The Handling of the Mechanistic Probe 5-Fluorouridine by the Pseudouridine Synthase TruA and Its Consistency with the Handling of the Same Probe by the Pseudouridine Synthases TruB and RluA

RNA containing 5-fluorouridine ((FU)-U-5) had previously been used to examine the mechanism of the pseudouridine synthase TruA, formerly known as pseudouridine synthase I [Gu et al. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 14270-14275]. From that work, it was reasonably concluded that the pseudouridine synthases proceed via a mechanism involving a Michael addition by an active site aspartic acid residue to the pyrimidine ring of uridine or (FU)-U-5. Those conclusions rested on the assumption that the hydrate of (FU)-U-5 was obtained after digestion of the product RNA and that hydration resulted from hydrolysis of the ester intermediate between the aspartic acid residue and (FU)-U-5. As reported here, O-18 labeling definitively demonstrates that ester hydrolysis does not give rise to the observed hydrated product and that digestion generates not the expected mononucleoside product but rather a dinucleotide between a hydrated isomer of (FU)-U-5 and the following nucleoside in RNA. The discovery that digestion products are dinucleotides accounts for the previously puzzling differences in the isolated products obtained following the action of the pseudouridine synthases TruB and RluA on (FU)-U-5 in RNA.