We have determined the solution structure of ribosomal protein L18 from Thermus thermophilus. L18 is a 12.5kDa protein of the large subunit of the ribosome and binds to both 5S and 23S rRNA. In the uncomplexed state L18 folds to a mixed α/β globular structure with a long disordered N-terminal region. We compared our high-resolution structure with RNA-complexed L18 from Haloarcula marismortui and T. thermophilus to examine RNA-induced as well as species-dependent structural differences. We also identified T. thermophilus S11 as a structural homologue and found that the structures of the RNA-recognition sites are conserved. Important features, for instance a bulge in the RNA-contacting β-sheet, are conserved in both proteins. We suggest that the L18 fold recognizes a specific RNA motif and that the resulting RNA—protein-recognition module is tolerant to variations in sequence.

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