Regulation of the phosphate regulon of Escherichia coli: properties of phoR deletion mutants and subcellular localization of PhoR protein

Effect of Varying Nitrate Concentrations on Denitrifying Phosphorus Uptake by DPAOs With a Molecular Insight Into Pho Regulon Gene Expression

Bacterial Pho regulon is a key regulator component in biological phosphorus-uptake. Poly-phosphate accumulating bacteria used in enhanced biological phosphorus removal (EBPR) system encounter negative regulation of the Pho regulon, resulting in reduced phosphorus-uptake from phosphorus-replete waste effluents. This study demonstrates possible trends of overcoming the PhoU negative regulation, resulting in excessive PO4 3--P uptake at varying concentrations of NO3 --N through denitrifying phosphorus removal process. We investigated the Pho regulon gene expression pattern and kinetic studies of P-removal by denitrifying phosphate accumulating organisms (DPAOs) which are able to remove both PO4 3--P and NO3 --N in single anoxic stage with the utilization of external carbon sources, without the use of stored polyhydroxyalkanoate (PHA) and without any anaerobic-aerobic or anaerobic-anoxic switches. Our study establishes that a minimum addition of 100 ppm NO3 --N leads to the withdrawal of the negative regulation of Pho regulon and results in ∼100% P-removal with concomitant escalated poly-phosphate accumulation by our established DPAO isolates and their artificially made consortium, isolated from sludge sample of PO4 3- -rich parboiled rice mill effluent, in a settling tank within 12 h of treatment. The same results were obtained when a phosphate rich effluent (stillage from distillery) mixed with a nitrate rich effluent (from explosive industry) was treated together in a single phase anoxic batch reactor, eliminating the need for alternating anaerobic/aerobic or anaerobic/anoxic switches for removing both the pollutants simultaneously. The highest poly-phosphate accumulation was observed to be more than 17% of cell dry weight. Our studies unequivocally establish that nitrate induction of Pho regulon is parallely associated with the repression of PhoU gene transcription, which is the negative regulator of Pho regulon. Based on earlier observations where similar nitrate mediated transcriptional repression was cited, we hypothesize the possible involvement of NarL/NarP transcriptional regulator proteins in PhoU repression. At present, we propose this denitrifying phosphorus removal endeavor as an innovative methodology to overcome the negative regulation of Pho regulon for accelerated unhindered phosphorus remediation from phosphate rich wastewater in India and the developing world where the stringency of EBPR and other reactors prevent their use due to financial reasons.

Protein synthesis controls phosphate homeostasis

In this study, Pontes et al. show that impaired protein synthesis alone triggers a Pi starvation response even when Pi is plentiful in the extracellular milieu in yeast and bacteria. Their findings identify a regulatory connection between protein synthesis and Pi homeostasis that is widespread in nature.

Phosphorus is an essential element assimilated largely as orthophosphate (Pi). Cells respond to Pi starvation by importing Pi from their surroundings. We now report that impaired protein synthesis alone triggers a Pi starvation response even when Pi is plentiful in the extracellular milieu. In the bacterium Salmonella enterica serovar Typhimurium, this response entails phosphorylation of the regulatory protein PhoB and transcription of PhoB-dependent Pi transporter genes and is eliminated upon stimulation of adenosine triphosphate (ATP) hydrolysis. When protein synthesis is impaired due to low cytoplasmic magnesium (Mg²⁺), Salmonella triggers the Pi starvation response because ribosomes are destabilized, which reduces ATP consumption and thus free cytoplasmic Pi. This response is transient because low cytoplasmic Mg²⁺ promotes an uptake in Mg²⁺ and a decrease in ATP levels, which stabilizes ribosomes, resulting in ATP consumption and Pi increase, thus ending the response. Notably, pharmacological inhibition of protein synthesis also elicited a Pi starvation response in the bacterium Escherichia coli and the yeast Saccharomyces cerevisiae. Our findings identify a regulatory connection between protein synthesis and Pi homeostasis that is widespread in nature.

