The C-terminal substrate binding domain of LysR-type transcriptional regulator LeuO, an activator of leucine synthesis operon, contains the type 2 periplasmic binding fold.LeuO, a LysR-type transcriptional regulator, was originally identified as an activator of the leucine synthesis operon (leuABCD). Subsequently, LeuO was found to be not a specific regulator of the leu gene but a global regulator of unrelated various genes. LeuO activates bglGFB (utilization of beta-D-glucoside) and represses cadCBA (lysine decarboxylation) and dsrA (encoding a regulatory small RNA for translational control of rpoS and hns). LeuO also regulates the yjjQ-bglJ operon which coding for a LuxR-type transcription factor. In Salmonella enterica serovar Typhi, LeuO is a positive regulator of ompS1 (encoding an outer membrane), ompS2 (encoding a pathogenicity determinant), and assT, while LeuO represses the expression of OmpX and Tpx. Both osmS1 and osmS2 influence virulence in the mouse model of Salmonella. In Vibrio cholerae, LeuO is involved in control of biofilm formation and in the stringent response. 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.