Purines in tRNAs required for recognition by ATP/CTP:tRNA nucleotidyltransferase from rabbit liver.

Recognition of tRNA by the enzyme ATP/CTP:tRNA nucleotidyltransferase from rabbit liver was studied using 12 tRNAs, previously treated with the chemical modifier diethylpyrocarbonate (DEP). Such chemically modified tRNAs were labeled with 32P by nucleotidyltransferase, using alpha-[32P]ATP as a cosubstrate. A carbethoxylated purine at position 57 in the psi-loop interfered with recognition of the tRNA in all instances. DEP-modified purines at other positions (58 in the psi-loop, 52 or 53 in the psi-stem, and 71-73 in the acceptor stem), also interfered with the interaction, but in only a few tRNAs. The mammalian enzyme was more similar to the homologous enzyme from yeast than that from bacteria, in its requirements for chemically unmodified purines. The extent of exclusion of modified bases from 32P-labeled material diminished as the concentration of enzyme increased, demonstrating that interference was not due to the inability of the chemically altered tRNA to refold into a recognizable conformation. The degree of purification of the enzyme did not affect the identity of bases that inhibited the reaction when modified.