The exchange properties of magnesium in Escherichia coli ribosomes.

Although some work has been done on the metabolism of the RNA and, to a lesser extent, of the protein in ribosomal particles (Roberts, 1960, 1961, 1962), little is known of their structure. The general physical characteristics of Escherichia coli ribosomes have been studied by Tissieres, Watson, Schlessinger & Hollingworth (1959), of yeast ribosomes by Chao & Schachman (1956), of peaseedling ribosomes by Ts'o, Bonner & Vinograd (1958) and of rat-liver ribosomes by Petermann, Hamilton, Balis, Samarth & Pecora (1958). The importance of magnesium in the structure of these particles is well known, and the dissociation of larger into small sub-units when magnesium is removed from them appears to be a general feature (Tissieres et al. 1959; Bolton, Hoyer & Ritter, 1958; Chao, 1957). Protein has also been implicated in this association (Morgan, Greenspan & Cunningham, 1963). Little attention has been paid to the properties of the magnesium bound to and involved in the structure of ribosomes. The binding of magnesium to Jensen-sarcoma ribosomes has been investigated by Petermann (1960), and shown to be dependent on the pH and ionic strength of the buffer. The exchange between free Mg2+ ions and rabbit-reticulocyte-microsomal magnesium has been studied by Edelman, Ts'o & Vinograd (1960), who found that 100% of the microsomal magnesium exchanged in about 30 min. The present work describes measurements of the exchangeability of the magnesium of some E. coli ribosomes. The results show that probably all the magnesium in E. coli ribosomes exchanges fairly quickly with free Mg2+ ions, but that the rate of exchange depends on the site of binding. The 50s ribosomes appear to be composed of 30 s sub-units. High Mg2+ ion concentrations are not essential for the stabilization of 70s ribosomes in crude extracts of E. coli.