Multistage unfolding of wheat germ ribosomal 5S RNA analyzed by differential scanning calorimetry

Unfolding of purified wheat germ ribosomal RNA has been studied by differential scanning calorimetry (DSC) from 15 to 95 degrees C, in the presence and absence of 100 mM NaCl and/or 10 mM MgCl2. The total enthalpy of melting varies from about 290 (no sodium or magnesium) to 480 kcal/mol (Mg2+ only) and precisely accounts for the number and types of base pairs deduced from prior Fourier-transform infrared experiments. The composite DSC curves are analyzed into four individual two-state melting stages. Both Na+ and Mg2+ shift the melting transitions to higher temperature; in addition, Mg2+ causes the individual transitions to merge (i.e., melt cooperatively) and leads to irreversible chain cleavage at high temperature. The results are analyzed according to three general secondary base-pairing models for eukaryotic 5S RNA.