The Hpk2-Rrp2 two-component regulatory system of Treponema denticola: a potential regulator of environmental and adaptive responses

Treponema denticola levels in the gingival crevice become elevated as periodontal disease develops. Oral treponemes may account for as much as 40% of the total bacterial population in the periodontal pocket. The stimuli that trigger enhanced growth of T. denticola, and the mechanisms associated with the transmission of these signals, remain to be defined. We hypothesize that the T. denticola open reading frames tde1970 (histidine kinase) and tde1969 (response regulator) constitute a functional two-component regulatory system that regulates, at least in part, responses to the changing environmental conditions associated with the development of periodontal disease. The results presented demonstrate that tde1970 and tde1969 are conserved, universal among T. denticola isolates and transcribed as part of a seven-gene operon in a growth-phase-dependent manner. tde1970 undergoes autophosphorylation and transfers phosphate to tde1969. Henceforth, the proteins encoded by these open reading frames are designated as Hpk2 and Rrp2 respectively. Hpk2 autophosphorylation kinetics were influenced by environmental conditions and by the presence or absence of a PAS domain. It can be concluded that Hpk2 and Rrp2 constitute a functional two-component system that contributes to environmental sensing.

Function of the N-terminal region of the phosphate-sensing histidine kinase, SphS, in Synechocystis sp. PCC 6803

In Synechocystis sp. PCC 6803 the histidine kinase SphS (sll0337) is involved in transcriptional activation of the phosphate (Pi)-acquisition system which includes alkaline phosphatase (AP). The N-terminal region of SphS contains both a hydrophobic region and a Per-Arnt-Sim (PAS) domain. The C-terminal region has a highly conserved transmitter domain. Immunological localization studies on heterologously expressed SphS in Escherichia coli indicate that the hydrophobic region is important for membrane localization. In order to evaluate the function of the N-terminal region of SphS, deletion mutants under the control of the native promoter were analysed for in vivo AP activity. Deletion of the N-terminal hydrophobic region resulted in loss of AP activity under both Pi-deficient and Pi-sufficient conditions. Substitution of the hydrophobic region of SphS with that from the Ni2+-sensing histidine kinase, NrsS, resulted in the same induction characteristics as SphS. Deletion of the PAS domain resulted in the constitutive induction of AP activity regardless of Pi availability. To characterize the PAS domain in more in detail, four amino acid residues conserved in the PAS domain were substituted with Ala. Among the mutants R121A constitutively expressed AP activity, suggesting that R121 is important for the function of the PAS domain. Our observations indicated that the presence of a transmembrane helix in the N-terminal region of SphS is critical for activity and that the PAS domain is involved in perception of Pi availability.

The extended N-terminal region of SphS is required for detection of external phosphate levels in Synechocystis sp. PCC 6803

A novel 47 amino acid extension at the N-terminus of the SphS histidine kinase has been identified in the cyanobacterium Synechocystis sp. PCC 6803. Here, we demonstrate this region is required for activation of the SphS-SphR phosphate-sensing two-component system under phosphate-limiting conditions and mutants lacking this extension do not show constitutive alkaline phosphatase activity when the negative regulator SphU is inactivated. We have also identified a putative membrane-associated domain within this region involved in control of the Pho regulon. In addition, there are two high-affinity ABC-type phosphate uptake systems in this organism. Our results demonstrate that the Pst1 system, but not the Pst2 system, is required for suppression of the Pho regulon under phosphate-sufficient conditions. Deletion of the pst1 operon and disruption of the membrane-spanning domain may both target the same control mechanism since constitutive alkaline phosphatase activity is similar in the double and single mutants.

Optimization of BLyS production and purification from Escherichia coli

B lymphocyte stimulator (BLyS) is a member of the tumor necrosis factor superfamily of cytokines. When the 152 amino acids of the C-terminus are associated into a homotrimer, this protein exhibits the ability to stimulate B cell proliferation and differentiation. Since numerous potential therapeutic indications have been identified for BLyS and other BLyS-derived products, large quantities of the protein are needed to further basic research and clinical trials. In this work, we have developed a high yield recombinant expression system that utilizes Escherichia coli as the host organism. Recombinant soluble BLyS (rsBLyS) production was achieved through the use of the phoA promoter system. This expression system, coupled to a semi-defined fermentation process, resulted in final purified yields of 435 mg/L of properly folded, trimeric, biologically active rsBLyS. This level of production is an 11-fold increase in volumetric yields compared to the process currently being used for clinical production. Furthermore, the increased rsBLyS production obtained from this process enabled the development of a conventional purification scheme that eliminated the use of a BLyS-affinity resin.

