Magnesium binding by Escherichia coli ribosomes

Abstract

Magnesium binding by ribosomes and ribosomal RNA from Escherichia coli has been studied as a function of the free magnesium concentration. At 10−2M-Mg2+ the 70 s and 100 s components bind at least three times more magnesium than they do at 10−4M-Mg2+ when they are completely dissociated into their 30 s and 50 s subunits. Between 10−4M- and 10−2M-magnesium, a pair of 30 s and 50 s particles take up about 1600 magnesium ions, thus approaching a ratio of bound Mg2+ to P of 0·5. Increasing concentrations of potassium decrease the extent of magnesium binding by competition for anionic sites. Magnesium is nevertheless bound preferentially, even at potassium to magnesium ratios greater than 10 to 1. Parallel studies have been carried out on the magnesium binding of ribosomal RNA. These studies support the suggestion of Ts'o and his co-workers that virtually all the magnesium bound by the ribosomes is bound by the RNA. Binding to a greater extent than one magnesium atom for every two RNA phosphates has been observed, but this effect appears to be correlated with the precipitation of the RNA at high ionic strengths. Almost all of the phosphate groups of the RNA must be available for magnesium binding in the completed ribosome, for removal of the protein moiety makes available at most 12% more binding sites.