Purification of glucose-inducible outer membrane protein OprB of Pseudomonas putida and reconstitution of glucose-specific pores

Anaerobic glucose uptake in Pseudomonas putida KT2440 in a bioelectrochemical system

Providing an anodic potential in a bio-electrochemical system to the obligate aerobe Pseudomonas putida enables anaerobic survival and allows the cells to overcome redox imbalances. In this setup, the bacteria could be exploited to produce chemicals via oxidative pathways at high yield. However, the absence of anaerobic growth and low carbon turnover rates remain as obstacles for the application of such an electro-fermentation technology. Growth and carbon turnover start with carbon uptake into the periplasm and cytosol. P. putida KT2440 has three native transporting systems for glucose, each differing in energy and redox demand. This architecture previously led to the hypothesis that internal redox and energy constraints ultimately limit cytoplasmic carbon utilization in a bio-electrochemical system. However, it remains largely unclear which uptake route is predominantly used by P. putida under electro-fermentative conditions. To elucidate this, we created three gene deletion mutants of P. putida KT2440, forcing the cells to exclusively utilize one of the routes. When grown in a bio-electrochemical system, the pathway mutants were heavily affected in terms of sugar consumption, current output and product formation. Surprisingly, however, we found that about half of the acetate formed in the cytoplasm originated from carbon that was put into the system via the inoculation biomass, while the other half came from the consumption of substrate. The deletion of individual sugar uptake routes did not alter significantly the secreted acetate concentrations among different strains even with different carbon sources. This means that the stoichiometry of the sugar uptake routes is not a limiting factor during electro-fermentation and that the low rates might be caused by other reasons, for example energy limitations or a yet-to-be-identified oxygen-dependent regulatory mechanism.

RB-TnSeq identifies genetic targets for improved tolerance of Pseudomonas putida towards compounds relevant to lignin conversion

Lignin-derived mixtures intended for bioconversion commonly contain high concentrations of aromatic acids, aliphatic acids, and salts. The inherent toxicity of these chemicals places a significant bottleneck upon the effective use of microbial systems for the valorization of these mixtures. Pseudomonas putida KT2440 can tolerate stressful quantities of several lignin-related compounds, making this bacterium a promising host for converting these chemicals to valuable bioproducts. Nonetheless, further increasing P. putida tolerance to chemicals in lignin-rich substrates has the potential to improve bioprocess performance. Accordingly, we employed random barcoded transposon insertion sequencing (RB-TnSeq) to reveal genetic determinants in P. putida KT2440 that influence stress outcomes during exposure to representative constituents found in lignin-rich process streams. The fitness information obtained from the RB-TnSeq experiments informed engineering of strains via deletion or constitutive expression of several genes. Namely, ΔgacAS, ΔfleQ, ΔlapAB, ΔttgR::P_tac:ttgABC, P_tac:PP_1150:PP_1152, ΔrelA, and ΔPP_1430 mutants showed growth improvement in the presence of single compounds, and some also exhibited greater tolerance when grown using a complex chemical mixture representative of a lignin-rich chemical stream. Overall, this work demonstrates the successful implementation of a genome-scale screening tool for the identification of genes influencing stress tolerance against notable compounds within lignin-enriched chemical streams, and the genetic targets identified herein offer promising engineering targets for improving feedstock tolerance in lignin valorization strains of P. putida KT2440.

Glucose transport via the pseudomonad porin OprB: implications for the design of Trojan Horse anti-infectives

Deciphering the transport through outer-membrane porins is crucial to understand how anti-infectives enter Gram-negative bacteria and perform their function. Here we elucidated the transport mechanism of substrates through the Pseudomonads sugar-specific porin OprB by means of multiscale modeling. We used molecular dynamics simulations to quantify the energetics of transport and thus a diffusion model to quantify the macroscopic flux of molecules through OprB. Our results show that Trp171 and several glutamate residues in the constriction region are key for the transport of glucose, the preferred natural substrate, through OprB. The unveiled transport mechanism suggests that 2-acetamido-1,2-dideoxynojirimycin (DNJ-NAc), an anti-infective structurally similar to glucose, can enter the cell via OprB. We quantified its energetics and macroscopic flux through OprB providing a comparative analysis with the natural substrate. Thus this pore can be considered as a promising gateway for exploiting the Trojan Horse strategy in pathogenic bacteria.

Analysis of Pseudomonas putida growth on non-trivial carbon sources using transcriptomics and genome-scale modelling

Pseudomonas putida is characterized by a versatile metabolism and stress tolerance traits that allow the bacterium to cope with different environmental conditions. In this work, the mechanisms that allow P. putida KT2440 to grow in the presence of four sole carbon sources (glucose, citrate, ferulic acid, serine) were investigated by RNA sequencing (RNA-seq) and genome-scale metabolic modelling. Transcriptomic data identified uptake systems for the four carbon sources, and candidates were subjected to preliminary experimental characterization by mutant strain growth to test their involvement in substrate assimilation. The OpdH and BenF-like porins were involved in citrate and ferulic acid uptake respectively. The citrate transporter (encoded by PP_0147) and the TctABC system were important for supporting cell growth in citrate; PcaT and VanK were associated with ferulic acid uptake; and the ABC transporter AapJPQM was involved in serine transport. A genome-scale metabolic model of P. putida KT2440 was used to integrate and analyze the transcriptomic data, identifying and confirming the active catabolic pathways for each carbon source. This study reveals novel information about transporters that are essential for understanding bacterial adaptation to different environments.

Effects of H-bonds on sugar binding to chitoporin from Vibrio harveyi

BACKGROUND

VhChiP is a sugar-specific-porin present in the outer membrane of the marine bacterium Vibrio harveyi and responsible for chitin uptake, with a high selectivity for chitohexaose.

METHODS

VhChiP and its mutants were expressed and purified from BL21 (DE3) Omp8 Rosetta strain. After reconstitution into planar lipid bilayers, the ion current fluctuations caused by chitohexaose entering the channel were measured in deuterium oxide and in water.

RESULTS

The role of hydrogen-bonding in sugar binding was investigated by comparing channel occlusion by chitohexaose in buffers containing H₂O and D₂O. The BLM results revealed the significant contribution of hydrogen bonding to the binding of chitohexaose in the constriction zone of VhChiP. Replacing H₂O as solvent by D₂O significantly decreased the on- and off-rates of sugar penetration into the channel. The importance of hydrogen bonding inside the channel was more noticeable when the hydrophobicity of the constriction zone was diminished by replacing Trp¹³⁶ with the charged residues Asp or Arg. The on- and off-rates decreased up to 2.5-fold and 4-fold when Trp¹³⁶ was replaced by Arg, or 5-fold and 3-fold for Trp¹³⁶ replacement by Asp, respectively. Measuring the on-rate at different temperatures and for different channel mutants revealed the activation energy for chitohexaose entrance into VhChiP channel.