Interdomain linkers of homologous response regulators determine their mechanism of action

OmpR and PhoB are response regulators that contain an N-terminal phosphorylation domain and a C-terminal DNA binding effector domain connected by a flexible interdomain linker. Phosphorylation of the N terminus results in an increase in affinity for specific DNA and the subsequent regulation of gene expression. Despite their sequence and structural similarity, OmpR and PhoB employ different mechanisms to regulate their effector domains. Phosphorylation of OmpR in the N terminus stimulates the DNA binding affinity of the C terminus, whereas phosphorylation of the PhoB N terminus relieves inhibition of the C terminus, enabling it to bind to DNA. Chimeras between OmpR and PhoB containing either interdomain linker were constructed to explore the basis of the differences in their activation mechanisms. Our results indicate that effector domain regulation by either N terminus requires its cognate interdomain linker. In addition, our findings suggest that the isolated C terminus of OmpR is not sufficient for a productive interaction with RNA polymerase.

Mutational analysis of the Escherichia coli PhoQ sensor kinase: differences with the Salmonella enterica serovar Typhimurium PhoQ protein and in the mechanism of Mg2+ and Ca2+ sensing

The PhoP-PhoQ two-component system plays a role in Mg2+ homeostasis and/or the virulence properties of a number of bacterial species. A Salmonella enterica serovar Typhimurium PhoQ sensor kinase mutant, in which the threonine at residue 48 in the periplasmic sensor domain is changed to an isoleucine, was shown previously to result in elevated expression of PhoP-activated genes and to affect mouse virulence, epithelial cell invasion, and sensitivity to macrophage killing. We characterized a complete set of proteins having amino acid substitutions at position 48 in the closely related Escherichia coli PhoQ protein. Numerous mutant proteins having amino acid substitutions with side chains of various sizes and characters displayed signaling phenotypes similar to that of the wild-type protein, indicating that interactions mediated by the wild-type threonine side chain are not required for normal protein function. Changes to amino acids with aromatic side chains had little impact on signaling in response to extracellular Mg2+ but resulted in reduced sensitivity to extracellular Ca2+, suggesting that the mechanisms of signal transduction in response to these two divalent cations are different. Surprisingly, the Ile48 protein displayed a defective phenotype rather than the hyperactive phenotype seen with the S. enterica serovar Typhimurium protein. We also describe a mutant PhoQ protein lacking the extracellular sensor domain with a defect in the ability to activate PhoP. The defect does not appear to be due to reduced autokinase activity but rather appears to be due to an effect on the stability of the aspartyl-phosphate bond of phospho-PhoP.

PAS domains: internal sensors of oxygen, redox potential, and light

PAS domains are newly recognized signaling domains that are widely distributed in proteins from members of the Archaea and Bacteria and from fungi, plants, insects, and vertebrates. They function as input modules in proteins that sense oxygen, redox potential, light, and some other stimuli. Specificity in sensing arises, in part, from different cofactors that may be associated with the PAS fold. Transduction of redox signals may be a common mechanistic theme in many different PAS domains. PAS proteins are always located intracellularly but may monitor the external as well as the internal environment. One way in which prokaryotic PAS proteins sense the environment is by detecting changes in the electron transport system. This serves as an early warning system for any reduction in cellular energy levels. Human PAS proteins include hypoxia-inducible factors and voltage-sensitive ion channels; other PAS proteins are integral components of circadian clocks. Although PAS domains were only recently identified, the signaling functions with which they are associated have long been recognized as fundamental properties of living cells.

