Reconstitution of maltose transport in Escherichia coli: conditions affecting import of maltose-binding protein into the periplasm of calcium-treated cells

A single intact ATPase site of the ABC transporter BtuCD drives 5% transport activity yet supports full in vivo vitamin B12 utilization

In all kingdoms of life, ATP binding cassette (ABC) transporters are essential to many cellular functions. In this large superfamily of proteins, two catalytic sites hydrolyze ATP to power uphill substrate translocation. A central question in the field concerns the relationship between the two ATPase catalytic sites: Are the sites independent of one another? Are both needed for function? Do they function cooperatively? These issues have been resolved for type I ABC transporters but never for a type II ABC transporter. The many mechanistic differences between type I and type II ABC transporters raise the question whether in respect to ATP hydrolysis the two subtypes are similar or different. We have addressed this question by studying the Escherichia coli vitamin B12 type II ABC transporter BtuCD. We have constructed and purified a series of BtuCD variants where both, one, or none of the ATPase sites were rendered inactive by mutation. We find that, in a membrane environment, the ATPase sites of BtuCD are highly cooperative with a Hill coefficient of 2. We also find that, when one of the ATPase sites is inactive, ATP hydrolysis and vitamin B12 transport by BtuCD is reduced by 95%. These exact features are also shared by the archetypical type I maltose ABC transporter. Remarkably, mutants that have lost 95% of their ATPase and transport capabilities still retain the ability to fully use vitamin B12 in vivo. The results demonstrate that, despite the many differences between type I and type II ABC transporters, the fundamental mechanism of ATP hydrolysis remains conserved.

Calcium chloride made E. coli competent for uptake of extraneous DNA through overproduction of OmpC protein

In the standard method of transformation of Escherichia coli with extraneous DNA, cells are made competent for DNA uptake by incubating in ice-cold 100 mM CaCl(2). Analysis of the whole protein profile of CaCl(2)-treated E. coli cells by the techniques of one- and two-dimensional gel electrophoresis, MALDI-MS and immunoprecipitation revealed overproduction of outer membrane proteins OmpC, OmpA and heat-shock protein GroEL. In parity, transformation efficiency of E. coli ompC mutant by plasmid pUC19 DNA was found to be about 40 % lower than that of the wild type strain. Moreover, in E. coli cells containing groEL-bearing plasmid, induction of GroEL caused simultaneous overproduction of OmpC. On the other hand, less OmpC was synthesized in E. coli groEL mutant compared to its wild type counterpart, by CaCl(2)-shock. From these results it can be suggested that in the process of CaCl(2)-mediated generation of competence, the heat-shock chaperone GroEL has specific role in DNA entry into the cell, possibly through the overproduced OmpC and OmpA porins.

Characterization of transmembrane segments 3, 4, and 5 of MalF by mutational analysis

MalF and MalG are the cytoplasmic membrane components of the binding protein-dependent ATP binding cassette maltose transporter in Escherichia coli. They are thought to form the transport channel and are thus of critical importance for the mechanism of transport. To study the contributions of individual transmembrane segments of MalF, we isolated 27 point mutations in membrane-spanning segments 3, 4, and 5. These data complement a previous study, which described the mutagenesis of membrane-spanning segments 6, 7, and 8. While most of the isolated mutations appear to cause assembly defects, L(323)Q in helix 5 could interfere more directly with substrate specificity. The phenotypes and locations of the mutations are consistent with a previously postulated structural model of MalF.

