Thermodynamic characterization of non-sequence-specific DNA-binding by the Sso7d protein from Sulfolobus solfataricus

We used isothermal titration calorimetry and fluorescence spectroscopy to investigate the thermodynamics of non-sequence-specific DNA-binding by the Sso7d protein from the archaeon Sulfolobus solfataricus. We report the Sso7d-poly(dGdC) binding thermodynamics as a function of buffer composition (Tris-HCl or phosphate), temperature (15 to 45 degrees C), pH (7.1 to 8.0), osmotic stress and solvent (H2O/(H2O)-H-2), and compare it to poly(dAdT) binding; and we have previously also reported the salt concentration dependence. Binding isotherms can be represented by the McGhee-von Hippel model for non-cooperative binding, with a binding site size of four to five DNA base-pairs and binding free energies in the range Delta G degrees approximate to -7 to Delta G degrees approximate to -10 kcal mol(-1), depending on experimental conditions. The non-specific nature of the binding is reflected in similar thermodynamics for binding to poly(dAdT) and poly(dGdC). The native lysine methylation of Sso7d has only minor effects on the binding thermodynamics. Sso7d binding to poly(dGdC) is endothermic at 25 degrees C with a binding enthalpy Delta H degrees approximate to 10 kcal mol(-1) in both phosphate and Tris-HCl buffers at pH 7.6, indicating that Delta H degrees does not include large contributions from coupled buffer ionization equilibria at this pH. The binding enthalpy is temperature dependent with a measured heat capacity change Delta C-o degrees = -0.25(+/-0.01) kcal mol(-1) K-1 and extrapolations of thermodynamic data indicate that the complex is heat stable with exothermic binding close to the growth temperature (75 to 80 degrees C) of S. solfataricus. Addition of neutral solutes (osmotic stress) has minor effects on Delta G degrees and the exchange of H2O for (H2O)-H-2 has only a small effect on Delta H degrees, consistent with the inference that complex formation is not accompanied by net changes in surface hydration. Thus, other mechanisms for the heat capacity change must be found. The observed thermodynamics is discussed in relation to the nature of non-sequence-specific DNA-binding by proteins. (C) 1998 Academic Press Limited.