Reactions at the 3' Terminus of Transfer Ribonucleic Acid

ATP (CTP) transfer RNA nucleotidyltransferase was purified to homogeneity from homogenates of rabbit liver. The final specific activity for AMP incorporation was approximately 250 µmoles per hour per mg of protein under optimal conditions. The ratio of CMP to AMP incorporation remained constant over an approximately 7000-fold purification. The two activities had identical properties on acrylamide gel electrophoresis and isoelectric focusing, and had identical rates of heat inactivation at several temperatures. The data suggest that a single protein incorporates AMP and CMP into the -CCA terminus of tRNA.