Cross-linking of single cysteine mutants of PhoP. (A and B) The indicated proteins (7.5 μM) were cross-linked as described in Materials and Methods to assess their ability to form BMH cross-linked dimers in the absence or presence of cognate DNA. Above the figure, the absence or presence of BMH and the oligonucleotide-based DNA probes is indicated by “−” or “+,” respectively. For the experimental results shown in panel A, 5 μM DR1,2 was included in the cross-linking reaction. For the experimental results shown in panel B, two oligonucleotide-based DNA probes, DR1,2 and NSP (a nonspecific DNA lacking any PhoP binding site), were used at 5 and 10 μM in lanes 3 and 4 and in lanes 5 and 6, respectively. The sizes of molecular mass markers are indicated in kilodaltons to the left of the figure. The PhoP monomer and PhoP dimer are indicated to the right as PhoP and (PhoP)2, respectively.