Anion-exchange mechanisms in bacteria

Context-dependent change in the fitness effect of (in)organic phosphate antiporter glpT during Salmonella Typhimurium infection

Salmonella enterica is a frequent cause of foodborne diseases, which is attributed to its adaptability. Even within a single host, expressing a gene can be beneficial in certain infection stages but neutral or even detrimental in others as previously shown for flagellins. Mutants deficient for the conserved glycerol-3-phosphate and phosphate antiporter glpT have been shown to be positively selected in nature, clinical, and laboratory settings. This suggests that different selective pressures select for the presence or absence of GlpT in a context dependent fashion, a phenomenon known as antagonistic pleiotropy. Using mutant libraries and reporters, we investigated the fitness of glpT-deficient mutants during murine orogastric infection. While glpT-deficient mutants thrive during initial growth in the gut lumen, where GlpT's capacity to import phosphate is disadvantageous, they are counter-selected by macrophages. The dichotomy showcases the need to study the spatial and temporal heterogeneity of enteric pathogens' fitness across distinct lifestyles and niches. Insights into the differential adaptation during infection may reveal opportunities for therapeutic interventions.

Phosphorus starvation response and PhoB-independent utilization of organic phosphate sources by Salmonella enterica

IMPORTANCE

Phosphorus (P) is the fifth most abundant element in living cells. This element is acquired mainly as inorganic phosphate (Pi, PO4 3-). In enteric bacteria, P starvation activates a two-component signal transduction system which is composed of the membrane sensor protein PhoR and its cognate transcription regulator PhoB. PhoB, in turn, promotes the transcription of genes that help maintain Pi homeostasis. Here, we characterize the P starvation response of the bacterium Salmonella enterica. We determine the PhoB-dependent and independent transcriptional changes promoted by P starvation and identify proteins enabling the utilization of a range of organic substrates as sole P sources. We show that transcription and activity of a subset of these proteins are independent of PhoB and Pi availability. These results establish that Salmonella enterica can maintain Pi homeostasis and repress PhoB/PhoR activation even when cells are grown in medium lacking Pi.

An intracellular phosphorus-starvation signal activates the PhoB/PhoR two-component system in Salmonella enterica

Bacteria acquire P primarily as inorganic orthophosphate (Pi, PO43-). Once internalized, Pi is rapidly assimilated into biomass during the synthesis of ATP. Because Pi is essential, but excessive ATP is toxic, the acquisition of environmental Pi is tightly regulated. In the bacterium Salmonella enterica (Salmonella), growth in Pi-limiting environments activates the membrane sensor histidine kinase PhoR, leading to the phosphorylation of its cognate transcriptional regulator PhoB and subsequent transcription of genes involved in adaptations to low Pi. Pi limitation is thought to promote PhoR kinase activity by altering the conformation of a membrane signaling complex comprised by PhoR, the multicomponent Pi transporter system PstSACB and the regulatory protein PhoU. However, the identity of the low Pi signal and how it controls PhoR activity remain unknown. Here we characterize the PhoB-dependent and independent transcriptional changes elicited by Salmonella in response to P starvation, and identify PhoB-independent genes that are required for the utilization of several organic-P sources. We use this knowledge to identify the cellular compartment where the PhoR signaling complex senses the Pi-limiting signal. We demonstrate that the PhoB and PhoR signal transduction proteins can be maintained in an inactive state even when Salmonella is grown in media lacking Pi. Our results establish that PhoR activity is controlled by an intracellular signal resulting from P insufficiency.

Multiple Optimal Phenotypes Overcome Redox and Glycolytic Intermediate Metabolite Imbalances in Escherichia coli pgi Knockout Evolutions

A mechanistic understanding of how new phenotypes develop to overcome the loss of a gene product provides valuable insight on both the metabolic and regulatory functions of the lost gene. The pgi gene, whose product catalyzes the second step in glycolysis, was deleted in a growth-optimized Escherichia coli K-12 MG1655 strain. The initial knockout (KO) strain exhibited an 80% drop in growth rate that was largely recovered in eight replicate, but phenotypically distinct, cultures after undergoing adaptive laboratory evolution (ALE). Multi-omic data sets showed that the loss of pgi substantially shifted pathway usage, leading to a redox and sugar phosphate stress response. These stress responses were overcome by unique combinations of innovative mutations selected for by ALE. Thus, the coordinated mechanisms from genome to metabolome that lead to multiple optimal phenotypes after the loss of a major gene product were revealed.IMPORTANCE A mechanistic understanding of how microbes are able to overcome the loss of a gene through regulatory and metabolic changes is not well understood. Eight independent adaptive laboratory evolution (ALE) experiments with pgi knockout strains resulted in eight phenotypically distinct endpoints that were able to overcome the gene loss. Utilizing multi-omics analysis, the coordinated mechanisms from genome to metabolome that lead to multiple optimal phenotypes after the loss of a major gene product were revealed.

Ugp and PitA participate in the selection of PHO-constitutive mutants

Mutations that cause the constitutive expression of the PHO regulon of Escherichia coli occur either in the pst operon or in the phoR gene, which encode, respectively, a high-affinity Pi transport system and a histidine kinase sensor protein. These mutations are normally selected on glycerol-2-phosphate (G2P) as the carbon source in the presence of excess Pi. The emergence of early PHO-constitutive mutants, which appear after growth for up to 48 h on selective medium, depends on the presence of phoA, which codes for a periplasmic alkaline phosphatase, while late mutants, which appear after 48 h, depend both on phoA and on the ugp operon, which encodes a glycerophosphodiester transport system. The emergence of the late mutants hints at an adaptive mutation process. PHO-constitutive phoR mutants appear only in a host that is mutated in pitA, which encodes an alternative Pi transport system that does not belong to the PHO regulon. The conserved Thr(217) residue in the PhoR protein is essential for PHO repression.

IMPORTANCE

One of the principal ways in which bacteria adapt to new nutrient sources is by acquiring mutations in key regulatory genes. The inability of E. coli to grow on G2P as a carbon source is used to select mutations that derepress the PHO regulon, a system of genes involved in the uptake of phosphorus-containing molecules. Mutations in the pst operon or in phoR result in the constitutive expression of the entire PHO regulon, including alkaline phosphatase, which hydrolyzes G2P. Here we demonstrate that the ugp operon, another member of the PHO regulon, is important for the selection of PHO-constitutive mutants under prolonged nutritional stress and that phoR mutations can be selected only in bacteria lacking pitA, which encodes a secondary Pi transport system.

