The nodulation (nod) genes in soil bacteria play important roles in the development of nodules. nod genes are involved in synthesis of Nod factors that are required for bacterial entry into root hairs. Thirteen nod genes have been identified and are classified into five transcription units: nodD, nodABCIJ, nodFEL, nodMNT, and nodO. NodD negatively auto-regulates expression of nodD gene, while other nod genes are inducible and positively regulated by NodD in the presence of flavonoids released by plant roots [, , ]. This substrate-binding domain has significant homology to the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
TrpI and indoleglycerol phosphate (InGP), are required to activate transcription of the trpBA, the genes for tryptophan synthase []. This substrate-binding domain of TrpI shows significant homology to the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
This entry represents the C-terminal substrate binding domain of LysR-type transcriptional regulator AmpR, whichis involved in control of the expression of beta-lactamase genes []. Beta-lactamases are responsible for bacterial resistance to beta-lactam antibiotics such as penicillins. AmpR regulates the expression of beta-lactamases in many enterobacterial strains and many other Gram-negative bacilli. AmpR is a key player in the intricate network of regulators that is responsible for mediating virulence and antibiotic resistance in P. aeruginosa []. The topology of this substrate-binding domain is most similar to that of the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
This entry represents the C-terminal substrate-domain of LysR-type transcriptional regulator NocR, which is involved in the catabolism of opines. Opines, such as octopine and nopaline, are low molecular weight compounds found in plant crown gall tumours that are produced by the parasitic bacterium Agrobacterium. There are at least 30 different opines identified so far. Opines are utilized by tumor-colonizing bacteria as a source of carbon, nitrogen, and energy. NocR positively regulates the catabolism of nopaline. Both nopaline and octopalin are arginine derivatives. In Agrobacterium tumefaciens, NocR regulates expression of the divergently transcribed nocB and nocR genes of the nopaline catabolism (noc) region [, ]. This substrate-binding domain shows significant homology to the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
LysR is the transcriptional activator of lysA, a diaminopimelate decarboxylase that catalyses the decarboxylation of diaminopimelate to produce lysine. The C-terminal substrate-binding domain of LysR shows significant homology to the type 2 periplasmic binding proteins (PBP2) []. The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
This entry represents the C-terminal substrate binding domain of LysR-type transcriptional regulator YidZ, which is involved in anaerobic NO protection []. This substrate-binding domain shows significant homology to the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
Galactose-binding protein regulator (GbpR), a member of the LysR family of bacterial transcriptional regulators, regulates the expression of chromosomal virulence gene chvE []. The chvE gene is involved in the uptake of specific sugars, in chemotaxis to these sugars, and in the VirA-VirG two-component signal transduction system. In the presence of an inducing sugar such as L-arabinose, D-fucose, or D-galactose, GbpR activates chvE expression, while in the absence of an inducing sugar, GbpR represses expression []. The topology of this substrate-binding domain is most similar to that of the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
In Escherichia coli, XapR is a positive regulator for the expression of xapA gene, encoding xanthosine phosphorylase, and xapB gene, encoding a polypeptide similar to the nucleotide transport protein NupG. As an operon, the expression of both xapA and xapB is fully dependent on the presence of both XapR and the inducer xanthosine. Expression of the xapR is constitutive but not auto-regulated, unlike many other LysR family proteins []. This substrate-binding domain shows significant homology to the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
This entry represents the substrate binding domain of BudR regulator, which is responsible for induction of the butanediol formation pathway under fermentative growth conditions. Three enzymes are involved in the production of 1 mol of 2,3 butanediol from the condensation of 2 mol of pyruvate with acetolactate and acetoin as intermediates: acetolactate synthetase, acetolactate decarboxylase, and acetoin reductase. In Klebsiella terrigena, BudR regulates the expression of the budABC operon genes, encoding these three enzymes of the butanediol pathway []. In many bacterial species, the use of this pathway can prevent intracellular acidification by diverting metabolism from acid production to the formation of neutral compounds (acetoin and butanediol). This substrate-binding domain has significant homology to the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
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).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
The IlvR is an activator of the upstream and divergently transcribed ilvD gene, which encodes dihydroxy acid dehydratase that participates in isoleucine, leucine, and valine biosynthesis. As in the case of other members of the LysR family, the expression of ilvR gene is repressed in the presence of its own gene product [, ]. This substrate-binding domain shows significant homology to the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
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).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
In soil bacterium Pseudomonas putida, the ortho-pathways of catechol and 3-chlorocatechol are central catabolic pathways that convert aromatic and chloroaromaric compounds to tricarboxylic acid (TCA) cycle intermediates. The 3-chlorocatechol-degradative pathway is encoded by clcABD operon, which requires the divergently transcribed clcR and an intermediate of the pathway, 2-chloromuconate, as an inducer for activation []. The topology of this substrate-binding domain is most similar to that of the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
This entry represents the C-terminal substrate binding domain of LysR-type transcriptional regulator BlaA, which is involved in control of the expression of beta-lactamase genes, blaA and blaB. Beta-lactamases are responsible for bacterial resistance to beta-lactam antibiotics such as penicillins. The blaA gene is located just upstream of blaB in the opposite direction and regulates the expression of the blaB. BlaA also negatively auto-regulates the expression of its own gene, blaA. BlaA (a constitutive class A penicllinase) belongs to the LysR family of transcriptional regulators, whereas BlaB (an inducible class C cephalosporinase or AmpC) can be referred to as a penicillin binding protein but it does not act as a beta-lactamase []. The topology of this substrate-binding domain is most similar to that of the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved insignal transduction [, , ].
