Because of previous data suggesting that aminoacyl-tRNA synthetases make a transient Michael adduct with a specific uridine residue in the tRNA structure, (Schoemaker, H.J.P., and Schimmel, P.R. (1977) Biochemistry 16, 5454-5460) attempts were made to find simple model systems in which this reaction might be studied in more detail. In the course of these investigations, it was found that Escherichia coli Ile-tRNA synthetase catalyzes cleavage of the glycosidic bond of 5-bromouridine. At pH 7.5, ambient temperatures, the turnover number is roughly 5/h. 5-Fluoro-, 5-chloro-, and 5-iodouridine are also cleaved in an analogous way by Ile-tRNA synthetase. In the case of uridine, conversion of uridine to uracil and ribose was also detected, but with a smaller turnover number. Three other E. coli and one mammalian aminoacyl-tRNA synthetases were also examined and all were found to catalyze glycosidic bond cleavage of 5-bromouridine. The data indicate that, in general, synthetases have a catalytic center that shows an unusual reactivity for uridine.