Influence of inorganic cations and histone proteins on the terbium(III)-nucleic acid interaction

We have studied the interaction of the fluorescent lanthanide, terbium(III) (Tb³⁺), with polynucleotides and linear and superhelical DNA, through employment of mono- and multivalent cations as competitive inhibitors. Increasingly effective competitive inhibition of the Tb³⁺-nucleic acid interaction was achieved, for the most part, in the cation order $\text{monovalent < divalent < tetravalent}$. The divalent cation Cu²⁺ proved to be an exception to this trend, and was the strongest competitive inhibitor of all cations tested, exhibiting an affinity for Tb³⁺ binding sites over twice that of Tb³⁺ itself. Unexpectedly, a narrow range of low sodium ion concentration (8–20 mM) was found to be effective in inducing localized unwinding or unstacking of linear and supercoiled DNA double helices, a phenomenon detectable through the use of both Tb³⁺ fluorescence enhancement and ultraviolet spectroscopy. Within a similar range of low sodium ion concentration, moreover, histone H1 was substantially more effective in displacing terbium ion from DNA than either histones H2B or H4, but at higher ionic strength, this difference was absent. These results further confirm the sensitivity and specificity of Tb³⁺ as a conformational probe of nucleic acids.