Characterization of transmembrane domains 6, 7, and 8 of MalF by mutational analysis

Oligonucleotide mutagenesis was used to isolate mutations in membrane-spanning segments 6, 7, and 8 of MalF. MalF is a cytoplasmic membrane component of the binding protein-dependent maltose transport system in Escherichia coli. The current structural model predicts eight transmembrane domains for MalF. Membrane-spanning segments 6, 7, and 8 of MalF flank or are part of the EAA-X3-G-X9-I-X-LP consensus region present in the cytoplasmic membrane subunits of the bacterial ABC transporter superfamily members. Mutations with two novel phenotypes with respect to substrate specificity of the maltose transport system were isolated. One mutant grew on minimal maltose media but not on media containing either maltoheptaose or maltoheptaose plus maltose and was thus termed dextrin dominant negative. The other class of mutations led to a maltose minus but maltoheptaose plus phenotype. Nine of the isolated mutations leading to changes in substrate specificity were tightly clustered on one face of the postulated transmembrane helix 6. A similar clustering of mutations was detected in transmembrane domain 7. The majority of mutations in membrane-spanning segment 7 led to a protease-sensitive or a conditional phenotype with respect to MalF function or both. Mutations in transmembrane domain 8 appeared to be more randomly distributed. The majority of mutations in membrane-spanning segment 8 caused a Mal+ Dex- phenotype. Six Mal+ suppressor mutations isolated to two mutations in transmembrane domain 7 changed amino acid residues in membrane-spanning segment 6 or 8.

C9-mediated killing of bacterial cells by transferred C5b-8 complexes: transferred C5b-9 complexes are nonbactericidal

The formation of the C5b-9 complex on the outer membrane of complement-sensitive cells of Escherichia coli results in inhibition of inner membrane function and the death of the cell. Cells bearing a precursor of the C5b-9 site, the C5b-8 complex, suffer no loss in viability. Antibiotic-sensitive, complement-sensitive donor cells bearing precursor C5b-8 complexes were incubated with equal numbers of antibiotic-resistant, complement-sensitive acceptor cells that had not been exposed to a complement source. This cell mixture was incubated with 5 mM EDTA for 5 min and then with calcium chloride (20 mM) for various times. The excess calcium ion concentration was effectively reduced with additional EDTA, and the cell mixture was washed and resuspended in buffer. The viability of the acceptor cells was assayed by plating on antibiotic-containing media. C9 was added to the mixture, and the mixture was incubated for 10 min at 37 degrees C and then plated as described above. It was found that the acceptor cells were killed by the addition of purified C9 only after incubation with donor cells bearing C5b-8 sites during the transfer procedure. This indicates that precursor C5b-8 sites that support C9-mediated killing could be transferred between cells. No loss in viability was detected for acceptor cells subjected to the procedure described above in the presence of donor cells bearing complete C5b-9 complexes, formed prior to mixing with acceptor cells for the transfer procedure.

Testing models for transport systems dependent on periplasmic binding proteins

A carrier model in which transport across the cytoplasmic membrane is mediated by a periplasmic binding protein (Krupka, R.M. (1992) Biochim. Biophys. Acta 1110, 1-10) is shown to account for many of the properties of these systems: (i) Michaelis-Menten kinetics; (ii) seemingly irreversible uptake; (iii) the absence of exchange transport and counter-transport; (iv) substrate half-saturation constants that in different systems may be lower or higher than the dissociation constant of the binding protein; (v) the high concentration of the binding protein in the periplasm and its weak association with the membrane component. The binding protein appears to function as a valve or rectifier that permits the substrate to enter the cell, but blocks exit in both the energized and de-energized states. The asymmetry depends on both the abruptness and the extent of the conformational change in the binding protein. Characteristically, these systems build up steep gradients across the membrane, circumstances in which such a valve might be important. In agreement with the mechanism, (a) the binding protein is missing in members of the same family of transporters that function in export of the substrate rather than import; and (b) in Gram-positive organisms, which have no periplasmic space, binding proteins function while anchored to the cytoplasmic membrane.