The acid-inducible asr gene in Escherichia coli: transcriptional control by the phoBR operon

Escherichia coli responds to external acidification (pH 4.0 to 5.0) by synthesizing a newly identified, approximately 450-nucleotide RNA component. At maximal levels of induction it is one of the most abundant small RNAs in the cell and is relatively stable bacterial RNA. The acid-inducible RNA was purified, and the gene encoding it, designated asr (for acid shock RNA), mapped at 35.98 min on the E. coli chromosome. Analysis of the asr DNA sequence revealed an open reading frame coding for a 111-amino-acid polypeptide with a deduced molecular mass of approximately 11.6 kDa. According to computer-assisted analysis, the predicted polypeptide contains a typical signal sequence of 30 amino acids and might represent either a periplasmic or an outer membrane protein. The asr gene cloned downstream from a T7 promoter was translated in vivo after transcription using a T7 RNA polymerase transcription system. Expression of a plasmid-encoded asr::lacZ fusion under a native asr promoter was reduced approximately 15-fold in a complex medium, such as Luria-Bertani medium, versus the minimal medium. Transcription of the chromosomal asr was abolished in the presence of a phoB-phoR (a two-component regulatory system, controlling the pho regulon inducible by phosphate starvation) deletion mutant. Acid-mediated induction of the asr gene in the Delta(phoB-phoR) mutant strain was restored by introduction of the plasmid with cloned phoB-phoR genes. Primer extension analysis of the asr transcript revealed a region similar to the Pho box (the consensus sequence found in promoters transcriptionally activated by the PhoB protein) upstream from the determined transcription start. The asr promoter DNA region was demonstrated to bind PhoB protein in vitro. We discuss our results in terms of how bacteria might employ the phoB-phoR regulatory system to sense an external acidity and regulate transcription of the asr gene.

Functional dissection of the transmitter module of the histidine kinase NtrB in Escherichia coli

Signal transduction by two-component systems involves phosphorylation and thereby activation of the response regulator by the cognate histidine kinase. Bifunctional histidine kinases have two opposing activities: depending on the environmental stimuli they either promote phosphorylation or stimulate the rapid dephosphorylation of the response regulator. To determine the mechanism of this switch, we analyzed the domain organization of the bifunctional histidine kinase NtrB. Based on sequence alignments with other histidine kinases and a deletion analysis, we defined three separate subdomains of the transmitter module, the H domain (amino acids 123-221), the N domain (amino acids 221-269), and the G domain (amino acids 269-349). The transmitter module, when separately expressed, exhibited a constitutive positive phenotype. In contrast, in the absence of the G domain, the H domain exhibits a constitutive negative phenotype. This negative regulatory activity of the H domain is inhibited by the G domain. The G domain could be physically uncoupled; when coexpressed with the H-N fragment, the constitutive positive phenotype of the transmitter was restored. We demonstrate, in vitro, that the constitutive negative phenotype of the fragments lacking the G domain is caused by stimulation of dephosphorylation of the response regulator NtrC-P. Based on our analysis, we suggest that the function of the sensor domain is to control the interaction of the H and G domains. If these subdomains interact, NtrB acts as a positive regulator; if they cannot interact, NtrB acts as a negative regulator.

The cytoplasmic kinase domain of PhoR is sufficient for the low phosphate-inducible expression of pho regulon genes in Bacillus subtilis

PhoP-PhoR, one of three two-component systems known to be required to regulate the pho regulon in Bacillus subtilis, directly regulates the alkaline phosphatase genes that are used as pho reporters. Biochemical studies showed that B. subtilis PhoR, purified from Escherichia coli, was autophosphorylated in vitro in the presence of ATP. Phosphorylated PhoR showed stability under basic conditions but not acidic conditions, indicating that the phosphorylation probably occurs on a conserved histidine residue. Phospho-PhoR phosphorylated its cognate response regulator, PhoP in vitro. B. subtilis phoR was placed in the Bacillus chromosome under the control of the Pspac promoter, which is IPTG inducible. The wild-type phoR, under either native promoter or Pspac promoter with IPTG induction, resulted in a similar level of alkaline phosphatase production. Under high phosphate conditions, strains containing wild-type phoR, or phoR mutant gene products that lacked either the periplasmic domain, or both N-terminal transmembrane PhoR mutant gene products that lacked either the periplasmic domain, or both N-terminal transmembrane PhoR sequences or various extended N-terminal sequences, showed no significant APase production. Under phosphate starvation conditions, in the presence of IPTG, all strains containing mutated phoR genes showed alkaline phosphatase induction patterns similar to that of the wild-type strain, although the fully induced level was lower in the mutants. The decrease in total alkaline phosphatase production in these mutant strains can be compensated completely or partially by increasing the copy number of the mutant phoR gene. These in vivo results suggest that the C-terminal kinase domain of PhoR is sufficient for the induction of alkaline phosphatase expression under phosphate-limited conditions, and that the regulation for repression of APase under phosphate-replete conditions remains intact.

