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Although we are still a long way from attaining an atomic-resolution structure of the ribosome, cryo-electron microscopy, in combination with angular reconstitution, is likely to yield three-dimensional maps with gradually increasing resolution. As exemplified by our current 23 A reconstruction, these maps will lead to progressive refinement of models of the ribosomal RNA.

The mRNA codon in the ribosomal A-site is recognized by aminoacyl-tRNA (aa-tRNA) in a ternary complex with elongation factor Tu (EF-Tu) and GTP. Here we report the 13 A resolution three-dimensional reconstruction determined by cryo-electron microscopy of the kirromycin-stalled codon-recognition complex. The structure of the ternary complex is distorted by binding of the tRNA anticodon arm in the decoding center. The aa-tRNA interacts with 16S rRNA, helix 69 of 23S rRNA and proteins S12 and L11, while the sarcin-ricin loop of 23S rRNA contacts domain 1 of EF-Tu near the nucleotide-binding pocket. These results provide a detailed snapshot view of an important functional state of the ribosome and suggest mechanisms of decoding and GTPase activation.