ARRANGEMENT OF TRANSFER-RNAS IN PRETRANSLOCATIONAL AND POSTTRANSLOCATIONAL RIBOSOMES REVEALED BY ELECTRON CRYOMICROSCOPY

The three-dimensional structure of the translating 70S E. coli ribosom e is presented in its two main conformations: the pretranslocational a nd the posttranslocational states. Using electron cryomicroscopy and a ngular reconstitution, structures at 20 Angstrom resolution were obtai ned, 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 tRN A fitted snugly against it at an angle of similar to 50 degrees, towar d the L7/L12 side of the ribosome. The E-site tRNA appears to lie betw een the side robe of the 30S subunit and the L1 protuberance.