Characterization of group B colicin-resistant mutants of Escherichia coli K-12: colicin resistance and the role of enterochelin

Iron Acquisition Systems of Gram-negative Bacterial Pathogens Define TonB-Dependent Pathways to Novel Antibiotics

Iron is an indispensable metabolic cofactor in both pro- and eukaryotes, which engenders a natural competition for the metal between bacterial pathogens and their human or animal hosts. Bacteria secrete siderophores that extract Fe3+ from tissues, fluids, cells, and proteins; the ligand gated porins of the Gram-negative bacterial outer membrane actively acquire the resulting ferric siderophores, as well as other iron-containing molecules like heme. Conversely, eukaryotic hosts combat bacterial iron scavenging by sequestering Fe3+ in binding proteins and ferritin. The variety of iron uptake systems in Gram-negative bacterial pathogens illustrates a range of chemical and biochemical mechanisms that facilitate microbial pathogenesis. This document attempts to summarize and understand these processes, to guide discovery of immunological or chemical interventions that may thwart infectious disease.

Plesiomonas shigelloides hugZ encodes an iron-regulated heme binding protein required for heme iron utilization

Plesiomonas shigelloides is an intestinal pathogen that uses heme as an iron source. The P. shigelloides heme utilization system consists of 10 genes, 7 of which permit heme transport and 3 of which are associated with utilization of heme as an iron source once it is inside the cell. The goal of this study was to examine hugZ, 1 of the 3 genes associated with utilization of heme iron. DPH8, a hugZ mutant, failed to grow to full cell density in media containing heme as the iron source, indicating that hugZ is required for heme iron utilization. Western blots using antibodies against Vibrio cholerae HutZ to detect the P. shigelloides HugZ indicated that hugZ encodes an iron-regulated cytoplasmic protein, which is absent in DPH8. A heme affinity bead assay performed on soluble protein fractions from P. shigelloides DPH8/pHUG24.5 (pHUG24.5 encodes hugZ) indicated that HugZ binds heme. Heme utilization was restored in DPH8 by hox1, which encodes the α-heme oxygenase from Synechocystis sp. strain PCC6803. However, HugZ did not exhibit α-heme oxygenase activity in an assay that detects the conversion of heme to the bilin functional group present in phycobiliproteins. These results do not rule out that HugZ exhibits another type of heme oxygenase activity not detected in the assay.

The colicin G, H and X determinants encode microcins M and H47, which might utilize the catecholate siderophore receptors FepA, Cir, Fiu and IroN

The colicin G producer Escherichia coli CA46, the colicin H producer E. coli CA58 and E. coli Nissle 1917 (DSM 6601) were shown to produce microcin H47 and the newly described microcin M. Both microcins were exported like colicin V by an RND-type export system, including TolC. The gene cluster encoding microcins H47 and M in strains CA46 and CA58 is nearly identical to that in strain DSM 6601, except that two additional genes are included. A Fur box identified in front of the microcin-encoding genes explained the observed iron regulation of microcin production. The catecholate siderophore receptors Fiu, Cir and FepA from E. coli and IroN, Cir and FepA from Salmonella were identified as receptors for microcins M, H47 and E492. IroN takes up the glucose-containing catecholate siderophore salmochelin, whose synthesis is encoded in the iro gene cluster found in Salmonella and certain, often uropathogenic, E. coli strains. A gene in this iro cluster, iroB, which encodes a putative glycosyltransferase, was also found in the microcin H47/M and microcin E492 gene clusters. These microcins could aid the producing strain in competing against enterobacteria that utilize catecholate siderophores.

Spectroscopic observations of ferric enterobactin transport

We characterized the uptake of ferric enterobactin (FeEnt), the native Escherichia coli ferric siderophore, through its cognate outer membrane receptor protein, FepA, using a site-directed fluorescence methodology. The experiments first defined locations in FepA that were accessible to covalent modification with fluorescein maleimide (FM) in vivo; among 10 sites that we tested by substituting single Cys residues, FM labeled W101C, S271C, F329C, and S397C, and all these exist within surface-exposed loops of the outer membrane protein. FeEnt normally adsorbed to the fluoresceinated S271C and S397C mutant FepA proteins in vivo, which we observed as quenching of fluorescence intensity, but the ferric siderophore did not bind to the FM-modified derivatives of W101C or F329C. These in vivo fluorescence determinations showed, for the first time, consistency with radioisotopic measurements of the affinity of the FeEnt-FepA interaction; K(d) was 0.2 nm by both methods. Analysis of the FepA mutants with AlexaFluor(680), a fluorescein derivative with red-shifted absorption and emission spectra that do not overlap the absorbance spectrum of FeEnt, refuted the possibility that the fluorescence quenching resulted from resonance energy transfer. These and other data instead indicated that the quenching originated from changes in the environment of the fluor as a result of loop conformational changes during ligand binding and transport. We used the fluorescence system to monitor FeEnt uptake by live bacteria and determined its dependence on ligand concentration, temperature, pH, and carbon sources and its susceptibility to inhibition by the metabolic poisons. Unlike cyanocobalamin transport through the outer membrane, FeEnt uptake was sensitive to inhibitors of electron transport and phosphorylation, in addition to its sensitivity to proton motive force depletion.

Ton-dependent colicins and microcins: modular design and evolution

Ton-dependent colicins and microcins are actively taken up into sensitive cells at the expense of energy which is provided by the proton motive force of the cytoplasmic membrane. The Ton system consisting of the proteins TonB, ExbB and ExbD is required for colicin and microcin import. Colicins as well as the outer membrane transport proteins contain proximal to the N-terminus a short sequence, called TonB box, which interacts with TonB and in which point mutants impair uptake. No TonB box is found in microcins. Colicins are composed of functional modules which during evolution have been interchanged resulting in new colicins. The modules define sites of interaction with the outer membrane transport genes, TonB, the immunity proteins, and the activity regions. Six TonB-dependent microcins with different primary structures are processed and exported by highly homologous proteins. Three of these microcins are modified in an unknown way and they have in common specificity for catecholate siderophore receptors.

Bacterial phage receptors, versatile tools for display of polypeptides on the cell surface

Four outer membrane proteins of Escherichia coli were examined for their capabilities and limitations in displaying heterologous peptide inserts on the bacterial cell surface. The T7 tag or multiple copies of the myc epitope were inserted into loops 4 and 5 of the ferrichrome and phage T5 receptor FhuA. Fluorescence-activated cell sorting analysis showed that peptides of up to 250 amino acids were efficiently displayed on the surface of E. coli as inserts within FhuA. Strains expressing FhuA fusion proteins behaved similarly to those expressing wild-type FhuA, as judged by phage infection and colicin sensitivity. The vitamin B(12) and phage BF23 receptor BtuB could display peptide inserts of at least 86 amino acids containing the T7 tag. In contrast, the receptors of the phages K3 and lambda, OmpA and LamB, accepted only insertions in their respective loop 4 of up to 40 amino acids containing the T7 tag. The insertion of larger fragments resulted in inefficient transport and/or assembly of OmpA and LamB fusion proteins into the outer membrane. Cells displaying a foreign peptide fused to any one of these outer membrane proteins were almost completely recovered by magnetic cell sorting from a large pool of cells expressing the relevant wild-type platform protein only. Thus, this approach offers a fast and simple screening procedure for cells displaying heterologous polypeptides. The combination of FhuA, along with with BtuB and LamB, should provide a comprehensive tool for displaying complex peptide libraries of various insert sizes on the surface of E. coli for diverse applications.

