Isomeric aminoacyl-tRNAs are both bound by elongation factor Tu

Recent suggestions that elongation factor Tu (EF-Tu) is specific for 2'-O-aminoacyl-tRNA, as compared with the 3'-isomer, prompted us to assay [3H]aminoacyl-tRNAs from Escherichia coli terminating in 2'- or 3'-deoxyadenosine for binding to EF-Tu to determine the possible positional specificity of the factor. Binding of modified aminaocyl-tRNAs to EF-Tu-GTP was measured both as a function of the ability of EF-Tu-GTP to diminish the rate of chemical deacylation of [3H]aminoacyl-tRNAs and by gel filtration of the individual ternary complexes. Fifteen different tRNA isoacceptors were tested by the deacylation procedure, including three (tRNAAsp, tRNACys, and tRNATyr) for which isomeric modified aminoacyl-tRNAs were available. All of the modified aminoacyl-tRNAs were protected fromdeacylation, although generally to a lesser extent than the corresponding unmodified species. Six modified tRNA isoacceptors (including tRNATrp and tRNATyr, for which both modified aminoacyl-tRNAs were accessible by enzymatic aminoacylation) were used in gel filtration experiments to permit direct measurement of the individual aminoacyl-tRNA-EF-Tu-GTP complexes. These experiments were also done in the presence of equimolar amounts of the corresponding unmodified [14C]aminoacyl-tRNAs, and the relative affinities for a limiting amount of EF-Tu-GTP were measured. The results were completely consistent with those obtained by the deacylation procedure and indicated that EF-Tu can bind to both positional isomers of aminoacyl-tRNA with no obvious preference for either.