Co-Regulation in Escherichia coli of a Novel Transport System for sn-Glycerol-3-Phosphate and Outer Membrane Protein Ic (e, E) with Alkaline Phosphatase and Phosphate-Binding Protein

Mutants constitutive for the novel outer membrane protein Ic (e or E) contained a recently discovered binding protein for sn-glycerol-3-phosphate. The corresponding parental strains missing the outer membrane protein Ic (e, E) were negative or strongly reduced in the synthesis of the binding protein. In addition, strains that were previously isolated as mutants constitutive for the sn-glycerol-3-phosphate transport system (ugp⁺ mutants) and that produced the novel periplasmic proteins GP1 to GP4 also synthesized a new outer membrane protein with the same electrophoretic mobility on sodium dodecyl sulfate-polyacrylamide gels as protein Ic. Screening of different ugp⁺ mutants revealed the existence of three types in respect to the four novel periplasmic proteins GP1, -2, -3, and -4: (i) one containing all four proteins; (ii) one containing only proteins GP1, -2, and -3; (iii) one containing only proteins GP1, -2, and -4. In confirmation of the data presented in the accompanying paper by Tommassen and Lugtenberg (J. Bacteriol. 143:151–157, 1980), we found that purified GP1 is identical to alkaline phosphatase, whereas purified GP3 has binding activity of inorganic phosphate and is identical to the phosphate-binding protein. Moreover, growth conditions that lead in a wild-type strain to the derepression of alkaline phosphatase synthesis also derepressed the synthesis of the sn-glycerol-3-phosphate-binding protein as well as the corresponding transport system. Thus, the new sn-glycerol-3-phosphate transport system is part of the alkaline phosphatase regulatory system.     

Characteristics of a Binding Protein-Dependent Transport System for sn-Glycerol-3-Phosphate in Escherichia coli That Is Part of the pho Regulon

The ugp-dependent transport system for sn-glycerol-3-phosphate has been characterized. The system is induced under conditions of phosphate starvation and in mutants that are constitutive for the pho regulon. The system does not operate in membrane vesicles and is highly sensitive toward osmotic shock. The participation of a periplasmic binding protein in the transport process can be deduced from the isolation of transport mutants that lack the binding protein. As with other binding protein-dependent transport systems, this protein appears to be necessary but not sufficient for transport activity. The isolation of mutants has become possible by selection for resistance against the toxic analog 3,4-dihydroxybutyl-1-phosphonate that is transported by the system. sn-Glycerol-3-phosphate transported via ugp cannot be used as the sole carbon source. Strains have been constructed that lack alkaline phosphatase and glycerol kinase. In addition, they are constitutive for the glp regulon and contain high levels of glycerol-3-phosphate dehydrogenase. Despite the fact that these strains exhibit high ugp-dependent transport activity for sn-glycerol-3-phosphate they are unable to grow on it as a sole source of carbon. However, when cells are grown on an alternate carbon source, ¹⁴C label from [¹⁴C]sn-glycerol-3-phosphate appears in phospholipids as well as in trichloroacetic acid-precipitable material. The incorporation of ¹⁴C label is strongly reduced when sn-glycerol-3-phosphate is the only carbon source. In the presence of an alternate carbon source, this inhibition is relieved, and sn-glycerol-3-phosphate transported by ugp can be used as the sole source of phosphate.     

Characterization of the Escherichia coli gene for 1-acyl-sn-glycerol-3-phosphate acyltransferase (pIsC)

Summary An Escherichia coli strain deficient in 1-acyl-sn-glycerol-3-phosphate acyltransferase activity has previously been isolated, and the gene (plsC) has been shown to map near min 65 on the chromosome. I precisely mapped the location of plsC on the chromosome, and determined its DNA sequence. plsC is located between parC and sufI, and is separated from sufI by 74 bp. Upstream of plsC is parC, separated by 233 bp, which includes an active promoter. parC, plsC, and sufI are all transcribed in the counterclockwise direction on the chromosome, possibly in an operon with multiple promoters. The amino-terminal sequence of the partially purified protein, combined with the DNA sequence, reveal 1-acyl-sn-glycerol-3-phosphate acyltransferase to be a 27.5 kDa highly basic protein. The plsC gene product, 1-acyl-sn-glycerol-3-phosphate acyltransferase, is localized to the cytoplasmic membrane of the cell. The amino-terminal sequence of the purified protein reveals the first amino acid to be a blocked methionine residue, most probably a formyl-methionine. The amino acid sequence of 1-acyl-sn-glycerol-3-phosphate acyltransferase has a short region of homology to two other E. coli acyltransferases that utilize acyl-acyl carrier protein as the acyl donor, sn-glycerol-3-phosphate acyltransferase and UDP-N-acetyl-glucosamine acyltransferase (involved in lipid A biosynthesis).     

