ABSTRACTtRNAPhe species from baker's yeast modified at the 3'-terminus in many cases are phenylalanylatable substrates. Out of several tRNAPhe species possessing a modified 3'-end that cannot be phenylalanylated, only two, tRNAPheC-C-2'dA and tRNAPheC-C-formycinoxi-reds are strong competitive inhibitors for tRNAPheC-C-A during phenylalanylation. In the ATP/PPi exchange, both these inhibitors reduce Vman, to about 25%o; but whereas tRNAPheCC_2'dA has no influence on Km ATP and Km Phe during ATP/PPj exchange, tRNAPheCC-formycinox i-red reduces Km AT! from 1430 IAM, found in the absence of tRNAPhe, to 230 jiM, and Km Phe, from 38 to 14 uM. The values found in the presence of tRNAPheC-C-CformycinOxi-re during ATP/PPi exchange, are identical with those determined in the phenylalanylation of tRNAPheC_C-A.All other tRNAPhe species carrying a modified 3'-end that cannot be phenylalanylated exhibit a mixed competitivenoncompetitive inhibition in the phenylalanylation reaction. In the ATP/PPi exchange, they do not influence Km ATP and K,, Phe and only weakly, if at all, Vmax.The results show that the 3'-adenosine of tRNAPhe cannot solely be a passive acceptor for phenylalanine, but must in addition play an active role during enzyme-substrate interaction. The data can be consistently explained by the hypothesis that the 3'-adenosine of tRNAPhe triggers a conformational change of the enzyme.In the living cell part of the genetic information stored in the DNA is translated into proteins. This is achieved by first transcribing the information into mRNAs and then translating the latter into peptide sequences on the ribosome. Prior to translation on the ribosome, the amino acids are activated by attachment to the 3'-terminal ribose of their particular tRNAs.The overall aminoacylation reaction is achieved according to tRNAaa + ATP + aa + E 2 aa-tRNAaa + AMNP + PPj + E, where E is the enzyme and aa is the amino acid. This reaction can be followed experimentally by the incorporation of radioactively labeled amino acid into tRNA.

tRNA nucleotidyl transferase has been purified 5000‐fold from commercial bakers' yeast. The isolation procedure includes ammonium sulfate fractionation and chromatography on DEAE‐cellulose, CM‐cellulose, Biogel and phosphocellulose. The enzyme is homogeneous on analytical centrifugation and by sodium dodecylsulfate gel electrophoresis. The molecular weight of the native and denatured enzyme Mc=Or is 71000 as determined by ultracentrifuge measurements and 70000 as determined by sodium dodecylsulfate gel electrophoresis. On micro isoelectric focusing the enzyme shows an isoelectric point at about pH 7.5. The enzyme catalyses the incorporation of ATP and CTP into the 3′‐end of tRNA, and, in the presence of pyrophosphate, the pyrophosphorolyses of adenylic and cytidylic acid residues from the 3′‐end of tRNA.

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