The C-terminal substrate binding domain of LysR-type transcriptional regulator metR, which regulates the expression of methionine biosynthetic genes, contains type 2 periplasmic binding fold.MetR, a member of the LysR family, is a positive regulator for the metA, metE, metF, and metH genes. The sulfur-containing amino acid methionine is the universal initiator of protein synthesis in all known organisms and its derivative S-adenosylmethionine (SAM) and autoinducer-2 (AI-2) are involved in various cellular processes. SAM plays a central role as methyl donor in methylation reactions, which are essential for the biosynthesis of phospholipids, proteins, DNA and RNA. The interspecies signaling molecule AI-2 is involved in cell-cell communication process (quorum sensing) and gene regulation in bacteria. Although methionine biosynthetic enzymes and metabolic pathways are well conserved in bacteria, the regulation of methionine biosynthesis involves various regulatory mechanisms. In Escherichia coli and Salmonella enterica serovar Typhimurium, MetJ and MetR regulate the expression of methionine biosynthetic genes. The MetJ repressor negatively regulates the E. coli met genes, except for metH. Several of these genes are also under the positive control of MetR with homocysteine as a co-inducer. In Bacillus subtilis, the met genes are controlled by S-box termination-antitermination system. 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.