This entry represents the C-terminal substrate binding domain of LysR-type transcriptional regulator (LTTR) MdcR that controls the expression of the malonate decarboxylase (mdc) genes []. Like other members of the LTTRs, MdcR is a positive regulatory protein for its target promoter and composed of two functional domains joined by a linker helix involved in oligomerization: an N-terminal HTH (helix-turn-helix) domain, which is responsible for the DNA-binding specificity, and a C-terminal substrate-binding domain, which is structurally homologous to the type 2 periplasmic binding proteins (PBP2) []. The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
CynR is a LysR-like transcriptional regulator of the cyn operon, which encodes genes that allow cyanate to be used as a sole source of nitrogen. The operon includes three genes in the following order: cynT (cyanate permease), cynS (cyanase), and cynX (a protein of unknown function) []. CynR negatively regulates its own expression independently of cyanate. CynR binds to DNA and induces bending of DNA in the presence or absence of cyanate, but the amount of bending is decreased by cyanate. CynR, as other LysR-type transcriptional regulators, is composed of two functional domains joined by a linker helix involved in oligomerization: an N-terminal HTH (helix-turn-helix) domain, which is responsible for the DNA-binding specificity, and a C-terminal substrate-binding domain, which is structurally homologous to the type 2 periplasmic binding proteins (PBP2) []. The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
In Escherichia coli, IlvY is required for the regulation of ilvC gene expression that encodes acetohydroxy acid isomeroreductase (AHIR), a key enzyme in the biosynthesis of branched-chain amino acids (isoleucine, valine, and leucine). The ilvGMEDA operon genes encode remaining enzyme activities required for the biosynthesis of these amino acids []. Activation of ilvC transcription by IlvY requires the additional binding of a co-inducer molecule (either alpha-acetolactate or alpha-acetohydoxybutyrate, the substrates for AHIR) to a preformed complex of IlvY protein-DNA []. Like many other LysR-family members, IlvY negatively auto-regulates the transcription of its own divergently transcribed ilvY gene in an inducer-independent manner []. This substrate-binding domain has significant homology to the type 2 periplasmic binding proteins (PBP2). The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
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).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
CysB is a transcriptional activator of genes involved in sulfate and thiosulfate transport, sulfate reduction, and cysteine synthesis. In Escherichia coli, the regulation of transcription in response to sulfur source is attributed to two transcriptional regulators, CysB and Cbl []. CysB, in association with Cbl, downregulates the expression of ssuEADCB operon which is required for the utilization of sulfur from aliphatic sulfonates, in the presence of cysteine []. Also, Cbl and CysB together directly function as transcriptional activators of tauABCD genes, which are required for utilization of taurine as sulfur source for growth []. Like many other members of the LTTR family, CysB is composed of two functional domains joined by a linker helix involved in oligomerization: an N-terminal HTH (helix-turn-helix) domain, which is responsible for the DNA-binding specificity, and a C-terminal substrate-binding domain, which is structurally homologous to the type 2 periplasmic binding proteins []. As also observed in the periplasmic binding proteins, the C-terminal domain of the bacterial transcriptional repressor undergoes a conformational change upon substrate binding which in turn changes the DNA binding affinity of the repressor []. The structural topology of this substrate-binding domain is most similar to that of the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
Cbl is a member of the LysR transcriptional regulators that comprise the largest family of prokaryotic transcription factor. Cbl shows high sequence similarity to CysB, the LysR-type transcriptional activator of genes involved in sulfate and thiosulfate transport, sulfate reduction, and cysteine synthesis []. In Escherichia coli, the function of Cbl is required for expression of sulfate starvation-inducible (ssi) genes, coupled with the biosynthesis of cysteine from the organic sulfur sources (sulfonates). The ssi genes include the ssuEADCB and tauABCD operons encoding uptake systems for organosulfur compounds, aliphatic sulfonates, and taurine []. The genes in these operons encode an ABC-type transport system required for uptake of aliphatic sulfonates and a desulfonation enzyme. Both Cbl and CysB require expression of the tau and ssu genes. Like many other members of the LTTR family, the Cbl is composed of two functional domains joined by a linker helix involved in oligomerization: an N-terminal HTH (helix-turn-helix) domain, which is responsible for the DNA-binding specificity, and a C-terminal substrate-binding domain, which is structurally homologous to the type 2 periplasmic binding proteins. As also observed in the periplasmic binding proteins, the C-terminal domain of the bacterial transcriptional repressor undergoes a conformational change upon substrate binding which in turn changes the DNA binding affinity of the repressor []. The structural topology of this substrate-binding domain is most similar to that of the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
This entry represents the C-terminal substrate-domain of OccR, which is a LysR-type transcriptional regulator of Agrobacterium tumefaciens that positively regulates the octopine catabolism operon of the Ti plasmid []. This substrate-binding domain shows significant homology to the type 2 periplasmic binding proteins (PBP2).The PBP2 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. Besides transport proteins, the PBP2 superfamily includes the substrate- binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction [, , ].
