Incorporation of Adenine Nucleotide into Ribonucleic Acid by Cytoplasmic Enzyme Preparations of Chick Embryos

Abstract

The incorporation of labeled precursors into ribonucleic acid with cell-free preparations of animal tissues has been observed in a number of laboratories (l-lo). Further studies with partially purified preparations indicate that the proximate precursors are nucleoside trirather than diphosphates (24). These enzyme preparations, unlike deoxyribonucleic acid polymerase (11-13) or polynucleotide phosphorylase (14)) incorporate adenine nucleotide into RNA almost exclusively at the terminal position of the polyribonucleotide chain adjacent to a cytosine monophosphate residue (2). The final product, adenosineCMP-CMP-RNA is the amino acid acceptor required for the transfer of aminoacyl residues to the site of their ultimate incorporation into proteins (15-18). Edmonds and Abrams (3), however, have provided some evidence for a separate enzyme, responsible for the incorporation of the adenine nucleotide moiety of ATP into internucleotide linkages in RNA. Herbert (10) has reported that this type of reaction is enhanced if cytoplasmic enzyme preparations are supplemented by nuclei.’ In the course of studies on the possible occurrence of polynucleotide phosphorylase in extracts of tissues of chick embryos, we investigated the exchange of labeled inorganic phosphate and pyrophosphate with ribonucleoside diand triphosphates. Although the rate of incorporation of phosphate into ribonucleoside diphosphates was rather low, that of inorganic pyrophosphate into ribonucleoside triphosphates appeared relatively high and independent of the presence of amino acids, but dependent on the presence of polynucleotide (19-21). The present communication examines the reactions involved in more detail. When the soluble fraction of homogenates of 14-day-old chick embryonic livers or hearts was purified by means of ammonium sulfate and calcium phosphate gel, an enzyme preparation was obtained which catalyzed the incorporation of the AMP moiety of ATP into RNA. The incorporated adenine nucleotide was found almost entirely at the terminal position of the polynucleotide chain. The reaction did not take place when ADP was substituted for ATP. The incorporation of adenine nucleotide into the interior of the polynucleotide chain could be shown to take place in the presence of a cruder enzyme preparation. This system required ATP as a substrate and was dependent on the presence of Mg++. Addi-