THE INCORPORATION OF AMINO ACIDS BY FREE POLYRIBOSOMES AND ROUGH ENDOPLASMIC RETICULUM IN A CELL‐FREE SYSTEM

The incorporation of amino acids into TCA * precipitable polypeptides has been studied in a cell‐free system isolated from rat liver. A new procedure is described for the isolation of all amino‐acyl‐tRNA synthetases necessary for optimal amino acid incorporation. Polyribosomes were present either as membrane‐bound or as free polyribosomes. Differences in incorporation between bound and free polyribosomes were observed. In contrast to free polyribosomes the incorporation of (14C)leucine in bound polyribosomes was not stimulated by addition of all the other non‐radioactive amino acids. The plateau for incorporation of (14C)leucine into bound polyribosomes was reached after 5 min and for free polyribosomes after 60 min. From these findings it was concluded that in vitro, apart from a limited translation, further translation of bound polyribosomes was blocked.