CONCLUSIONS

Hydrogen-bonds contribute to sugar permeation.

Chitosugar translocation by an unexpressed monomeric protein channel

The outer membrane protein channel EcChiP, associated with a silent gene in E. coli, is a monomeric chitoporin. In a glucose-deficient environment, E. coli can express the ChiP gene to exploit chitin degradation products. Single-channel small ion current measurements, which reveal the dynamics of single sugar molecules trapped in channel, are used here to study the exotic transport of chitosugars by E. coli. Molecules escape from the channel on multiple timescales. Voltage-dependent trapping rates observed for charged chitosan molecules, as well as model calculations, indicate that the rapid escape processes are those in which the molecule escapes back to the side of the membrane from which it originated. The probability that a sugar molecule is translocated through the membrane is thus estimated from the current data and the dependence of this translocation probability on the length of the chitosugar molecule and the applied voltage analyzed. The described method for obtaining the translocation probability and related molecular translocation current is applicable to other transport channels.

Engineering sucrose metabolism in Pseudomonas putida highlights the importance of porins

Using agricultural wastes as a substrate for biotechnological processes is of great interest in industrial biotechnology. A prerequisite for using these wastes is the ability of the industrially relevant microorganisms to metabolize the sugars present therein. Therefore, many metabolic engineering approaches are directed towards widening the substrate spectrum of the workhorses of industrial biotechnology like Escherichia coli, yeast or Pseudomonas putida. For instance, neither xylose or arabinose from cellulosic residues, nor sucrose, the main sugar in waste molasses, can be metabolized by most E. coli and P. putida wild types. We evaluated a new, so far uncharacterized gene cluster for sucrose metabolism from Pseudomonas protegens Pf‐5 and showed that it enables P. putida to grow on sucrose as the sole carbon and energy source. Even when integrated into the genome of P. putida, the resulting strain grew on sucrose at rates similar to the rate of the wild type on glucose – making it the fastest growing, plasmid‐free P. putida strain known so far using sucrose as substrate. Next, we elucidated the role of the porin, an orthologue of the sucrose porin ScrY, in the gene cluster and found that in P. putida, a porin is needed for sucrose transport across the outer membrane. Consequently, native porins were not sufficient to allow unlimited growth on sucrose. Therefore, we concluded that the outer membrane can be a considerable barrier for substrate transport, depending on strain, genotype and culture conditions, all of which should be taken into account in metabolic engineering approaches. We additionally showed the potential of the engineered P. putida strains by growing them on molasses with efficiencies twice as high as obtained with the wild‐type P. putida. This can be seen as a further step towards the production of low‐value chemicals and biofuels with P. putida from alternative and more affordable substrates in the future.

Xanthomonas citri ssp. citri requires the outer membrane porin OprB for maximal virulence and biofilm formation

Xanthomonas citri ssp. citri (Xcc) causes canker disease in citrus, and biofilm formation is critical for the disease cycle. OprB (Outer membrane protein B) has been shown previously to be more abundant in Xcc biofilms compared with the planktonic state. In this work, we showed that the loss of OprB in an oprB mutant abolishes bacterial biofilm formation and adherence to the host, and also compromises virulence and efficient epiphytic survival of the bacteria. Moreover, the oprB mutant is impaired in bacterial stress resistance. OprB belongs to a family of carbohydrate transport proteins, and the uptake of glucose is decreased in the mutant strain, indicating that OprB transports glucose. Loss of OprB leads to increased production of xanthan exopolysaccharide, and the carbohydrate intermediates of xanthan biosynthesis are also elevated in the mutant. The xanthan produced by the mutant has a higher viscosity and, unlike wild-type xanthan, completely lacks pyruvylation. Overall, these results suggest that Xcc reprogrammes its carbon metabolism when it senses a shortage of glucose input. The participation of OprB in the process of biofilm formation and virulence, as well as in metabolic changes to redirect the carbon flux, is discussed. Our results demonstrate the importance of environmental nutrient supply and glucose uptake via OprB for Xcc virulence.

Global genetic capacity for mixotrophy in marine picocyanobacteria

The assimilation of organic nutrients by autotrophs, a form of mixotrophy, has been demonstrated in the globally abundant marine picocyanobacterial genera Prochlorococcus and Synechococcus. However, the range of compounds used and the distribution of organic compound uptake genes within picocyanobacteria are unknown. Here we analyze genomic and metagenomic data from around the world to determine the extent and distribution of mixotrophy in these phototrophs. Analysis of 49 Prochlorococcus and 18 Synechococcus isolate genomes reveals that all have the transporters necessary to take up amino acids, peptides and sugars. However, the number and type of transporters and associated catabolic genes differ between different phylogenetic groups, with low-light IV Prochlorococcus, and 5.1B, 5.2 and 5.3 Synechococcus strains having the largest number. Metagenomic data from 68 stations from the Tara Oceans expedition indicate that the genetic potential for mixotrophy in picocyanobacteria is globally distributed and differs between clades. Phylogenetic analyses indicate gradual organic nutrient transporter gene loss from the low-light IV to the high-light II Prochlorococcus. The phylogenetic differences in genetic capacity for mixotrophy, combined with the ubiquity of picocyanobacterial organic compound uptake genes suggests that mixotrophy has a more central role in picocyanobacterial ecology than was previously thought.

Correlated trapping of sugar molecules by the trimeric protein channel chitoporin

The protein channel chitoporin (ChiP), which is used by marine bacteria to translocate selected sugar molecules through the outer cell membrane, is studied via single channel current measurements in water and heavy water sugar solutions. The dynamic trapping and escape probabilities of sugar molecules from different monomers in the trimeric channel are characterized, including their dependence on channel orientation and sensitivity to a deuterium isotope effect. A detailed analysis of stochastic current fluctuations reveals that the trapping properties of chitoporin exhibit memory effects: the rate of trapping transitions depends on the previous sequence of transitions; and intermonomer correlations: the average trapping rate of an unblocked monomer is larger when its neighboring monomers are blocked. The latter, likely resulting from rapid re-trapping of recently escaped sugar molecules, is considered as a possible design strategy to enhance sugar transport.

