Escherichia coli tRNA₄^Arg(UCU) induces a constrained conformation of the crucial Ω-loop of arginyl-tRNA synthetase

Previous investigations show that tRNA^Arg-induced conformational changes of arginyl-tRNA synthetase (ArgRS) Ω-loop region (Escherichia coli (E. coli), Ala451-Ala457) may contribute to the productive conformation of the enzyme catalytic core, and E. coli tRNA₂ ^Arg(ICG)-bound and -free conformations of the Ω-loop exchange at an intermediate rate on NMR timescale. Herein, we report that E. coli ArgRS catalyzes tRNA₂^Arg(ICG) and tRNA₄ ^Arg(UCU) with similar efficiencies. However, ¹⁹F NMR spectroscopy of 4-fluorotryptophan-labeled E. coli ArgRS reveals that the tRNA₄^Arg(UCU)-bound and -free conformations of the Ω-loop region interconvert very slowly and the lifetime of bound conformation is much longer than 0.33ms. Therefore, tRNA₄ ^Arg(UCU) differs from tRNA₂^Arg(ICG) in the conformation-exchanging rate of the Ω-loop. Comparative structure model of E. coli ArgRS is presented to rationalize these ¹⁹F NMR data. Our ¹⁹F NMR and catalytic assay results suggest that the tRNA ^Arg-induced conformational changes of Ω-loop little contribute to the productive conformation of ArgRS catalytic core.