Three tRNAs on the ribosome slow translation elongation.

During protein synthesis, the ribosome simultaneously binds up to three different transfer RNA (tRNA) molecules. Among the three tRNA binding sites, the regulatory role of the exit (E) site, where deacylated tRNA spontaneously dissociates from the translational complex, has remained elusive. Here we use two donor-quencher pairs to observe and correlate both the conformation of ribosomes and tRNAs as well as tRNA occupancy. Our results reveal a partially rotated state of the ribosome wherein all three tRNA sites are occupied during translation elongation. The appearance and lifetime of this state depend on the E-site tRNA dissociation kinetics, which may vary among tRNA species and depends on temperature and ionic strength. The 3-tRNA partially rotated state is not a proper substrate for elongation factor G (EF-G), thus inhibiting translocation until the E-site tRNA dissociates. Our result presents two parallel kinetic pathways during translation elongation, underscoring the ability of E-site codons to modulate the dynamics of protein synthesis.