Previous investigations show that tRNA(Arg)-induced conformational changes of arginyl-tRNA synthetase (ArgRS) Omega-loop region (Escherichia coli (E coli), Ala451-Ala457) may contribute to the productive conformation of the enzyme catalytic core, and E. coli tRNA(2)(Arg)(ICG)-bound and -free conformations of the Omega-loop exchange at an intermediate rate on NMR timescale. Herein, we report that E. coli ArgRS catalyzes tRNA(2)(Arg)(ICG) and tRNA(4)(Arg)(UCU) with similar efficiencies. However, F-19 NMR spectroscopy of 4-fluorotryptophan-labeled E. coli ArgRS reveals that the tRNA(4)(Arg)(UCU)-bound and -free conformations of the Omega-loop region interconvert very slowly and the lifetime of bound conformation is much longer than 0.33ms. Therefore, tRNA(4)(Arg) (UCU) differs from tRNA(2)(Arg)(ICG) in the conformation-exchanging rate of the Omega-loop. Comparative structure model of E. coli ArgRS is presented to rationalize these F-19 NMR data. Our F-19 NMR and catalytic assay results suggest that the tRNA(Arg)-induced conformational changes of Omega-loop little contribute to the productive conformation of ArgRS catalytic core. (C) 2003 Elsevier Inc. All rights reserved.