Bacillus subtilis PhoP binds to the phoB tandem promoter exclusively within the phosphate starvation-inducible promoter

Several gene products, including three two-component systems, make up a signal transduction network that controls the phosphate starvation response in Bacillus subtilis. Epistasis experiments indicate that PhoP, a response regulator, is furthest downstream of the known regulators in the signaling pathway that regulates Pho regulon genes. We report the overexpression, purification, and use of PhoP in investigating its role in Pho regulon gene activation. PhoP was a substrate for both the kinase and phosphatase activities of its cognate sensor kinase, PhoR. It was not phosphorylated by acetyl phosphate. Purified phosphorylated PhoP (PhoPP) had a half-life of approximately 2.5 h, which was reduced to about 15 min by addition of the same molar amount of *PhoR (the cytoplasmic region of PhoR). ATP significantly increased phosphatase activity of *PhoR on PhoPP. In gel filtration and cross-linking studies, both PhoP and PhoPP were shown to be dimers. The dimerization domain was located within the 135 amino acids at the N terminus of PhoP. Phosphorylated or unphosphorylated PhoP bound to one of the alkaline phosphatase gene promoters, the phoB promoter. Furthermore, PhoP bound exclusively to the -18 to -73 region (relative to the transcriptional start site +1) of the phosphate starvation-inducible promoter (Pv) but not to the adjacent developmentally regulated promoter (Ps). These data corroborate the genetic data for phoB regulation and suggest that activation of phoB is via direct interaction between PhoP and the phoB promoter. Studies of the phosphorylation, oligomerization, and DNA binding activity of the PhoP protein demonstrate that its N-terminal phosphorylation and dimerization domain and its C-terminal DNA binding domain function independently of one another, distinguishing PhoP from other response regulators, such as PhoB (Escherichia coli) and NtrC.

The activation of PhoB by acetylphosphate

PhoB is a response-regulator protein from Escherichia coli that controls an adaptive response to limiting phosphate. It is activated by autophosphorylation of a conserved aspartate residue within its regulatory domain. Its primary phospho-donor is its cognate histidine kinase PhoR; however, it also becomes phosphorylated when incubated with acetylphosphate. To further characterize its activation, PhoB was considered to be an acetylphosphatase whose enzymatic mechanism involves a phospho-enzyme intermediate. The kinetic constants for autophosphorylation were determined using 32P-and fluorescence-based assays and indicated that PhoB has a K(m) for acetylphosphate of between 7 and 8 mM. These constants are not consistent with an in vivo role for acetylphosphate in the normal control of the Pho regulon. In addition, when PhoB was phosphorylated by acetylphosphate it eluted from a high-performance liquid chromatography (HPLC) size-exclusion column in two peaks. The larger form of PhoB eluted from the column in a similar manner to a chemically cross-linked dimer of PhoB. The smaller form of PhoB is a monomer. Phosphorylated PhoB bound pho-box DNA approximately 10 times tighter than PhoB. These observations show that PhoB forms a dimer when phosphorylated and suggest that the characteristics of activated PhoB result from its dimerization.

Topology of the PhoR protein of Escherichia coli and functional analysis of internal deletion mutants

The PhoR protein of Escherichia coli K-12 belongs to a family of structurally related sensor-kinases that regulate responses to environmental stimuli. These proteins are often located in the inner membrane with two membrane-spanning segments that are separated by a periplasmic domain, which is supposed to sense the environmental stimuli. However, the hydrophobicity plot of PhoR suggests a somewhat different topology in which a large periplasmic domain is lacking and an extended cytoplasmic domain is present besides the kinase domain. In protease-accessibility experiments and by using phoR-phoA gene fusions, the topology of PhoR was investigated and the absence of a large periplasmic domain was confirmed. Furthermore, the function of the extended cytoplasmic domain was studied by creating internal deletions. The mutations in this domain resulted in a constitutive expression of the pho regulon, indicating that the mutant PhoR proteins are locked in their kinase function. We propose that this extended cytoplasmic domain functions by sensing an internal signal that represses the kinase function of the PhoR protein.

