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Equilibrium measurements of cognate and noncognate interactions between aminoacyl transfer RNA synthetases and transfer RNA.

The interaction of Escherichia coli isoleucyl-tRNA synthetase with its cognate and five noncognate tRNAs, and of yeast valyl-tRNA synthetase with its cognate and four noncognate tRNAs, has been measured directly by fluorescence quenching. The cognate associations are strongest (association constant of 10(8) M-1 or more at pH 5.5, 17 degrees). A wide variation is found in the strengths of the noncognate interactions; these have association constants smaller than that of these cognate association by a factor of less than 10 to over 10(4), depending on the enzyme-t-RNA pair. A more detailed study of the cognate isoleucyl-tRNA synthetase-tRNAIle association suggests that the strength of the interaction is markedly sensitive to a pH-dependent transition in the enzyme centered at pH 6 on the other hand, Mg2+-induced structural changes in tRNAIle at 17 degrees in low salt do not greatly affect the availability of the nucleic acid's receptor sites for enzyme...