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Single molecule structural dynamics of ribosome before and during translocation

We employ single‐molecule TIRF spectroscopy to study ribosomal structural dynamics over the first two elongation cycles, using complexes containing either Cy3‐labeled L11 and Cy5‐labeled tRNAs to monitor dynamics between L11 and P‐ or A‐site tRNAs (Lt complexes) or Cy3 and Cy5 labeled tRNAs to monitor dynamics between adjacent tRNAs (tt complexes). Each pretranslocation (PRE) complex has two FRET states, classical and hybrid, with similar transition rates between the two states. EF‐G.GTP converts PRE complexes to postranslocation complexes, giving a single new FRET peak in each case. Translocation occurs from both classical and hybrid states with similar probabilities, with no detection of a translocation intermediate (lifetime > 0.1 s) within our time resolution. EF‐G binding to the PRE complex strongly reduces fluctuations between classical and hybrid states prior to translocation. Moreover, a slow relaxation process seen in Lt complexes following the main translocation step is suppressed in the presence of the next cognate ternary complex, probably due to stabilizing interactions with the L11 region of the ribosome. Supported by GM080376 (NIH); 70NANB7H7011 (NIST)