EFFECTS OF MAGNESIUM-IONS ON RIBOSOMES - A FLUORESCENCE STUDY

Fluorescence intensity measurements of ethidium bromide (EB) bound to ribosomal RNA (rRNA) in suspensions of 30S and 50S subunits, of 70S ri bosomal particles and of protein-free extracted rRNA are presented. Ch anges in the intercalation of EB reflect changes in conformation and d egree of exposure of rRNA. The effect of removal of magnesium ions on the binding of EB is compared in protein-free rRNA and in ribosomal pa rticles by a Scatchard plot analysis. In free ribosomal RNA the number of bound EBs do not depend on magnesium content, only the association constant is affected. In intact 70S particles and both in the separat ed 50S and 30S subunits the presence of magnesium greatly reduces bind ing of EB and no saturation of the fluorescence intensity with rRNA co ncentration is observed, preventing a Scatchard plot analysis. Removal of magnesium restores a strong EB intercalation. Then magnesium ions induce a conformational change in the 70S particles as well as in the separated subunits. The different behavior of the free-rRNA and of the ribosomal particles indicates that ribosomal proteins are relevant to the structural changes induced by magnesium ions. The comparison of t he number of excluded sites and of the association constant in the 30S , 50S subunits and in the 70S particles indicates that even without Mg 2+ ions the two subunits still interact, at variance with the commonly shared opinion that subunits dissociation takes place at low magnesiu m concentration.