Mechanisms of toxicity of hydroxylated polybrominated diphenyl ethers (HO-PBDEs) determined by toxicogenomic analysis with a live cell array coupled with mutagenesis in Escherichia coli

Results of previous studies have indicated that 6-HO-BDE-47, the addition of the hydroxyl (HO) group to the backbone of BDE-47, significantly increased the toxicity of the chemical compared to its postulated precursor analogues, BDE-47 and 6-MeO-BDE-47. However, whether such a result is conserved across polybrominated diphenyl ether (PBDE) congeners was unknown. Here, cytotoxicity of 32 PBDE analogues (17 HO-PBDEs and 15 MeO-PBDEs) was further tested and the underlying molecular mechanism was investigated. A total of 14 of the 17 HO-PBDEs inhibited growth of Escherichia coli during 4 or 24 h durations of exposure, but none of the MeO-PBDEs was cytotoxic at the concentrations tested. 6-HO-BDE-47 and 2-HO-BDE-28 were most potent with 4 h median effect concentrations (EC50) of 12.13 and 6.25 mg/L, respectively, which trended to be lesser with a longer exposure time (24 h). Expression of 30 modulated and validated genes by 6-HO-BDE-47 in a previous study was also observed after exposure to other HO-PBDE analogues. For instance, uhpT was upregulated by 13 HO-PBDEs, and three rRNA operons (rrnA, rrnB, and rrnC) were downregulated by 8 HO-PBDEs. These unanimous responses suggested a potential common molecular signaling modulated by HO-PBDEs. To explore new information on mechanisms of action, this work was extended by testing the increased susceptibility of 182 mutations of transcriptional factors (TFs) and 22 mutations as genes modulated by 6-HO-BDE-47 after exposure to 6-HO-BDE-47 at the 4 h IC50 concentration. Although a unanimous upregulation of uhpT was observed after exposure to HO-PBDEs, no significant shift in sensitivity was observed in uhpT-defective mutants. The 54 genes, selected by cut-offs of 0.35 and 0.65, were determined to be responsible for "organic acid/oxoacid/carboxylic acid metabolic process" pathways, which supported a previous finding.

Depletion of glycolytic intermediates plays a key role in glucose-phosphate stress in Escherichia coli

In bacteria like Escherichia coli, the accumulation of glucose-6-phosphate (G6P) or its analogs such as α-methyl glucoside-6-phosphate (αMG6P) results in stress that appears in the form of growth inhibition. The small RNA SgrS is an essential part of the response that helps E. coli combat glucose-phosphate stress; the growth of sgrS mutants during stress caused by αMG is significantly impaired. The cause of this stress is not currently known but may be due to either toxicity of accumulated sugar-phosphates or to depletion of metabolic intermediates. Here, we present evidence that glucose-phosphate stress results from depletion of glycolytic intermediates. Addition of glycolytic compounds like G6P and fructose-6-phosphate rescues the αMG growth defect of an sgrS mutant. These intermediates also markedly decrease induction of the stress response in both wild-type and sgrS strains grown with αMG, implying that cells grown with these intermediates experience less stress. Moreover, αMG transport assays confirm that G6P relieves stress even when αMG is taken up by the cell, strongly suggesting that accumulated αMG6P per se does not cause stress. We also report that addition of pyruvate during stress has a novel lethal effect on the sgrS mutant, resulting in cell lysis. The phosphoenolpyruvate (PEP) synthetase PpsA, which converts pyruvate to PEP, can confer resistance to pyruvate-induced lysis when ppsA is ectopically expressed in the sgrS mutant. Taken as a whole, these results provide the strongest evidence thus far that depletion of glycolytic intermediates is at the metabolic root of glucose-phosphate stress.

Simulation of spontaneous substrate binding revealing the binding pathway and mechanism and initial conformational response of GlpT

Glycerol 3-phosphate transporter (GlpT) mediates the import of glycerol 3-phosphate (G3P) using the gradient of inorganic phosphate (P(i)). To study the process and mechanism of substrate binding and to investigate the protein's initial response, we performed equilibrium simulations of wild-type GlpT and several of its mutant forms in membranes in the presence of all physiologically relevant substrates (P(i)(-), P(i)(2-), G3P(-), and G3P(2-)). The simulations capture spontaneous substrate binding of GlpT, driven by the positive electrostatic potential of the lumen. K80 is found to act as a "hook" making the first encounter with the substrate and guiding it toward the binding site, where it binds tightly to R45, a key binding site residue that acts as a "fork" holding the substrate. R269 establishes no direct contact with the substrate during the simulations, a surprising behavior given its structural pseudosymmetry to R45. In all substrate-bound systems, partial closing of the cytoplasmic half of GlpT was observed. The substrate appears to stabilize the partially occluded state, as in the two apo simulations either no closing was observed or the protein reverted to its open form toward the end of the simulation, whereas in all substrate-bound systems, a stable partially closed state was produced. Along with the modulation of the periplasmic salt bridge network, these substrate-induced events destabilize the periplasmic half while inducing a closure in the cytoplasmic half, thus capturing the early stages of the proposed rocker-switch mechanism in GlpT.

Structure-function study of the glucose-6-phosphate transporter, an eukaryotic antiporter deficient in glycogen storage disease type Ib

Glycogen storage disease type Ib is caused by deficiencies in the glucose-6-phosphate transporter (G6PT), a phosphate (P(i))-linked antiporter capable of homologous (P(i):P(i)) and heterologous (G6P:P(i)) exchanges similar to the bacterial hexose-6-phosphate transporter, UhpT. Protease protection and glycosylation scanning assays have suggested that G6PT is anchored to the endoplasmic reticulum by 10 transmembrane domains. However, recent homology modeling proposed that G6PT may contain 12 helices and that amino acids essential for the functions of UhpT also play important roles in G6PT. Site-directed mutagenesis and in vitro expression assays demonstrated that only one of the four residues critical for UhpT activity is essential in G6PT. Furthermore, glycosylation scanning and protease sensitivity assays showed that the 10-domain model of G6PT is more probable than the 12-domain UhpT-like model.

Salt-bridge dynamics control substrate-induced conformational change in the membrane transporter GlpT

Active transport of substrates across cytoplasmic membranes is of great physiological, medical and pharmaceutical importance. The glycerol-3-phosphate (G3P) transporter (GlpT) of the E. coli inner membrane is a secondary active antiporter from the ubiquitous major facilitator superfamily that couples the import of G3P to the efflux of inorganic phosphate (P(i)) down its concentration gradient. Integrating information from a novel combination of structural, molecular dynamics simulations and biochemical studies, we identify the residues involved directly in binding of substrate to the inward-facing conformation of GlpT, thus defining the structural basis for the substrate-specificity of this transporter. The substrate binding mechanism involves protonation of a histidine residue at the binding site. Furthermore, our data suggest that the formation and breaking of inter- and intradomain salt bridges control the conformational change of the transporter that accompanies substrate translocation across the membrane. The mechanism we propose may be a paradigm for organophosphate:phosphate antiporters.

Ins and outs of major facilitator superfamily antiporters

The major facilitator superfamily (MFS) represents the largest group of secondary active membrane transporters, and its members transport a diverse range of substrates. Recent work shows that MFS antiporters, and perhaps all members of the MFS, share the same three-dimensional structure, consisting of two domains that surround a substrate translocation pore. The advent of crystal structures of three MFS antiporters sheds light on their fundamental mechanism; they operate via a single binding site, alternating-access mechanism that involves a rocker-switch type movement of the two halves of the protein. In the sn-glycerol-3-phosphate transporter (GlpT) from Escherichia coli, the substrate-binding site is formed by several charged residues and a histidine that can be protonated. Salt-bridge formation and breakage are involved in the conformational changes of the protein during transport. In this review, we attempt to give an account of a set of mechanistic principles that characterize all MFS antiporters.

