Secondary and tertiary structural changes in γδ resolvase: Comparison of the wild‐type enzyme, the I11OR mutant, and the C‐terminal DNA binding domain in solution

γδ Resolvase is a site‐specific DNA recombinase (M_r 20.5 kDa) in Escherichia coli that shares homology with a family of bacterial resolvases and invertases. We have characterized the secondary and tertiary structural behavior of the cloned DNA binding domain (DBD) and a dimerization defective mutant in solution. Low‐salt conditions were found to destabilize the tertiary structure of the DBD dramatically, with concomitant changes in the secondary structure that were localized near the hinge regions between the helices. The molten ertiary fold appears to contribute significantly to productive DNA interactions and supports a mechanism of DNA‐induced folding of the tertiary structure, a process that enables the DBD to adapt in conformation for each of the three imperfect palindromic sites. At high salt concentrations, the monomeric I11OR resolvase shows a minimal perturbation to the three helices of the DBD structure and changes in the linker segment in comparison to the cloned DBD containing the linker. Comparative analysis of the NMR spectra suggest that the I11OR mutant contains a folded catalytic core of ∼60 residues and that the segment from residues 100 to 149 are devoid of regular structure in the I11OR resolvase. No increase in the helicity of the linker region of I11OR resolvase occurs on binding DNA. These results support a subunit rotation model of strand exchange that involves the partial unfolding of the catalytic domains.