Kinetics of transport systems dependent on periplasmic binding proteins

Rate equations are derived for a transport model involving a water-soluble binding protein outside the plasma membrane. On addition of the substrate, the conformation of the binding protein changes; the complex then combines with the membrane carrier, transferring the substrate to the carrier site. The free binding protein leaves and the carrier shifts inward, releasing the substrate inside the cell. Exit follows the reverse path. The predicted behaviour is as follows. (i) Uptake does not necessarily conform to Michaelis-Menten kinetics. (ii) In both the energized and de-energized states, the maximum rate of exit is far lower than that of entry; the asymmetry is determined by the conformational change in the binding protein, which is independent of the energy state of the system. (iii) Exchange transport is inhibited by external substrate and is extremely slow; consequently counter-transport is not expected. (iv) The half-saturation constant in uptake can differ from the dissociation constant of the binding protein. (v) The maximum rate of uptake depends on the intrinsic substrate affinity of the membrane carrier relative to that of the binding protein. (vi) The maximum rate of uptake and the substrate half-saturation constant depend on the concentration of the binding protein.

The malX malY operon of Escherichia coli encodes a novel enzyme II of the phosphotransferase system recognizing glucose and maltose and an enzyme abolishing the endogenous induction of the maltose system

Mutants lacking MalK, a subunit of the binding protein-dependent maltose-maltodextrin transport system, constitutively express the maltose genes. A second site mutation in malI abolishes the constitutive expression. The malI gene (at 36 min on the linkage map) codes for a typical repressor protein that is homologous to the Escherichia coli LacI, GalR, or CytR repressor (J. Reidl, K. Römisch, M. Ehrmann, and W. Boos, J. Bacteriol. 171:4888-4899, 1989). We now report that MalI regulates an adjacent and divergently oriented operon containing malX and malY. MalX encodes a protein with a molecular weight of 56,654, and the deduced amino acid sequence of MalX exhibits 34.9% identity to the enzyme II of the phosphototransferase system for glucose (ptsG) and 32.1% identity to the enzyme II for N-acetylglucosamine (nagE). When constitutively expressed, malX can complement a ptsG ptsM double mutant for growth on glucose. Also, a delta malE malT(Con) strain that is unable to grow on maltose due to its maltose transport defect becomes Mal+ after introduction of malI::Tn10 and the plasmid carrying malX. MalX-mediated transport of glucose and maltose is likely to occur by facilitated diffusion. We conclude that malX encodes a phosphotransferase system enzyme II that can recognize glucose and maltose as substrates even though these sugars may not represent the natural substrates of the system. The second gene in the operon, malY, encodes a protein of 43,500 daltons. Its deduced amino acid sequence exhibits weak homology to aminotransferase sequences. The presence of plasmid-encoded MalX alone was sufficient for complementing growth on glucose in a ptsM ptsG glk mutant, and the plasmid-encoded MalY alone was sufficient to abolish the constitutivity of the mal genes in a malK mutant. The overexpression of malY in a strain that is wild type with respect to the maltose genes strongly interferes with growth on maltose. This is not the case in a malT(Con) strain that expresses the mal genes constitutively. We conclude that malY encodes an enzyme that degrades the inducer of the maltose system or prevents its synthesis.

Genetic analysis of membrane protein topology by a sandwich gene fusion approach

We describe a cloning vector that allows the construction of phoA sandwich fusions in which mature alkaline phosphatase is inserted into target proteins. In contrast to previous fusions obtained using the TnphoA transposon, the entire amino acid sequence of the target protein is present in the fusion product. We have constructed a series of sandwich fusions of alkaline phosphatase to the multispanning cytoplasmic membrane protein MalF. Despite the fact that the alkaline phosphatase was tethered to MalF at both its N and its C terminus, the enzyme exhibited high activity when it was fused to a periplasmic domain of the membrane protein. Cells harboring an alkaline phosphatase sandwich fusion to the end of the first membrane-spanning segment of MalF exhibited both MalF and alkaline phosphatase activity. When alkaline phosphatase was inserted into a cytoplasmic domain of MalF, its specific activity was very low. Our results suggest that the alkaline phosphatase activity of phoA sandwich fusions provides a more sensitive monitor than previous methods of the cellular localization of the domain of the target protein to which the enzyme is fused. Thus, the sandwich fusion approach can give a more accurate picture of membrane protein topology.

