AMINO ACID ACTIVATION AND TRANSFER TO RIBONUCLEIC ACIDS IN THE CELL NUCLEUS

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Abstract
A pH 5 enzyme fraction catalyzing an amino-acid-dependent exchange of pyrophosphate with adenosine triphosphate (ATP) (a test for amino acid activation) has been found in calf thymus nuclei prepared in both sucrose solutions and in nonaqueous media. Similar activating enzymes occur in chicken kidney nuclei prepared in nonaqueous media. Twelve L-amino acids have been shown to stimulate pyrophosphate-ATP exchange in the presence of the "pH 5 fraction" from calf thymus nuclei while D-amino acids have no such effect. Deoxypentose nucleic acid (DNA) isolated from calf thymus nuclei which have been incubated with DL-leucine-2-C14 and are actively incorporating amino acids into their proteins contains a negligible amount of labeled amino acid. Ribose nucleic acid (RNA) isolated from calf thymus nuclei incubated under the same conditions contains a significant amount of labeled amino acid which is rapidly released by dilute alkali or RNAase but not by acid. Chloramphenicol (0.0067 M) inhibits nearly completely the incorporation of amino acid into protein of calf thymus nuclei but has no effect on its incorporation into nuclear RNA.