Energetics of Sequence-Specific Protein−DNA Association: Conformational Stability of the DNA Binding Domain of Integrase Tn916 and Its Cognate DNA Duplex

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Abstract
Sequence-specific DNA recognition by bacterial integrase Tn916 involves structural rearrangements of both the protein and the DNA duplex. Energetic contributions from changes of conformation, thermal motions and soft vibrational modi of the protein, the DNA, and the complex significantly influence the energetic profile of protein−DNA association. Understanding the energetics of such a complicated system requires not only a detailed calorimetric investigation of the association reaction but also of the components in isolation. Here we report on the conformational stability of the integrase Tn916 DNA binding domain and its cognate 13 base pair target DNA duplex. Using a combination of temperature and denaturant induced unfolding experiments, we find that the 74-residue DNA binding domain is compact and unfolds cooperatively with only small deviation from two-state behavior. Scanning calorimetry reveals an increase of the heat capacity of the native protein attributable to increased thermal fluctuations. From the combined calorimetric and spectroscopic experiments, the parameters of protein unfolding are Tm = 43.8 ± 0.3 °C, ΔHm = 255 ± 18 kJ mol-1, ΔSm = 0.80 ± 0.06 kJ mol-1, and ΔCp = 5.0 ± 0.8 kJ K-1 mol-1. The DNA target duplex displays a thermodynamic signature typical of short oligonucleotide duplexes: significant heat absorption due to end fraying and twisting precedes cooperative unfolding and dissociation. The parameters for DNA unfolding and dissociation are ΔHm = 335 ± 4 kJ mol-1 and ΔCp = 2.7 ± 0.9 kJ K-1 mol-1. The results reported here have been instrumental in interpreting the thermodynamic features of the association reaction of the integrase with its 13 base pair target DNA duplex reported in the accompanying paper [Milev et al. (2003) Biochemistry42, 3481−3491].

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