Structural Studies Reveal the Role of Helix 68 in the Elongation Step of Protein Biosynthesis.

The ribosome, a multicomponent assembly consisting of RNA and proteins, is a pivotal macromolecular machine that translates the genetic code into proteins. The large ribosomal subunit rRNA helix 68 (H68) is a key element in the protein synthesis process, as it coordinates the coupled movements of the actors involved in translocation, including the tRNAs and L1 stalk. Examination of cryo-electron microscopy (cryo-EM) structures of ribosomes incubated for various time durations at physiological temperatures led to the identification of functionally relevant H68 movements. These movements assist the transition of the L1 stalk between its open and closed states. H68 spatial flexibility and its significance to the protein synthesis process were confirmed through its effective targeting with antisense PNA oligomers. Our results suggest that H68 is actively involved in ribosome movements that are central to the elongation process. IMPORTANCE The mechanism that regulates the translocation step in ribosomes during protein synthesis is not fully understood. In this work, cryo-EM techniques used to image ribosomes from Staphylococcus aureus after incubation at physiological temperature allowed the identification of a conformation of the helix 68 that has never been observed so far. We then propose a mechanism in which such helix, switching between two different conformations, actively coordinates the translocation step, shedding light on the dynamics of ribosomal components. In addition, the relevance of helix 68 to ribosome function and its potential as an antibiotic target was proved by inhibiting Staphylococcus aureus ribosomes activity in vitro using oligomers with sequence complementarity.