Conformational switch in the ribosomal protein S1 guides unfolding of structured RNAs for translation initiation.

Initiation of bacterial translation requires that the ribosome-binding site in mRNAs adopts single-stranded conformations. In Gram-negative bacteria the ribosomal protein S1 (rS1) is a key player in resolving of structured elements in mRNAs. However, the exact mechanism of how rS1 unfolds persistent secondary structures in the translation initiation region (TIR) is still unknown. Here, we show by NMR spectroscopy that Vibrio vulnificus rS1 displays a unique architecture of its mRNA-binding domains, where domains D3 and D4 provide the mRNA-binding platform and cover the nucleotide binding length of the full-length rS1. D5 significantly increases rS1's chaperone activity, although it displays structural heterogeneity both in isolation and in presence of the other domains, albeit to varying degrees. The heterogeneity is induced by the switch between the two equilibrium conformations and is triggered by an order-to-order transition of two mutually exclusive secondary structures (β-strand-to-α-helix) of the 'AERERI' sequence. The conformational switching is exploited for melting of structured 5'-UTR's, as the conformational heterogeneity of D5 can compensate the entropic penalty of complex formation. Our data thus provides a detailed understanding of the intricate coupling of protein and RNA folding dynamics enabling translation initiation of structured mRNAs.