Identification of sn-Glycerol-1-phosphate Dehydrogenase Activity from Genomic Information on a Hyperthermophilic Archaeon,Sulfolobus tokodaii Strain 7

sn-Glycerol-1-phosphate dehydrogenase is responsible for the formation of sn-glycerol-1-phosphate, the backbone of membrane phospholipids of Archaea. This activity had never been detected in cell-free extract of Sulfolobus sp. Here we report the detection of this activity on the thermostable ST0344 protein of Sulfolobus tokodaii expressed in Escherichia coli, which was predicted from genomic information on S. tokodaii. This is another line of evidence for the general mechanism of sn-glycerol-1-phosphate formation by the enzyme.     

Isolation and characterisation of a maize cDNA that complements a 1-acyl sn-glycerol-3-phosphate acyltransferase mutant of Escherichia coli and encodes a protein which has similarities to other acyltransferases

We selected cDNA plasmid clones that corrected the temperature-sensitive phenotype of Escherichia coli strain JC201, which is deficient in 1-acyl-sn-glycerol-3-phosphate acyltransferase activity. A plasmid-based maize endosperm cDNA library was used for complementation and a plasmid that enabled the cells to grow at 44°C on ampicillin was isolated. Addition of this plasmid (pMAT1) to JC201 restored 1-acyl-sn-glycerol-3-phosphate acyltransferase activity to the cells. Total phospholipid labelling showed that the substrate for the enzyme, lysophosphatidic acid, accumulated in JC201 and was further metabolised to phosphatidylethanolamine in complemented cells. Membranes isolated from such cells were able to convert lysophosphatidic acid to phosphatidic acid in acyltransferase assays. The cDNA insert of pMAT1 contains one long open reading frame of 374 amino acids which encodes a protein of relative molecular weight 42 543. The sequence of this protein is most similar to SLC1, which is thought to be able to acylate glycerol at the sn-2 position during synthesis of inositol-containing lipids. Homologies between the SLC1 protein, the 1-acyl-sn-glycerol-3-phosphate acyltransferase of E. coli (PlsC) and the maize ORF were found with blocks of conserved amino acids, whose spacing was conserved between the three proteins, identifiable.     

Coupling of ethanol metabolism to lipid biosynthesis: labelling of the glycerol moieties of sn-glycerol-3-phosphate,a phosphatidic acid and a phosphatidylcholine in liver of rats given [1,1-2H2]ethanol

The mechanism behind ethanol-induced fatty liver was investigated by administration of [1,1-²H₂]ethanol to rats and analysis of intermediates in lipid biosynthesis. Phosphatidic acid and phosphatidylcholine were isolated by chromatography on a lipophilic anion exchanger and molecular species were isolated by high-performance liquid chromatography in a non-aqueous system. The glycerol moieties of palmitoyl-linoleoylphosphatidic acid, the corresponding phosphatidylcholine and free sn-glycerol-3-phosphate were analysed by GC/MS of methyl ester t-butyldimethylsilyl derivatives. The deuterium labelling in the glycerol moiety of the phosphatidic acid was 2–3-times higher than in free sn-glycerol-3-phosphate, indicating that a specific pool of sn-glycerol-3-phosphate was used for the synthesis of phosphatidic acid in liver. The results indicate that NADH formed during ethanol oxidation is used in the formation of a pool of sn-glycerol-3-phosphate that gives rise to triacylglycerol and possibly fatty liver.     