Identification and Functional Characterization of a Novel OprD-like Chitin Uptake Channel in Non-chitinolytic Bacteria

Chitoporin from the chitinolytic marine Vibrio has been characterized as a trimeric OmpC-like channel responsible for effective chitin uptake. In this study we describe the identification and characterization of a novel OprD-like chitoporin (so-called EcChiP) from Escherichia coli The gene was identified, cloned, and functionally expressed in the Omp-deficient E. coli BL21 (Omp8) Rosetta strain. On size exclusion chromatography, EcChiP had an apparent native molecular mass of 50 kDa, as predicted by amino acid sequencing and mass analysis, confirming that the protein is a monomer. Black lipid membrane reconstitution demonstrated that EcChiP could readily form stable, monomeric channels in artificial phospholipid membranes, with an average single channel conductance of 0.55 ± 0.01 nanosiemens and a slight preference for cations. Single EcChiP channels showed strong specificity, interacting with long chain chitooligosaccharides but not with maltooligosaccharides. Liposome swelling assays indicated the bulk permeation of neutral monosaccharides and showed the size exclusion limit of EcChiP to be ∼200-300 Da for small permeants that pass through by general diffusion while allowing long chain chitooligosaccharides to pass through by a facilitated diffusion process. Taking E. coli as a model, we offer the first evidence that non-chitinolytic bacteria can activate a quiescent ChiP gene to express a functional chitoporin, enabling them to take up chitooligosaccharides for metabolism as an immediate source of energy.

Chitoporin from the Marine Bacterium Vibrio harveyi: PROBING THE ESSENTIAL ROLES OF TRP136 AT THE SURFACE OF THE CONSTRICTION ZONE

VhChiP is a sugar-specific porin present in the outer membrane of the marine bacterium Vibrio harveyi. VhChiP is responsible for the uptake of chitin oligosaccharides, with particular selectivity for chitohexaose. In this study, we employed electrophysiological and biochemical approaches to demonstrate that Trp(136), located at the mouth of the VhChiP pore, plays an essential role in controlling the channel's ion conductivity, chitin affinity, and permeability. Kinetic analysis of sugar translocation obtained from single channel recordings indicated that the Trp(136) mutations W136A, W136D, W136R, and W136F considerably reduce the binding affinity of the protein channel for its best substrate, chitohexaose. Liposome swelling assays confirmed that the Trp(136) mutations decreased the rate of bulk chitohexaose permeation through the VhChiP channel. Notably, all of the mutants show increases in the off-rate for chitohexaose of up to 20-fold compared with that of the native channel. Furthermore, the cation/anion permeability ratio Pc/Pa is decreased in the W136R mutant and increased in the W136D mutant. This demonstrates that the negatively charged surface at the interior of the protein lumen preferentially attracts cationic species, leading to the cation selectivity of this trimeric channel.

A proteomic approach of SigX function in Pseudomonas aeruginosa outer membrane composition

SigX is one of the 19 extracytoplasmic function sigma factors that have been predicted in the human opportunistic pathogen Pseudomonas aeruginosa genome. SigX is involved in the transcription of oprF, encoding the major outer membrane protein OprF, a pleiotropic porin that contributes to the maintaining of the wall structure, and is essential to P. aeruginosa virulence. This study aimed to get further insights into the functions of SigX. We performed here an outer membrane subproteome of a sigX mutant. Proteomic investigations revealed lower production of 8 porins among which 4 gated channels involved in iron or hem uptake, OprF, and the three substrate-specific proteins OprD, OprQ and OprE. On the other side, the glucose-specific porin OprB and the lipid A 3-O-deacylase that is involved in LPS modification were up-regulated. Our results indicate that SigX may be involved in the control and/or regulation of the outer membrane composition.

BIOLOGICAL SIGNIFICANCE

A proteomic approach was used herein to get further insights into SigX functions in P. aeruginosa. The data presented here suggest that SigX is involved in the outer membrane protein composition, and could be linked to a regulatory network involved in OM homeostasis.

Single-molecule trapping dynamics of sugar-uptake channels in marine bacteria

Stochastic fluctuations of ion current through one chitoporin (ChiP) channel within a bilayer lipid membrane in sugar solution are analyzed. These reflect single-molecule dynamics, which indicate that ChiP has multiple binding sites for sugar and exploits interactions between bound molecules to direct sugar passage. Since ChiP is used by marine bacteria, this is likely an adaptive strategy to enhance sugar translocation from rough water.

A Nostoc punctiforme sugar transporter necessary to establish a Cyanobacterium-plant symbiosis

In cyanobacteria-plant symbioses, the symbiotic nitrogen-fixing cyanobacterium has low photosynthetic activity and is supplemented by sugars provided by the plant partner. Which sugars and cyanobacterial sugar uptake mechanism(s) are involved in the symbiosis, however, is unknown. Mutants of the symbiotically competent, facultatively heterotrophic cyanobacterium Nostoc punctiforme were constructed bearing a neomycin resistance gene cassette replacing genes in a putative sugar transport gene cluster. Results of transport activity assays using (14)C-labeled fructose and glucose and tests of heterotrophic growth with these sugars enabled the identification of an ATP-binding cassette-type transporter for fructose (Frt), a major facilitator permease for glucose (GlcP), and a porin needed for the optimal uptake of both fructose and glucose. Analysis of green fluorescent protein fluorescence in strains of N. punctiforme bearing frt::gfp fusions showed high expression in vegetative cells and akinetes, variable expression in hormogonia, and no expression in heterocysts. The symbiotic efficiency of N. punctiforme sugar transport mutants was investigated by testing their ability to infect a nonvascular plant partner, the hornwort Anthoceros punctatus. Strains that were specifically unable to transport glucose did not infect the plant. These results imply a role for GlcP in establishing symbiosis under the conditions used in this work.

Chitoporin from Vibrio harveyi, a channel with exceptional sugar specificity

Chitoporin (VhChiP) is a sugar-specific channel responsible for the transport of chitooligosaccharides through the outer membrane of the marine bacterium Vibrio harveyi. Single channel reconstitution into black lipid membrane allowed single chitosugar binding events in the channel to be resolved. VhChiP has an exceptionally high substrate affinity, with a binding constant of K = 5.0 × 10(6) M(-1) for its best substrate (chitohexaose). The on-rates of chitosugars depend on applied voltages, as well as the side of the sugar addition, clearly indicating the inherent asymmetry of the VhChiP lumen. The binding affinity of VhChiP for chitohexaose is 1-5 orders of magnitude larger than that of other known sugar-specific porins for their preferred substrates. Thus, VhChiP is the most potent sugar-specific channel reported to date, with its high efficiency presumably reflecting the need for the bacterium to take up chitin-containing nutrients promptly under turbulent aquatic conditions to exploit them efficiently as its sole source of energy.

