F-19 NMR-STUDIES OF THE THERMAL UNFOLDING OF 5-FLUOROURACIL-SUBSTITUTED ESCHERICHIA-COLI VALINE TRANSFER-RNA

F-19 NMR spectroscopy was used to monitor the thermal unfolding of E. coli tRNA(Val) labeled by incorporation of 5-fluorouracil (FUra). With rising temperatures, resonances in the F-19 NMR spectrum of (FUra)tRNA(Val) gradually shift towards the central region of the spectrum and merge into a single broad peak above 85-degrees-C. FU55 and FU12 are the first to shift, beginning at temperatures below 40-degrees-C, which suggests that the initial steps of thermal denaturation of tRNA(Val) involve disruption of the tertiary interactions between the D- and T-arms. The acceptor stem and the FU64-G50 wobble base pair in the T-stem are particularly stable to thermal denaturation. A temperature-dependent splitting of the F-19 resonance assigned to FU64, at temperatures above 40-degrees-C, suggests that the T-arm of (FUra)tRNA(Val) exists in two conformations in slow exchange on the NMR time scale.