Kinetic evidence is consistent with the rocker-switch mechanism of membrane transport by GlpT

Secondary active transport of substrate across the cell membrane is crucial to many cellular and physiological processes. The crystal structure of one member of the secondary active transporter family, the sn-glycerol-3-phosphate (G3P) transporter (GlpT) of the inner membrane of Escherichia coli, suggests a mechanism for substrate translocation across the membrane that involves a rocker-switch-type movement of the protein. This rocker-switch mechanism makes two specific predictions with respect to kinetic behavior: the transport rate increases with the temperature, whereas the binding affinity of the transporter to a substrate is temperature-independent. In this work, we directly tested these two predictions by transport kinetics and substrate-binding experiments, integrating the data on this single system into a coherent set of observations. The transport kinetics of the physiologically relevant G3P-phosphate antiport reaction were characterized at different temperatures using both E. coli whole cells and GlpT reconstituted into proteoliposomes. Substrate-binding affinity of the transporter was measured using tryptophan fluorescence quenching in detergent solution. Indeed, the substrate transport velocity of GlpT increased dramatically with temperature. In contrast, neither the apparent Michaelis constant (Km) nor the apparent substrate-binding dissociation constant (Kd) showed temperature dependence. Moreover, GlpT-catalyzed G3P translocation exhibited a completely linear Arrhenius function with an activation energy of 35.2 kJ mol-1 for the transporter reconstituted into proteoliposomes, suggesting that the substrate-loaded transporter is delicately poised between the inward- and outward-facing conformations. When these results are taken together, they are in agreement with a rocker-switch mechanism for GlpT.

Analysis of global control of Escherichia coli carbohydrate uptake

Background

Global control influences the regulation of many individual subsystems by superimposed regulator proteins. A prominent example is the control of carbohydrate uptake systems by the transcription factor Crp in Escherichia coli. A detailed understanding of the coordination of the control of individual transporters offers possibilities to explore the potential of microorganisms e.g. in biotechnology.

Results

An o.d.e. based mathematical model is presented that maps a physiological parameter – the specific growth rate – to the sensor of the signal transduction unit, here a component of the bacterial phosphotransferase system (PTS), namely EIIA^Crr. The model describes the relation between the growth rate and the degree of phosphorylation of EIIA ^crr for a number of carbohydrates by a distinctive response curve, that differentiates between PTS transported carbohydrates and non-PTS carbohydrates. With only a small number of kinetic parameters, the model is able to describe a broad range of experimental steady-state and dynamical conditions.

Conclusion

The steady-state characteristic presented shows a relationship between the growth rate and the output of the sensor system PTS. The glycolytic flux that is measured by this sensor is a good indicator to represent the nutritional status of the cell.

Altered oxyanion selectivity in mutants of UhpT, the Pi-linked sugar phosphate carrier of Escherichia coli

In Escherichia coli, the UhpT transporter catalyzes the electroneutral accumulation of sugar 6-phosphate by exchange with internal inorganic phosphate (Pi). The substrate specificity of UhpT is regulated at least in part by constituents of an Asp388-Lys391 intrahelical salt bridge, and mutations that remove one but not both of these residues alter UhpT preference for organophosphate substrates. Using site-directed mutagenesis, we examined the role played by these two positions in the selection of the oxyanion countersubstrate. We show that derivatives having aliphatic or polar residues at positions 388 and 391 are gain-of-function mutants capable of transporting SO4 as well as Pi. These oxyanions share similar structures but differ significantly in the presence of a proton(s) on Pi. Our findings therefore lead us to suggest that the Asp388-Lys391 ion pair acts normally as a filter that prevents substrates lacking a proton that can be donated from occupying the UhpT active site.

Homology modeling of the human microsomal glucose 6-phosphate transporter explains the mutations that cause the glycogen storage disease type Ib

Glycogen storage disease type Ib is caused by mutations in the glucose 6-phosphate transporter (G6PT) in the endoplasmic reticulum membrane in liver and kidney. Twenty-eight missense and two deletion mutations that cause the disease were previously shown to reduce or abolish the transporter's activity. However, the mechanisms by which these mutations impair transport remain unknown. On the basis of the recently determined crystal structure of its Escherichia coli homologue, the glycerol 3-phosphate transporter, we built a three-dimensional structural model of human G6PT by homology modeling. G6PT is proposed to consist of 12 transmembrane alpha-helices that are divided into N- and C-terminal domains, with the substrate-translocation pore located between the two domains and the substrate-binding site formed by R28 and K240 at the domain interface. The disease-causing mutations were found to occur at four types of positions: (I) in the substrate-translocation pore, (II) at the N-/C-terminal domain interface, (III) in the interior of the N- and C-terminal domains, and (IV) on the protein surface. Whereas class I mutations affect substrate binding directly, class II mutations, mostly involving changes in side chain size, charge, or both, hinder the conformational change required for substrate translocation. On the other hand, class III and class IV mutations, often introducing a charged residue into a helix bundle or at the protein-lipid interface, probably destabilize the protein. These results also suggest that G6PT operates by a similar antiport mechanism as its E. coli homologue, namely, the substrate binds at the N- and C-terminal domain interface and is then transported across the membrane via a rocker-switch type of movement of the two domains.

Structure and mechanism of the glycerol-3-phosphate transporter from Escherichia coli

The major facilitator superfamily represents the largest group of secondary membrane transporters in the cell. Here we report the 3.3 angstrom resolution structure of a member of this superfamily, GlpT, which transports glycerol-3-phosphate into the cytoplasm and inorganic phosphate into the periplasm. The amino- and carboxyl-terminal halves of the protein exhibit a pseudo two-fold symmetry. Closed off to the periplasm, a centrally located substrate-translocation pore contains two arginines at its closed end, which comprise the substrate-binding site. Upon substrate binding, the protein adopts a more compact conformation. We propose that GlpT operates by a single-binding site, alternating-access mechanism through a rocker-switch type of movement.

Functional characterization of cysteine residues in GlpT, the glycerol 3-phosphate transporter of Escherichia coli

In Escherichia coli, the GlpT transporter, a member of the major facilitator superfamily, moves external glycerol 3-phosphate (G3P) into the cytoplasm in exchange for cytoplasmic phosphate. Study of intact cells showed that both GlpT and HisGlpT, a variant with an N-terminal six-histidine tag, are inhibited (50% inhibitory concentration approximately 35 microM) by the hydrophilic thiol-specific agent p-mercurichlorobenzosulfonate (PCMBS) in a substrate-protectable fashion; by contrast, two other thiol-directed probes, N-maleimidylpropionylbiocytin (MPB) and [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET), have no effect. Use of variants in which the HisGlpT native cysteines are replaced individually by serine or glycine implicates Cys-176, on transmembrane helix 5 (TM5), as the major target for PCMBS. The inhibitor sensitivity of purified and reconstituted HisGlpT or its cysteine substitution derivatives was found to be consistent with the findings with intact cells, except that a partial response to PCMBS was found for the C176G mutant, suggesting the presence of a mixed population of both right-side-out (RSO) (resistant) and inside-out (ISO) (sensitive) orientations after reconstitution. To clarify this issue, we studied a derivative (P290C) in which the RSO molecules can be blocked independently due to an MPB-responsive cysteine in an extracellular loop. In this derivative, comparisons of variants with (P290C) and without (P290C/C176G) Cys-176 indicated that this residue shows substrate-protectable inhibition by PCMBS in the ISO orientation in proteoliposomes. Since PCMBS gains access to Cys-176 from both periplasmic and cytoplasmic surfaces of the protein (in intact cells and in a reconstituted ISO orientation, respectively) and since access is unavailable when the substrate is present, we propose that Cys-176 is located on the transport pathway and that TM5 has a role in lining this pathway.

