Ionic Effects on the Ribosomal Quaternary Structure

Escherichia coli ribosomes, in various ionic conditions, were examined by ultracentrifugation analysis and melting studies. It has been shown that: The Tris ion acts as a monovalent cation and is indistinguishable from the potassium ion as far as its influence on the stability of the ribosome is concerned. The ribosomal stability, as measured by the melting temperature (Tm), and the ribosomal conformation, as observed by ultracentrifugation analysis, are only dependent on R, the ratio of the mono to the divalent ions concentrations. The evolution of the ribosome as R increases is characterized by three phases: in the first one, the ribosome is in its most compact form (70 S); in the second one, the ribosomal subunits undergo a progressive loosening; and in the third, they are subject to a stepwise unfolding. The hinge between the first and the second phases corresponds to the ionic conditions required for the correct functioning of the ribosomes in protein synthesis in vitro, and also for successful ribosomal reconstitutions.