Structural basis for outer membrane sugar uptake in pseudomonads

Substrate-specific outer membrane channels of gram-negative bacteria mediate uptake of many small molecules, including carbohydrates. The mechanism of sugar uptake by enterobacterial channels, such as Escherichia coli LamB (maltoporin), has been characterized in great detail. In pseudomonads and related organisms, sugar uptake is not mediated by LamB but by OprB channels. Beyond the notion that OprB channels seem to prefer monosaccharides as substrates, very little is known about OprB-mediated sugar uptake. Here I report the X-ray crystal structure of an OprB channel from Pseudomonas putida F1. The structure shows that OprB forms a monomeric, 16-stranded β-barrel with a constriction formed by extracellular loops L2 and L3. The side chains of two highly conserved arginine residues (Arg(83) and Arg(110)) and a conserved glutamate (Glu(106)) line the channel constriction and interact with a bound glucose molecule. Liposome swelling uptake assays show a strong preference for monosaccharide transport over disaccharides. Moreover, substrates with a net negative charge are disfavored by the channel, probably due to the negatively charged character of the constriction. The architecture of the eyelet and the absence of a greasy slide provide an explanation for the observed specificity of OprB for monosaccharides rather than the oligosaccharides preferred by LamB and related enterobacterial channels.

A novel host-responsive sensor mediates virulence and type III secretion during Pseudomonas aeruginosa-host cell interactions

Sensitive sensory mechanisms are instrumental in affording Pseudomonas aeruginosa the capacity to establish diverse yet severe human infections, which can manifest themselves in long-term untreatable disease. The ability of P. aeruginosa to tightly regulate gene expression and virulence factor production, in response to activation of these sensory components, enables the pathogen to sustain infection despite the host immune response and aggressive antibiotic treatment. Although a number of factors are recognized as playing a role in early infection, very little is known regarding the sensors involved in this process. In this study, we identified P. aeruginosa PA3191 as a novel host-responsive sensor that plays a key role during P. aeruginosa-host interactions and is required for optimum colonization and dissemination in a mouse model of infection. We demonstrated that PA3191 contributed to modulation of the type III secretion system (T3SS) in response to host cells and T3SS-inducing conditions in vitro. PA3191 (designated GtrS) acted in concert with the response regulator GltR to regulate the OprB transport system and subsequently carbon metabolism. Through this signal transduction pathway, T3SS activation was mediated via the RsmAYZ regulatory cascade and involved the global anaerobic response regulator Anr.

The ColRS system is essential for the hunger response of glucose-growing Pseudomonas putida

Background

The survival of bacteria largely depends on signaling systems that coordinate cell responses to environmental cues. Previous studies on the two-component ColRS signal system in Pseudomonas putida revealed a peculiar subpopulation lysis phenotype of colR mutant that grows on solid glucose medium. Here, we aimed to clarify the reasons for the lysis of bacteria.

Results

We present evidence that the lysis defect of P. putida colR mutant is linked to hunger response. A subpopulation prone to lysis was located in the periphery of bacterial cultures growing on solid medium. Cell lysis was observed in glucose-limiting, but not in glucose-rich conditions. Furthermore, lysis was also alleviated by exhaustion of glucose from the medium which was evidenced by a lower lysis of central cells compared to peripheral ones. Thus, lysis takes place at a certain glucose concentration range that most probably provides bacteria a hunger signal. An analysis of membrane protein pattern revealed several hunger-induced changes in the bacterial outer membrane: at glucose limitation the amount of OprB1 channel protein was significantly increased whereas that of OprE was decreased. Hunger-induced up-regulation of OprB1 correlated in space and time with the lysis of the colR mutant, indicating that hunger response is detrimental to the colR-deficient bacteria. The amount of OprB1 is controlled post-transcriptionally and derepression of OprB1 in glucose-limiting medium depends at least partly on the carbon catabolite regulator protein Crc. The essentiality of ColR in hunger response can be bypassed by reducing the amount of certain outer membrane proteins. In addition to depletion of OprB1, the lysis defect of colR mutant can be suppressed by the down-regulation of OprF levels and the hindering of SecB-dependent protein secretion.

Conclusions

We show that Pseudomonas putida growing on solid glucose medium adapts to glucose limitation through up-regulation of the sugar channel protein OprB1 that probably allows enhanced acquisition of a limiting nutrient. However, to survive such hunger response bacteria need signalling by the ColRS system. Hence, the ColRS system should be considered a safety factor in hunger response that ensures the welfare of the cell membrane during the increased expression of certain membrane proteins.

Repression of the glucose-inducible outer-membrane protein OprB during utilization of aromatic compounds and organic acids in Pseudomonas putida CSV86

Pseudomonas putida CSV86 shows preferential utilization of aromatic compounds over glucose. Protein analysis and [¹⁴C]glucose-binding studies of the outer membrane fraction of cells grown on different carbon sources revealed a 40 kDa protein that was transcriptionally induced by glucose and repressed by aromatics and succinate. Based on 2D gel electrophoresis and liquid chromatography-tandem mass spectrometry analysis, the 40 kDa protein closely resembled the porin B of P. putida KT2440 and carbohydrate-selective porin OprB of various Pseudomonas strains. The purified native protein (i) was estimated to be a homotrimer of 125 kDa with a subunit molecular mass of 40 kDa, (ii) displayed heat modifiability of electrophoretic mobility, (iii) showed channel conductance of 166 pS in 1 M KCl, (iv) permeated various sugars (mono-, di- and tri-saccharides), organic acids, amino acids and aromatic compounds, and (v) harboured a glucose-specific and saturable binding site with a dissociation constant of 1.3 µM. These results identify the glucose-inducible outer-membrane protein of P. putida CSV86 as a carbohydrate-selective protein OprB. Besides modulation of intracellular glucose-metabolizing enzymes and specific glucose-binding periplasmic space protein, the repression of OprB by aromatics and organic acids, even in the presence of glucose, also contributes significantly to the strain's ability to utilize aromatics and organic acids over glucose.

