By analyzing aminoacylation activities of variants of isoleucine tRNAs and glutamic acid tRNA from Escherichia coli, it was found that the anticodons are the major determinants for "identities" of these tRNAs. It was also shown that the post-transcriptional modifications are essential to aminoacylation of these tRNAs. Interactions between the tRNAs and the aminoacyl-tRNA synthetases were analyzed by NMR spectroscopy.