Cryo-EM study of an archaeal 30S initiation complex gives insights into evolution of translation initiation

Archaeal translation initiation occurs within a macromolecular complex containing the small ribosomal subunit (30S) bound to mRNA, initiation factors aIF1, aIF1A and the ternary complex aIF2:GDPNP:Met-tRNA(i)(Met). Here, we determine the cryo-EM structure of a 30S:mRNA:aIF1A:aIF2:GTP:Met-tRNA(i)(Met) complex from Pyrococcus abyssi at 3.2 angstrom resolution. It highlights archaeal features in ribosomal proteins and rRNA modifications. We find an aS21 protein, at the location of eS21 in eukaryotic ribosomes. Moreover, we identify an N-terminal extension of archaeal eL41 contacting the P site. We characterize 34 N-4-acetylcytidines distributed throughout 16S rRNA, likely contributing to hyperthermostability. Without aIF1, the 30S head is stabilized and initiator tRNA is tightly bound to the P site. A network of interactions involving tRNA, mRNA, rRNA modified nucleotides and C-terminal tails of uS9, uS13 and uS19 is observed. Universal features and domain-specific idiosyncrasies of translation initiation are discussed in light of ribosomal structures from representatives of each domain of life. Coureux et al. describe a cryo-EM structure of a 30S initiation complex of Pyrococcus abyssi at 3.2 angstrom resolution. The structure uncovers a novel archaeal ribosomal protein aS21, N-terminal extension of eL41 and brings insights into base modifications of the rRNA.