Different Species of 70S Ribosomes of Escherichia coli and their Dissociation into Subunits

1 Mg2+-dependent dissociation of Escherichia coli ribosomes has been studied at various Mg2+ concentrations ranging from 10 to 0.5 mM. The extent of dissociation was determined by sucrose gradient centrifugation. 2 Of all 70 S ribosomes present in standard iS30 extracts about 50% dissociate in a relatively narrow range of Mg2+ concentration (between 4 and 2 mM), whereas the remainder dissociate between 10 and 4 mM Mg2+. 3 Puromycin at a concentration (0.1 mg/ml) which releases about 90% of nascent polypeptide chains after polypeptide synthesis does not influence the Mg2+-dependent dissociation curve of iS30 ribosomes, whereas it has a significant effect on the dissociation of 70 S ribosomes bearing peptidyl-tRNA (so called complexed 70 S ribosomes). Very high concentrations of puromycin (1 mg/ml) in addition to chain release also cause dissociation of ribosomes at 10 mM Mg2+. 4 Twenty-five per cent of the complexed 70 S particles, isolated after MS2-RNA programmed polypeptide synthesis, dissociate between 4 and 2 mM Mg2+. The remainder 75% dissociate between 2 and 0.5 mM Mg2+. 5 The distribution of the various classes of free and complexed 70 S ribosomes in iS30 extracts before and after MS2-RNA directed polypeptide synthesis and before and after puromycin action has been determined.