Mutations in the Escherichia coli receptor FepA reveal residues involved in ligand binding and transport

FepA is the Escherichia coli outer membrane receptor for ferric enterobactin, colicin D and colicin B. The transport processes through FepA are energy-dependent, relying on the periplasmic protein TonB to interact with FepA. Through this interaction, TonB tranduces energy derived from the cytoplasmic membrane across the periplasmic space to FepA. In this study, random mutagenesis strategies were used to define residues of FepA important for its function. Both polymerase chain reaction (PCR)-generated random mutations in the N-terminal 180 amino acids of FepA and spontaneous chromosomal fepA mutations were selected by resistance to colicin B. The PCR mutagenesis strategy targeted the N-terminus because it forms a plug inside the FepA barrel that is expected to be involved in ligand binding, ligand transport, and interaction with TonB. We report the characterization of 15 fepA missense mutations that were localized to three regions of the FepA receptor. The first region was a stretch of eight amino acids referred to as the TonB box. The second region included extracellular loops of both the barrel and the plug. A third region formed a cluster near the barrel wall around positions 75 and 126 of the plug. These mutations provide initial insight into the mechanisms of ligand binding and transport through the FepA receptor.

Characterization of in vitro interactions between a truncated TonB protein from Escherichia coli and the outer membrane receptors FhuA and FepA

High-affinity iron uptake in gram-negative bacteria depends upon TonB, a protein which couples the proton motive force in the cytoplasmic membrane to iron chelate receptors in the outer membrane. To advance studies on TonB structure and function, we expressed a recombinant form of Escherichia coli TonB lacking the N-terminal cytoplasmic membrane anchor. This protein (H(6)-'TonB; M(r), 24,880) was isolated in a soluble fraction of lysed cells and was purified by virtue of a hexahistidine tag located at its N terminus. Sedimentation experiments indicated that the H(6)-'TonB preparation was almost monodisperse and the protein was essentially monomeric. The value found for the Stokes radius (3.8 nm) is in good agreement with the value calculated by size exclusion chromatography. The frictional ratio (2.0) suggested that H(6)-'TonB adopts a highly asymmetrical form with an axial ratio of 15. H(6)-'TonB captured both the ferrichrome-iron receptor FhuA and the ferric enterobactin receptor FepA from detergent-solubilized outer membranes in vitro. Capture was enhanced by preincubation of the receptors with their cognate ligands. Cross-linking assays with the purified proteins in vitro demonstrated that there was preferential interaction between TonB and ligand-loaded FhuA. Purified H(6)-'TonB was found to be stable and thus shows promise for high-resolution structural studies.

Characterization of the Plesiomonas shigelloides genes encoding the heme iron utilization system

Plesiomonas shigelloides is a gram-negative pathogen which can utilize heme as an iron source. In previous work, P. shigelloides genes which permitted heme iron utilization in a laboratory strain of Escherichia coli were isolated. In the present study, the cloned P. shigelloides sequences were found to encode ten potential heme utilization proteins: HugA, the putative heme receptor; TonB and ExbBD; HugB, the putative periplasmic binding protein; HugCD, the putative inner membrane permease; and the proteins HugW, HugX, and HugZ. Three of the genes, hugA, hugZ, and tonB, contain a Fur box in their putative promoters, indicating that the genes may be iron regulated. When the P. shigelloides genes were tested in E. coli K-12 or in a heme iron utilization mutant of P. shigelloides, hugA, the TonB system genes, and hugW, hugX, or hugZ were required for heme iron utilization. When the genes were tested in a hemA entB mutant of E. coli, hugWXZ were not required for utilization of heme as a porphyrin source, but their absence resulted in heme toxicity when the strains were grown in media containing heme as an iron source. hugA could replace the Vibrio cholerae hutA in a heme iron utilization assay, and V. cholerae hutA could complement a P. shigelloides heme utilization mutant, suggesting that HugA is the heme receptor. Our analyses of the TonB system of P. shigelloides indicated that it could function in tonB mutants of both E. coli and V. cholerae and that it was similar to the V. cholerae TonB1 system in the amino acid sequence of the proteins and in the ability of the system to function in high-salt medium.

Exchangeability of N termini in the ligand-gated porins of Escherichia coli

The ferric siderophore transporters of the Gram-negative bacterial outer membrane manifest a unique architecture: Their N termini fold into a globular domain that lodges within, and physically obstructs, a transmembrane porin beta-barrel formed by their C termini. We exchanged and deleted the N termini of two such siderophore receptors, FepA and FhuA, which recognize and transport ferric enterobactin and ferrichrome, respectively. The resultant chimeric proteins and empty beta-barrels avidly bound appropriate ligands, including iron complexes, protein toxins, and viruses. Thus, the ability to recognize and discriminate these molecules fully originates in the transmembrane beta-barrel domain. Both the hybrid and the deletion proteins also transported the ferric siderophore that they bound. The FepA constructs showed less transport activity than wild type receptor protein, but the FhuA constructs functioned with turnover numbers that were equivalent to wild type. The mutant proteins displayed the full range of transport functionalities, despite their aberrant or missing N termini, confirming (Braun, M., Killmann, H., and Braun, V. (1999) Mol. Microbiol. 33, 1037-1049) that the globular domain within the pore is dispensable to the siderophore internalization reaction, and when present, acts without specificity during solute uptake. These and other data suggest a transport process in which siderophore receptors undergo multiple conformational states that ultimately expel the N terminus from the channel concomitant with solute internalization.

Binding of ferric enterobactin by the Escherichia coli periplasmic protein FepB

The periplasmic protein FepB of Escherichia coli is a component of the ferric enterobactin transport system. We overexpressed and purified the binding protein 23-fold from periplasmic extracts by ammonium sulfate precipitation and chromatographic methods, with a yield of 20%, to a final specific activity of 15,500 pmol of ferric enterobactin bound/mg. Periplasmic fluid from cells overexpressing the binding protein adsorbed catecholate ferric siderophores with high affinity: in a gel filtration chromatography assay the K(d) of the ferric enterobactin-FepB binding reaction was approximately 135 nM. Intrinsic fluorescence measurements of binding by the purified protein, which were more accurate, showed higher affinity for both ferric enterobactin (K(d) = 30 nM) and ferric enantioenterobactin (K(d) = 15 nM), the left-handed stereoisomer of the natural E. coli siderophore. Purified FepB also adsorbed the apo-siderophore, enterobactin, with comparable affinity (K(d) = 60 nM) but did not bind ferric agrobactin. Polyclonal rabbit antisera and mouse monoclonal antibodies raised against nearly homogeneous preparations of FepB specifically recognized it in solid-phase immunoassays. These sera enabled the measurement of the FepB concentration in vivo when expressed from the chromosome (4,000 copies/cell) or from multicopy plasmids (>100,000 copies/cell). Overexpression of the binding protein did not enhance the overall affinity or rate of ferric enterobactin transport, supporting the conclusion that the rate-limiting step of ferric siderophore uptake through the cell envelope is passage through the outer membrane.

