Identification of the catalytic nucleophile of tRNA (m5U54) methyltransferase

A covalent complex between tRNA (m5U54)methyltransferase, 5-fluorouridine tRNA(Phe), and S-adenosyl-L-[methyl-3H]methionine was formed in vitro and purified. Previously, it was shown that in this complex the 6-position of fluorouridine-54 is covalently linked to a catalytic nucleophile and the 5-position is bound to the transferred methyl group of AdoMet [Santi, D. V., & Hardy, L. W. (1987) Biochemistry 26, 8599-8606]. Proteolysis of the complex generated a [3H]methyl-FUtRNA-bound peptide, which was purified by 7 M urea-15% polyacrylamide gel electrophoresis. The peptide component of the complex was sequenced by gas-phase Edman degradation and found to contain two cysteines. The tritium was shown to be associated with Cys 324 of the methyltransferase, which unequivocally identifies this residue as the catalytic nucleophile.