Visualization of tRNA movements on the Escherichia coli 70S ribosome during the elongation cycle

Three-dimensional cryomaps have been reconstructed for tRNA-ribosome comple xes in pre- and posttranslocational states at 17-Angstrom resolution. The p ositions of tRNAs in the A and P sites in the pretranslocational complexes and in the P and E sites in the posttranslocational complexes have been det ermined. Of these, the P-site tRNA position is the same as seen earlier in the initiation-like fMet-tRNA(f)(Met)-ribosome complex, where it was visual ized with high accuracy. Now, the positions of the A- and E-site tRNAs are determined with similar accuracy. The positions of the CCA end of the tRNAs at the A site are different before and after peptide bond formation. The r elative positions of anticodons of P- and E-site tRNAs in the posttransloca tional state are such that a codon-anticodon interaction at the E site appe ars feasible.