Arrangement of tRNAs in pre- and posttranslocational ribosomes revealed by electron cryomicroscopy

Abstract

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 Angstrom 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 similar to 50 degrees, toward the L7/L12 side of the ribosome. The E-site tRNA appears to lie between the side robe of the 30S subunit and the L1 protuberance.