Characterization of a tonB mutation in Erwinia chrysanthemi 3937: TonB(Ech) is a member of the enterobacterial TonB family

The pectinolytic enterobacterium Erwinia chrysanthemi 3937 causes a systemic disease in its natural host, the African violet (Saintpaulia: ionantha). It produces two structurally unrelated siderophores, chrysobactin and achromobactin. Chrysobactin makes a large contribution to invasive growth of the bacterium in its host. Insertion mutants of a chrysobactin-defective strain were constructed and screened on the universal CAS-agar medium used for siderophore detection. A set of mutants affected in the production of achromobactin were identified. This paper describes a mutant affected in the transport of all the ferrisiderophores used by the bacterium as iron sources. Molecular analysis revealed that the insertion mutation disrupts the tonB gene. The predicted Er. chrysanthemi TonB protein has a molecular mass of 27600 Da and shares 20-58% identity with the TonB proteins from 20 other bacterial species. The pathogenicity of the tonB mutant was assessed by inoculation of African violets. The impairment in the spread of symptoms was similar in the tonB mutant to that in chrysobactin-defective mutants. However, the pectinolytic activity, the major pathogenicity determinant in Er. chrysanthemi, appeared to be stimulated twofold in the tonB mutant.

Copurification of the FpvA ferric pyoverdin receptor of Pseudomonas aeruginosa with its iron-free ligand: implications for siderophore-mediated iron transport

The Pseudomonas aeruginosa FpvA receptor is a TonB-dependent outer membrane transport protein that catalyzes uptake of ferric pyoverdin across the outer membrane. Surprisingly, FpvA expressed in P. aeruginosa grown in an iron-deficient medium copurifies with a ligand X that we have characterized by UV, fluorescence, and mass spectrometry as being iron-free pyoverdin (apo-PaA). PaA was absent from FpvA purified from a PaA-deficient P. aeruginosa strain. The properties of ligand binding in vitro revealed very similar affinities of apo-PaA and ferric-PaA to FpvA. Fluorescence resonance energy transfer was used to study in vitro the formation of the FpvA-PaA-Fe complex in the presence of PaA-Fe or citrate-Fe. The circular dichroism spectrum of FpvA indicated a 57% beta-structure content typical of porins and in agreement with the 3D structures of the siderophore receptors FhuA and FepA. In the absence of the protease's inhibitors, a truncated form of FpvA lacking 87 amino acids at its N-terminus was purified. This truncated form still bound PaA, and its beta-sheet content was conserved. This N-terminal region displays significant homology to the N-terminal periplasmic extensions of FecA from Escherichia coli and PupB from Pseudomonas putida, which were previously shown to be involved in signal transduction. This suggests a similar function for FpvA. The mechanism of iron transport in P. aeruginosa via the pyoverdin pathway is discussed in the light of all these new findings.

Ferritin mutants of Escherichia coli are iron deficient and growth impaired, and fur mutants are iron deficient

Escherichia coli contains at least two iron storage proteins, a ferritin (FtnA) and a bacterioferritin (Bfr). To investigate their specific functions, the corresponding genes (ftnA and bfr) were inactivated by replacing the chromosomal ftnA and bfr genes with disrupted derivatives containing antibiotic resistance cassettes in place of internal segments of the corresponding coding regions. Single mutants (ftnA::spc and bfr::kan) and a double mutant (ftnA::spc bfr::kan) were generated and confirmed by Western and Southern blot analyses. The iron contents of the parental strain (W3110) and the bfr mutant increased by 1.5- to 2-fold during the transition from logarithmic to stationary phase in iron-rich media, whereas the iron contents of the ftnA and ftnA bfr mutants remained unchanged. The ftnA and ftnA bfr mutants were growth impaired in iron-deficient media, but this was apparent only after the mutant and parental strains had been precultured in iron-rich media. Surprisingly, ferric iron uptake regulation (fur) mutants also had very low iron contents (2.5-fold less iron than Fur+ strains) despite constitutive expression of the iron acquisition systems. The iron deficiencies of the ftnA and fur mutants were confirmed by Mössbauer spectroscopy, which further showed that the low iron contents of ftnA mutants are due to a lack of magnetically ordered ferric iron clusters likely to correspond to FtnA iron cores. In combination with the fur mutation, ftnA and bfr mutations produced an enhanced sensitivity to hydroperoxides, presumably due to an increase in production of "reactive ferrous iron." It is concluded that FtnA acts as an iron store accommodating up to 50% of the cellular iron during postexponential growth in iron-rich media and providing a source of iron that partially compensates for iron deficiency during iron-restricted growth. In addition to repressing the iron acquisition systems, Fur appears to regulate the demand for iron, probably by controlling the expression of iron-containing proteins. The role of Bfr remains unclear.

Selectivity of ferric enterobactin binding and cooperativity of transport in gram-negative bacteria

The ligand-gated outer membrane porin FepA serves Escherichia coli as the receptor for the siderophore ferric enterobactin. We characterized the ability of seven analogs of enterobactin to supply iron via FepA by quantitatively measuring the binding and transport of their 59Fe complexes. The experiments refuted the idea that chirality of the iron complex affects its recognition by FepA and demonstrated the necessity of an unsubstituted catecholate coordination center for binding to the outer membrane protein. Among the compounds we tested, only ferric enantioenterobactin, the synthetic, left-handed isomer of natural enterobactin, and ferric TRENCAM, which substitutes a tertiary amine for the macrocyclic lactone ring of ferric enterobactin but maintains an unsubstituted catecholate iron complex, were recognized by FepA (Kd approximately 20 nM). Ferric complexes of other analogs (TRENCAM-3,2-HOPO; TREN-Me-3,2-HOPO; MeMEEtTAM; MeME-Me-3,2-HOPO; K3MECAMS; agrobactin A) with alterations to the chelating groups and different net charge on the iron center neither adsorbed to nor transported through FepA. We also compared the binding and uptake of ferric enterobactin by homologs of FepA from Bordetella bronchisepticus, Pseudomonas aeruginosa, and Salmonella typhimurium in the native organisms and as plasmid-mediated clones expressed in E. coli. All the transport proteins bound ferric enterobactin with high affinity (Kd </= 100 nM) and transported it at comparable rates (>/=50 pmol/min/10(9) cells) in their own particular membrane environments. However, the FepA and IroN proteins of S. typhimurium failed to efficiently function in E. coli. For E. coli, S. typhimurium, and P. aeruginosa, the rate of ferric enterobactin uptake was a sigmoidal function of its concentration, indicating a cooperative transport reaction involving multiple interacting binding sites on FepA.