Osmolability of Escherichia coli and modification of [125I]ampicillin-binding by competence induction for uptake of transforming DNA

The regimen conferring competence for uptake of transforming DNA is shown to render Escherichia coli osmolabile. Three different K-12 strains were exposed to the standard procedure of competence induction, i.e. incubation in the presence of 0.1 M Ca2+ or Mg2+ for 50 min at 0 degrees C, interrupted by a heat shock for 5 min at 37 degrees C. Upon osmotic challenge of competent cells formation of protoplasts was observed in approximately 2% of the treated cells. Incubation of competent cells of strain W1485 in phosphate-buffered saline for 1, 2, and 3 h reduced the viable counts to 67, 58, and 41%, respectively. Competence induction with divalent cations altered the affinity of penicillin-binding proteins (PBPs) for [125I]ampicillin. In isolated cell envelopes the presence of Ca2+ and Mg2+ stimulated the binding of [125I]ampicillin to PBPs 1, 3, 4, 5, and 6, whereas the binding to PBP 2 remained unchanged. The binding to PBP 1 C was inhibited by 0.23 M Ca2+. In living cells the binding to PBPs 1, 3, and 4 was enhanced, while the binding to PBP 8 was inhibited. Newly [125I]ampicillin-labelled proteins of Mr 55,000 and 45,000 were apparent, especially after competence induction with Ca2+. Interaction of divalent cations with PBPs is suggested to contribute to osmolability of competent cells. Disintegration of the cell wall may be necessary for uptake of transforming DNA.

Fusion of small unilamellar vesicles with viable EDTA-treated Escherichia coli cells

Fusion characteristics of EDTA-treated Escherichia coli cells with small unilamellar vesicles were investigated, using a membrane fusion assay based on resonance energy transfer. Ca2+-EDTA treatments of Escherichia coli O111:B4 (wild type), E. coli C600 (rough), and E. coli D21f2 (deep rough) which permeabilize the outer membrane by inducing the release of lipopolysaccharide and outer membrane proteins resulted in fusion activity of the intact and viable bacteria with small unilamellar vesicles. No fusion activity was observed when the EDTA treatment was omitted. Fusion could be elicited at low pH and by a combination of a higher pH and Ca2+. The low-pH-induced fusion was composed of a fast and a slow reaction. The latter and the Ca2+-induced fusion could be completely inhibited by trypsin treatments of the EDTA-treated cells, which also resulted in the simultaneous disappearance of two outer membrane protein bands (50 and 58 kilodaltons) and the appearance of proteins banding at 22, 52, and 54 kilodaltons. The most efficient fusion was obtained with negatively charged liposomes composed of cardiolipin. In contrast to the Ca2+-induced fusion, fusion was observed at low pH with small unilamellar vesicles containing lipids with decreased negative charge (phosphatidylserine). Fluorescent and phase-contrast microscopy revealed that essentially all bacteria were engaged in fusion. We propose that a Ca2+-EDTA treatment of E. coli cells results in the appearance of phospholipids and the exposure of a protein(s) in the outer leaflet of the outer membrane, both of which could mediate fusion with liposomes.

Putative structure and functions of a poly-beta-hydroxybutyrate/calcium polyphosphate channel in bacterial plasma membranes

A poly-beta-hydroxybutyrate complex extracted from the plasma membranes of genetically competent Escherichia coli contained polyhydroxybutyrate:polyphosphate:calcium in molar ratios approximating 1:1:0.5. The chain length of the polyhydroxybutyrate was estimated as 120-200 subunits, and that of the polyphosphate was estimated as 130-170 subunits. The extracted complex, when incorporated into liposomes, exhibited a lipid phase transition in the same temperature range as that of the membrane complex in whole cells as well as the same properties of irreversibility, lability, and sensitivity to chelating buffers. Space-filling molecular models and molecular energy minimization methods (Charmm) were used to develop and evaluate a plausible structure for the complex. It is proposed that the polyhydroxybutyrate forms an exolipophilic-endopolarophilic helix around an inner framework helix of calcium polyphosphate. The calcium ions link the two polymers by forming ionic bonds with phosphoryl oxygens of the polyphosphate and ion-dipole bonds with the ester carbonyl oxygens of the polyhydroxybutyrate. This symmetrical structure forms a channel through the membrane and may play a role in the transport of calcium, phosphate, and DNA.

