The C-terminal substrate binding domain of LysR-type transcriptional regulator ToxR regulates the expression of the toxoflavin biosynthesis genes; contains the type 2 periplasmic bindinig fold.
In soil bacterium Burkholderia glumae, ToxR regulates the toxABCDE and toxFGHI operons in the presence of toxoflavin as a coinducer. Additionally, the expression of both operons requires a transcriptional activator, ToxJ, whose expression is regulated by the TofI or TofR quorum-sensing system. The biosynthesis of toxoflavin is suggested to be synthesized in a pathway common to the synthesis of riboflavin. 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.