Quantitation of glycerophosphorylcholine by flow injection analysis using immobilized enzymes

A method for quantitating glycerophosphorylcholine by flow injection analysis is reported in the present paper. Glycerophosphorylcholine phosphodiesterase and choline oxidase, immobilized on controlled porosity glass beads, are packed in a small reactor inserted in a flow injection manifold. When samples containing glycerophosphorylcholine are injected, glycerophosphorylcholine is hydrolyzed into choline and sn-glycerol-3-phosphate. The free choline produced in this reaction is oxidized to betain and hydrogen peroxide. Hydrogen peroxide is detected amperometrically.Quantitation of glycerophosphorylcholine in samples containing choline and phosphorylcholine is obtained inserting ahead of the reactor a small column packed with a mixed bed ion exchange resin. The time needed for each determination does not exceed one minute.The present method, applied to quantitate glycerophosphorylcholine in samples of seminal plasma, gave results comparable with those obtained using the standard enzymatic- spectrophotometric procedure.An alternative procedure, making use of co-immobilized glycerophosphorylcholine phosphodiesterase and glycerol-3-phosphate oxidase for quantitating glycerophosphorylcholine, glycerophosphorylethanolamine and glycerophosphorylserine is also described.Abbreviations GPC sn-glycerol-3-phosphorylcholine - GPE sn-glycerol-3-phosphorylethanolamine - GPS sn-glycerol-3-phosphorylserine - GPA sn-glycerol-3-phosphoric acid - PDE glycerophosphorylcholine-phosphodiesterase - GPA-Ox glycerophosphate oxidase - Cho-Ox choline oxidase     

<Emphasis Type="Italic">Escherichia coli tat</Emphasis> mutant strains are able to transport maltose in the absence of an active <Emphasis Type="Italic">malE</Emphasis> gene

The twin-arginine transport (Tat) system is a prokaryotic protein transport system. Escherichia coli mutants in this pathway show a defect in cell separation during cell division, resulting in destabilization and permeability of the outer membrane. Maltose uptake is catalysed by a membrane-bound transporter of the ATP binding cassette (ABC) superfamily, where MalE is the essential periplasmic binding protein component. Here, we report that tat mutants are unexpectedly able to transport maltose in the absence of malE. This observation is specific to the MalE component since co-inactivation of malF, which encodes one of the channel components of the transporter, completely abolishes maltose transport even when the Tat system is inactivated. Genetic repair of the outer membrane leaky phenotype of the tat mutant strain re-established the absolute requirement for MalE in maltose uptake. In addition, we demonstrate that phenotypic repair of the outer membrane defect of the tat strain can also be achieved chemically by the inclusion of high concentrations of calcium or magnesium in the growth medium.     

Only one gene is required for the glpT-dependent transport of sn-glycerol-3-phosphate in Escherichia coli

Summary Deletion and point mutants defective in the glpT-dependent sn-glycerol-3-phosphate transport system were isolated and located on the Escherichia coli chromosome. They mapped in glpT in the clockwise order gyrA, glpA, glpT at around 48 min on the Escherichia coli linkage map. The mutations within glpT were ordered by deletion mapping, three factor crosses, and by crosses involving transducing bacteriophages carrying glpT-lac operon fusions. Results obtained using these fusion phages indicated that glpT is transcribed in the counterclockwise direction on the E. coli linkage map.Complementation analysis using these mutants revealed only one complementation group. Thus, one gene is necessary and sufficient for the proton motive force-dependent sn-glycerol-3-phosphate transport system.     

Transport of iron across the outer membrane

Summary The TonB protein is involved in energy-coupled receptor-dependent transport processes across the outer membrane. The TonB protein is anchored in the cytoplasmic membrane but exposed to the periplasmic space. To fulfill its function, it has to couple the energy-providing metabolism in the cytoplasmic membrane with regulation of outer membrane receptor activity. Ferrichrome and albomycin transport, uptake of colicin M, and infection by the phages T1 and80 occur via the same receptor, the FhuA protein in the outer membrane. Therefore, this receptor is particularly suitable for the study of energy-coupled TonB-dependent transport across the outer membrane. Ferrichrome, albomycin and colicin M bind to the FhuA receptor but are not released into the periplasmic space of unenergized cells, ortonB mutants. In vivo interaction between FhuA and TonB is suggested by the restoration of activity of inactive FhuA proteins, bearing amino acid replacements in the TonB box, by TonB derivatives with single amino acid substitutions. Point mutations in thefhuA gene are suppressed by point mutations in thetonB gene. In addition, naturally occurring degradation of the TonB protein and its derivatives is preferentially prevented in vivo by FhuA and FhuA derivatives where functional interaction takes place. It is proposed that in the energized state, TonB induces a conformation in FhuA which leads to the release of the FhuA-bound compounds into the periplasmic space. Activation of FhuA by TonB depends on the ExbBD proteins in the cytoplasmic membrane. They can be partially replaced by the TolQR proteins which show strong sequence similarity to the ExbBD proteins. A physical interaction of these proteins with the TonB protein is suggested by TonB stabilization through ExbB and TolQR. We propose a permanent or reversible complex in the cytoplasmic membrane composed of the TonB protein and the ExbBD/TolQR proteins through which TonB is energized.     