Pyridoxal 5-phosphate inhibition of substrate selectivity mutants of UhpT, the sugar 6-phosphate carrier of Escherichia coli

In the sugar phosphate transporter UhpT, gain-of-function derivatives that prefer phosphoenolpyruvate (PEP) as substrate have an uncompensated lysine residue on transmembrane segment 11. We show here that these variants are also highly susceptible to substrate-protectable inhibition by covalent modification of lysine with pyridoxal 5-phosphate. The chemical requirements of this interaction provide evidence that the gain-of-function phenotype results from the pairing of the uncompensated lysines in these mutants with the anionic carboxyl group of PEP.

Evidence for glucose-6-phosphate transport in rat liver microsomes

The existence of glucose-6-phosphate transport across the liver microsomal membrane is still controversial. In this paper, we show that S3483, a chlorogenic acid derivative known to inhibit glucose-6-phosphatase in intact microsomes, caused the intravesicular accumulation of glucose-6-phosphate when the latter was produced by glucose-6-phosphatase from glucose and carbamoyl-phosphate. S3483 also inhibited the conversion of glucose-6-phosphate to 6-phosphogluconate occurring inside microsomes in the presence of electron acceptors (NADP or metyrapone). These data indicate that liver microsomal membranes contain a reversible glucose-6-phosphate transporter, which furnishes substrate not only to glucose-6-phosphatase, but also to hexose-6-phosphate dehydrogenase.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

Gleanings of a chemiosmotic eye

In 1961, an inventive Englishman, named Peter Mitchell, proposed a radically novel hypothesis to explain how energy is conserved during respiration and photosynthesis, and applied to the generation of ATP and other kinds of functional work. The chemiosmotic hypothesis sparked an intense controversy that lasted for 15 years. Today, Mitchell's conception of proton currents and their role in phosphorylation and active transport is generally accepted, and has ramified into many corners of cellular physiology. His most profound contribution may have been to introduce spatial direction into biochemistry, and thereby transform our perception of the relationship between molecules and cells.

High-yield expression and functional analysis of Escherichia coli glycerol-3-phosphate transporter

The glycerol-3-phosphate (G3P) transporter, GlpT, from Escherichia coli mediates G3P and inorganic phosphate exchange across the bacterial inner membrane. It possesses 12 transmembrane alpha-helices and is a member of the Major Facilitator Superfamily. Here we report overexpression, purification, and characterization of GlpT. Extensive optimization applied to the DNA construct and cell culture has led to a protocol yielding approximately 1.8 mg of the transporter protein per liter of E. coli culture. After purification, this protein binds substrates in detergent solution, as measured by tryptophan fluorescence quenching, and its dissociation constants for G3P, glycerol-2-phosphate, and inorganic phosphate at neutral pH are 3.64, 0.34, and 9.18 microM, respectively. It also shows transport activity upon reconstitution into proteoliposomes. The phosphate efflux rate of the transporter in the presence of G3P is measured to be 29 micromol min(-1) mg(-1) at pH 7.0 and 37 degrees C, corresponding to 24 mol of phosphate s(-1) (mol of protein)(-1). In addition, the glycerol-3-phosphate transporter is monomeric and stable over a wide pH range and in the presence of a variety of detergents. This preparation of GlpT provides ideal material for biochemical, biophysical, and structural studies of the glycerol-3-phosphate transporter.

Towards the molecular mechanism of prokaryotic and eukaryotic multidrug transporters

Due to their ability to extrude structurally dissimilar cytotoxic drugs out of the cell, multidrug transporters are able to reduce the cytoplasmic drug concentration, and, hence, are able to confer drug resistance on human cancer cells and pathogenic microorganisms. This review will focus on the molecular properties of two bacterial multidrug transporters, the ATP-binding cassette transporter LmrA and the proton motive force-dependent major facilitator superfamily transporter LmrP, which each represent a major class of multidrug transport proteins encountered in pro- and eukaryotic cells. In spite of the structural differences between LmrA and LmrP, the molecular bases of their drug transport activity may turn out to be more similar than might currently appear.

Transmembrane segment 11 of UhpT, the sugar phosphate carrier of Escherichia coli, is an alpha-helix that carries determinants of substrate selectivity

In Escherichia coli, transport of hexose 6-phosphates is mediated by the P(i)-linked antiport carrier, UhpT, a member of the major facilitator superfamily. We showed earlier that Lys(391), a member of an intrahelical salt bridge (Asp(388)/Lys(391)) in the eleventh transmembrane segment (TM11) of this transporter, can function as a determinant of substrate selectivity (Hall, J. A., Fann, M.-C., and Maloney, P. C. (1999) J. Biol. Chem. 274, 6148-6153). Here, we examine in detail the role of TM11 in setting substrate preference. Derivatives having an uncompensated cationic charge at either position 388 or 391 (the D388C, D388V, or D388K/K391C variants) are gain-of-function mutants in which phosphoenolpyruvate, not sugar 6-phosphate, is the preferred organic substrate. By contrast, when an uncompensated anionic charge is placed at position 388 (K391C), we observed behavior consistent with an increased preference for monovalent rather than divalent sugar 6-phosphate. Because positions 388 and 391 lie deep within the UhpT hydrophobic sector, these findings suggested that an extended length of TM11 may be accessible to external substrates and probes. To explore this issue, we used a panel of TM11 single cysteine variants to examine the transport of glucose 6-phosphate in the presence and absence of the membrane-impermeant, thiol-reactive agent p-chloromercuribenzosulfonate (PCMBS). Accessibility to PCMBS, together with the pattern of substrate protection against PCMBS inhibition, leads us to conclude that TM11 spans the membrane as an alpha-helix, with approximately two-thirds of its surface lining a substrate translocation pathway. We suggest that this feature is a general property of carrier proteins in the major facilitator superfamily and that for this reason residues in TM11 will serve to carry determinants of substrate selectivity.

Effect of altered spacing between uhpT promoter elements on transcription activation

Many bacterial promoters possess multiple sites for binding of transcriptional activator proteins. The uhpT promoter, which controls expression of the sugar phosphate transport system in Escherichia coli, possesses multiple sites for its specific activator protein, UhpA, and a single site for binding of the global regulator, the catabolite gene activator protein (CAP). The binding of UhpA to the uhpT promoter was determined by DNase protection assays; UhpA displayed different affinities for the target sites. The upstream or strong sites, between positions -80 and -50, exhibited a higher affinity for UhpA than did the downstream or weak sites, between positions -50 and -32, adjoining the RNA polymerase-binding site. Phosphorylation of UhpA strongly increased its affinity for both sites. To examine the possible roles of the two sets of UhpA-binding sites, a series of insertion and deletion mutations were introduced at the boundary between them, as suggested from the positions that were protected by UhpA against hydroxyl radical cleavage. Deletions extended in the direction of the weak sites. The insertion or deletion of one helical turn of DNA resulted in the loss of promoter activity and of occupancy by UhpA of the remaining weak-site sequences but was accompanied by normal occupancy of the strong site and no change in the gel retardation behavior of the promoter fragments. However, the deletion of two helical turns of DNA, i.e., 20, 21, or 22 bp, resulted in the novel appearance of UhpA-independent expression and in an additional level of expression that was dependent on UhpA but independent of an inducing signal. The UhpA-independent promoter activity was shown to result from activation by CAP at its more proximal position. UhpA-dependent activity under noninducing conditions appears to result from the binding of unphosphorylated UhpA to the strong sites, which are now in the position normally occupied by the weak sites. Thus, regulated phosphorylation of the response regulator UhpA enhances its occupancy of the weak sites where favorable contacts can allow the binding of RNA polymerase to the promoter.

