The C-terminal substrate binding domain of LysR-type transcriptional regulator XapR involved in xanthosine catabolism, contains the type 2 periplasmic binding fold.In Escherichia coli, XapR is a positive regulator for the expression of xapA gene, encoding xanthosine phosphorylase, and xapB gene, encoding a polypeptide similar to the nucleotide transport protein NupG. As an operon, the expression of both xapA and xapB is fully dependent on the presence of both XapR and the inducer xanthosine. Expression of the xapR is constitutive but not auto-regulated, unlike many other LysR family proteins. 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.