PhoE protein pore of the outer membrane of Escherichia coli K12 is a particularly efficient channel for organic and inorganic phosphate

This study was undertaken to investigate the proposed in vivo pore function of PhoE protein, an Escherichia coli K12 outer membrane protein induced by growth under phosphate limitation, and to compare it with those of the constitutive pore proteins OmpF and OmpC. Appropriate mutant strains were constructed containing only one of the proteins PhoE, OmpF or OmpC, or none of these proteins at all. By measuring rates of nutrient uptake at low solute concentrations, the proposed pore function of PhoE protein was confirmed as the presence of the protein facilitates the diffusion of P_i through the outer membrane, such that a pore protein deficient strain behaves as a K_m mutant. Comparison of the rates of permeation of P_i, glycerol 3-phosphate and glucose 6-phosphate through pores formed by PhoE, OmpF and OmpC proteins shows that PhoE protein is the most effective pore in facilitating the diffusion of P_i and phosphorus-containing compounds. The three types of pores were about equally effective in facilitating the permeation of glucose and arsenate. Possible reasons for the preference for P_i and P_i-containing solutes are discussed.     

Sites of phospholipid biosynthesis during induction of intracytoplasmic membrane formation in Rhodopseudomonas sphaeroides

A rapid, gratuitous and cell-division uncoupled induction of intracytoplasmic photosynthetic membrane formation was demonstrated in low-aeration suspensions of chemotrophically grown Rhodopseudomonas sphaeroides. Despite a nearly 2-fold increase in phospholipid levels, no significant increases were detected in the specific activities of CDP-1,2-diacyl-sn-glycerol:sn-glycerol-3-phosphate phosphatidyltransferase (phosphatidylglycerophosphate synthase, EC 2.7.8.5) and CDP-1,2-diacyl-sn-glycerol:L-serine O-phosphatidyltransferase (phosphatidylserine synthase, EC 2.7.8.8), the first committed enzymes of anionic and zwitterionic phospholipid biosyntheses, respectively. The distribution of phosphatidylglycerophosphate and phosphatidylserine synthase activities after rate-zone sedimentation of cell-free extracts indicated that intracytoplasmic membrane phospholipids were synthesized mainly within distinct domains of the conserved cytoplasmic membrane. Labeling studies with ³²P_i and L-[³H]phenylalanine suggested that preexisting phospholipid was utilized initially as the matrix for insertion of intracytoplasmic membrane protein that was synthesized and assembled de novo during induction.Abbreviations BChl bacteriochlorophyll a - B800-850, B875 peripheral and core light-harvesting BChl-protein complexes, respectively, identified by near-IR absorption maxima This paper is dedicated to Professor Gerhart Drews on the occasion of his sixtieth birthday     

Role of the receptor for bacteriophage lambda in the functioning of the maltose chemoreceptor of Escherichia coli.

Chemotaxis towards maltose is specifically defective in many strains of Escherichia coli carrying mutations affecting lamB, the gene coding for the outer membrane receptor for bacteriophage lambda. However, with one exception, the most extreme effect of lamB mutants on the maltose response as determined in the capillary assay is a shift to higher sugar concentrations and a reduction in the number of bacteria accumulated to about 25% of the wild-type level. The severity of the taxis defect is strongly correlated with reduced ability of the cells to take up the maltose present at 1 and 10 muM. Evidence presented here and in the accompanying paper indicates that the lambda receptor is involved in the transport of maltose at these concentrations. The effects of lamB mutations on maltose taxis can be explained by postulating that the high-affinity maltose transport system in which the lambda receptor participates transfers maltose from the surrounding medium across the outer membrane and into the periplasmic space. If the maltose chemoreceptor detects sugar present in the periplasmic space, and not molecules external to the outer membrane, then defective transport of low concentrations of maltose into the periplasm would result in the observed apparent reduction in the sensitivity of the maltose receptor. Thus, the lambda receptor protein would participate in maltose chemorecepton only indirectly through its role in maltose transport.     

