Sequence features of a tRNA that are recognized by aminoacyl-tRNA synthetases. (a) The two-dimensional and (b) three-dimensional structures of tRNA, showing the nucleotides recognized by only one (orange residues) or by two or more (blue residues) aminoacyl-tRNA synthetases. Nucleotides shown in purple are common to all tRNAs and therefore cannot be used to distinguish among them; nucleotides shown in black are positions in the tRNA sequence that are not specificity determinants.