The C-terminal substrate binding domain of LysR-type transcriptional regulator TdcA, which is involved in the degradation of L-serine and L-threonine, contains the type 2 periplasmic binding fold.TdcA, a member of the LysR family, activates the expression of the anaerobically-regulated tdcABCDEFG operon which is involved in the degradation of L-serine and L-threonine to acetate and propionate, respectively. The tdc operon is comprised of one regulatory gene tdcA and six structural genes, tdcB to tdcG. The expression of the tdc operon is affected by several transcription factors including the cAMP receptor protein (CRP), integration host factor (IHF), histone-like protein (HU), and the operon specific regulators TdcA and TcdR. TcdR is divergently transcribed from the operon and encodes a small protein that is required for efficient expression of the Escherichia coli tdc operon. This substrate-binding domain shows significant homology to the type 2 periplasmic binding proteins (PBP2), which are responsible for the uptake of a variety of substrates such as phosphate, sulfate, polysaccharides, lysine/arginine/ornithine, and histidine. The PBP2 bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap. After binding their specific ligand with high affinity, they can interact with a cognate membrane transport complex comprised of two integral membrane domains and two cytoplasmically located ATPase domains. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis.