EFFECTS OF MAGNESIUM AND TEMPERATURE ON THE CONFORMATION AND REASSOCIATION OF ESCHERICHIA-COLI AND SULFOLOBUS-SOLFATARICUS RIBOSOMES

The structural response of the ribosomes of the extremely thermophilic archaeon Sulfolobus solfataricus was analysed and compared to that of the mesophilic (E. coli) ribosomes by assaying ethidium bromide (EB) binding to the native 70S particles as a function of magnesium concent ration. We found that the thermophilic ribosomes bound more EB than th eir mesophilic counterparts; on the other hand, inhibition of EB bindi ng by Mg2+ ions was more effective in the E. coli 70S particle. In Sul folobus, the separated 30S and 50S subunits and the 70S particle bound the drug in a similar fashion, whereas the E. coli 70S had a reduced number of binding sites with respect to the subunits. Light scattering measurements as a function of Mg2+ concentration were carried out at various temperatures to study the interaction between the ribosomal su bunits from the thermophilic and the mesophilic bacteria. As expected, the association of ribosomal subunits in E. coli was magnesium depend ent and could be observed also at low temperature. By contrast, the in teraction between Sulfolobus ribosomal subunits was obligatorily depen dent upon both magnesium ions and a temperature of at least 80 degrees C, close to the physiological optimum for cell growth (87 degrees C).