Equilibrium DNA binding of Sac7d protein from the hyperthermophile Sulfolobus acidocaldarius: Fluorescence and circular dichroism studies

The thermodynamics of the binding of the Sac7d protein of Sulfolobus acidocaldarius to double-stranded DNA has been characterized using spectroscopic signals arising from both the protein and the DNA. Ligand binding density function analysis has been used to demonstrate that the fractional change in protein intrinsic tryptophan fluorescence quenching that occurs upon DNA binding is equal to the fraction of protein bound. Reverse titration data have been fit directly to the McGhee-von Hippel model [McGhee, J., & von Hippel, P. (1974) J. Mol. Biol. 86, 469-489] using nonlinear regression. Sac7d binds noncooperatively to poly(dGdC). poly(dGdC) with an intrinsic affinity of 6.5 x 10(6) M(-1) and a site size of 4 base pairs in 1 mM KH2PO4 and 50 mM KCI (pH 6.8). Some binding sequence preference is noted, with the binding to poly(dIdC). poly(dIdC) over 10-fold stronger than to poly(dAdT). poly(dAdT). The binding is largely driven by the polyelectrolyte effect and is consistent with a release of 4.4 monovalent cations from DNA upon complex formation or the formation of 5 ion pairs at the protein-DNA interface. Extrapolation of salt back-titration data to 1 M KCI indicates a -2.2 kcal/mol nonelectrostatic contribution to the binding free energy. A van't Hoff analysis of poly(dGdC). poly(dGdC) binding shows that the binding enthalpy is approximately zero and the process is entropically driven. The affinity decreases slightly between pH 5.4 and 8.0. There is no significant difference between the binding parameters of recombinant Sac7d and native Sac7 proteins, indicating that methylation of the native protein has no effect on the DNA binding function. The binding of Sac7d to various DNAs leads to a significant increase in the DNA long-wavelength circular dichroism (CD) band, the intensity of which shows a sigmoidal dependence on Sac7d concentration. The sigmoidal CD binding isotherm can be quantitatively modeled by a conformational transition in the DNA that is cooperatively induced when protein monomers are bound within a given number of base pairs, ranging from zero for poly(dIdC). poly(dIdC) to 8 or less for poly(dAdG). poly(dCdT).