Phosphate starvation-inducible proteins of Bacillus subtilis: proteomics and transcriptional analysis

The phosphate starvation response in Bacillus subtilis was analyzed using two-dimensional (2D) polyacrylamide gel electrophoresis of cell extracts and supernatants from phosphate-starved cells. Most of the phosphate starvation-induced proteins are under the control of sigma(B), the activity of which is increased by energy depletion. In order to define the proteins belonging to the Pho regulon, which is regulated by the two-component regulatory proteins PhoP and PhoR, the 2D protein pattern of the wild type was compared with those of a sigB mutant and a phoR mutant. By matrix-assisted laser desorption ionization-time of flight mass spectrometry, two alkaline phosphatases (APases) (PhoA and PhoB), an APase-alkaline phosphodiesterase (PhoD), a glycerophosphoryl diester phosphodiesterase (GlpQ), and the lipoprotein YdhF were identified as very strongly induced PhoPR-dependent proteins secreted into the extracellular medium. In the cytoplasmic fraction, PstB1, PstB2, and TuaD were identified as already known PhoPR-dependent proteins, in addition to PhoB, PhoD, and the previously described PstS. Transcriptional studies of glpQ and ydhF confirmed the strong PhoPR dependence. Northern hybridization and primer extension experiments showed that glpQ is transcribed monocistronically from a sigma(A) promoter which is overlapped by four putative TT(A/T)ACA-like PhoP binding sites. Furthermore, ydhF might be cotranscribed with phoB initiating from the phoB promoter. Only a small group of proteins remained phosphate starvation inducible in both phoR and sigB mutant and did not form a unique regulation group. Among these, YfhM and YjbC were controlled by sigma(B)-dependent and unknown PhoPR-independent mechanisms. Furthermore, YtxH and YvyD seemed to be induced after phosphate starvation in the wild type in a sigma(B)-dependent manner and in the sigB mutant probably via sigma(H). YxiE was induced by phosphate starvation independently of sigma(B) and PhoPR.

Separate contributions of UhpA and CAP to activation of transcription of the uhpT promoter of Escherichia coli

Activation of promoters by multiple transcription factors might occur through favorable contacts of the activators with themselves or RNA polymerase, or by changes in DNA geometry that enhance formation of the transcription complex. Transcription of the Escherichia coli uhpT gene, encoding the organophosphate transporter, requires the response regulator UhpA and is stimulated by the global regulator protein CAP. CAP binds to the uhpT promoter at a single site, centered at -103.5 bp relative to the start of transcription, and UhpA binds to multiple sites between positions -80 and -32. Overexpression of UhpA did not reduce the degree of CAP stimulation of uhpT-lacZ expression, showing that CAP action is more complex than enhancement of the binding of UhpA. Footprinting experiments demonstrated that UhpA and CAP modestly stimulated each other's binding to the uhpT promoter, but did not affect the positioning of the binding sites. An in vitro transcription system was used to examine the contribution of each transcription factor at the uhpT promoter. Action of UhpA and CAP proteins was not affected by template supercoiling. Kinetic analyses of productive and abortive initiation showed that CAP acted both to stabilize by fivefold the open promoter complexes formed in the presence of UhpA and to enhance by twofold the rate of their formation. These results indicate that open complex formation requires UhpA and that CAP stabilizes the open complex.

Altered substrate selectivity in a mutant of an intrahelical salt bridge in UhpT, the sugar phosphate carrier of Escherichia coli

Site-directed and second site suppressor mutagenesis identify an intrahelical salt bridge in the eleventh transmembrane segment of UhpT, the sugar phosphate carrier of Escherichia coli. Glucose 6-phosphate (G6P) transport by UhpT is inactivated if cysteine replaces either Asp388 or Lys391 but not if both are replaced. This suggests that Asp388 and Lys391 are involved in an intrahelical salt bridge and that neither is required for normal UhpT function. This interpretation is strengthened by the finding that mutations at Lys391 (K391N, K391Q, and K391T) are recovered as revertants of the inactive D388C variant. Further work shows that although the D388C variant is null for G6P transport, movement of 32Pi by homologous Pi/Pi exchange is unaffected. This raises the possibility that this derivative may have latent function, a possibility confirmed by showing that D388C is a gain-of-function mutation in which phosphoenolpyruvate (PEP) is the preferred substrate. Added study of the Pi/Pi exchange shows that in wild type UhpT this partial reaction is readily blocked by G6P but not PEP. By contrast, in the D388C variant, Pi/Pi exchange is unaffected by G6P but is inhibited by both PEP and 3-phosphoglycerate. These latter substrates are used by PgtP, a related Pi-linked antiporter, which lacks the Asp388-Lys391 salt bridge but has instead an uncompensated arginine at position 391. For this reason, we conclude that in both UhpT and PgtP position 391 can serve as a determinant of substrate selectivity by acting as a receptor for the anionic carboxyl brought into the translocation pathway by PEP.

Functional symmetry of UhpT, the sugar phosphate transporter of Escherichia coli

UhpT, the sugar phosphate transporter of Escherichia coli, acts to exchange internal inorganic phosphate for external hexose 6-phosphate. Because of this operational asymmetry, we studied variants in which right-side-out (RSO) or inside-out (ISO) orientations could be analyzed independently to ask whether the inward- and outward-facing UhpT surfaces have different substrate specificities. To study the RSO orientation, we constructed a histidine-tagged derivative, His10K291C/K294N, in which the sole external tryptic cleavage site (Lys294) had been removed. Functional assay as well as immunoblot analysis showed that trypsin treatment of proteoliposomes containing His10K291C/K294N led to loss of about 50% of the original population, reflecting retention of only the RSO population. To study the ISO orientation, we used a His10V284C derivative, in which a newly inserted external cysteine (Cys284) conferred sensitivity to the thiol-reactive agent, 3-(N-maleimidylpropionyl)biocytin. In this case, 3-(N-maleimidylpropionyl)biocytin treatment of proteoliposomes containing His10V284C gave about a 60% loss of activity, and immunodetection of biotin showed parallel modification of an equivalent fraction of the original population. Together, such findings indicate that the UhpT RSO and ISO orientations are in about equal proportion in proteoliposomes and that a single population can be generated by exposure of these derivatives to the appropriate agent. This allowed us to study proteoliposomes with UhpT functioning in RSO orientation (His10K291C/K294N) or ISO orientation (His10V284C) with respect to the kinetics of glucose 6-phosphate transport by phosphate-loaded proteoliposomes and also the inhibitions found with 2-deoxy-glucose 6-phosphate, mannose 6-phosphate, galactose 6-phosphate, fructose 6-phosphate, and inorganic phosphate. We found no significant differences in the behavior of UhpT in its different orientations, indicating that the transporter possesses an overall functional symmetry.

