – interact with nascent pre-ribosomal RNA have so far been poorly understood. We have combined biochemical and structural biology approaches with ensembles of RNA-protein cross-linking to elucidate the essential function of both complexes. Here we show that UtpA contains a large composite RNA binding site and captures the 5´ end of pre-ribosomal RNA. UtpB forms an extended structure that binds early pre-ribosomal intermediates in close proximity to key architectural sites such as an RNA duplex formed by the 5´ ETS and U3 snoRNA as well as the 3´ boundary of 18S rRNA. Both complexes therefore act as vital RNA chaperones to initiate eukaryotic ribosome assembly.