The C-terminal substrate binding domain of LysR-type trnascriptional regulator AlsR, which regulates acetoin formation under stationary phase growth conditions; contains the type 2 periplasmic binding fold.
AlsR is responsible for activating the expression of the acetoin operon (alsSD) in response to inducing signals such as glucose and acetate. Like many other LysR family proteins, AlsR is transcribed divergently from the alsSD operon. The alsS gene encodes acetolactate synthase, an enzyme involved in the production of acetoin in cells of stationary-phase. AlsS catalyzes the conversion of two pyruvate molecules to acetolactate and carbon dioxide. Acetolactate is then converted to acetoin at low pH by acetolactate decarboxylase which encoded by the alsD gene. Acetoin is an important physiological metabolite excreted by many microorganisms grown on glucose or other fermentable carbon sources. 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.