Thermal Denaturation of Ribosomes

In dilute buffer of Tris-acetate (0.001 M, pH 7.2) where the nuclease-free ribosomes from Escherichia coli MRE 600 can be heated without aggregation, reversible ~23% hyperchromicity can be observed with ultraviolet spectrophotometry or by optical rotation (α_Dline). The T_mis 62°. Mg²⁺elevates the T_mwhereas EDTA, urea, salt, and various organic reagents lower it. Analysis of ribosomal ash by emission spectrography revealed zinc and nickel in significant amounts and magnesium, calcium, and iron in smaller amounts. The sedimentation constants of natural ribosomes in Tris-acetate (0.001 m) are 24 S + 42 S. However, after heating to 65° and cooling their S values dropped to 17 S + 24 S (“heated particles”). Viscometric analysis showed that the conformation of heated particles is between that of natural ribosomes and an open ribosomal structure which exists at 65°. Reversible interconversion between the “open-molecule” and “heated-particle” conformations was found in repeated cycle of heating and cooling, as was measured by spectrophotometric, viscometric and optical rotation methods. Examination of the composition of “heated particles” showed that the ribosomal ribonucleic acid remained intact and that the particles retain virtually all their proteins. On the basis of these findings a model of thermal denaturation of ribosomes is proposed.