This entry represents the periplasmic binding domain type 2 (PBP2) found in the arginine-binding proteins from Chlamydiae trachomatis (CT-ArtJ) and pneumoniae (CPn-ArtJ) and closely related proteins. CT- and CPn-ArtJ are shown to have different immunogenic properties despite a high sequence similarity. The ArtJ proteins display the PBP2 fold organized in two α-β domains with arginine-binding region at their interface [].
GlnH belongs to the type 2 periplasmic-binding fold protein (PBP2) superfamily, whose members are involved in chemotaxis and uptake of nutrients and other small molecules from the extracellular space as a primary receptor []. PBP2 typically comprises of two globular subdomains connected by a flexible hinge and 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 receptor cytoplasmically-located ATPase domains. This interaction triggersthe ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis [, ].
This superfamily represents a structural motif found at the C-terminal of penicillin-binding proteins 4 (PBP4) and 5 (PBP5), as well as at the C-terminal of D-Ala-D-Ala carboxypeptidase A, a member of the MEROPS S11 peptidase family (PBP4 and PBP5 also belong to this peptidase family). These domains share a similar structure, consisting of a β-sandwich of six strands in two sheets [, ].Penicillin-binding proteins are beta-lactam antibiotic-sensitive bacterial enzymes required for the growth and maintenance of the peptidoglycan layer of the bacterial cell wall that protects the cell from osmotic stress. PBP4 functions as a transpeptidase, acting co-operatively with PBP2 in staphylococcal cell wall biosynthesis and susceptibility to antimicrobial agents [].
This entry includes TRAP transporters that binds to ketoacids such as pyruvate and alpha-ketobutyrate, xylulose, and other unknown ligands [, , ]. TRAP transporters are a large family of solute transporters ubiquitously found in bacteria and archaea. They are comprised of a periplasmic substrate-binding protein (SBP; often called the P subunit) and two unequally sized integral membrane components: a large transmembrane subunit involved in the translocation process (the M subunit) and a smaller membrane of unknown function (the Q subunit). The driving force of TRAP transporters is provided by electrochemical ion gradients (either protons or sodium ions) across the cytoplasmic membrane, rather than ATP hydrolysis. This substrate-binding domain belongs to the type 2 periplasmic binding fold protein superfamily (PBP2). The PBP2 proteins are typically comprised of two globular subdomains connected by a flexible hinge and bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap [].
This entry includes a lactate binding TRAP transporter TTHA0766 and related proteins []. TRAP transporters are a large family of solute transporters ubiquitously found in bacteria and archaea. They are comprised of a periplasmic substrate-binding protein (SBP; often called the P subunit) and two unequally sized integral membrane components: a large transmembrane subunit involved in the translocation process (the M subunit) and a smaller membrane of unknown function (the Q subunit). The driving force of TRAP transporters is provided by electrochemical ion gradients (either protons or sodium ions) across the cytoplasmic membrane, rather than ATP hydrolysis. This substrate-binding domain belongs to the type 2 periplasmic binding fold protein superfamily (PBP2). The PBP2 proteins are typically comprised of two globular subdomains connected by a flexible hinge and bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap [].
This entry represents the substrate-binding domain found in ProX from the hyperthermophilic archaeon Archaeoglobus fulgidus and its related proteins. AfProX is involved in uptake of compatible solutes such as the trimethylammonium compound glycine betaine and the dimethylammonium compound proline betaine, but the relative substrate preference is not known. To counteract the efflux of water, many microorganisms accumulate the compatible solutes for a sustained adjustment to high osmolarity surroundings []. AfProX belongs to the type 2 periplasmic binding fold protein superfamily (PBP2). The PBP2 proteins are typically comprised of two globular subdomains connected by a flexible hinge and bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap [].
