Direct iodination of specific residues in crystals of yeast formylatable methionine-accepting transfer ribonucleic acid.

Crystals of yeast formylatable methionine-accepting transfer ribonucleic acid (tRNAfMet) were iodinated by using a modification of Commerford's procedure [Commerford, S.L. (1971) Biochemistry 10, 1993]. Chromatographic analysis of nuclease digestion products showed that radioactive iodine binds covalently to the 5 position of three nucleotide residues: U8, C73, and C74. These three iodine substitutions were assigned to three peaks in a difference Fourier synthesis comparing the iodinated derivative with native tRNAfMet. In this way the positions of U8, C73, and C74 were marked in the crystal structure of yeast tRNAfMet, providing guidepoints for the interpretation of a 4.5-A electron density map.