Crystal structure of domain A of Thermus flavus 5S rRNA and the contribution of water molecules to its structure.

This is the first high resolution crystal structure of an RNA molecule made by solid phase chemical synthesis and representing a natural RNA. The structure of the domain A of Thermus flavus ribosomal 5S RNA is refined to R = 18% at 2.4 A including 159 solvent molecules. Most of the 2'-hydroxyl groups as well as the phosphate oxygens are involved either in specific hydrogen bonds in intermolecular contacts or to solvent molecules. The two U-G and G-U base-pairs are stabilized by H-bonds supplied via three water molecules to compensate for the lack of base-pair hydrogen bonds. The structure shows for the first time in detail the importance of highly ordered internal water in stabilizing an RNA structure.