C-terminal substrate binding domain of LysR-type transcriptional regulator CysL, which activates the transcription of the cysJI operon encoding sulfite reductase, contains the type 2 periplasmic binding fold.
CysL, also known as YwfK, is a regular of sulfur metabolism in Bacillus subtilis. Sulfur is required for the synthesis of proteins and essential cofactors in all living organism. Sulfur can be assimilated either from inorganic sources (sulfate and thiosulfate), or from organic sources (sulfate esters, sulfamates, and sulfonates). CysL activates the transcription of the cysJI operon encoding sulfite reductase, which reduces sulfite to sulfide. Both cysL mutant and cysJI mutant are unable to grow using sulfate or sulfite as the sulfur source. Like other LysR-type regulators, CysL also negatively regulates its own transcription. In Escherichia coli, three LysR-type activators are involved in the regulation of sulfur metabolism: CysB, Cbl and MetR. 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.