A gene on chromosome 11q23 coding for a putative glucose- 6-phosphate translocase is mutated in glycogen-storage disease types Ib and Ic

Glycogen-storage diseases type I (GSD type I) are due to a deficiency in glucose-6-phosphatase, an enzymatic system present in the endoplasmic reticulum that plays a crucial role in blood glucose homeostasis. Unlike GSD type Ia, types Ib and Ic are not due to mutations in the phosphohydrolase gene and are clinically characterized by the presence of associated neutropenia and neutrophil dysfunction. Biochemical evidence indicates the presence of a defect in glucose-6-phosphate (GSD type Ib) or inorganic phosphate (Pi) (GSD type Ic) transport in the microsomes. We have recently cloned a cDNA encoding a putative glucose-6-phosphate translocase. We have now localized the corresponding gene on chromosome 11q23, the region where GSD types Ib and Ic have been mapped. Using SSCP analysis and sequencing, we have screened this gene, for mutations in genomic DNA, from patients from 22 different families who have GSD types Ib and Ic. Of 20 mutations found, 11 result in truncated proteins that are probably nonfunctional. Most other mutations result in substitutions of conserved or semiconserved residues. The two most common mutations (Gly339Cys and 1211-1212 delCT) together constitute approximately 40% of the disease alleles. The fact that the same mutations are found in GSD types Ib and Ic could indicate either that Pi and glucose-6-phosphate are transported in microsomes by the same transporter or that the biochemical assays used to differentiate Pi and glucose-6-phosphate transport defects are not reliable.

Major facilitator superfamily

The major facilitator superfamily (MFS) is one of the two largest families of membrane transporters found on Earth. It is present ubiquitously in bacteria, archaea, and eukarya and includes members that can function by solute uniport, solute/cation symport, solute/cation antiport and/or solute/solute antiport with inwardly and/or outwardly directed polarity. All homologous MFS protein sequences in the public databases as of January 1997 were identified on the basis of sequence similarity and shown to be homologous. Phylogenetic analyses revealed the occurrence of 17 distinct families within the MFS, each of which generally transports a single class of compounds. Compounds transported by MFS permeases include simple sugars, oligosaccharides, inositols, drugs, amino acids, nucleosides, organophosphate esters, Krebs cycle metabolites, and a large variety of organic and inorganic anions and cations. Protein members of some MFS families are found exclusively in bacteria or in eukaryotes, but others are found in bacteria, archaea, and eukaryotes. All permeases of the MFS possess either 12 or 14 putative or established transmembrane alpha-helical spanners, and evidence is presented substantiating the proposal that an internal tandem gene duplication event gave rise to a primordial MFS protein prior to divergence of the family members. All 17 families are shown to exhibit the common feature of a well-conserved motif present between transmembrane spanners 2 and 3. The analyses reported serve to characterize one of the largest and most diverse families of transport proteins found in living organisms.

Major facilitator superfamily

The major facilitator superfamily (MFS) is one of the two largest families of membrane transporters found on Earth. It is present ubiquitously in bacteria, archaea, and eukarya and includes members that can function by solute uniport, solute/cation symport, solute/cation antiport and/or solute/solute antiport with inwardly and/or outwardly directed polarity. All homologous MFS protein sequences in the public databases as of January 1997 were identified on the basis of sequence similarity and shown to be homologous. Phylogenetic analyses revealed the occurrence of 17 distinct families within the MFS, each of which generally transports a single class of compounds. Compounds transported by MFS permeases include simple sugars, oligosaccharides, inositols, drugs, amino acids, nucleosides, organophosphate esters, Krebs cycle metabolites, and a large variety of organic and inorganic anions and cations. Protein members of some MFS families are found exclusively in bacteria or in eukaryotes, but others are found in bacteria, archaea, and eukaryotes. All permeases of the MFS possess either 12 or 14 putative or established transmembrane alpha-helical spanners, and evidence is presented substantiating the proposal that an internal tandem gene duplication event gave rise to a primordial MFS protein prior to divergence of the family members. All 17 families are shown to exhibit the common feature of a well-conserved motif present between transmembrane spanners 2 and 3. The analyses reported serve to characterize one of the largest and most diverse families of transport proteins found in living organisms.

Purification of UhpT, the sugar phosphate transporter of Escherichia coli

To purify UhpT, the sugar phosphate carrier of Escherichia coli, we constructed a variant (HisUhpT) in which 10 tandem histidine residues were placed at the UhpT N terminus and then used Ni(2+)-agarose affinity chromatography of detergent-solubilized proteins. Membrane vesicles from a strain overexpressing His-UhpT were extracted at pH 7.4 with either 1.5% n-octyl-beta-D-glucopyranoside (octylglucoside) or 1.5% n-dodecyl-beta-D-maltoside (dodecylmaltoside) in 200 mM sodium chloride, 100 mM potassium phosphate, 50 mM glucose 6-phosphate, 10-20% glycerol, 0.2% E. coli phospholipid, and 5 mM beta-mercaptoethanol. After the detergent extract was applied to a Ni(2+)-agarose column, nonspecifically bound material was removed by washing at pH 7 with the same buffer also containing 50 mM imidazole. Purified HisUhpT was released subsequently, when sodium chloride was replaced with 300 mM imidazole or 100 mM EDTA, giving an overall yield of about 25 micrograms HisUhpT/mg vesicle protein. Whether eluted by imidazole or EDTA in either octylglucoside or dodecylmaltoside, purified HisUhpT showed a specific activity of 2.5-3 mumol/min per milligram of protein as monitored by [14C]glucose 6-phosphate transport by proteoliposomes loaded with 100 mM potassium phosphate. This corresponded to a calculated turnover number near 20 s-1 for the heterologous exchange of external sugar phosphate with internal phosphate. At low temperature (4 degrees C) HisUhpT retained full activity in either octylglucoside or dodecylmaltoside; however, at elevated temperature (> or = 23 degrees C), the protein displayed a marked lability in octylglucoside (t1/2 = 11 min), but not in dodecylmaltoside (t1/2 > or = 200-300 min).

Primary structure and properties of the Na+/glucose symporter (Sg1S) of Vibrio parahaemolyticus

Previously, we cloned and sequenced a DNA fragment from Vibrio parahaemolyticus and found four open reading frames (ORFs). Here, we clearly demonstrate that one of the ORFs, ORF1, is the gene (sglS) encoding a Na+/glucose symporter (SglS). We characterize the Na+/glucose symporter produced in Escherichia coli mutant (JM1100) cells which lack original glucose transport activity and galactose transport activity. We also show that phlorizin, a potent inhibitor of the SGLT1 Na+/glucose symporter of animal cells, inhibited glucose transport, but not galactose transport, via the SglS system.

Protein phosphorylation affects binding of the Escherichia coli transcription activator UhpA to the uhpT promoter

Expression of the Escherichia coli sugar phosphate transporter UhpT is induced by extracellular glucose 6-phosphate through a transmembrane signaling process dependent on the sensor kinase UhpB and the UhpT homolog, UhpC. These proteins are thought to regulate the phosphorylation of the transcription activator, UhpA. To examine the effect of protein phosphorylation on the binding of UhpA to target sequences in the uhpT promoter region, the UhpA protein was overexpressed and purified. Purified UhpA was phosphorylated by acetyl phosphate in a reaction that was dependent on Mg2+ and on the presence of aspartate 54, the site of phosphorylation in homologous response regulators. Gel electrophoretic mobility shift and DNase I and hydroxyl radical protection assays showed that UhpA bound specifically to the region of the uhpT promoter extending from -80 to -50 bp, relative to the transcription start site. At higher concentrations of UhpA, binding was extended to the -32 region. Binding to the -64 element exhibited positive cooperativity and was stimulated severalfold by phosphorylation of UhpA, whereas extension to the downstream region was more strongly affected by phosphorylation. The consensus sequences for the high affinity UhpA-binding sites in the -64 element and for the downstream, low affinity sites are proposed. The pattern of in vitro binding by UhpA agreed with the in vivo observations that phosphorylation-independent assembly of the transcription initiation complex can occur at elevated concentrations of UhpA.