Biphasic binding kinetics between FepA and its ligands

The Escherichia coli FepA protein is an energy- and TonB-dependent, ligand-binding porin that functions as a receptor for the siderophore ferric enterobactin and colicins B and D. We characterized the kinetic and thermodynamic parameters associated with the initial, energy-independent steps in ligand binding to FepA. In vivo experiments produced Kd values of 24, 185, and 560 nM for ferric enterobactin, colicin B, and colicin D, respectively. The siderophore and colicin B bound to FepA with a 1:1 stoichiometry, but colicin D bound to a maximum level that was 3-fold lower. Preincubation with ferric enterobactin prevented colicin B binding, and preincubation with colicin B prevented ferric enterobactin binding. Colicin B release from FepA was unexpectedly slow in vivo, about 10-fold slower than ferric enterobactin release. This slow dissociation of the colicin B.FepA complex facilitated the affinity purification of FepA and FepA mutants with colicin B-Sepharose. Analysis of a fluorescent FepA derivative showed that ferric enterobactin and colicin B adsorbed with biphasic kinetics, suggesting that both ligands bind in at least two distinct steps, an initial rapid stage and a subsequent slower step, that presumably establishes a transport-competent complex.

Double mutagenesis of a positive charge cluster in the ligand-binding site of the ferric enterobactin receptor, FepA

Siderophores and colicins enter bacterial cells through TonB-dependent outer membrane proteins. Using site-directed substitution mutagenesis, we studied ligand recognition by a prototypic Escherichia coli siderophore receptor, FepA, that binds the iron chelate ferric enterobactin and colicins B and D. These genetic experiments identified a common binding site for two of the three ligands, containing multiple positive charges, within cell surface residues of FepA. Elimination of single residues in this region did not impair the adsorption or transport of ferric enterobactin, but double mutagenesis in the charge cluster identified amino acids (Arg-286 and Arg-316) that participate in siderophore binding and function in FepA-mediated killing by colicins B and D. Ferric enterobactin binding, furthermore, prevented covalent modification of FepA within this domain by either a fluorescent probe or an arginine-specific reagent, corroborating the involvement of this site in ligand recognition. These results identify, for the first time, residues in a TonB-dependent outer membrane protein that participate in ligand binding. They also explain the competition between ferric enterobactin and the colicins on the bacterial cell surface: all three ligands interact with the same arginine residues within FepA during their penetration through the outer membrane.

Colicins B and Ia as novel counterselective agents in interspecies conjugal DNA transfers from colicin-sensitive Escherichia coli donors to other gram-negative recipient species

Colicins are powerful counterselective agents in conjugal DNA transfers using colicin-sensitive Escherichia coli donor strains and colicin-resistant recipient species. We have constructed a high-level constitutive expression system for the bacteriocin and immunity genes of channel-forming colicins B and Ia to optimize production of these colicins as molecular biology reagents.

Role of enterobactin and intracellular iron in cell lethality during near-UV irradiation in Escherichia coli

In Escherichia coli, fur mutants that constitutively express their native iron chelating agent, enterobactin, are significantly more sensitive to near-UV radiation (NUV) than wild type, An entA mutant, which is incapable of synthesizing enterobactin, is equal to wild type in resistance to NUV irradiation. However, the addition of Fe+3 enterobactin but not AI+3 enterobactin to entA cell suspensions just prior to irradiation results in an increased sensitivity to NUV irradiation. A fes mutant, which is unable to reduce and release iron from enterobactin, is significantly more sensitive to NUV irradiation than wild type. The addition of nontoxic levels of H2O2 (5 microM) just prior to irradiation significantly increases sensitivity of both fur and fes mutants. These results suggest that one mechanism by which NUV irradiation leads to cell lethality is by creating a transient iron overload, producing very favorable conditions for the production of highly deleterious free radicals through a variety of mechanisms that lead to oxidative stress and DNA damage including lethal and mutagenic lesions. These results are consistent with the hypothesis that enterobactin is an endogenous chromophore for NUV and contributes to cell lethality via the destruction of its ligand, releasing Fe+2 into the cytoplasm to catalyze the production of highly reactive hydroxyl radicals and other toxic oxygen species via the Haber-Weiss reaction.

The activity of microcin E492 from Klebsiella pneumoniae is regulated by a microcin antagonist

Microcin E492 is a polypeptide antibiotic that is produced and excreted by Klebsiella pneumoniae. Different growth conditions of the producer strain affect microcin activity. The production of a microcin antagonist is responsible for the changes in microcin activity. The microcin antagonist is induced when cells are iron-deprived, resulting in a low microcin activity. The microcin antagonist was purified using a procedure developed for the isolation of a catechol-type siderophore, and its activity was titrated using purified microcin. The inhibitory effect of the microcin antagonist is not observed when this compound is forming a complex with iron. The same inhibitory effect on microcin activity was obtained using purified enterochelin from Escherichia coli. The microcin antagonist was identified as enterochelin through thin-layer chromatography.

The peptide antibiotic microcin 25 is imported through the TonB pathway and the SbmA protein

Selection of spontaneous mutants for insensitivity to the peptide antibiotic microcin 25 led to the isolation of five categories of mutants. Phenotypic and mapping studies showed the mutations to be located in the fhuA, exb, tonB, and sbmA genes. The latter encodes a cytoplasmic membrane protein which is also required for the penetration of microcin B17.

Effect of precultivation conditions on colicin susceptibility in Escherichia coli

The effect of 20 colicins and cloacin was studied after various precultivations. Nutrient agar supplemented with subinhibitory concentration of EDTA used for precultivation or elevating the growth-temperature of the inoculum from 37 degrees C to 42 degrees C increased the susceptibility of wild-type (smooth) Escherichia coli strains to the inhibitory action of some colicins. There were great differences among the colicins in respect to these effects. In case of rough mutants, their sensitivities did not change or eventually decrease after EDTA or heat pretreatment. The LPS pattern in SDS-PAGE of smooth cells grown in EDTA-containing nutrient medium changed in some degree towards the rough character. In case of precultivation at 42 degrees C this change was less considerable. It is supposed that both factors applied during precultivation have influence on colicin sensitivity by means of the change of receptor activity caused by LPS modification.

Mechanism of tonB-dependent transport of KP-736, a 1,5-dihydroxy-4-pyridone-substituted cephalosporin, into Escherichia coli K-12 cells

The mechanism of transport of KP-736, a novel cephalosporin with a 1,5-dihydroxy-4-pyridone moiety at the C-7 position, into the Escherichia coli K-12 cell was investigated by determining the susceptibilities of iron transport mutants to KP-736. The tonB mutant showed a higher degree of resistance to KP-736, indicating that KP-736 was incorporated into E. coli cells via the tonB-dependent iron transport system. The product of the exbB gene was also necessary for the maximal antibacterial potency of KP-736. Cir-lacking and Fiu-lacking mutants showed a moderate level of resistance to KP-736. However, mutants lacking any one of the proteins FepA, FecA, FhuA, and FhuE did not show any increased resistance to KP-736. Two types of spontaneous mutants (e.g., KT1004 and KT1011) could be isolated from cir and fiu mutants by selection for KP-736 resistance and showed the same level of resistance to KP-736 as a tonB mutant. KT1004 showed tonB phenotypes, resistance to phage phi 80, and loss of FecA, whereas KT1011 did not. KT1011 lost the ability to express both Cir and Fiu proteins. These results indicate that the Cir and Fiu outer membrane proteins are involved specifically in the tonB-dependent transport process of KP-736. Against OmpF- and OmpC-deficient transformants producing various groups of beta-lactamases, KP-736 was more effective than the other cephalosporins tested.

