When the CAU anticodon of a tRNAMet was modified to UAC, the anticodon for tRNAVal, valine aminoacyl-tRNA synthetase, recognized the altered tRNAMet and added valine rather than methionine to it. When the converse modification was made, the altered tRNAValcontaining a CAU anticodon (rather than UAC) was recognized and activated by methionine aminoacyl-tRNA synthetase. What do these data suggest about the mechanism by which aminoacyl-tRNA synthetases recognize their cognate tRNAs