Sequence–function relationships in MalG, an inner membrane protein from the maltose transport system in Escherichia coli

The maIG gene encodes a hydrophobic cytoplasmic membrane protein which is required for the energy-dependent transport of maltose and maltodextrins in Escherichia coli. The MalG protein, together with MalF and MalK proteins, forms a multimeric complex in the membrane consisting of two MalK subunits for each MalF and MalG subunit. Fifteen mutations have been isolated in malG by random linker insertion mutagenesis. Two regions essential for maltose transport have been identified. In particular, a hydro philic region containing the peptidic motif EAA—G———I-LP, highly conserved among inner membrane proteins from binding protein-dependent transport systems, is essential for maltose transport. The results also show that several regions of MalG are not essential for function. A region (residues 30–50) encompassing the first predicted transmembrane segment and the first periplasmic loop in MalG may be modified extensively with little effect on maltose transport and no effect on the stability and the localization of the protein. A region located at the middle of the protein (residues 153–157) is not essential for the function of the protein. A region, essential for maltodextrin utilization but not for maltose transport, has been identified near the C-terminus of the protein.     

Dual Mechanisms of Metabolite Acquisition by the Obligate Intracytosolic Pathogen Rickettsia prowazekii Reveal Novel Aspects of Triose Phosphate Transport

Rickettsia prowazekii is an obligate intracytosolic pathogen and the causative agent of epidemic typhus fever in humans. As an evolutionary model of intracellular pathogenesis, rickettsiae are notorious for their use of transport systems that parasitize eukaryotic host cell biochemical pathways. Rickettsial transport systems for substrates found only in eukaryotic cell cytoplasm are uncommon among free-living microorganisms and often possess distinctive mechanisms. We previously reported that R. prowazekii acquires triose phosphates for phospholipid biosynthesis via the coordinated activities of a novel dihydroxyacetone phosphate transport system and an sn-glycerol-3-phosphate dehydrogenase (K. M. Frohlich et al., J. Bacteriol. 192:4281–4288, 2010). In the present study, we have determined that R. prowazekii utilizes a second, independent triose phosphate acquisition pathway whereby sn-glycerol-3-phosphate is directly transported and incorporated into phospholipids. Herein we describe the sn-glycerol-3-phosphate and dihydroxyacetone phosphate transport systems in isolated R. prowazekii with respect to kinetics, energy coupling, transport mechanisms, and substrate specificity. These data suggest the existence of multiple rickettsial triose phosphate transport systems. Furthermore, the R. prowazekii dihydroxyacetone phosphate transport systems displayed unexpected mechanistic properties compared to well-characterized triose phosphate transport systems from plant plastids. Questions regarding possible roles for dual-substrate acquisition pathways as metabolic virulence factors in the context of a pathogen undergoing reductive evolution are discussed.     

The role of the Escherichia coli λ receptor in the transport of maltose and maltodextrins

The λ receptor is a peptidoglycan-associated integral protein that spans the outer membrane. Beside its function in phage λ adsorption it participates in transport. The latter function can be summarized as follows: (1) Receptor allows the nonspecific permeation of small molecules other than maltose and maltodextrins (in close analogy to a molecular sieve). Here the only criterion for selectivity is size and it has the properties of an unspecific pore. In this respect, it is similar to the outer membrane proteins Ia, Ib, and Ic, the porins. (2) It is a binding protein for maltodextrins. Binding affinity is low but increases by a factor of 500 as the chain length of the maltodextrins increases. In contrast, the affinity of the periplasmic maltose-binding protein for maltose and maltodextrins is similarly high (in the μM range). (3) In the in vitro system of liposomes, the λ receptor facilitates specifically the diffusion of maltodextrins that exceed the size limit given by its porin function. This clearly demonstrates that the λ receptor alone is able to specifically overcome the permeability barrier of the outer membrane for maltodextrins. (4) From the genetic and kinetic analysis of maltose and maltodextrin transport, it can be concluded that the λ receptor interacts with the periplasmic maltose-binding protein. (5) Electron microscopic studies indicate a location for the maltose-binding protein in the outer cell envelope. This location is dependent on the presence of the λ receptor.     