Epitope insertions define functional and topological features of the Escherichia coli ferric enterobactin receptor

The outer membrane protein FepA of Escherichia coli is the receptor for the ferric enterobactin siderophore complex and colicins B and D. A foreign antigenic determinant inserted into selected FepA sites allowed mutational analysis of receptor function and in situ immunological tracking of specific protein domains with respect to the bacterial cell compartment. Immunoblot analysis of bacterial proteins using an epitope-specific antibody detected the peptide determinant in the receptor fusions. The impact of the insertions on FepA function was examined by ferric enterobactin-mediated iron uptake experiments and colicin sensitivity tests. In all cases, FepA retained biological activity despite introduction of the foreign sequence. To further develop the topological model of FepA, the peptide-specific antibody was used to localize epitope-carrying FepA domains in intact bacterial cells and their isolated membranes. One epitope resided in a region on the exterior of the cell, at the surface of the FepA protein, while other epitopes appeared to be localized to the periplasm or within the outer membrane.

Cloning and sequencing of the Haemophilus influenzae exbB and exbD genes

The exbB and exbD genes of Haemophilus influenzae (Hi) were cloned and sequenced. The deduced Hi ExbB and ExbD proteins possessed 27 and 28% amino-acid identity (56 and 58% relatedness) with the Escherichia coli ExbB and ExbD proteins, respectively; two proteins which function as TonB accessory proteins during biopolymer transport. Plasmid-encoded Hi exbB and exbD partially complemented an E. coli exbB/exbD mutation.

Characterization of the receptor and translocator domains of colicin N

Intact colicin N and various colicin derivatives, including a natural fragment lacking the first 36 amino-acid residues, a chymotryptic fragment lacking the first 66 amino acids and a thermolytic fragment comprising residues 183-387, were used to locate the regions involved in colicin-N uptake by sensitive Escherichia coli cells. Two separate domains of the molecule participate in colicin-N entry. Specific binding to OmpF receptor site requires a region located between residues 67-182. A N-terminal domain, located between residues 17-66, is involved during the translocation step after binding to receptor. Two sub-regions, residues 17-36 and residues 37-36, can be defined in this domain. The location and interactions between these domains are discussed in comparison to other colicins which use similar cell components for their uptake.

Evolutionary relationship of uptake systems for biopolymers in Escherichia coli: cross-complementation between the TonB-ExbB-ExbD and the TolA-TolQ-TolR proteins

Escherichia coli possesses two energy-coupled import systems through which substances of low concentration and of a size too large to permit diffusion through the porins are translocated across the outer membrane. Group B colicins, ferric siderophores and vitamin B12 are taken up via the TonB-ExbB-ExbD, group A colicins via the TolA-TolQ-TolR system. Cross-complementation between the two systems was demonstrated in that tolQ tolR mutants transformed with plasmids carrying exbB exbD became sensitive to group A colicins, and exbB exbD mutants transformed with plasmid-encoded tolQ tolR became sensitive to group B colicins. TolQ-TolR interacted through TonB, and ExbB-ExbD interacted through TolA with the outer membrane receptors and colicins. Activity of ExbB ExbD via TolA was higher in cells lacking TonB, and activity of TolQ TolR via TonB was increased when TolA was missing. The very distinct TolA and TonB proteins mediate exclusive interaction with group A and group B receptors, respectively. ExbB-TolR and ExbD-TolQ mixtures showed little if any complementation of exbB exbD and tolQ tolR mutants indicating coevolution of ExbB with ExbD and TolQ with TolR. Sequence homology and mutual functional substitution of ExbB-ExbD and TolQ-TolR suggest the evolution of the two import systems from a single import system.

Cloning and characterization of the Vibrio cholerae genes encoding the utilization of iron from haemin and haemoglobin

Vibrio cholerae can utilize haemin or haemoglobin as its sole source of iron. Four haem utilization mutants of a classical strain of V. cholerae were isolated. These mutations were complemented with pHUT1, a cosmid clone isolated from a library of wild-type CA401 DNA. Two independent Tn5 insertions into the cloned sequence disrupted function in all of the complemented mutants. Escherichia coli 1017 transformed with pHUT1 failed to utilize haemin as an iron source; a second plasmid containing a different cloned fragment of V. cholerae DNA (pHUT3) was required in addition to pHUT1 to reconstitute the system in E. coli. Minicell analysis and SDS-PAGE of protein fractions indicate that pHUT10 (a subclone of pHUT1) encodes a 26 kDa inner membrane protein, and pHUT3 encodes a 77 kDa outer membrane protein. Loss of either protein by Tn5 mutagenesis abolishes haem utilization in E. coli. An E. coli hemA mutant that cannot synthesize porphyrins was transformed with the recombinant plasmids to determine whether the plasmids encoded the ability to transport the porphyrin as well as the iron. The transformants grew aerobically in media containing haemin, whereas the parental strain was unable to grow under these conditions. This indicates that V. cholerae haem-iron utilization genes allow transport of the entire haem moiety into the cell.

The excC gene of Escherichia coli K-12 required for cell envelope integrity encodes the peptidoglycan-associated lipoprotein (PAL)

The excC mutants of Escherichia coli are hypersensitive to drugs such as cholic acid and release periplasmic proteins into the extracellular medium. A 1884 bp fragment carrying the excC gene was isolated and sequenced. It contains the 3' end of the tolB gene which maps at min 17 on the E. coli map and an open reading frame which encodes the 18,748 Da ExcC protein. The protein is composed of a hydrophobic region of 22 residues and displayed an overall hydrophilic configuration. It was shown that the ExcC protein is indeed the PAL (peptidoglycan-associated lipoprotein) described by Mizuno (1979). The pal gene had not yet been characterized on the E. coli linkage map since no obvious phenotype could be identified for mutations in this gene. A topologic analysis of the PAL protein using PAL-PhoA translational fusions showed that PAL is associated with the outer membrane only by its N-terminal moiety. The carboxy-terminal part of the protein is necessary for correct interaction of PAL with the peptidoglycan layer.

The tonB gene of Serratia marcescens: sequence, activity and partial complementation of Escherichia coli tonB mutants

The TonB protein plays a key role in the energy-coupled transport of iron siderophores, of vitamin B12, and of colicins of the B-group across the outer membrane of Escherichia coli. In order to obtain more data about which of its particular amino acid sequences are necessary for TonB function, we have cloned and sequenced the tonB gene of Serratia marcescens. The nucleotide sequence predicts an amino acid sequence of 247 residues (Mr 27,389), which is unusually proline-rich and contains the tandem sequences (Glu-Pro)5 and (Lys-Pro)5. In contrast to the TonB proteins of E. coli and Salmonella typhimurium, translation of the S. marcescens TonB protein starts at the first methionine residue of the open reading frame, which is the only amino acid removed during TonB maturation and export. Only the N-terminal sequence is hydrophobic, suggesting its involvement in anchoring the TonB protein to the cytoplasmic membrane. The S. marcescens tonB gene complemented an E. coli tonB mutant with regard to uptake of iron siderophores, and sensitivity to phages T1 and phi 80, and to colicins B and M. However, an E. coli tonB mutant transformed with the S. marcescens tonB gene remained resistant to colicins Ia and Ib, to colicin B derivatives carrying the amino acid replacements Val/Ala and Val/Gly at position 20 in the TonB box, and they exhibited a tenfold lower activity with colicin D. In addition, the S. marcescens TonB protein did not restore T1 sensitivity of an E. coli exbB tolQ double mutant, as has been found for the overexpressed E. coli TonB protein, indicating a lower activity of the S. marcescens TonB protein. Although the S. marcescens TonB protein was less prone to proteolytic degradation, it was stabilized in E. coli by the ExbBD proteins. In E. coli, TonB activity of S. marcescens depended either on the ExbBD or the TolQR activities.

