A circular dichroism comparative study of isolated and phage R17 RNA reveals in both cases the same degree of base pairing. However, thermal circular dichroism melting profiles exhibit the presence of free energy of interaction between RNA and capsid protein. It is apparent that the capsid stabilizes the RNA structure with and without the addition of Mg. A close RNA capsid association is also derived from pH titration circular dichroism studies. The pH melting of the RNA starts to occur about 0.5 pH unit higher with and without the addition of Mg than the acid denaturation of isolated RNA. A direct correlation between bathochromic CD peak shift of the main position band and loss of survivors is noted for the thermal melting as well as pH titration experiments. It is suggested that the heat and pH induced conformational alterations of R17 RNA coinciding with loss of infectivity occur after an alteration of nucleic acid-capsid protein interaction.


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