We report the crystal structure of a fragment of Sinorhizobium meliloti DctD, a bacterial enhancer binding protein, at 1.7 Å. The fragment contains the protein's two‐component receiver module and adjacent linker, which in the native protein joins the receiver domain to a σ54‐dependent ATPase domain. The structure reveals a novel dimerization surface, which sequence analysis indicates is common to 4.5% of the known two‐component receiver domains. Genetic, biochemical, and structural data for amino acid substitution variants indicate that the dimer is necessary to inhibit the basal activity of the ATPase domain. The dimerization element is thus needed to maintain the “off” state, and changes within it may signal activation. Analytical ultracentrifugation data for the phosphorylated fragment of DctD appear to rule out the simple model that signaling is mediated via monomerization of the receiver domain.
The Rhizobium meliloti DctD two-component receiver domain was expressed in Escherichia coli and purified to homogeneity. Crystals were obtained using the hanging-drop vapor-diffusion geometry with ammonium phosphate as the precipitant. The crystals diffract to 2.3 A and exhibit the symmetry of space group I222 or I212121. The unit-cell dimensions are a = 59.0, b = 58.6 and c = 169.8 A. The asymmetric unit contains a dimer and the crystals have a Vm of 2.16 A3 Da-1.
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