The Ribosome Cycle in a Reconstituted Cell-Free System from Reticulocytes

subunits (RSU) have been identified in the cyto-plasm of a number of animal cells (Girard et al., 1965; Ristow and Kohler, 1966; Bishop, 1966; Henshaw et al., 1965). Polyribosomes bear nascent peptide chains and are thought to be the site of protein synthesis in the cell (Gicrer, 1963; Marks et al., 1962; Warner et al., 1963; Wettstein et al., 1963). The reason for the existence of the other two classes of ribosomes is not completely clear. Exchange of RSU with monoribosomes and polyri-bosomes has been shown to occur during protein synthesis in bacteria and in animal cells (Man-giarotti and Schlessinger, 1967; Kaempfer et al., 1968; Hogan and Korner, 1968). During protein synthesis in a bacterial cell-free system ribosomes