Regulation of glucose metabolism in Pseudomonas: the phosphorylative branch and entner-doudoroff enzymes are regulated by a repressor containing a sugar isomerase domain

In Pseudomonas putida, genes for the glucose phosphorylative pathway and the Entner-Doudoroff pathway are organized in two operons; one made up of the zwf, pgl, and eda genes and another consisting of the edd, glk, gltR2, and gltS genes. Divergently with respect to the edd gene is the gap-1 gene. Expression from P(zwf), P(edd), and P(gap) is modulated by HexR in response to the availability of glucose in the medium. To study the regulatory process in greater detail we purified HexR and showed that it is a monomer in solution. Electrophoretic mobility shift assays and isothermal titration calorimetry assays were done showing that HexR recognizes the P(edd), P(zwf), and P(gap-1) promoters with affinity in the nanomolar range. DNA footprinting assays identified the binding site between +30 and +1 at P(zwf), between +16 and +41 at P(edd), and between -6 and +18 at P(gap-1). Based on DNA sequence alignment of the target sites and isothermal titration calorimetry data, two monomers of HexR bind to a pseudopalindrome with a consensus sequence of 5'-TTGTN(7-8)ACAA-3'. Binding of the Entner-Doudoroff pathway intermediate 2-keto-3-deoxy-6-phosphogluconate to HexR released the repressor from its target operators, whereas other chemicals such as glucose, glucose 6-phosphate, and 6-phosphogluconate did not induce complex dissociation. The phosphorylated effector is likely to be recognized by a sugar isomerase domain located at the C-terminal end of HexR, whereas the helix-turn-helix DNA binding domain of HexR exhibits high similarity to proteins of the RpiR family of regulators.

Metabolic flux analysis of a phenol producing mutant of Pseudomonas putida S12: verification and complementation of hypotheses derived from transcriptomics

The physiological effects of genetic and transcriptional changes observed in a phenol producing mutant of the solvent-tolerant Pseudomonas putida S12 were assessed with metabolic flux analysis. The upregulation of a malate/lactate dehydrogenase encoding gene could be connected to a flux increase from malate to oxaloacetate. A mutation in the pykA gene decreased in vitro pyruvate kinase activity, which is consistent with a lower flux from phosphoenolpyruvate to pyruvate. Changes in the oprB-1, gntP and gnuK genes, encoding a glucose-selective porin, gluconokinase and a gluconate transporter respectively, altered the substrate uptake profile. Metabolic flux analysis furthermore revealed cellular events not predicted by the transcriptome analysis. Gluconeogenic formation of glucose-6-phosphate from triose-3-phosphate was abolished, in favour of increased phosphoenolpyruvate production. An increased pentose phosphate pathway flux resulted in higher erythrose-4-phosphate production. Thus, the availability of these two central phenol precursors was improved. Furthermore, metabolic fluxes were redistributed such that the overall TCA cycle flux was unaffected and energy production increased. Engineering P. putida S12 for phenol production has yielded a strain that channels carbon fluxes to previously unfavourable routes to reconcile the drain on metabolites required for phenol production, while maintaining basal flux levels through central carbon metabolism.

ColRS two-component system prevents lysis of subpopulation of glucose-grown Pseudomonas putida

ColRS two-component system is well conserved in pseudomonads, but its exact role has remained obscure. Here, we report that Pseudomonas putida deficient in ColR experiences serious carbon source-specific stress that leads to the lysis of a subpopulation of bacteria growing on solid glucose medium. We observed that on glucose medium colR-deficient bacteria aggregated, produced a Congo Red-binding substance and had enhanced membrane permeability. Detection of a large amount of cytoplasmic beta-galactosidase and other proteins as well as chromosomal DNA in the growth medium of a colR mutant indicated that cell lysis took place if ColR was absent. Investigation of colony morphology revealed concavities in the centre of the colonies of colR mutant suggesting that cell lysis occurred mainly in the areas of the highest cell density. Analysis of bacteria at a single cell level by flow cytometry showed that population of glucose-grown colR-deficient cells was heterogeneous. In addition to the wild type-like population, we detected a subpopulation of cells with damaged membrane permeable to propidium iodide. Interestingly, inactivation of oprB1 encoding a glucose porin eliminated the cell lysis as well as autoaggregation and membrane leakiness of a colR mutant indicating that glucose influx could be responsible for membrane stress in the absence of ColRS system.

A set of activators and repressors control peripheral glucose pathways in Pseudomonas putida to yield a common central intermediate

Pseudomonas putida KT2440 channels glucose to the central Entner-Doudoroff intermediate 6-phosphogluconate through three convergent pathways. The genes for these convergent pathways are clustered in three independent regions on the host chromosome. A number of monocistronic units and operons coexist within each of these clusters, favoring coexpression of catabolic enzymes and transport systems. Expression of the three pathways is mediated by three transcriptional repressors, HexR, GnuR, and PtxS, and by a positive transcriptional regulator, GltR-2. In this study, we generated mutants in each of the regulators and carried out transcriptional assays using microarrays and transcriptional fusions. These studies revealed that HexR controls the genes that encode glucokinase/glucose 6-phosphate dehydrogenase that yield 6-phosphogluconate; the genes for the Entner-Doudoroff enzymes that yield glyceraldehyde-3-phosphate and pyruvate; and gap-1, which encodes glyceraldehyde-3-phosphate dehydrogenase. GltR-2 is the transcriptional regulator that controls specific porins for the entry of glucose into the periplasmic space, as well as the gtsABCD operon for glucose transport through the inner membrane. GnuR is the repressor of gluconate transport and gluconokinase responsible for the conversion of gluconate into 6-phosphogluconate. PtxS, however, controls the enzymes for oxidation of gluconate to 2-ketogluconate, its transport and metabolism, and a set of genes unrelated to glucose metabolism.

Modulation of glucose transport causes preferential utilization of aromatic compounds in Pseudomonas putida CSV86

Pseudomonas putida CSV86 utilizes aromatic compounds in preference to glucose and coutilizes aromatics and organic acids. Protein analysis of cells grown on different carbon sources, either alone or in combination, revealed that a 43-kDa periplasmic-space protein was induced by glucose and repressed by aromatics and succinate. Two-dimensional gel electrophoresis and liquid chromatography-tandem mass spectrometry analysis identified this protein as closely resembling the sugar ABC transporter of Pseudomonas putida KT2440. A partially purified 43-kDa protein showed glucose binding activity and was specific for glucose. The results demonstrate that the aromatic- and organic acid-mediated repression of a periplasmic-space glucose binding protein and consequent inhibition of glucose transport are responsible for this strain's ability to utilize aromatics and organic acids in preference to glucose.

