Structural evidence explains the residue distributions for triplet U519/R18/D22.

<p>In E.coli, the values of RNA U519/L22 R18/L22 D22 are U, Arg (R) and Asp (D), respectively. A hydrogen bond network in E.coli goes from the side chain of D22 to the side chain of R18 to the phosphate atom of U519. (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030022#pone-0030022-g004" target="_blank">Figure 4A</a>), and explains the tight coupling seen in the distribution (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030022#pone-0030022-g003" target="_blank">Figure 3</a>). The triplet from the archeon haloarcula marismortui represents a shift from pyrimidine to purine, with the values of U519/R18/D22 at G, Lys (K) and Arg (R), respectively. A structural alignment of the crystal structure from both species reveals that the hydrogen bonds are broken when the RNA is a purine and the residues farther apart (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030022#pone-0030022-g004" target="_blank">Figure 4B</a>). This data suggests that the change in packing to accommodate a larger RNA side chain influences the packing between the L22 and 23S protein in such a way that this hydrogen bond network is broken.