Abstract: The deletion of nucleoside 37 from yeast tRNAPhe was accomplished in a three-step procedure that involved (i) specific depurination of this purine under acidic conditions and removal of the carbohydrate moiety with aniline-2-aminopyridine, (ii) removal of phosphate monoesters from the 5'-half-molecule with alkaline phosphatase, and (iii) resealing of the anticodon loop with T4 RNA ligase. The course of the resealing reaction was monitored by polyacrylamide gel electrophoresis and found to be complete within a few hours when incubated at 37 degrees C in the presence of 215 units/ml of RNA ligase. Although the half-molecules used to prepare the modified tRNA had substantial phenylalanine acceptance when assayed at low ionic strength, the modified species itself was essentially devoid of phenylalanine acceptance. We conclude that the anticodon loop of tRNAPhe is involved in the activation of this tRNA and that both the presence of specific nucleotides in this loop and their ability to assume an appropriate spatial or conformational arrangement may be important for enzyme recognition.