- An efficient procedure for the replacement of the anticodon and the adjacent hypermodified nucleotide (residues 34-37) of yeast tRNAPhe with any desired oligoribonucleotide sequence has been developed. The four residues are removed by chemical cleavage at Y-37 and partial ribonuclease A digestion at U-33. An oligonucleotide is inserted in three steps by using T4 RNA ligase and T4 polynucleotide kinase. When different oligonucleotides are inserted, both the size of the loop and the sequence of nucleotides in the anticodon region of this tRNA can be varied. The ability of the different anticodon loop substituted tRNAs to be aminoacylated by yeast phenylalanyl-tRNA synthetase is dependent upon the sequence of the oligonucleotide inserted. This suggests that there is an important interaction between the anticodon region of yeast tRNAPhe and its synthetase.