Specific binding of KorA to its operator-binding site. (A) Specific KorA-O_A* contacts in the crystal. Arg48 and Gln53 of the recognition helix (α4) of one KorA monomer form specific hydrogen bonds to G4, T7, C10 and T11, in one half-site of the symmetric operator sequence. (B) In vitro binding assay of purified wt KorA and mutants binding to DNA. The 436-bp fragment carries the class I O_A site (korAp) of plasmid RP4, the other fragments serve as competitor DNA; 7.5 pmol of wt and each mutant protein were applied. The DNA mixture contained 0.15 pmol of each fragment. O_A, DNA fragment containing O_A; O_A°, complex of O_A with wt KorA; M, 100-bp DNA ladder. (C) Calorimetric titration of KorA with 18-bp O_A. The panel shows the integrated heat released after correction of dilution (data points, squares) and the curve of best fit (red) for one KorA dimer binding to a single O_A site.