Sequence of the mglB gene from Escherichia coli K12: comparison of wild-type and mutant galactose chemoreceptors

The mglB gene of Escherichia coli codes for a galactose-binding protein (GBP) that serves both as the galactose chemoreceptor and as the recognition component of the beta-methylgalactoside transport system. The mglB551 mutation eliminates the chemotactic function of GBP without altering its transport or substrate-binding properties. To investigate the interaction between GBP and Trg, the chemotactic signal transducer for galactose, we sequenced the mglB genes from wild-type and mglB551 mutant strains. The mutation causes the replacement of Gly74 of GBP by Asp. This residue is located in alpha-Helix III at the tip of the P domain in the GBP tertiary structure farthest removed from the substrate-binding cleft between the P and Q domains. We conclude that Helix III must be part of, or at least adjacent to, the recognition site for Trg. Our sequence also included part of the mglA gene, which is immediately distal to mglB. The amino acid sequence deduced for the beginning of the MglA protein showed homology with a family of polypeptides that contain an ATP-binding site and are components of binding-protein-dependent transport systems.

The effect of polymyxin B nonapeptide (PMBN) on transformation

The effect of the outer membrane permeabilizing polycation, polymyxin B nonapeptide (PMBN) on the transformation of E. coli HB101 with pBR322 plasmid DNA was investigated. Pretreatment of cells with PMBN (followed by suspending the cells in PMBN-free medium) did not stimulate the development of competence induced by the calcium heat pulse. In the absence of calcium-ions, a high PMBN concentration (1 mM) was able to induce a low transformation frequency provided that PMBN was not removed before the addition of DNA.

3-Azi-1-methoxybutyl D-maltooligosaccharides specifically bind to the maltose/maltooligosaccharide-binding protein of Escherichia coli and can be used as photoaffinity labels

Maltooligosaccharides with two to six (alpha 1-4)-linked glucose residues, carrying at their reducing end a 3-azi-1-methoxybutyl group in either alpha or in beta glycosidic linkage, were synthesized. These maltooligosaccharide analogues inhibit maltose uptake via the maltose-binding-protein-dependent transport system in Escherichia coli. The concentration of half-maximal inhibition of maltose transport, at 15 nM concentration, decreases with increasing chain length of the analogue, levelling off at 40 microM after a chain length of four glucose residues in the alpha series and at 350 microM after a chain length of three glucose residues in the beta series. The inhibition of maltose transport occurs at the level of the periplasmic maltose-binding protein. 3-Azi-1-methoxybutyl alpha-D-[3H]maltotrioside was bound by the maltose-binding protein with a Kd of 0.18 mM. Irradiation at 350 nm of purified maltose-binding protein in the presence of 4 microM of this substrate labeled the protein covalently; labeling was prevented by 1 mM maltose. Using a crude preparation of periplasmic proteins two proteins were labeled, the maltose-binding protein and alpha-amylase. Thus, 3-azi-1-methoxybutyl alpha-D-maltooligosaccharides are potent photoaffinity labels for proteins with maltooligosaccharides-binding sites.

Osmoregulation of the maltose regulon in Escherichia coli

The maltose regulon consists of four operons that direct the synthesis of proteins required for the transport and metabolism of maltose and maltodextrins. Expression of the mal genes is induced by maltose and maltodextrins and is dependent on a specific positive regulator, the MalT protein, as well as on the cyclic AMP-catabolite gene activator protein complex. In the absence of an exogenous inducer, expression of the mal regulon was greatly reduced when the osmolarity of the growth medium was high; maltose-induced expression was not affected, and malTc-dependent expression was only weakly affected. Mutants lacking MalK, a cytoplasmic membrane protein required for maltose transport, expressed the remaining mal genes at a high level, presumably because an internal inducer of the mal system accumulated; this expression was also strongly repressed at high osmolarity. The repression of mal regulon expression at high osmolarity was not caused by reduced expression of the malT, envZ, or crp gene or by changes in cellular cyclic AMP levels. In strains carrying mutations in genes encoding amylomaltase (malQ), maltodextrin phosphorylase (malP), amylase (malS), or glycogen (glg), malK mutations still led to elevated expression at low osmolarity. The repression at high osmolarity no longer occurred in malQ mutants, however, provided that glycogen was present.