Role of PhoU in phosphate transport and alkaline phosphatase regulation

The negative regulatory function of PhoU in alkaline phosphatase (AP) was suggested by the behavior of K10 phoU35 carrying a missense mutation whose product was detected by immunoblotting. To define more clearly the regulatory function of this protein for the synthesis of AP, we constructed a null mutation. The constitutive synthesis of AP in this phoU deletion strain confirmed the negative role of PhoU. However, the expression of the PhoU protein from an isopropyl-beta-D-thiogalactopyranoside-inducible promoter had no effect on the repression of AP synthesis. Furthermore, the involvement of PhoU in free-Pi uptake was demonstrated. These results provide evidence that PhoU participates in Pi transport and in the regulatory role of the phosphate-specific transport system.

Gene regulation of plasmid- and chromosome-determined inorganic ion transport in bacteria

Regulation of chromosomally determined nutrient cation and anion uptake systems shows important similarities to regulation of plasmid-determined toxic ion resistance systems that mediate the outward transport of deleterious ions. Chromosomally determined transport systems result in accumulation of K+, Mg2+, Fe3+, Mn2+, PO4(3-), SO4(2-), and additional trace nutrients, while bacterial plasmids harbor highly specific resistance systems for AsO2-, AsO4(3-), CrO4(2-), Cd2+, Co2+, Cu2+, Hg2+, Ni2+, SbO2-, TeO3(2-), Zn2+, and other toxic ions. To study the regulation of these systems, we need to define both the trans-acting regulatory proteins and the cis-acting target operator DNA regions for the proteins. The regulation of gene expression for K+ and PO4(3-) transport systems involves two-component sensor-effector pairs of proteins. The first protein responds to an extracellular ionic (or related) signal and then transmits the signal to an intracellular DNA-binding protein. Regulation of Fe3+ transport utilizes the single iron-binding and DNA-binding protein Fur. The MerR regulatory protein for mercury resistance both represses and activates transcription. The ArsR regulatory protein functions as a repressor for the arsenic and antimony(III) efflux system. Although the predicted cadR regulatory gene has not been identified, cadmium, lead, bismuth, zinc, and cobalt induce this system in a carefully regulated manner from a single mRNA start site. The cadA Cd2+ resistance determinant encodes an E1(1)-1E2-class efflux ATPase (consisting of two polypeptides, rather than the one earlier identified). Cadmium resistance is also conferred by the czc system (which confers resistances to zinc and cobalt in Alcaligenes species) via a complex efflux pump consisting of four polypeptides. These two cadmium efflux systems are not otherwise related. For chromate resistance, reduced cellular accumulation is again the resistance mechanism, but the regulatory components are not identified. For other toxic heavy metals (with few exceptions), there exist specific plasmid resistances that remain relatively terra incognita for future exploration of bioinorganic molecular genetics and gene regulation.

Dual regulation of the ugp operon by phosphate and carbon starvation at two interspaced promoters

The ugp operon of Escherichia coli includes genes involved in the uptake of sn-glycerol-3-phosphate and glycerophosphoryl diesters and belongs to the pho regulon which is induced by phosphate limitation. This operon has two transcriptional initiation sites, as determined by S1 nuclease mapping of the in vivo transcripts. The downstream promoter has multiple copies of the pho box, the consensus sequence shared by the pho promoters; the upstream promoter has a consensus sequence for the promoters regulated by cyclic AMP and its receptor protein, CRP. PhoB protein, which is the transcriptional activator for the pho regulon, protected the regulatory region with the pho boxes in DNase I footprinting experiments and activated transcription from the downstream promoter in vitro. Studies with transcriptional fusions between ugp and a promoterless gene for chloramphenicol acetyltransferase show that the upstream promoter is induced by carbon starvation in a manner that required the cya and crp genes. PhoB protein may act as a repressor for this upstream promoter, which also overlaps the upstream third pho box. The downstream promoter was induced by phosphate starvation and requires the PhoB protein for its activation as do the other pho regulon promoters. These results suggest that the two promoters function alternately in responding to phosphate or carbon starvation, thus providing the cell with a means to adapt to these physiological stresses.

