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The “ribosome cycle” proposed by Mangiarotti and Schlessinger (1966) suggested that one 30 S and one 50 S ribosome of exponentially growing cells periodically couple from a free pool into an initiation monosome on messenger RNA (mRNA). Each 30 S–50 S couple then moves across mRNA forming the corresponding protein chain. When the protein chain is complete, the monosome leaves the mRNA and dissociates to 30 S and 50 S particles that rejoin the free pool. The particles free in cells were shown to be in rapid exchange with particles in polyribosomes (Mangiarotti and Schlessinger, 1967). With the demonstration of subunit exchange in vivo (Kaempfer, Meselson, and Raskas, 1968) and in vitro (Kaempfer, 1968; Guthrie and Nomura, 1968), and with increasingly detailed analysis of the requirements for entry of subunits into polyribosomes (Schlessinger, Mangiarotti, and Apirion, 1967; for a review, see Lengyel and Soll, 1969), it is clear that 30...
Schlessinger et al. (Wed,) studied this question.