Lateral diffusion of proteins in the periplasm of Escherichia coli

We have introduced biologically active, fluorescently labeled maltose-binding protein into the periplasmic space of Escherichia coli and measured its lateral diffusion coefficient by the fluorescence photobleaching recovery method. Diffusion of this protein in the periplasm was found to be surprisingly low (lateral diffusion coefficient, 0.9 X 10(-10) cm2 s-1), about 1,000-fold lower than would be expected for diffusion in aqueous medium and almost 100-fold lower than for an equivalent-size protein in the cytoplasm. Galactose-binding protein, myoglobin, and cytochrome c were also introduced into the periplasm and had diffusion coefficients identical to that determined for the maltose-binding protein. For all proteins nearly 100% recovery of fluorescence was obtained after photobleaching, indicating that the periplasm is a single contiguous compartment surrounding the cell. These data have considerable implications for periplasmic structure and for the role of periplasmic proteins in transport and chemotaxis.

Interspecific reconstitution of maltose transport and chemotaxis in Escherichia coli with maltose-binding protein from various enteric bacteria

In Escherichia coli, the periplasmic maltose-binding protein (MBP), the product of the malE gene, is the primary recognition component of the transport system for maltose and maltodextrins. It is also the maltose chemoreceptor, in which capacity it interacts with the signal transducer Tar (taxis to aspartate and some repellents). In studies of the maltose system in other members of the family Enterobacteriaceae, we found that MBP is produced by Salmonella typhimurium, Klebsiella pneumoniae, Enterobacter aerogenes, and Serratia marcescens. MBP from all of these species cross-reacted with antibody against the E. coli protein and had a similar molecular weight (about 40,000). The Shigella flexneri and Proteus mirabilis strains we examined did not synthesize MBP. The isoelectric points of MBP from different species varied from the acid extreme of E. coli (4.8) to the basic extreme of E. aerogenes (8.9). All species with MBP transported maltose with high affinity, although the Vmax for K. pneumoniae was severalfold lower than that for the other species. Maltose chemotaxis was observed only in E. coli and E. aerogenes. In S. typhimurium LT2, Tar was completely inactive in maltose taxis, although it signaled normally in response to aspartate. MBP isolated from all five species could be used to reconstitute maltose transport and taxis in a delta malE strain of E. coli after permeabilization of the outer membrane with calcium.

Characterization of the Salmonella typhimurium mgl operon and its gene products

In Salmonella typhimurium and Escherichia coli the high-affinity galactose transport system, which contains a periplasmic galactose-binding protein as an essential component, is encoded by the mgl genes. The entire mgl region of S. typhimurium is contained on a 6.3-kilobase EcoRI restriction fragment, which has been cloned into plasmid vectors. We determined the extent of the mgl region on this fragment by Tn5 mutagenesis, examination of lacZ fusions to mgl genes, and subcloning smaller restriction fragments. Polyacrylamide gel electrophoresis of protein preparations derived from strains carrying different plasmids was used to identify the mgl gene products. We conclude that the mgl operon consists of four genes that form a single transcription unit: mglB, mglA, mglE, and mglC. The mglB gene codes for galactose-binding protein (33,000 daltons), mglA codes for a membrane-bound protein of 51,000 daltons, and mglC codes for a 29,000-dalton membrane protein. The mglE product was less well characterized. Its existence was inferred from a mglE-lacZ protein fusion located between mglA and mglC. In addition, the coupled transcription-translation in vitro system indicated that mglE codes for a 21,000-dalton protein.

