The effect of the C-C mismatch on RNA structure. (A) Solution NMR structure (slate blue) of an RNA hairpin containing a C-C mismatch (PDB ID 2RPT; (Tavares et al., 2009)) shows that the cytosines (red) deviate from co-planarity and induce a large deviation of RNA conformation from that of an A-form double helix (gray). (B) The C-C mismatch (red) in the 70S P site does not disrupt the A-form RNA geometry of the codon-anticodon helix, which resembles the (CCG)n-repeat double helix (purple; PDB ID 4E59; (Kiliszek et al., 2012)). In the (CCG)n-repeat double helix, the C-C mismatch is stabilized by interactions with the flanking G-C pairs, which are part of the crystal-lattice-stabilized system of the stacked base pairs. 16S rRNA is cyan; mRNA is yellow, P-site tRNA is green with the exception of the C-C mismatch, which is shown in red.