Structural analysis of spermine and magnesium ion binding to yeast phenylalanine transfer RNA

Refinement of the diffraction data at 2.5-A resolution from orthorhombic crystals of yeast tRNAPhe has proceeded to the point where spermine and magnesium ions can be located in the difference electron density map. Two spermine molecules are found: one is located in the major groove at one end of the anticodon stem; the other is near the variable loop and curls around phosphate 10 in a region where the polynucleotide chain takes a sharp turn. Four distinct magnesium ions have been identified: one in the anticodon loop, two in the D loop, and one coordinated with phosphates 8, 9, 11, and 12, where the polynucleotide chain is coiled. The conformation of the anticodon stem and loop is stabilized by the cations at the end of the molecule. The positions of these ions may be related to aspects of the biological activity of tRNA. The spermine and magnesium ions appear to be important in maintaining the overall folding of the tRNA molecule.