Ca2+-induced permeabilization of the Escherichia coli outer membrane: comparison of transformation and reconstitution of binding-protein-dependent transport

Ca2+ treatment renders the outer membrane of Escherichia coli reversibly permeable for macromolecules. We investigated whether Ca2+-induced uptake of exogenous protein into the periplasm occurs by mechanisms similar to Ca2+-induced uptake of DNA into the cytoplasm during transformation. Protein import through the outer membrane was monitored by measuring reconstitution of maltose transport after the addition of shock fluid containing maltose-binding protein. DNA import through the outer and inner membrane was measured by determining the efficiency of transformation with plasmid DNA. Both processes were stimulated by increasing Ca2+ concentrations up to 400 mM. Plasmolysis was essential for a high efficiency; reconstitution and transformation could be stimulated 5- and 40-fold, respectively, by a high concentration of sucrose (400 mM) in cells incubated with a suboptimal Ca2+ concentration (50 mM). The same divalent cations that promote import of DNA (Ca2+, Ba2+, Sr2+, Mg2+, and Ni2+) also induced import of protein. Ca2+ alone was found to be inefficient in promoting reconstitution; successive treatment with phosphate and Ca2+ ions was essential. Transformation also was observed in the absence of phosphate, but could be stimulated by pretreatment with phosphate. The optimal phosphate concentrations were 100 mM and 1 to 10 mM for reconstitution and transformation, respectively. Heat shock, in which the cells are rapidly transferred from 0 to 42 degrees C, affected the two processes differently. Incubation of cells at 0 degrees C in Ca2+ alone allows rapid entry of protein, but not of DNA. Transformation was observed only when exogenous DNA was still present during the heat shock. Shock fluid containing maltose-binding protein inhibited transformation (with 6 microgram of DNA per ml, half-maximal inhibition occurred at around 300 microgram of shock fluid per ml). DNA inhibited reconstitution (with 5 microgram of shock fluid per ml, half-maximal inhibition occurred at around 3 mg of DNA per ml).

Effect of Reactor Turbulence on the Binding-Protein-Mediated Aspartate Transport System in Thin Wastewater Biofilms

This research documents an effect of reactor turbulence on the ability of gram-negative wastewater biofilm bacteria to actively transport l -aspartate via a binding-protein-mediated transport system. Biofilms which were not preadapted to turbulence and which possessed two separate and distinct aspartate transport systems (systems 1 and 2) were subjected to a turbulent flow condition in a hydrodynamically defined closed-loop reactor system. A shear stress treatment of 3.1 N · m −2 for 10 min at a turbulent Reynolds number (Re = 11,297) inactivated the low-affinity, high-capacity binding-protein-mediated transport system (system 2) and resolved the high-affinity, low-capacity membrane-bound proton symport system (system 1). The K t and V max values for the resolved system were statistically similar to K t and V max values for system 1 when system 2 was inactivated either by osmotic shock or arsenate, two treatments which are known to inactivate binding-protein-mediated transport systems. We hypothesize that shear stress disrupts system 2 by deforming the outer membranes of the firmly adhered gram-negative bacteria.

The in vivo formation of nonbilayer lipid phase in E. coli membranes during the development of Ca2-dependent competence

31P-NMR studies on E. coli cells reveal the in vivo formation of nonbilayer lipid structures, presumably of H11 phase, during Ca2-dependent competence induction. The data suggest the involvement of these structures in the exogenous DNA transfer into the cells during genetic transformation and transfection. The suggestion is supported by in vitro experiments in which the liposomes composed of different phospholipid species bind 14C-DNA in DNase and wash resistant form in conditions promoting the hexagonal phase formation.