Ferric hydroxamate binding protein FhuD from Escherichia coli: mutants in conserved and non-conserved regions

Uptake of iron complexes into the Gram-negative bacterial cell requires highly specific outer membrane receptors and specific ATP-dependent (ATP-Binding-Cassette (ABC)) transport systems located in the inner membrane. The latter type of import system is characterized by a periplasmic binding protein (BP), integral membrane proteins, and membrane-associated ATP-hydrolyzing proteins. In Gram-positive bacteria lacking the periplasmic space, the binding proteins are lipoproteins tethered to the cytoplasmic membrane. To date, there is little structural information about the components of ABC transport systems involved in iron complex transport. The recently determined structure of the Escherichia coli periplasmic ferric siderophore binding protein FhuD is unique for an ABC transport system (Clarke et al. 2000). Unlike other BP's, FhuD has two domains connected by a long -helix. The ligand binds in a shallow pocket between the two domains. In vivo and in vitro analysis of single amino acid mutants of FhuD identified several residues that are important for proper functioning of the protein. In this study, the mutated residues were mapped to the protein structure to define special areas and specific amino acid residues in E. coli FhuD that are vital for correct protein function. A number of these important residues were localized in conserved regions according to a multiple sequence alignment of E. coli FhuD with other BP's that transport siderophores, heme, and vitamin B₁₂. The alignment and structure prediction of these polypeptides indicate that they form a distinct family of periplasmic binding proteins.     

Towards the genetic engineering of triacylglycerols of defined fatty acid composition: major changes in erucic acid content at the sn-2 position affected by the introduction of a 1-acyl-sn-glycerol-3-phosphate acyltransferase from Limnanthes douglasii into oil seed rape

A cDNA encoding a 1-acyl-sn-glycerol-3-phosphate acyltransferase from Limnanthes douglasii was introduced into oil seed rape (Brassica napus) under the control of a napin promoter. Seed triacylglycerols from transgenic plants were analysed by reversed-phase HPLC and trierucin was detected at a level of 0.4% and 2.8% in two transgenic plants but was not found in untransformed rape seed. Total fatty acid composition analysis of seeds from these selected plants revealed that the erucic acid content was no higher than the maximum found in the starting population. Analysis of fatty acids at the sn-2 position showed no erucic acid in untransformed rape but in the selected transgenic plants 9% (mol/mol) and 28.3% (mol/mol) erucic acid was present. These results conclusively demonstrate that the gene from L. douglasii encodes a 1-acyl-sn-glycerol-3-phosphate acyltransferase which can function in rape and incorporate erucic acid at the sn-2 position of triacylglycerols in seed. Additional modifications may further increase levels of trierucin.     

Novel stereoconfiguration in lyso-bis-phosphatidic acid of cultured BHK-cells

Lyso-bis-phosphatidic acid purified from cultured hamster kidney fibroblast cells (BHK-cells) was subjected to strong alkaline hydrolysis. The hydrolysate contained phosphorus, free glycerol, total glycerol, α-glycerophosphate, β-glycerophosphate and sn-glycerol-3-phosphate in mole ratios of 1.0:1.0:1.9:0.4:0.6:0.02. The absence of sn-glycerol-3-phosphate indicates that the backbone of this lipid has the uncommon structure of 1-sn-glycerophosphoryl-1′-sn-glycerol. Consequently, the biosynthesis and the degradation of this lipid must differ from the other known mammalian glycerophospholipids.     

A C-terminal truncation results in high-level expression of the functional photoreceptor sensory rhodopsin I in the archaeon Halobacterium salinarium

TonB protein functions as an energy transducer, coupling cytoplasmic membrane electrochemical potential to the active transport of vitamin B₁₂ and Fe(III)–siderophore complexes across the outer membrane of Escherichia coli and other Gram-negative bacteria. Accumulated evidence indicates that TonB is anchored in the cytoplasm, but spans the periplasmic space to interact physically with outer membrane receptors. It has been presumed that this ability is caused by a conserved (Glu–Pro)_n–(Lys–Pro)_m repeat motif, predicted to assume a rigid, linear conformation of sufficient length to reach the outer membrane. Based on in vitro studies with synthetic peptides and purified FhuA outer membrane receptor, it has been suggested that this region contains a site that directly binds outer membrane receptors and is essential for energy transduction. We have found a TonB lacking the (Glu–Pro)_n–(Lys–Pro)_m, repeat motif (TonBΔ(66–100)). TonBΔ(66–100) is fully capable of irreversible 80 adsorption, except under physiological circumstances where the periplasmic space is expanded. Based on the ability of TonBΔ(66–100) to interact with outer membrane receptors and components of the energy transduction apparatus under normal physiological conditions, it is evident that the TonB proline-rich region has no role in energy transduction other than to provide a physical extension sufficient to reach the outer membrane.