The C-terminal substrate binding domain of LysR-type transcriptional regulators involved in the chlorocatechol catabolism, contains the type 2 periplasmic binding fold.
This CD includes the substrate binding domain of LysR-type regulators CbnR, ClcR and TfdR, which are involved in the regulation of chlorocatechol breakdown. The chlorocatechol-degradative pathway is often found in bacteria that can use chlorinated aromatic compounds as carbon and energy sources. CbnR is found in the 3-chlorobenzoate degradative bacterium Ralstonia eutropha NH9 and forms a tetramer. CbnR activates the expression of the cbnABCD genes, which are responsible for the degradation of chlorocatechol converted from 3-chlorobenzoate and are transcribed divergently from cbnR. In soil bacterium Pseudomonas putida, the 3-chlorocatechol-degradative pathway is encoded by clcABD operon, which requires the divergently transcribed clcR for activation. TfdR is involved in the activation of tfdA and tfdB gene expression. These genes encode enzymes for the conversion of 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol. The topology of this substrate-binding domain is most similar to that of 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.