The A, P, and E sites of the ribosome. (a) The A, P, and E sites in relation to the bound mRNA. Both ribosomal subunits are shown. AA indicates an amino acid bound to tRNA. (b) A representation of the E. coli 70S ribosome crystal structure with bound tRNAs—representing aminoacyl-tRNA, peptidyl-tRNA, and free tRNA, respectively—in the A, P, and E sites, viewed from the 30S interface.