Convergent peripheral pathways catalyze initial glucose catabolism in Pseudomonas putida: genomic and flux analysis

In this study, we show that glucose catabolism in Pseudomonas putida occurs through the simultaneous operation of three pathways that converge at the level of 6-phosphogluconate, which is metabolized by the Edd and Eda Entner/Doudoroff enzymes to central metabolites. When glucose enters the periplasmic space through specific OprB porins, it can either be internalized into the cytoplasm or be oxidized to gluconate. Glucose is transported to the cytoplasm in a process mediated by an ABC uptake system encoded by open reading frames PP1015 to PP1018 and is then phosphorylated by glucokinase (encoded by the glk gene) and converted by glucose-6-phosphate dehydrogenase (encoded by the zwf genes) to 6-phosphogluconate. Gluconate in the periplasm can be transported into the cytoplasm and subsequently phosphorylated by gluconokinase to 6-phosphogluconate or oxidized to 2-ketogluconate, which is transported to the cytoplasm, and subsequently phosphorylated and reduced to 6-phosphogluconate. In the wild-type strain, glucose was consumed at a rate of around 6 mmol g(-1) h(-1), which allowed a growth rate of 0.58 h(-1) and a biomass yield of 0.44 g/g carbon used. Flux analysis of (13)C-labeled glucose revealed that, in the Krebs cycle, most of the oxalacetate fraction was produced by the pyruvate shunt rather than by the direct oxidation of malate by malate dehydrogenase. Enzymatic and microarray assays revealed that the enzymes, regulators, and transport systems of the three peripheral glucose pathways were induced in response to glucose in the outer medium. We generated a series of isogenic mutants in one or more of the steps of all three pathways and found that, although all three functioned simultaneously, the glucokinase pathway and the 2-ketogluconate loop were quantitatively more important than the direct phosphorylation of gluconate. In physical terms, glucose catabolism genes were organized in a series of clusters scattered along the chromosome. Within each of the clusters, genes encoding porins, transporters, enzymes, and regulators formed operons, suggesting that genes in each cluster coevolved. The glk gene encoding glucokinase was located in an operon with the edd gene, whereas the zwf-1 gene, encoding glucose-6-phosphate dehydrogenase, formed an operon with the eda gene. Therefore, the enzymes of the glucokinase pathway and those of the Entner-Doudoroff pathway are physically linked and induced simultaneously. It can therefore be concluded that the glucokinase pathway is a sine qua non condition for P. putida to grow with glucose.

Characterization of OpdH, a Pseudomonas aeruginosa porin involved in the uptake of tricarboxylates

The Pseudomonas aeruginosa outer membrane is intrinsically impermeable to many classes of antibiotics, due in part to its relative lack of general uptake pathways. Instead, this organism relies on a large number of substrate-specific uptake porins. Included in this group are the 19 members of the OprD family, which are involved in the uptake of a diverse array of metabolites. One of these porins, OpdH, has been implicated in the uptake of cis-aconitate. Here we demonstrate that this porin may also enable P. aeruginosa to take up other tricarboxylates. Isocitrate and citrate strongly and specifically induced the opdH gene via a mechanism involving derepression by the putative two-component regulatory system PA0756-PA0757. Planar bilayer analysis of purified OpdH demonstrated that it was a channel-forming protein with a large single-channel conductance (230 pS in 1 M KCl; 10-fold higher than that of OprD); however, we were unable to demonstrate the presence of a tricarboxylate binding site within the channel. Thus, these data suggest that the requirement for OpdH for efficient growth on tricarboxylates was likely due to the specific expression of this large-channel porin under particular growth conditions.

The ColRS two-component system regulates membrane functions and protects Pseudomonas putida against phenol

As reported, the two-component system ColRS is involved in two completely different processes. It facilitates the root colonization ability of Pseudomonas fluorescens and is necessary for the Tn4652 transposition-dependent accumulation of phenol-utilizing mutants in Pseudomonas putida. To determine the role of the ColRS system in P. putida, we searched for target genes of response regulator ColR by use of a promoter library. Promoter screening was performed on phenol plates to mimic the conditions under which the effect of ColR on transposition was detected. The library screen revealed the porin-encoding gene oprQ and the alginate biosynthesis gene algD occurring under negative control of ColR. Binding of ColR to the promoter regions of oprQ and algD in vitro confirmed its direct involvement in regulation of these genes. Additionally, the porin-encoding gene ompA(PP0773) and the type I pilus gene csuB were also identified in the promoter screen. However, it turned out that ompA(PP0773) and csuB were actually affected by phenol and that the influence of ColR on these promoters was indirect. Namely, our results show that ColR is involved in phenol tolerance of P. putida. Phenol MIC measurement demonstrated that a colR mutant strain did not tolerate elevated phenol concentrations. Our data suggest that increased phenol susceptibility is also the reason for inhibition of transposition of Tn4652 in phenol-starving colR mutant bacteria. Thus, the current study revealed the role of the ColRS two-component system in regulation of membrane functionality, particularly in phenol tolerance of P. putida.

Engineering of solvent-tolerant Pseudomonas putida S12 for bioproduction of phenol from glucose

Efficient bioconversion of glucose to phenol via the central metabolite tyrosine was achieved in the solvent-tolerant strain Pseudomonas putida S12. The tpl gene from Pantoea agglomerans, encoding tyrosine phenol lyase, was introduced into P. putida S12 to enable phenol production. Tyrosine availability was a bottleneck for efficient production. The production host was optimized by overexpressing the aroF-1 gene, which codes for the first enzyme in the tyrosine biosynthetic pathway, and by random mutagenesis procedures involving selection with the toxic antimetabolites m-fluoro-dl-phenylalanine and m-fluoro-l-tyrosine. High-throughput screening of analogue-resistant mutants obtained in this way yielded a P. putida S12 derivative capable of producing 1.5 mM phenol in a shake flask culture with a yield of 6.7% (mol/mol). In a fed-batch process, the productivity was limited by accumulation of 5 mM phenol in the medium. This toxicity was overcome by use of octanol as an extractant for phenol in a biphasic medium-octanol system. This approach resulted in accumulation of 58 mM phenol in the octanol phase, and there was a twofold increase in the overall production compared to a single-phase fed batch.

Genomic insights into the iron uptake mechanisms of the biomining microorganism Acidithiobacillus ferrooxidans

Commercial bioleaching of copper and the biooxidation of gold is a cost-effective and environmentally friendly process for metal recovery. A partial genome sequence of the acidophilic, bioleaching bacterium Acidithiobacillus ferrooxidans is available from two public sources. This information has been used to build preliminary models that describe how this microorganism confronts unusually high iron loads in the extremely acidic conditions (pH 2) found in natural environments and in bioleaching operations. A. ferrooxidans contains candidate genes for iron uptake, sensing, storage, and regulation of iron homeostasis. Predicted proteins exhibit significant amino acid similarity with known proteins from neutrophilic organisms, including conservation of functional motifs, permitting their identification by bioinformatics tools and allowing the recognition of common themes in iron transport across distantly related species. However, significant differences in amino acid sequence were detected in pertinent domains that suggest ways in which the periplasmic and outer membrane proteins of A. ferrooxidans maintain structural integrity and relevant protein-protein contacts at low pH. Unexpectedly, the microorganism also contains candidate genes, organized in operon-like structures that potentially encode at least 11 siderophore systems for the uptake of Fe(III), although it does not exhibit genes that could encode the biosynthesis of the siderophores themselves. The presence of multiple Fe(III) uptake systems suggests that A. ferrooxidans can inhabit aerobic environments where iron is scarce and where siderophore producers are present. It may also help to explain why it cannot tolerate high Fe(III) concentrations in bioleaching operations where it is out-competed by Leptospirillum species.

