Comparison of 70S ribosome crystal structures showing the ribosomal A site (decoding center) in the presence of cognate tRNA, release factor RF2 or the preceding P-site C-C mismatch. In panels A–D, 23S ribosomal RNA is shown in gray, 16S rRNA in cyan; mRNA in yellow, P-site tRNA in green. The C-C mismatch is indicated in red (panels C and D). (A) Conformation of the decoding center in the presence of cognate tRNAPhe (magenta) bound to the A site (PDB ID 2J00; (Selmer et al., 2006)). (B) Conformation of the decoding center in the presence of release factor RF2 (magenta) bound in response to a UAA stop codon (PDB ID 3F1E; (Korostelev et al., 2008)). (C) Conformation of the vacant decoding center in the 70S C-C mismatch complex (this work). (D) Fo-Fc simulated-annealing omit map (gray) shows unbiased density of the decoding center (this work).