Cross talk to the phosphate regulon of Escherichia coli by PhoM protein: PhoM is a histidine protein kinase and catalyzes phosphorylation of PhoB and PhoM-open reading frame 2

Transcription of the genes in the phosphate regulon in Escherichia coli is activated by PhoB protein, which is phosphorylated by PhoR protein under phosphate-limiting conditions. In the absence of the phoR function, the genes in the phosphate regulon are expressed constitutively and the expression is dependent on the function of phoM and phoB. We constructed a plasmid with a lacZ'-'phoM fusion gene, which encoded a hybrid protein (PhoM1206) in which the hydrophobic amino-terminal half of the native PhoM was replaced by beta-galactosidase. The phoM1206 gene could complement the phoM mutation in vivo. We purified PhoM1206 from the overproducing strain carrying the plasmid; it was autophosphorylated at a histidine residue in the presence of ATP, and the phospho-PhoM1206 phosphorylated PhoB. PhoM1206 could also transphosphorylate the product of phoM-orf2, which is structurally homologous to phoB and located immediately upstream of phoM. Although PhoR1084 that lacked the hydrophobic amino-terminal region of the native PhoR protein transphosphorylated PhoB, it could not phosphorylate PhoM-open reading frame 2. Therefore, cross talk by protein phosphorylation appears to occur from PhoM to PhoB but not from PhoR to PhoM-open reading frame 2.

From cell membrane to nucleotides: the phosphate regulon in Escherichia coli

Most of the essential cellular components, like nucleic acids, lipids and sugars, are phosphorylated. The phosphate equilibrium in Escherichia coli is regulated by the phosphate (Pi) input from the surrounding medium. Some 90 proteins are synthesized at an increased rate during Pi starvation and the global control of the cellular metabolism requires cross-talk with other regulatory mechanisms. Since the Pi concentration is normally low in E. coli's natural habitat, these cells have devised a mechanism for synthesis of about 15 proteins to accomplish two specific functions: transport of Pi and its intracellular regulation. The synthesis of these proteins is controlled by two genes (the phoB-phoR operon), involving both negative and positive functions. PhoR protein is a histidine protein kinase, induced in Pi starvation and is a transmembrane protein. It phosphorylates the regulator protein PhoB which is also Pi starvation-induced. The PhoB phosphorylated form binds specifically to a DNA sequence of 18 nucleotides (the pho Box), which is part of the promoters of the Pho genes. The genes controlled by phoB constitute the Pho regulon. The repression of phoA (the gene encoding alkaline phosphatase) by high Pi concentrations in the medium requires the presence of an intact Pst operon (pstS, pstC, pstA, pstB and phoU) and phoR. The products of pstA and pstC are membrane bound, whereas the product of pstS is periplasmic and PstB and PhoU proteins are cytoplasmic. The function of the PhoU protein may be regulated by cofactor nucleotides and may be involved in signaling the activation of the regulon via PhoR.

Molecular aspects of phosphate transport in Escherichia coli

Escherichia coli transports inorganic phosphate (Pi) by the low-affinity transport system, Pit. When the level of the external Pi is lower than 20 microM, another transport system, Pst, is induced with a Kt of 0.25 microM. An outer-membrane porin, PhoE, with a Km of about 1 microM is also induced. The outer membrane allows the intake of organic phosphates which are degraded to Pi by phosphatases in the periplasm. The Pi-binding protein will capture the free Pi produced in the periplasm and direct it to the transmembrane channel of the cytoplasmic membrane. The channel consists of two proteins, PstA and PstC, which have six and five transmembrane helices, respectively. On the cytoplasmic side of the membrane the channel is linked to the PstB protein, which carries a nucleotide (probably ATP)-binding site. PstB probably provides the energy required by the channel to free Pi. The Pst system has two functions in E. coli: (i) the transport of Pi, and (ii) the negative regulation of the phosphate regulon (a complex of 20 proteins mostly related to organic phosphate transport). It is remarkable that these two functions are not related, since the repressibility of the regulon depends on the integral structure of Pst (PiBP + PstA + PstC + PstB) and not on the Pi transported. Another gene of the pst operon, phoU, produces a protein involved in the negative regulation of the Pho regulon, but the mechanism of this function has not been explained. Thus the regulatory function of the Pst system remains obscure. Its basal level, present when Pi is abundant, is sufficient to repress the Pho regulon but the negative regulatory function is lost upon Pi starvation.