FIG. 6.

Possible symmetries of dimerization of OmpR/PhoB subfamily transcription factors when bound to direct repeat DNA recognition sequences. Response regulators, such as DrrB, that contain an extensive domain interface, most likely dimerize with an asymmetric orientation of their regulatory domains. Twofold rotational symmetry of the regulatory domains is unlikely, as it requires nonequivalent monomers within the dimer (gray). Response regulators such as DrrD, in which the regulatory and DNA-binding domains lack a fixed orientation, can potentially dimerize with either rotationally symmetric or asymmetric orientations of their regulatory domains. If the nature of the interface changes upon phosphorylation, then additional combinations of dimerization mechanisms are possible (dashed arrows).