The non-specific role of Mg2+ in ribosomal subunit association: Kinetics and equilibrium in the presence of other divalent metal ions

The effects of Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+ and Zn2+ on the kinetics and equilibrium of the association of vacant “tight” ribosomal subunits from Escherichia coli were studied. Increments of Mg2+, Ca2+, Sr2+ and, by and large, Ba2+, to ribosomes dissociated to 30 S and 50 S particles at 1.2 mm-Mg2+ (60 mm-M2+, pH 7.5, 25°C) produce nearly indistinguishable association curves, with midpoints at 1.8 mm total M2+ and complete association to 70 S particles at 4 to 5 mm total M2+ . The association rate constants at 1 mm-Mg2+, 2 mM-M2+ are similar (0.5 × 106 to 0.9 × 106 m−1 s−1), as are the dissociation rate constants at 1 mm-(Mg2+ + M2+) (0.2 to 0.4 s−1). Mn2+ and Zn2+ increase the degree of association, as well as further aggregation (Zn2+ especially), at lower concentrations than the alkaline earth ions. Co2+ and Ni2+ produce lower degrees of association, by promoting dissociation of the 70 S particle : the association rate constants at 1 mm-Mg2+, 2 mm-M2+ for the transition metal ions are all grouped at 2 × 106 to 3 × 106 m−1 s−1. Ni2+ also causes a slower inactivation of one or both subunits.