Bindings of RNAse to denatured and native deoxyribonucleic acids

The binding of pancreatic ribonuclease‐A by denatured DNA, native DNA, poly‐dA, and poly‐dT, has been studied by a gel filtration method. With denatured DNA at pH 7.5, ionic strength 0.053M, there is one binding site per 12 nucleotides and the equilibrium binding constant per site is 9.7 × 104 l./mole. The binding constant increases by a factor of 8 as the pH is decreased from 8 to 7. The strength of the binding of denatured DNA increases with decreasing ionic strength. At pH 7.5, native DNA binds about ⅛ as strongly as does denatured DNA. The binding affinity increases in the order poly‐dA, denatured DNA, and poly‐dT. These results support the view that the binding of denatured DNA involves both electrostatic interactions between the negatively charged polynucleotide and the positively charged protein, and an interaction of the protein with a pyrimidine residue of the denatured DNA, and thus that the binding is basically similar to that between RNAse and its substrate RNA.