FIG 6 .

Model for ArcA-P binding to a three-DR binding site. The orientation and protein-protein contacts between the N- and C-terminal domains within an ArcA-P dimer are based on crystallographic data from ArcA and PhoB, respectively (15, 16). Energetically favorable contacts are indicated in blue, while contacts likely to be less favorable are indicated in red. We propose that two ArcA-P dimers bind to a three-DR site in a cooperative manner; the first dimer binds to DR1 and DR2, and a second dimer binds to DR3 and adjacent nonspecific sequences. A favorable energetic contribution from the interaction between ArcA-P dimers is likely required to overcome the poor binding affinity of an ArcA-P dimer to DR3 and adjacent nonspecific sequence. Alternatively, it is possible that dimerization is not required for binding to DR3; ArcA-P may bind to DR3 as a monomer. Potential regions of interaction between ArcA-P molecules in both scenarios are marked with question marks