Functional analysis of a C-terminally altered TonB protein of Escherichia coli

Deletions within the 3'-end of the cloned Escherichia coli tonB gene were constructed and recombined into the chromosome. Deletions affecting the last eight C-terminal amino acids (aa) yielded functionally active TonB, whereas deletion of the last 15 C-terminal aa, which removed the C-terminal hydrophobic region of TonB, abolished TonB function entirely. Distribution of TonB within cell compartments was not significantly affected by any of the deletions. All deletion derivatives were less stable than wild-type TonB. A major degradation product of approx. 29 kDa was observed for all TonB derivatives, indicating that it was an N-terminal fragment. The possibility is discussed that the C-terminal hydrophobic region is involved in the intramolecular interaction rather than in the membrane association of TonB.

Antibodies to iron-regulated outer membrane proteins of coliform bacteria isolated from bovine intramammary infections

Expression of iron-regulated outer membrane proteins (OMP) by Escherichia coli and Klebsiella pneumoniae initially isolated from bovine intramammary infections (IMI) was investigated. Additionally, the presence of antibodies in bovine serum and mammary secretion directed against the iron-regulated OMP was examined. Outer membrane proteins were separated by sodium-dodecyl polyacrylamide electrophoresis. Detection of immunoglobulin G directed against OMP was by immunoblotting. All Gram-negative bacteria expressed iron-regulated OMP when grown in skim milk or trypticase soy broth plus iron chelator, alpha-alpha'-dipyridyl. Immunoglobulin G directed against the iron-regulated OMP, as well as the major OMP and several other proteins, was detected in serum and milk of lactating cows with or without Gram-negative bacterial IMI. Antibody against the iron-regulated OMP was detected also in colostrum, secretion from the involuted gland, and in newborn calf serum 4 days after ingesting colostrum.

Interaction of lactoferrin with Escherichia coli cells and correlation with antibacterial activity

It has been established that the antimicrobial activity of lactoferrin towards Escherichia coli is enhanced by a direct contact between the protein and the microbial cell and that, in the case of E. coli K-12 strains, an antibacterial activity of lactoferrin unrelated to iron withdrawal is present. Evidence is now reported that lactoferrin binds to surface structures expressed in E. coli K-12 strains grown in either an "excess" or "stress" of iron. Under the experimental conditions used, lactoferrin binding both in the apo and in the iron-saturated form yields a maximum of 1.6 X 10(5) bound molecules/E. coli K-12 cell; the amount of lactoferrin bound does not depend on the expression of the iron-regulated outer membrane proteins. In contrast, lactoferrin does not bind to E. coli clinical isolates. Apo-lactoferrin (at 500 micrograms/ml in a chemically defined medium) inhibits the growth of E. coli K-12 strains but not of clinical isolates. These findings suggest that the antibacterial activity of the protein could be associated to its binding to the cell surface.

Current views on chloroplast protein import and hypotheses on the origin of the transport mechanism

Most chloroplastic proteins are synthesized as precursors in the cytosol prior to their transport into chloroplasts. These precursors are generally synthesized in a form that is larger than the mature form found inside chloroplasts. The extra amino acids, called transit peptides, are present at the amino terminus. The transit peptide is necessary and sufficient to recognize the chloroplast and induce movement of the attached protein across the envelope membranes. In this review, we discuss the primary and secondary structure of transit peptides, describe what is known about the import process, and present some hypotheses on the evolutionary origin of the import mechanism.

Import-defective colicin B derivatives mutated in the TonB box

The pore-forming colicin B is taken up into Escherichia coli by a receptor and TonB-dependent process. The receptor and colicin B both contain a similar amino acid sequence, close to the N-terminal end, termed the TonB box. Point mutations were introduced into the TonB-box region of the colicin B structural gene cba resulting in colicin B derivatives which were partially or totally inactive against E. coli cells. All derivatives still bound to the receptor. An inactive derivative killed cells when translocated across the outer membrane by osmotic shock treatment, and formed pores in planar lipid bilayer membranes identical to the wild-type colicin. Some of the mutations were partially suppressed by mutations in the tonB structural gene. It was concluded that the TonB-box mutations define a region that is involved in the uptake of colicin B across the outer membrane.

Surface topology of the Escherichia coli K-12 ferric enterobactin receptor

Monoclonal antibodies (MAb) were raised to the Escherichia coli K-12 ferric enterobactin receptor, FepA, and used to identify regions of the polypeptide that are involved in interaction with its ligands ferric enterobactin and colicins B and D. A total of 11 distinct FepA epitopes were identified. The locations of these epitopes within the primary sequence of FepA were mapped by screening MAb against a library of FepA::PhoA fusion proteins, a FepA deletion mutant, and proteolytically modified FepA. These experiments localized the 11 epitopes to seven different regions within the FepA polypeptide, including residues 2 to 24, 27 to 37, 100 to 178, 204 to 227, 258 to 290, 290 to 339, and 382 to 400 of the mature protein. Cell surface-exposed epitopes of FepA were identified and discriminated by cytofluorimetry and by the ability of MAb that recognize them to block the interaction of FepA with its ligands. Seven surface epitopes were defined, including one each in regions 27 to 37, 204 to 227, and 258 to 290 and two each in regions 290 to 339 and 382 to 400. One of these, within region 290 to 339, was recognized by MAb in bacteria containing intact (rfa+) lipopolysaccharide (LPS); all other surface epitopes were susceptible to MAb binding only in a strain containing a truncated (rfaD) LPS core, suggesting that they are physically shielded by E. coli K-12 LPS core sugars. Antibody binding to FepA surface epitopes within region 290 to 339 or 382 to 400 inhibited killing by colicin B or D and the uptake of ferric enterobactin. In addition to the FepA-specific MAb, antibodies that recognized other outer membrane components, including Cir, OmpA, TonA, and LPS, were identified. Immunochemical and biochemical characterization of the surface structures of FepA and analysis of its hydrophobicity and amphilicity were used to generate a model of the ferric enterobactin receptor's transmembrane strands, surface peptides, and ligand-binding domains.

The structurally related exbB and tolQ genes are interchangeable in conferring tonB-dependent colicin, bacteriophage, and albomycin sensitivity

Double exbB tolQ mutants of Escherichia coli were completely resistant to bacteriophages T1 and phi 80, in contrast to strains with exbB or tolQ mutations, which were sensitive. Cells carrying mutations in exbB were partially tolerant to colicins B, D, and M and became fully tolerant by the introduction of tolQ mutations. This suggested involvement of both exbB and tolQ in tonB-dependent uptake.

