Part of the tripartite ATP-independent periplasmic (TRAP) transport system DctPQM involved in C4-dicarboxylates uptake. The complex comprises the extracytoplasmic solute receptor protein DctP, and the two transmembrane proteins DctQ and DctM. Expression is maximal in early exponential growth phase and declines with cell density to reach a plateau in the stationary growth phase. Induced by the C(4)-dicarboxylates succinate, fumarate and malate. Positively regulated by RpoN and the DctB/DctD two-component system. Negatively regulated by DctA. The dctA-dctPQM double mutant shows no growth on malate and fumarate and residual growth on succinate. The DctPQM carrier is more efficient than the DctA carrier for the utilization of succinate at micromolar concentrations, whereas DctA is the major transporter at millimolar concentrations. Belongs to the TRAP transporter small permease family.