Hydration of transfer RNA molecules: a crystallographic study.

Four crystal structures of transfer RNA molecules were refined at 3 A resolution with the inclusion of the solvent molecules found in the difference maps: yeast tRNA-phe in the orthorhombic form, yeast tRNA-phe in the monoclinic form and yeast tRNA-asp in the A and B forms. Over 100 solvent molecules were located in each tRNA crystal. Several hydration schemes are found repeatedly in the 4 crystals. The tertiary interactions in the corner of the L-shaped molecule attract numerous solvent molecules which bridge the ribose hydroxyl O(2') atoms, base exocyclic atoms and phosphate anionic oxygen atoms. Conservation of bases leads to conservative localized hydration patterns. Several solvent molecules are found stabilizing unusual base pairs like the G-U pairs and those involving the pseudouridine base. Water bridges between the O(2') and the exocyclic atom O2 of pyrimidines or the N3 atom of purines are common. Water bridges occur frequently between successive anionic oxygen atoms of each strand as well as between N7 or other exocyclic atoms of successive bases in the major groove. Magnesium ions or spermine molecules are found to bind in the major groove of tRNA helices without specific interactions.