Involvement of ExbB and TonB in transport across the outer membrane of Escherichia coli: phenotypic complementation of exb mutants by overexpressed tonB and physical stabilization of TonB by ExbB

The exb locus in Escherichia coli consists of two genes, termed exbB and exbD. Exb functions are related to TonB function in that most TonB-dependent processes are enhanced by Exb. Like tonB mutants, exb mutants were resistant to colicin M and albomycin but, in contrast to tonB mutants, showed only reduced sensitivity to colicins B and D. Overexpressed tonB on the multicopy vector pACYC177 largely restored the sensitivity of exb mutants to colicins B, D, and M but only marginally increased sensitivity to albomycin. Suppression of the btuB451 mutation in the structural gene for the vitamin B12 outer membrane receptor protein by a mutation in tonB occurred only in an exb+ strain. Degradation of the unstable overproduced TonB protein was prevented by overproduced ExbB protein. The ExbB protein also stabilized the ExbD protein. Pulse-chase experiments with radiolabeled ferrichrome revealed release of ferrichrome from exbB, tonB, and fhuC mutants, showing that ferrichrome had not crossed the cytoplasmic membrane. It is concluded that the ExbB and ExbD proteins contribute to the activity of TonB and, like TonB, are involved in receptor-dependent transport processes across the outer membrane.

Import of biopolymers into Escherichia coli: nucleotide sequences of the exbB and exbD genes are homologous to those of the tolQ and tolR genes, respectively

Escherichia coli with mutations in the exb region are impaired in outer membrane receptor-dependent uptake processes. They are resistant to the antibiotic albomycin and exhibit reduced sensitivity to group B colicins. A 2.2-kilobase-pair DNA fragment of the exb locus was sequenced. It contained two open reading frames, designated exbB and exbD, which encoded polypeptides of 244 and 141 amino acids, respectively. Both proteins were found in the cytoplasmic membrane. They showed strong homologies to the TolQ and TolR proteins, respectively, which are involved in uptake of group A colicins and infection by filamentous bacteriophages. exbB and exbD were required to complement exb mutations. Osmotic shock treatment rendered exb mutants sensitive to colicin M, which was taken as evidence that the ExbB and ExbD proteins are involved in transport processes across the outer membrane. It is concluded that the exb- and tol-dependent systems originate from a common uptake system for biopolymers.

The Escherichia coli enterobactin biosynthesis gene, entD: nucleotide sequence and membrane localization of its protein product

The nucleotide sequence of the Escherichia coli enterobactin biosynthesis gene entD has been determined. entD specifies a predicted 23579 Dalton protein containing several helical regions, a transmembrane segment and one positively charged domain. The EntD polypeptide was overexpressed and identified in electrophoretic gels as a membrane protein. Although results of conventional membrane fractionation techniques were inconclusive, protease accessibility studies provided evidence that EntD domains are exposed on the inner leaflet of the cytoplasmic membrane. The presence of repetitive extragenic palindromic (REP) sequences within the fepA-entD intercistronic region was confirmed. Lack of a canonical promoter and an iron control region 5' to entD, along with RNA hybridization data, suggest that an iron-regulated transcript contains both fepA and entD.

Iron-regulated outer membrane proteins of Escherichia coli K-12 and mechanism of action of catechol-substituted cephalosporins

Selected aminothiazolyl-oxime cephalosporin congeners substituted at C-3' with a catechol moiety were used to probe the basis of the enhanced antibacterial activity against Escherichia coli K-12 often associated with chemical modifications of this type. Evidence is presented for a tonB-dependent illicit transport of the compounds across the outer membrane of E. coli K-12, the process involving jointly and specifically the Fiu and Cir iron-regulated outer membrane proteins. Thus, both tonB and fiu cir mutants showed a comparably reduced susceptibility to the probe compounds, whereas mutants singularly lacking any one of the six iron-regulated outer membrane proteins (Fiu, FepA, FecA, FhuA, FhuE, and Cir) or lacking any combination of any two of these proteins (except Fiu plus Cir) did not show this resistance. Mutants devoid of all six iron-regulated outer membrane proteins were no more resistant to the probe compounds than fiu cir or tonB strains. In addition to the latter genes, the products of the exbB and possibly the exbC loci were necessary for maximal antibacterial potency. A dependence of antibacterial activity on the level of expression of the uptake system components was noted. Comparison of penicillin-binding protein target affinity with antibacterial activity suggested a possible periplasmic accumulation of active compounds by E. coli K-12. Free vicinal hydroxyl groups of the catechol residue were a primary chemical requirement for recognition by the uptake pathway and thus for high antibacterial activity.

The membrane channel-forming colicin A: synthesis, secretion, structure, action and immunity

The study of colicin release from producing cells has revealed a novel mechanism of secretion. Instead of a built-in 'tag', such as a signal peptide containing information for secretion, the mechanism employs coordinate expression of a small protein which causes an increase in the envelope permeability, resulting in the release of the colicin as well as other proteins. On the other hand, the mechanism of entry of colicins into sensitive cells involves the same three stages of protein translocation that have been demonstrated for various cellular organelles. They first interact with receptors located at the surface of the outer membrane and are then transferred across the cell envelope in a process that requires energy and depends upon accessory proteins (TolA, TolB, TolC, TolQ, TolR) which might play a role similar to that of the secretory apparatus of eukaryotic and prokaryotic cells. At this point, the type of colicin described in this review interacts specifically with the inner membrane to form an ion channel. The pore-forming colicins are isolated as soluble proteins and yet insert spontaneously into lipid bilayers. The three-dimensional structures of some of these colicins should soon become available and site-directed mutagenesis studies have now provided a large number of modified polypeptides. Their use in model systems, particularly those in which the role of transmembrane potential can be tested for polypeptide insertion and ionic channel gating, constitutes a powerful handle with which to improve our understanding of the dynamics of protein insertion into and across membranes and the molecular basis of membrane excitability. In addition, their immunity proteins, which exist only in one state (membrane-inserted) will also contribute to such an understanding.

DNA supercoiling and the anaerobic and growth phase regulation of tonB gene expression

We show that several interacting environmental factors influence the topology of intracellular DNA. Negative supercoiling of DNA in vivo is increased by anaerobic growth and is also influenced by growth phase. The tonB promoter of Escherichia coli and Salmonella typhimurium was found to be highly sensitive to changes in DNA supercoiling. Expression was increased by novobiocin, an inhibitor of DNA gyrase, and was decreased by factors which increase DNA superhelicity. Expression of the plasmid-encoded tonB gene was enhanced by gamma delta insertions in cis in a distance- and orientation-independent fashion. Both the res site and the TnpR protein of gamma delta, which is known to function as a type I topoisomerase, were required for this activation. tonB expression increased during the growth cycle and was reduced by anaerobiosis. There was excellent correlation between tonB expression from a plasmid and the level of supercoiling of that plasmid under a wide range of conditions. The chromosomal tonB gene was regulated in a manner identical to that of the plasmid-encoded gene. Thus, the physiological regulation of tonB expression in response to anaerobiosis and growth phase appears to be mediated by environmentally induced changes in DNA superhelicity.

