Part of the binding-protein-dependent transport system for uptake of C4-dicarboxylates. Responsible for growth on fumarate and succinate but not malate. Is not directly involved in C4-dicarboxylate uptake, but plays a sensory role in the DctS/DctR two-component system which regulates the expression of the dctA C4-dicarboxylate transporter. Belongs to the bacterial solute-binding protein 7 family.