Differentiation of DctA and DcuS function in the DctA/DcuS sensor complex of Escherichia coli: function of DctA as an activity switch and of DcuS as the C4‐dicarboxylate sensor

The C₄‐dicarboxylate responsiveness of the sensor kinase DcuS is only provided in concert with C₄‐dicarboxylate transporters DctA or DcuB. The individual roles of DctA and DcuS for the function of the DctA/DcuS sensor complex were analysed. (i) Variant DctA(S380D) in the C₄‐dicarboxylate site of DctA conferred C₄‐dicarboxylate sensitivity to DcuS in the DctA/DcuS complex, but was deficient for transport and for growth on C₄‐dicarboxylates. Consequently transport activity of DctA is not required for its function in the sensor complex. (ii) Effectors like fumarate induced expression of DctA/DcuS‐dependent reporter genes (dcuB–lacZ) and served as substrates of DctA, whereas citrate served only as an inducer of dcuB–lacZ without affecting DctA function. (iii) Induction of dcuB–lacZ by fumarate required 33‐fold higher concentrations than for transport by DctA (K_m = 30 μM), demonstrating the existence of different fumarate sites for both processes. (iv) In titration experiments with increasing dctA expression levels, the effect of DctA on the C₄‐dicarboxylate sensitivity of DcuS was concentration dependent. The data uniformly show that C₄‐dicarboxylate sensing by DctA/DcuS resides in DcuS, and that DctA serves as an activity switch. Shifting of DcuS from the constitutive ON to the C₄‐dicarboxylate responsive state, required presence of DctA but not transport by DctA.