Systemic virulence of Erwinia chrysanthemi 3937 requires a functional iron assimilation system

In Erwinia chrysanthemi, conditions of iron starvation initiate production of a catechol-type siderophore and enhance production of three outer membrane polypeptides. Twenty-two mutants affected in the different stages of this iron assimilation system were isolated by mini-Mu insertion mutagenesis. All of them failed to induce systemic soft rot on axenically grown Saintpaulia plants. From the siderophore auxotrophs and the iron uptake mutants, clones having recovered the missing function(s) were isolated by using the in vivo cloning vector pULB113 (RP4::mini-Mu). An R-prime plasmid containing a ca. 35.5-kilobase-pair DNA insert was identified. Restoration of the iron functions restored partially, if not completely, the virulence of the parental strain.

Molecular characterization of the Escherichia coli enterobactin cistron entF and coupled expression of entF and the fes gene

The Escherichia coli entF gene, which encodes the serine-activating enzyme involved in enterobactin synthesis, has been localized to a 4.7-kilobase-pair DNA fragment inserted in the vector pBR328. This recombinant molecule, pITS32, restored the ability of an entF mutant to grow on low-iron medium and to produce enterobactin. Examination of its translation products by minicell and electrophoretic analyses revealed a protein of approximately 160,000 daltons, which we identified as the EntF protein. A small DNA segment from pITS32 containing the translational start site for entF allowed the low constitutive expression of beta-galactosidase when cloned (pITS301) upstream of the lacZ structural gene in the vector pMC1403. In contrast, a clone (pITS312) containing the identical entF-lacZ fusion and a larger region upstream of entF including the entire fes gene and extending into the fepA gene (whose transcription is in the opposite direction relative to entF) expressed beta-galactosidase in high yet inducible amounts in response to fluctuations in the metabolic iron concentration. Transposon insertion mutations in the fes gene but not an insertion near the 5' region of fepA in pITS312 reduced this high inducible expression to the low constitutive level seen for pITS301. These observations are most readily explained by the presence of a regulatory region located upstream of fes which mediates the iron-regulated expression of a transcript that includes the fes and entF genes.

Genetic organization of multiple fep genes encoding ferric enterobactin transport functions in Escherichia coli

Three genes were shown to provide functions specific for ferric enterobactin transport in Escherichia coli: fepA encoded the outer membrane receptor, fepB produced a periplasmic protein, and the fepC product was presumably a component of a cytoplasmic membrane permease system for this siderophore. A 10.6-kilobase-pair E. coli chromosomal EcoRI restriction fragment containing the fepB and fepC genes was isolated from a genomic library constructed in the vector pBR328. Both cistrons were localized on this clone (pITS24) by subcloning and deletion and insertion mutagenesis to positions that were separated by approximately 2.5 kilobases. Within this region, insertion mutations defining an additional ferric enterobactin transport gene (fepD) were isolated, and polarity effects from insertions into fepB suggested that fepD is encoded downstream on the same transcript. A 31,500-dalton FepC protein and a family of FepB polypeptides ranging from 34,000 to 37,000 daltons were identified in E. coli minicells, but the product of fepD was not detectable by this system. Another insertion mutation between entF and fepC was also shown to disrupt iron transport via enterobactin and thus defined the fepE locus; fepE weakly expressed a 43,000-dalton protein in minicells. It is proposed that these newly identified genes, fepD and fepE, provide functions which act in conjunction with the fepC product to form the ferric enterobactin-specific cytoplasmic membrane permease. An additional 44,000-dalton protein was identified and shown to be expressed from a gene that is situated between fepB and entE and that is transcribed in the direction opposite that of fepB. Although the function of this protein is uncharacterized, its membrane location suggests that it too may function in iron transport.

Ferric-coprogen receptor FhuE of Escherichia coli: processing and sequence common to all TonB-dependent outer membrane receptor proteins

Iron transport via siderophores requires outer membrane receptor proteins and the TonB protein. The FhuE protein of Escherichia coli functions as the receptor for ferric coprogen and ferric-rhodotorulic acid. A chromosomal DNA fragment bearing the fhuE gene was cloned into pACYC184. The gene was localized by insertion mutagenesis by using the transposon Tn1000. Expression in minicells revealed a FhuE precursor with an apparent molecular weight of 82,000 and a FhuE protein with a molecular weight of 76,000. The transcription polarity of the fhuE gene was deduced from the size of truncated polypeptides derived from Tn1000 insertions, which were mapped by restriction analysis. The processing of truncated precursors that were synthesized by insertion mutants was strongly reduced even when the insertion site was close to the carboxy terminus of the FhuE protein. It is concluded that either the efficient insertion of proFhuE into the cytoplasmic membrane or the rate of cleavage of the signal peptide requires a particular conformation of the proFhuE protein, which is only formed by the complete primary structure. The amino-terminal amino acid sequence deduced from the nucleotide sequence was confirmed by gas-phase sequencing of the precursor and the mature form, which were separated by electrophoresis on polyacrylamide gels. The precursor contained an unusually long signal peptide of 36 amino acids. The amino-terminal end of the mature form contained the sequence Glu-Thr-Val Ile-Val. A pentapeptide starting with either Glu or Asp, followed by Thr, and two uncharged residues ending with Val were found in all outer membrane receptor proteins that were constituents of TonB-dependent transport systems.

E-0702, a new cephalosporin, is incorporated into Escherichia coli cells via the tonB-dependent iron transport system

E-0702, a new cephalosporin with a potent antipseudomonal action, was synthesized. In the study of the mode of action of this antibiotic in Escherichia coli, it was found that mutants which acquired resistance to E-0702 were isolated spontaneously and could be shown to be susceptible to its closely related derivatives, E-0702-060 and E-0702-061, and other representative beta-lactam antibiotics. In these mutants, no increased production of beta-lactamase was detectable. No apparent differences between the resistant mutants and the parental strains were observed in the affinity of E-0702 for penicillin-binding proteins. Furthermore, no significant reduction in or loss of both OmpF and OmpC porin proteins in the outer membrane was observed. The mutation was mapped to the tonB gene, which is known to be essential for the iron transport system of bacteria. The bactericidal action of E-0702 was rapidly expressed against iron-starved cells in which the iron transport system was induced, whereas the bactericidal action against iron-supplemented cells was ineffective. It is suggested that E-0702 is incorporated into bacterial cells as a chelator of iron via the tonB-dependent iron transport system, after which its strong and rapid bactericidal action is manifested.

Cloning and expression of the exbB gene of Escherichia coli K-12

The exbB locus of Escherichia coli is involved in the uptake of certain iron(III) siderophore compounds, of vitamin B12 and of certain colicins. Outer membrane receptor proteins are essential constituents of the corresponding uptake systems. The DNA carrying the exbB locus was cloned into pACYC184 and subcloned into pUC18. With the use of insertion mutagenesis employing transposon Tn1000 and by deletion analysis, the exbB locus was confined to a 1.9 kb DNA fragment. An in vitro transcription/translation system and minicells programmed by exbB+ plasmids expressed a protein with an apparent molecular weight of 26,000. One plasmid, designated pKE7, expressed this protein to an extent that it became a prominent band in the membrane fraction of transformants. In contrast, chromosomally encoded ExbB protein could not be detected. The plasmid-encoded ExbB protein was mainly localized in the cytoplasmic membrane. Ferrichrome transport in exbB mutants was restored by exbB+ plasmids. Moderate overexpression of ExbB resulted in an enhanced ferrichrome transport, strong overexpression reduced the transport rate compared to a wild-type strain. The ExbB function shares some properties with the TonB function.