A FLUORESCENCE SPECTROSCOPIC STUDY OF SUBSTRATE-INDUCED CONFORMATIONAL-CHANGES IN GLUTAMINYL-TRANSFER RNA-SYNTHETASE

Glutaminyl-tRNA synthetase from Escherichia coli is a member of a subg roup of aminoacyl-tRNA synthetases that do not catalyze ATP-PP(i) exch ange in the absence of the cognate tRNA. Such behavior suggests confor mational changes upon substrate binding. Two different fluorescent pro bes, pyrenylmaleimide and acrylodan, were used to specifically label a nonessential sulfhydryl group of GlnRS. Conformational changes induce d by substrates were studied using glutaminyl-tRNA synthetase labeled with these two environment-sensitive probes. ATP was shown to cause a significant conformational change that alters the mode of binding to t RNA(Gln) to GlnRS. The alteration of the salt sensitivity pattern of t RNA(Gln) binding to GlnRS by ATP supports this. Binding of tRNA(Gln) c auses a conformational change that may be different in nature for the ATP/GlnRS complex and free GlnRS. Hydrodynamic parameters deduced from fluorescence polarization studies and the use of a noncovalent probe indicate that the ATP-induced conformational change may not be global in character.