Role of Pseudomonas putida tol-oprL gene products in uptake of solutes through the cytoplasmic membrane

Proteins of the Tol-Pal (Tol-OprL) system play a key role in the maintenance of outer membrane integrity and cell morphology in gram-negative bacteria. Here we describe an additional role for this system in the transport of various carbon sources across the cytoplasmic membrane. Growth of Pseudomonas putida tol-oprL mutant strains in minimal medium with glycerol, fructose, or arginine was impaired, and the growth rate with succinate, proline, or sucrose as the carbon source was lower than the growth rate of the parental strain. Assays with radiolabeled substrates revealed that the rates of uptake of these compounds by mutant cells were lower than the rates of uptake by the wild-type strain. The pattern and amount of outer membrane protein in the P. putida tol-oprL mutants were not changed, suggesting that the transport defect was not in the outer membrane. Consistently, the uptake of radiolabeled glucose and glycerol in spheroplasts was defective in the P. putida tol-oprL mutant strains, suggesting that there was a defect at the cytoplasmic membrane level. Generation of a proton motive force appeared to be unaffected in these mutants. To rule out the possibility that the uptake defect was due to a lack of specific transporter proteins, the PutP symporter was overproduced, but this overproduction did not enhance proline uptake in the tol-oprL mutants. These results suggest that the Tol-OprL system is necessary for appropriate functioning of certain uptake systems at the level of the cytoplasmic membrane.

Microarray analysis of Pseudomonas aeruginosa quorum-sensing regulons: effects of growth phase and environment

Bacterial communication via quorum sensing (QS) has been reported to be important in the production of virulence factors, antibiotic sensitivity, and biofilm development. Two QS systems, known as the las and rhl systems, have been identified previously in the opportunistic pathogen Pseudomonas aeruginosa. High-density oligonucleotide microarrays for the P. aeruginosa PAO1 genome were used to investigate global gene expression patterns modulated by QS regulons. In the initial experiments we focused on identifying las and/or rhl QS-regulated genes using a QS signal generation-deficient mutant (PAO-JP2) that was cultured with and without added exogenous autoinducers [N-(3-oxododecanoyl) homoserine lactone and N-butyryl homoserine lactone]. Conservatively, 616 genes showed statistically significant differential expression (P </= 0.05) in response to the exogenous autoinducers and were classified as QS regulated. A total of 244 genes were identified as being QS regulated at the mid-logarithmic phase, and 450 genes were identified as being QS regulated at the early stationary phase. Most of the previously reported QS-promoted genes were confirmed, and a large number of additional QS-promoted genes were identified. Importantly, 222 genes were identified as being QS repressed. Environmental factors, such as medium composition and oxygen availability, eliminated detection of transcripts of many genes that were identified as being QS regulated.

Hydroxyectoine is superior to trehalose for anhydrobiotic engineering of Pseudomonas putida KT2440

Anhydrobiotic engineering aims to increase the level of desiccation tolerance in sensitive organisms to that observed in true anhydrobiotes. In addition to a suitable extracellular drying excipient, a key factor for anhydrobiotic engineering of gram-negative enterobacteria seems to be the generation of high intracellular concentrations of the nonreducing disaccharide trehalose, which can be achieved by osmotic induction. In the soil bacterium Pseudomonas putida KT2440, however, only limited amounts of trehalose are naturally accumulated in defined high-osmolarity medium, correlating with relatively poor survival of desiccated cultures. Based on the enterobacterial model, it was proposed that increasing intracellular trehalose concentration in P. putida KT2440 should improve survival. Using genetic engineering techniques, intracellular trehalose concentrations were obtained which were similar to or greater than those in enterobacteria, but this did not translate into improved desiccation tolerance. Therefore, at least for some populations of microorganisms, trehalose does not appear to provide full protection against desiccation damage, even when present at high concentrations both inside and outside the cell. For P. putida KT2440, it was shown that this was not due to a natural limit in desiccation tolerance since successful anhydrobiotic engineering was achieved by use of a different drying excipient, hydroxyectoine, with osmotically preconditioned bacteria for which 40 to 60% viability was maintained over extended periods (up to 42 days) in the dry state. Hydroxyectoine therefore has considerable potential for the improvement of desiccation tolerance in sensitive microorganisms, particularly for those recalcitrant to trehalose.

The gene bglH present in the bgl operon of Escherichia coli, responsible for uptake and fermentation of beta-glucosides encodes for a carbohydrate-specific outer membrane porin

The cryptic gene bglH from the Escherichia coli chromosome was cloned into a tacOP-driven expression vector. The resulting plasmid was transferred into the porin-deficient E. coli strain KS26 and the protein was expressed by addition of IPTG. The BglH protein was localized in the outer membrane. It was purified to homogeneity using standard methods. Reconstitution experiments with lipid bilayer membranes defined BglH as a channel-forming component, i.e. it is an outer membrane porin. The single-channel conductance of BglH (560 pS in 1 M KCl) was only one-third of that of the general diffusion porins of E. coli outer membrane. The presence of carbohydrates in the aqueous phase led to a dose-dependent block of ion transport through the channel, similar to that found for LamB (maltoporin) of E. coli and Salmonella typhimurium, which means that BglH is a porin specific for the uptake of carbohydrates. The binding constants of a variety of different carbohydrates were calculated from titration experiments of the BglH-induced membrane conductance. The tightest binding was observed with the aromatic beta-D-glucosides arbutin and salicin, and with gentibiose and cellobiose. Binding of maltooligosaccharides to BglH was in contrast to their binding to LamB in that it was much weaker, indicating that the binding site of BglH for carbohydrates is different from that of LamB (maltoporin). The kinetics of cellopentaose binding to BglH was investigated using the carbohydrate-induced current noise and was compared with that of cellopentaose binding to LamB (maltoporin) and ScrY (sucroseporin).

