Turnover of nucleic acids in a non-multiplying animal cell.
It was suggested to us by Dr R. B. Loftfield that the most likely explanation for the excess of labelling in the presence of L-[14C]leucine is the presence of small quantities of other amino acids. This point does, however, need further examination. The importance of cell-sap RNA in the specific selection of amino acids before their incorporation into the peptide-chain sequence is further emphasized by our experiments with glutamic acid and glutamine. It has been shown by Sansom & Barry (1958) that asparagine and glutamine are incorporated into milk protein independently from aspartic acid and glutamic acid respectively. We were able to show that the binding sites on cell-sap RNA can distinguish between glutamine and glutamic acid. Though the instability of glutamine made it impossible to obtain quantitative information for this amino acid it seems likely that the cellsap RNA-coupling process is the point in protein synthesis at which the distinction occurs between the dicarboxylic amino acids and their amides.