Protein Synthesis by Free and Bound Paramecium Ribosomes in Vivo and in Vitro

Abstract

SYNOPSIS. Cytoplasmic extracts of Paramecium aurelia were fractionated by density gradient centrifugation. The gradient fractions were characterized by chemical analysis and electron microscopy. Membrane‐bound ribosomes were separated from free polyribosomes and the ability of each of these forms to incorporate C14‐leucine into protein was tested. Incorporation was measured in both in vivo and in vitro systems, and similar results were obtained in both types of experiment except that there was little release of soluble labelled protein in the in vitro system. Paramecium appears to synthesize most of its protein on free polyribosomes but membrane‐bound ribosomes constitute an important protein synthetic fraction, perhaps accounting for as much as 30% of the total synthesis. When isolated in the in vitro system, increasing concentration of the ribosome fraction gave increased incorporation, but increasing concentration of the membrane fraction gave decreased incorporation after a critical value. This inhibitory effect can be removed by adding excess cytoplasmic‐supernatant to the system. The nature of the association of ribosomes with membranes is discussed.