This entry represents the substrate binding domainfound in the putative ABC transporter substrate-binding lipoprotein YvgL. It is a ModA-like protein that belong to the PBP2 superfamilyof periplasmic binding proteins that differ in size and ligand specificity, but have similar tertiary structures consisting of two globular subdomains connected by a flexible hinge []. They have been shown to bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap [].ModA proteins, which serve as initial receptors in the ABC transport of molybdate in eubacteria and archaea []. Bacteria and archaea import molybdenum and tungsten from the environment in the form of the oxyanions molybdate (MoO(4) (2-)) and tungstate (WO(4) (2-)). After binding molybdate and tungstate with high affinity, they interact with a cognate membrane transport complex comprised of two integral membrane domains and two cytoplasmically located ATPase. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis [].
This entry represents the periplasmic binding protein type 2 (PBP2) domain found in cyclohexadienyl dehydratase PheC []. Proteins containing this domain catalyze the decarboxylation of prephenate to phenylpyruvate in the alternative phenylalanine biosynthesis pathway in some proteobacteria and archaea []. The PheC proteins belong to the PBP2 superfamily of periplasmic binding proteins that differ in size and ligand specificity, but have similar tertiary structures consisting of two globular subdomains connected by a flexible hinge. They have been shown to bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap []. Since the PheC proteins are so similar to periplasmic binding proteins, (PBP), it is evolutionary plausible that several pre-existing PBP proteins might have been recruited to perform the enzymatic function.
In general, ModA serves as initial receptors in the ABC transport of molybdate in eubacteria and archaea. Bacteria and archaea import molybdenum and tungsten from the environment in the form of the oxyanions molybdate (MoO(4) (2-)) and tungstate (WO(4) (2-)). After binding molybdate with high affinity, they interact with a cognate membrane transport complex comprised of two integral membrane domains and two cytoplasmically located ATPase. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis [].In contrast to the structure of the two ModA homologues from Escherichia coli and Azotobacter vinelandii, where the oxygen atoms are tetrahedrally arranged around the metal centre, the structure of Pyrococcus furiosus ModA/WtpA (PfModA) has shown that a binding site for molybdate and tungstate is where the central metal atom is in a hexacoordinate configuration. This octahedral geometry was rather unexpected [].The ModA proteins belong to the PBP2 superfamily of periplasmic binding proteins that differ in size and ligand specificity, but have similar tertiary structures consisting of two globular subdomains connected by a flexible hinge. They have been shown to bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap [, , ].This entry represents a domain found in the ModA protein from Azotobacter vinelandii and its homologues.
This entry includes a domain found in nitrate (Nrt) and bicarbonate (Cmp) receptors. This domain is found in eubacterial periplasmic-binding proteins that serve as initial receptors in the ABC transport of bicarbonate, nitrate, taurine, or a wide range of aliphatic sulfonates []. After binding its ligand with high affinity, it interacts 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 energised by ATP hydrolysis. These binding proteins belong to the PBP2 superfamily of periplasmic-binding proteins that differ in size and ligand specificity, but have similar tertiary structures consisting of two globular subdomains connected by a flexible hinge. They have been shown to bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap [].In cyanobacteria, nitrate transport takes place through NRT system, a multicomponent ABC transporter. NRT consists of 4 proteins: a periplasmic substrate-binding protein (NrtA) involved in the specific and high affinity binding of nitrate and nitrite. NrtB is a hydrophobic protein with structural similarities to integral membrane subunits of ABC transporters []. It is thought to form a pore across the membrane to allow the translocation of nitrate and nitrite. Finally, NrtC and NrtD are proposed to form a heterodimer associated to the inner side of the cytoplasmic membrane, and to be responsible of energising the transport system via ATP hydrolysis. In NrtC the binding site for ATP in found at the N-terminal [].The cmpA, cmpB, cmpC, and cmpD genes are strongly similar to the genes encoding the nitrate/nitrite transporter, nrtA, nrtB, nrtC, and nrtD, respectively. NrtB and CmpB are hydrophobic proteins with structural similarities to the integral membrane components of ABC transporters. CmpC and CmpD are ATP-binding cassette proteins strongly similar to NrtC and NrtD, respectively. CmpA is a cytoplasmic membrane protein, which is 46.5% identical to NrtA that functions as the membrane-anchored substrate (nitrate and nitrite)-binding protein []. The similarity of CmpA to NrtA and its involvement in HCO(3)(-) uptake suggest that CmpA is the substrate-binding protein of the HCO(3)(-) transporter [].
