A Soluble Ribonucleic Acid Intermediate in Protein Synthesis

The cell-free rat liver system in which Cl*-amino acids are incorporated irreversibly into cY-peptide linkage in protein has been used in our laboratories for a number of years as a measure of protein synthesis. The essential components of this system are the microsomal ribonucleoprotein particles, certain enzymes derived from the soluble protein fraction, adenosine triphosphate, guanosine dior triphosphate, and a nucleoside triphosphate-generating system (l-3). The ribonucleoprotein particles of the microsomes appear to be the actual site of peptide condensation. The soluble enzymes and ATP’ have been found to effect the initial carboxyl activation of the amino acids (4). The role of GTP is not yet understood, although the present paper sheds light on its probable locus of action. Much evidence has accumulated in the past 8 years, beginning with the studies of Caspersson (5) and Brachet (6), implicating a role for cellular RNA in protein synthesis. The intermediate stages between amino acid activation and final incorporation into protein in the rat liver in vitro system offered us unexplored regions in which to seek more direct evidence for a chemical association of RNA and amino acids. A preliminary report of such an association has recently been presented by us (7). There it was shown that the RNA of a particular fraction of the cytoplasm hitherto uncharacterized became labeled with Cl*-amino acids in t,he presence of ATP and the amino acid-activating enzymes, and that this labeled RNA subsequently was able to transfer the amino acid to microsomal protein