Melting of local ordered structures in yeast 5S ribosomal RNA in aqueous salts.

Equilibrium and kinetics of thermal melting of yeast 5S ribosomal RNA in aqueous NaCl with or without Mg2+ were investigated by differential thermal melting and temperature jump methods. Two peaks (1 and 2) and a shoulder were observed in each of melting curves at ionic strength I=0.002-0.5 and linearity between each of melting temperatures T1m and T2m and log I was found at I=0.01-0.5 in the Mg2+-free solution. The local structures were found to be stabilized considerably by Mg2+. The temperature jump measurements gave the kinetic melting curve of the structure 1 at I=0.03 without Mg2+ or with 0.5 mM Mg2+. The kinetic Tm coincided well with the corresponding static Tm. For the structure 1, various parameters were calculated from the kinetic data, which indicated a double helical character of the structure 1. In terms of the values of Tm, G-C content, and enthalpy change of the transition of the structure 1 or 2, appropriateness of each of the secondary structure models of eukaryotic 5S RNA proposed previously was discussed.