The C-terminal substrate domain of LysR-like regulator LrhA (LysR homologue A) and that of closely related homologs, contains the type 2 periplasmic binding fold.
This CD represents the LrhA subfamily of LysR-like bacterial transcriptional regulators, including LrhA, HexA, PecT, and DgdR. LrhA is involved in control of the transcription of flagellar, motility, and chemotaxis genes by regulating the synthesis and concentration of FlhD(2)C(2), the master regulator for the expression of flagellar and chemotaxis genes. The LrhA protein has strong homology to HexA and PecT from plant pathogenic bacteria, in which HexA and PecT act as repressors of motility and of virulence factors, such as exoenzymes required for lytic reactions. DgdR also shares similar characteristics to those of LrhA, HexA and PecT. 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.