Regulation of Escherichia coli adenylate cyclase activity during hexose phosphate transport

In Escherichia coli, cAMP levels vary with the carbon source used in the culture medium. These levels are dependent on the cellular concentration of phosphorylated EnzymeIIAglc, a component of the glucose-phosphotransferase system, which activates adenylate cyclase (AC). When cells are grown on glucose 6-phosphate (Glc6P), the cAMP level is particularly low. In this study, we investigated the mechanism leading to the low cAMP level when Glc6P is used as the carbon source, i.e. the mechanism preventing the activation of AC by phosphorylated EnzymeIIAglc. Glc6P is transported via the Uhp system which is inducible by extracellular Glc6P. The Uhp system comprises a permease UhpT and three proteins UhpA, UhpB and UhpC which are necessary for uhpT gene transcription. Controlled expression of UhpT in the absence of the regulatory proteins (UhpA, UhpB and UhpC) allowed us to demonstrate that (i) the Uhp regulatory proteins do not prevent the activation of AC by direct interaction with EnzymeIIAglc and (ii) an increase in the amount of UhpT synthesized (corresponding to an increase in the amount of Glc6P transported) correlates with a decrease in the cAMP level. We present data indicating that Glc6P per se or its degradation is unlikely to be responsible for the low cAMP level. It is concluded that the level of cAMP in the cell is determined by the flux of Glc6P through UhpT.

Repressible cation-phosphate symporters in Neurospora crassa

The filamentous fungus Neurospora crassa possesses two nonhomologous high-affinity phosphate permeases, PHO-4 and PHO-5. We have isolated separate null mutants of these permeases, allowing us to study the remaining active transporter in vivo in terms of phosphate uptake and sensitivity to inhibitors. The specificity for the cotransported cation differs for PHO-4 and PHO-5, suggesting that these permeases employ different mechanisms for phosphate translocation. Phosphate uptake by PHO-4 is stimulated 85-fold by the addition of Na+, which supports the idea that PHO-4 is a Na(+)-phosphate symporter. PHO-5 is unaffected by Na+ concentration but is much more sensitive to elevated pH than is PHO-4. Presumably, PHO-5 is a H(+)-phosphate symporter. Na(+)-coupled symport is usually associated with animal cells. The finding of such a system in a filamentous fungus is in harmony with the idea that the fungal and animal kingdoms are more closely related to each other than either is to the plant kingdom.

Transcription activation at the Escherichia coli uhpT promoter by the catabolite gene activator protein

Transport and utilization of sugar phosphates in Escherichia coli depend on the transport protein encoded by the uhpT gene. Transmembrane induction of uhpT expression by external glucose 6-phosphate is positively regulated by the promoter-specific activator protein UhpA and the global regulator catabolite gene activator protein (CAP). Activation by UhpA requires a promoter element centered at -64 bp, relative to the start of transcription, and activation by CAP requires a DNA site centered at position -103.5. This DNA site binds the cyclic AMP-CAP complex in vitro, and its deletion from the promoter reduces transcription activity to 7 to 9% of the wild-type level. Ten uhpT promoter derivatives with altered spacing between the DNA site for CAP and the remainder of the promoter were constructed. Their transcription activities indicated that the action of CAP at this promoter is dependent on proper helical phasing of promoter elements, with CAP binding on the same face of the helix as RNA polymerase does. Five CAP mutants defective in transcription activation at class I and class II CAP-dependent promoters but not defective in DNA binding or DNA bending (positive control mutants) were tested for the ability to activate transcription. These CAPpc mutants exhibited little or no defect in transcription activation at uhpT, indicating that CAP action at uhpTp involves a different mechanism than that which is used for its action at other classes of CAP-dependent promoters.

A phosphate-repressible, high-affinity phosphate permease is encoded by the pho-5+ gene of Neurospora crassa

The pho-5+ gene of Neurospora crassa, which encodes a high-affinity phosphate permease, has been cloned and analyzed. The deduced ORF of 1707 nucleotides is interrupted by a single 63-nt intron and codes for a protein of 569 amino acids (aa). This aa sequence has 48% identity with the high-affinity phosphate transporter of Saccharomyces cerevisiae, PHO84. The pho-5 null mutants have no obvious phenotype. Strains which contain a null mutation in pho-4, which encodes an additional high-affinity phosphate permease [Bowman et al., J. Bacteriol. 153 (1983) 292-296], also have no obvious phenotype. However, strains containing mutations in both pho-5 and pho-4 are unable to grow under phosphate-restrictive conditions.

Ion efflux systems involved in bacterial metal resistances

Studying metal ion resistance gives us important insights into environmental processes and provides an understanding of basic living processes. This review concentrates on bacterial efflux systems for inorganic metal cations and anions, which have generally been found as resistance systems from bacteria isolated from metal-polluted environments. The protein products of the genes involved are sometimes prototypes of new families of proteins or of important new branches of known families. Sometimes, a group of related proteins (and presumedly the underlying physiological function) has still to be defined. For example, the efflux of the inorganic metal anion arsenite is mediated by a membrane protein which functions alone in Gram-positive bacteria, but which requires an additional ATPase subunit in some Gram-negative bacteria. Resistance to Cd2+ and Zn2+ in Gram-positive bacteria is the result of a P-type efflux ATPase which is related to the copper transport P-type ATPases of bacteria and humans (defective in the human hereditary diseases Menkes' syndrome and Wilson's disease). In contrast, resistance to Zn2+, Ni2+, Co2+ and Cd2+ in Gram-negative bacteria is based on the action of proton-cation antiporters, members of a newly-recognized protein family that has been implicated in diverse functions such as metal resistance/nodulation of legumes/cell division (therefore, the family is called RND). Another new protein family, named CDF for 'cation diffusion facilitator' has as prototype the protein CzcD, which is a regulatory component of a cobalt-zinc-cadmium resistance determinant in the Gram-negative bacterium Alcaligenes eutrophus. A family for the ChrA chromate resistance system in Gram-negative bacteria has still to be defined.

Molecular genetics of carbon-phosphorus bond cleavage in bacteria

Phosphonates (Pn) are a large class of organophosphorus molecules that have direct carbon-phosphorus (C-P) bonds in place of the carbon-oxygen-phosphorus ester bond. In bacteria two pathways exist for Pn breakdown for use as a P source: the phosphonatase and C-P lyase pathways. These pathways differ both in regard to their substrate specificity and their cleavage mechanism. The phosphonatase pathway acts on the natural Pn alpha-aminoethylphosphonate (AEPn). In a two-step process it leads to cleavage of the C-P bond by a hydrolysis reaction requiring an adjacent carbonyl group. In contrast the C-P lyase pathway has a broad substrate specificity. It leads to cleavage of substituted Pn (such as AEPn) as well as unsubstituted Pn by a mechanism involving redox or radical chemistry. Due to its broad substrate specificity, the C-P lyase pathway is generally thought to be responsible for the breakdown of Pn herbicides (such as glyphosate) by bacteria. As a way to gain a more in-depth understanding of these Pn degradative pathways, their respective genes have been isolated and characterized. In the absence of a biochemical assay for the C-P lyase pathway such molecular approaches have been especially valuable. The roles of individual genes have been inferred from DNA sequence analysis and mutational effects. Genes for the C-P lyase pathway exist in a fourteen-gene operon that appears to encode both a binding protein-dependent Pn transporter and a C-P lyase. Genes for the phosphonatase pathway also exist in a gene cluster containing Pn uptake and degradative genes. A combination of biochemistry, molecular biology, and molecular genetics approaches has provided more detailed understanding of the mechanisms of C-P bond cleavage. Such basic information may provide a new handle for improvement of Pn degradation capabilities in bacteria, or in other cells in which the respective genes may be introduced and expressed.