Maltose-binding protein does not modulate the activity of maltoporin as a general porin in Escherichia coli

Maltoporin (lambda receptor) is part of the maltose transport system in Escherichia coli and is necessary for the facilitated diffusion of maltose and maltodextrins across the outer membrane. Maltoporin also allows the diffusion of nonmaltodextrin substrates, albeit with less efficiency. The preference of maltoporin for maltodextrins in vivo is thought to be the result of an interaction of maltoporin with the maltose-binding protein, the malE gene product. In a recent report Heuzenroeder and Reeves (J. Bacteriol. 144:431-435, 1980) suggested that this interaction establishes a gating mechanism which inhibits the diffusion of nonmaltodextrin substrates, such as lactose. To reinvestigate this important conclusion, we constructed ompR malTc strains carrying either the malE+ gene, the nonpolar malE444 deletion, or the malE254 allele, which specifies an interaction-deficient maltose-binding protein. Lactose uptake was measured at different concentrations below the Km of this transport system and under conditions where transport was limited by the diffusion through maltoporin. We found no difference in the kinetics of lactose uptake irrespective of the malE allele. We conclude that the maltose-binding protein does not modulate the activity of maltoporin as a general outer membrane porin.

Temperature-sensitive catabolite activator protein in Escherichia coli BUG6

BUG6 is a temperature-sensitive cell division mutant which forms filaments at the nonpermissive temperature. Synthesis of the maltose- and galactose-binding protein-dependent transport systems is also temperature sensitive in BUG6. Using operon and protein fusions of the maltose transport genes to lacZ, we observed that the temperature-sensitive control of the maltose transport system in BUG6 occurs at the transcriptional level. By P1-mediated transductions, we found that BUG6 contains two independent temperature-sensitive mutations. One maps between 2 and 3 min on the Escherichia coli linkage map, in close proximity to the fts-envA region. This mutation is responsible for temperature-sensitive cell division. The other mutation maps at 73 min in crp, the structural gene of the catabolite activator protein. The latter could be complemented by a hybrid plasmid carrying the wild-type crp as the only gene on a 0.9-kilobase HindIII-AluI restriction fragment. The mutation in crp alone was found to be responsible for the temperature-sensitive synthesis of the maltose transport system. Although it causes a complete block of transcription of the maltose transport genes at 41 degrees C, this mutation had only a marginal effect on the transcription of the lac operon.

Transposon Tn10-dependent expression of the lamB gene in Escherichia coli

Among Tn10 insertions isolated in or near the malB region of Escherichia coli, one (zjb-729::Tn10) mapped between malK and lamB or late in malK and allowed MalT-independent expression of lamB. Tn10-dependent expression of a lamB-lacZ protein fusion was 25% of the expression of the fusion from the malK-lamB operon promoter in malTc constitutive strains. The maltoporin content of a strain carrying this Tn10 was about 20% that of a malTc malB+ strain. Transport of maltose at concentrations of below 10(-6) M was reduced about threefold. When maltoporin was present at about 50% of the level of malTc malB+ strains, maltose transport was largely restored. We conclude that maltoporin is not rate limiting for maltose transport in wild-type cells but becomes rate limiting when the ratio of maltoporin to other maltose transport components is reduced more than twofold.

Reconstitution of maltose chemotaxis in Escherichia coli by addition of maltose-binding protein to calcium-treated cells of maltose regulon mutants

Maltose chemotaxis was reconstituted in delta malE cells lacking maltose-binding protein (MBP). Purified MBP was introduced into intact cells during incubation with 250 mM CaCl2 in Tris-hydrochloride buffer at 0 degrees C. After removal of extracellular CaCl2 and MBP, chemotaxis was measured with tethered bacteria in a flow chamber or with free-swimming cells in a capillary assay. About 20% of tethered cells responded to 10(-4) M maltose; the mean response times were about half those of CaCl2-treated wild-type cells (100 s as opposed to 190 s). In capillary tests, the maltose response of reconstituted cells was between 15 and 40% of the aspartate response, about the same percentage as in wild-type cells. The best reconstitution was seen with 0.5 to 1 mM MBP in the reconstitution mixture, which is similar to the periplasmic MBP concentration estimated for maltose-induced wild-type cells. Strains containing large deletions of the malB region and malT mutants lacking the positive regulator gene of the mal regulon also could be reconstituted for maltose chemotaxis, showing that no product of the mal regulon other than MBP is essential for maltose chemotaxis.