A dimeric two‐component receiver domain inhibits the σ54‐dependent ATPase in DctD

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 σ 54 ‐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.