Solute transport and energy transduction in bacteria

In bacteria two forms of metabolic energy are usually present, i.e. ATP and transmembrane ion-gradients, that can be used to drive the various endergonic reactions associated with cellular growth. ATP can be formed directly in substrate level phosphorylation reactions whereas primary transport processes can generate the ion-gradients across the cytoplasmic membrane. The two forms of metabolic energy can be interconverted by the action of ion-translocating ATPases. For fermentative organisms it has long been thought that ion-gradients could only be generated at the expense of ATP hydrolysis by the F0F1-ATPase. In the present article, an overview is given of the various secondary transport processes that form ion-gradients at the expense of precursor (substrate) and/or end-product concentration gradients. The metabolic energy formed by these chemiosmotic circuits contributes to the 'energy status' of the bacterial cell which is particularly important for anaerobic/fermentative organisms.

Electrogenic glutamine uptake by Peptostreptococcus anaerobius and generation of a transmembrane potential

Peptostreptococcus anaerobius converted glutamine stoichiometrically to ammonia and pyroglutamic acid, and the Eadie-Hofstee plot of glutamine transport was biphasic. High-affinity, sodium-dependent glutamine transport (affinity constant [Kt] of 1.5 microM) could be driven by the chemical gradient of sodium, and more than 20 mM sodium was required for half-maximal velocity. High-affinity glutamine transport was not stimulated or inhibited by a membrane potential (delta psi). Low-affinity glutamine transport had a rate which was directly proportional to the external glutamine concentration, required less than 100 microM sodium, and was inhibited strongly by a delta psi. Cells which were treated with N,N-dicyclohexylcarbodiimide to inhibit the F1F0 ATPase still generated a delta psi but did so only if the external glutamine concentration was greater than 15 mM. Low-affinity glutamine uptake could not be saturated by as much as 200 mM glutamine, but glutamine-1 accounts for only a small fraction of the total glutamine at physiological pH values (pH 6 to 7). On the basis of these results, it appeared that the low-affinity glutamine transport was an electrogenic mechanism which was converting a chemical gradient of glutamine-1 into a delta psi. Other mechanisms of delta psi generation (electrogenic glutamine-pyroglutamate or -ammonium exchange) could not be demonstrated.

The family of organo-phosphate transport proteins includes a transmembrane regulatory protein

This review article briefly summarizes aspects of our current understanding of the Uhp sugar phosphate transport system in enteric bacteria, particularly the mode of genetic regulation of its synthesis. This regulation occurs by a process that involves an example of the very widespread and ever-growing group of so-called two-component bacterial regulatory systems, a mechanism of response to environmental signals that employs phosphate transfer reactions between constituent proteins. Of emphasis here is the unusual involvement in transmembrane signaling of the UhpC protein which is related in sequence and structure to some transport proteins, including the very protein whose synthesis it helps regulate.

Energy transduction in lactic acid bacteria

In the discovery of some general principles of energy transduction, lactic acid bacteria have played an important role. In this review, the energy transducing processes of lactic acid bacteria are discussed with the emphasis on the major developments of the past 5 years. This work not only includes the biochemistry of the enzymes and the bioenergetics of the processes, but also the genetics of the genes encoding the energy transducing proteins. The progress in the area of carbohydrate transport and metabolism is presented first. Sugar translocation involving ATP-driven transport, ion-linked cotransport, heterologous exchange and group translocation are discussed. The coupling of precursor uptake to product product excretion and the linkage of antiport mechanisms to the deiminase pathways of lactic acid bacteria is dealt with in the second section. The third topic relates to metabolic energy conservation by chemiosmotic processes. There is increasing evidence that precursor/product exchange in combination with precursor decarboxylation allows bacteria to generate additional metabolic energy. In the final section transport of nutrients and ions as well as mechanisms to excrete undesirable (toxic) compounds from the cells are discussed.

Carbohydrate transport in bacteria under environmental conditions, a black box?

A typical eubacterium carries a battery of substrate transport systems which are the ultimate pacemakers for growth. These systems reflect a billion year old selection for coping with rapidly changing conditions in the environment and each of them is optimised for specific growth conditions. Metabolic pathways in combination with transport systems can be interpreted as transient sensory systems, where a transport system corresponds to a sensor for external stimuli. Characteristics is a tightly linked common control between a carbohydrate metabolic pathway and the corresponding transport system. Many of the observed growth phenomena are a direct result of adaptation and regulation of transport capacity to rapid changes in environmental conditions. Some of the better understood examples are discusses. Nevertheless, knowledge on bacterial carbohydrate transport under environmental conditions as documented in the literature is still scarce.

Probing the structure of the Neurospora crassa plasma membrane H(+)-ATPase

The structure of the Neurospora crassa plasma membrane H(+)-ATPase has been investigated using a variety of chemical and physiochemical techniques. The transmembrane topography of the H(+)-ATPase has been elucidated by a direct, protein chemical approach. Reconstituted proteoliposomes containing purified H(+)-ATPase molecules oriented predominantly with their cytoplasmic surface facing outward were treated with trypsin, and the numerous peptides released were purified by HPLC and subjected to amino acid sequence analysis. In this way, seventeen released peptides were unequivocally identified as located on the cytoplasmic side of the membrane, and numerous intervening segments could be inferred to be cytoplasmically located by virtue of the fact that they are too short to cross the membrane and return between sequences established to be cytoplasmically located. Additionally, three large membrane-embedded segments of the H(+)-ATPase were isolated using our recently developed methods for purifying hydrophobic peptides, and identified by amino acid sequence analysis. This information established the topographical location of virtually all of the 919 residues in the H(+)-ATPase molecule, allowing the formulation of a reasonably detailed model for the transmembrane topography of the H(+)-ATPase polypeptide chain. Separate studies of the cysteine chemistry of the H(+)-ATPase have demonstrated the existence of a single disulfide bridge in the molecule, linking the NH2- and COOH-terminal membrane-embedded domains. And, analyses of the circular dichroism and infrared spectra of the purified H(+)-ATPase have elucidated the secondary structure composition of the molecule. A first-generation model for the tertiary structure of the H(+)-ATPase based on this information and other considerations is presented.

Cloning and sequencing of the melB gene encoding the melibiose permease of Salmonella typhimurium LT2

The nucleotide sequence of the melB gene coding for the Na+ (Li+)/melibiose symporter of Salmonella typhimurium LT2 was determined, and its amino acid sequence was deduced. It consists of 1428 bp, corresponding to a protein of 476 amino acid residues (calculated molecular weight 52,800). The amino acid sequence is homologous to that of the melibiose permease of Escherichia coli K12, with 85% identical residues. All, except one, of the amino acid residues that have been reported to be important for cation or substrate recognition in the melibiose permease of E. coli are conserved in the melibiose permease of S. typhimurium. In addition, part of the sequence resembles the lactose permease of Streptococcus thermophilus, the animal glucose transporter (GLUT1), the plasmid-coded raffinose permease (RafB), and the NADH-ubiquinone oxidoreductase chain 4 (Nuo4) of Aspergillus amstelodami.