The C-terminal substrate binding of LysR-type symbiotic regulator SyrM, which activates expression of nodulation gene NodD3, contains the type 2 periplasmic binding fold.
Rhizobium is a nitrogen fixing bacteria present in the roots of leguminous plants, which fixes atmospheric nitrogen to the soil. Most Rhizobium species possess multiple nodulation (nod) genes for the development of nodules. For example, Rhizobium meliloti possesses three copies of nodD genes. NodD1 and NodD2 activate nod operons when Rhizobium is exposed to inducers synthesized by the host plant, while NodD3 acts independent of plant inducers and requires the symbiotic regulator SyrM for nod gene expression. SyrM activates the expression of the regulatory nodulation gene nodD3. In turn, NodD3 activates expression of syrM. In addition, SyrM is involved in exopolysaccharide synthesis. 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.