Channel specificity and secondary structure of the glucose-inducible porins of Pseudomonas spp

The OprB porin-mediated glucose transport system was investigated in Pseudomonas chlororaphis, Burkholderia cepacia, and Pseudomonas fluorescens. Kinetic studies of [U-14C]glucose uptake revealed an inducible system of low Km values (0.3-5 microM) and high specificity for glucose. OprB homologs were purified and reconstituted into proteoliposomes. The porin function and channel preference for glucose were demonstrated by liposome swelling assays. Examination of the periplasmic glucose-binding protein (GBP) components by Western immunoblotting using P. aeruginosa GBP-specific antiserum revealed some homology between P. aeruginosa GBP and periplasmic proteins from P. fluorescens and P. chlororaphis but not B. cepacia. Circular dichroism spectropolarimetry of purified OprB-like porins from the three species revealed beta sheet contents of 31-50% in agreement with 40% beta sheet content for the P. aeruginosa OprB porin. These findings suggest that the high-affinity glucose transport system is primarily specific for glucose and well conserved in the genus Pseudomonas although its outer membrane component may differ in channel architecture and specificity for other carbohydrates.

Bacterial genetic loci implicated in the Pseudomonas putida GR12-2R3--canola mutualism: identification of an exudate-inducible sugar transporter

Pseudomonas putida GR12-2R3 promotes the emergence and growth of diverse plant species. Analyses of TnphoA insertion mutations are revealing bacterial characteristics pertinent to the plant-microbe interaction. Pseudomonas putida PG269 is a TnphoA insertion derivative of GR12-2R3 that expresses canola seed exudate-inducible alkaline phosphatase (PhoA) activity. It promoted the growth of canola roots, as well as strain GR12-2R3, and outgrew its parent when they were cocultured in the presence of canola roots or in liquid seed exudate medium. (In contrast, mutant PG126 failed to promote canola root growth and was outgrown by its parent strain.) The PhoA activity of strain PG269 was induced by glucosamine and other sugars; glucosamine inhibited the growth of strain GR12-2R3 and stimulated the growth of strain PG269. Strain PG269 contained two TnphoA insertions: seiA1::TnphoA and seiB1::TnphoA. Strain PG312, which contained only insertion seiA1::TnphoA, shared all aspects of the PG269 phenotype, except the ability to outcompete strain GR12-2R3 during coculture. Insertion seiA1::TnphoA interrupted an open reading frame related in sequence to members of the MalF family of sugar transporter subunits. The PhoA-inducing fraction of canola seed exudate was hydrophilic, low in molecular weight, and heat stable. It cochromatographed with basic amino acids and amino sugars, and was inactivated by strains GR12-2R3 and PG269. Gene seiA may encode a subunit of an ABC transporter with broad specificity for glucose and related sugars whose expression can be induced by exudate sugars.

Isolation and characterization of a family of porin proteins from Helicobacter pylori

Two-dimensional gel electrophoresis was used to identify heat-modifiable outer membrane proteins, which were candidates for porins, from Helicobacter pylori membrane preparations. Four such proteins with apparent molecular masses of 48, 49, 50, and 67 kDa were isolated. The four proteins copurified together after selective detergent solubilizations followed by anion-exchange chromatography, and each protein was ultimately purified to homogeneity by gel purification. These proteins were then tested for pore-forming ability with a planar lipid bilayer model membrane system. All four proteins appeared to be present as monomers, and they formed pores with low single-channel conductances in 1.0 M KCl of 0.36, 0.36, 0.30, and 0.25 nS, respectively, for the 48-, 49-, 50-, and 67-kDa proteins which we propose to designate HopA, HopB, HopC, and HopD. N-terminal amino acid sequence analyses showed a high degree of homology among all four proteins, and it appears that these proteins constitute a family of related porins in H. pylori.

Cloning and nucleotide sequence of the Pseudomonas aeruginosa glucose-selective OprB porin gene and distribution of OprB within the family Pseudomonadaceae

OprB is a glucose-selective porin known to be produced by Pseudomonas aeruginosa and Pseudomonas putida. We have cloned and sequenced the oprB gene of P. aeruginosa and obtained expression of OprB in Escherichia coli. The mature protein consists of 423 amino acid residues with a deduced molecular mass of 47597 Da. Several clusters of amino acid residues, potentially involved in the structure or function of the protein, were identified. An area of regional homology with E. coli LamB was also identified. Carbohydrate-inducible proteins, potentially homologous to OprB, were identified in several rRNA homology-group-I pseudomonads by sodium dodecyl sulfate/polyacrylamide gel electrophoresis analysis, Western immunoblotting and N-terminal amino acid sequencing. These species also contained DNA that hybridized to a P. aeruginosa oprB gene probe.

Biophysical characterization of OprB, a glucose-inducible porin of Pseudomonas aeruginosa

OprB, a glucose-inducible porin of P. aeruginosa, was characterized by black lipid bilayer analysis and circular dichroism spectroscopy. Black lipid bilayer analysis of OprB revealed a single-channel conductance of 25 pS, the presence of a glucose binding site with a Ks for glucose of 380 +/- 40 mM, and the formation of channels with a strong selection for anions. Analysis of P. aeruginosa OprB circular dichroism spectra revealed a high beta sheet content (40%) which is within the range of that determined for other porins. Values obtained from black lipid bilayer analysis were compared to those previously obtained for OprB of P. putida [Saravolac et al. (1991). J. Bacteriol. 173, 4970-4976] and indicated extensive similarities in the single-channel conductance and glucose-binding properties of these two porins. Immunological and amino terminal sequence analysis revealed a high degree of homology. Of the first 14 amino terminal residues, 12 were identical. A major difference between the two porins was found in their ion selectivity. Whereas P. aeruginosa OprB is anion selective, P. putida OprB and other carbohydrate selective porins are known to be cation selective.

Purification of Aeromonas hydrophila major outer-membrane proteins: N-terminal sequence analysis and channel-forming properties

Four outer-membrane proteins of Aeromonas hydrophila were purified and their N-terminal sequences and channel-forming properties were determined. Three could be matched with proteins from other species. One was a maltoporin, as its level increased when cells were grown in maltose-containing media, and the channel it formed was blocked by maltose. Another was like OmpF and OmpC of Escherichia coli, except that its channel fluctuated much more rapidly. The third protein, which was produced in low-phosphate medium, exhibited several properties of the general anion porin PhoE. The fourth showed no similarity to any known proteins. It had a unique N-terminus and it formed small sharply-defined cation-selective channels. Two other proteins which corresponded to OmpW of Vibrio cholerae and E. coli OmpA were partly characterized.