Arrangement of tRNAs in Pre- and Posttranslocational Ribosomes Revealed by Electron Cryomicroscopy

The three-dimensional structure of the translating 70S E. coli ribosome is presented in its two main conformations: the pretranslocational and the posttranslocational states. Using electron cryomicroscopy and angular reconstitution, structures at 20 Å resolution were obtained, which, when compared with our earlier reconstruction of “empty” ribosomes, showed densities corresponding to tRNA molecules—at the P and E sites for posttranslocational ribosomes and at the A and P sites for pretranslocational ribosomes. The P-site tRNA lies directly above the bridge connecting the two ribosomal subunits, with the A-site tRNA fitted snugly against it at an angle of ∼50°, toward the L7/L12 side of the ribosome. The E-site tRNA appears to lie between the side lobe of the 30S subunit and the L1 protuberance.