Biosynthesis of the outer membrane receptor for vitamin B12, E colicins, and bacteriophage BF23 by Escherichia coli: kinetics of phenotypic expression after the introduction of bfe+ and bfe alleles

TonB-Dependent Transport Across the Bacterial Outer Membrane

TonB-dependent transporters (TBDTs) are present in all gram-negative bacteria and mediate energy-dependent uptake of molecules that are too scarce or large to be taken up efficiently by outer membrane (OM) diffusion channels. This process requires energy that is derived from the proton motive force and delivered to TBDTs by the TonB-ExbBD motor complex in the inner membrane. Together with the need to preserve the OM permeability barrier, this has led to an extremely complex and fascinating transport mechanism for which the fundamentals, despite decades of research, are still unclear. In this review, we describe our current understanding of the transport mechanism of TBDTs, their potential role in the delivery of novel antibiotics, and the important contributions made by TBDT-associated (lipo)proteins.

Energy-dependent immunity protein release during tol-dependent nuclease colicin translocation

Nuclease colicins bind their target receptor in the outer membrane of sensitive cells in the form of a high affinity complex with their cognate immunity proteins. Upon cell entry the immunity protein is lost from the complex by means that are poorly understood. We have developed a sensitive fluorescence assay that has enabled us to study the molecular requirements for immunity protein release. Nuclease colicins use members of the tol operon for their translocation across the outer membrane. We have demonstrated that the amino-terminal 80 residues of the colicin E9 molecule, which is the region that interacts with TolB, are essential for immunity protein release. Using tol deletion strains we analyzed the cellular components necessary for immunity protein release and found that in addition to a requirement for tolB, the tolA deletion strain was most affected. Complementation studies showed that the mutation H22A, within the transmembrane segment of TolA, abolishes immunity protein release. Investigation of the energy requirements demonstrated that the proton motive force of the cytoplasmic membrane is critical. Taken together these results demonstrate for the first time a clear energy requirement for the uptake of a nuclease colicin complex and suggest that energy transduced from the cytoplasmic membrane to the outer membrane by TolA could be the driving force for immunity protein release and concomitant translocation of the nuclease domain.

Investigating early events in receptor binding and translocation of colicin E9 using synchronized cell killing and proteolytic cleavage

Enzymatic colicins such as colicin E9 (ColE9) bind to BtuB on the cell surface of Escherichia coli and rapidly recruit a second coreceptor, either OmpF or OmpC, through which the N-terminal natively disordered region (NDR) of their translocation domain gains entry into the cell periplasm and interacts with TolB. Previously, we constructed an inactive disulfide-locked mutant ColE9 (ColE9(s-s)) that binds to BtuB and can be reduced with dithiothreitol (DTT) to synchronize cell killing. By introducing unique enterokinase (EK) cleavage sites in ColE9(s-s), we showed that the first 61 residues of the NDR were inaccessible to cleavage when bound to BtuB, whereas an EK cleavage site inserted at residue 82 of the NDR remained accessible. This suggests that most of the NDR is occluded by OmpF shortly after binding to BtuB, whereas the extreme distal region of the NDR is surface exposed before unfolding of the receptor-binding domain occurs. EK cleavage of unique cleavage sites located in the ordered region of the translocation domain or in the distal region of the receptor-binding domain confirmed that these regions of ColE9 remained accessible at the E. coli cell surface. Lack of EK cleavage of the DNase domain of the cell-bound, oxidized ColE9/Im9 complex, and the rapid detection of Alexa Fluor 594-labeled Im9 (Im9(AF)) in the cell supernatant following treatment of cells with DTT, suggested that immunity release occurred immediately after unfolding of the colicin and was not driven by binding to BtuB.

Killing of E coli cells by E group nuclease colicins

The process by which the endonuclease domain of colicin E9 is translocated across the outer membrane, the periplasmic space and the cytoplasmic membrane to reach the cytoplasm of E. coli cells, resulting in DNA degradation and cell death, is a unique event in prokaryotic biology. Although considerable information is known about the role of the BtuB outer membrane receptor, as well as the mostly periplasmic Tol proteins that are essential for the translocation process, the precise nature of the interactions between colicin E9 and these proteins remains to be elucidated. In this review, we consider our current understanding of the key events in this process, concentrating on recent findings concerning receptor-binding, translocation and the mechanism of cytotoxicity.

A 76-residue polypeptide of colicin E9 confers receptor specificity and inhibits the growth of vitamin B12-dependent Escherichia coli 113/3 cells

The mechanism by which E colicins recognize and then bind to BtuB receptors in the outer membrane of Escherichia coli cells is a poorly understood first step in the process that results in cell killing. Using N- and C-terminal deletions of the N-terminal 448 residues of colicin E9, we demonstrated that the smallest polypeptide encoded by one of these constructs that retained receptor-binding activity consisted of residues 343-418. The results of the in vivo receptor-binding assay were supported by an alternative competition assay that we developed using a fusion protein consisting of residues 1-497 of colicin E9 fused to the green fluorescent protein as a fluorescent probe of binding to BtuB in E. coli cells. Using this improved assay, we demonstrated competitive inhibition of the binding of the fluorescent fusion protein by the minimal receptor-binding domain of colicin E9 and by vitamin B12. Mutations located in the minimum R domain that abolished or reduced the biological activity of colicin E9 similarly affected the competitive binding of the mutant colicin protein to BtuB. The sequence of the 76-residue R domain in colicin E9 is identical to that found in colicin E3, an RNase type E colicin. Comparative sequence analysis of colicin E3 and cloacin DF13, which is also an RNase-type colicin but uses the IutA receptor to bind to E. coli cells, revealed significant sequence homology throughout the two proteins, with the exception of a region of 92 residues that included the minimum R domain. We constructed two chimeras between cloacin DF13 and colicin E9 in which (i) the DNase domain of colicin E9 was fused onto the T+R domains of cloacin DF13; and (ii) the R domain and DNase domain of colicin E9 were fused onto the T domain of cloacin DF13. The killing activities of these two chimeric colicins against indicator strains expressing BtuB or IutA receptors support the conclusion that the 76 residues of colicin E9 confer receptor specificity. The minimum receptor-binding domain polypeptide inhibited the growth of the vitamin B12-dependent E. coli 113/3 mutant cells, demonstrating that vitamin B12 and colicin E9 binding is mutually exclusive.

Quantification of group A colicin import sites

Pore-forming colicins are soluble bacteriocins which form voltage-gated ion channels in the inner membrane of Escherichia coli. To reach their target, these colicins first bind to a receptor located on the outer membrane and then are translocated through the envelope. Colicins are subdivided into two groups according to the envelope proteins involved in their translocation: group A colicins use the Tol proteins; group B colicins use the proteins TonB, ExbB, and ExbD. We have previously shown that a double-cysteine colicin A mutant which possesses a disulfide bond in its pore-forming domain is translocated through the envelope but is unable to form a channel in the inner membrane (D. Duché, D. Baty, M. Chartier, and L. Letellier, J. Biol. Chem. 269:24820-24825, 1994). Measurements of colicin-induced K+ efflux reveal that preincubation of the cells with the double-cysteine mutant prevents binding of colicins of group A but not of group B. Moreover, we show that the mutant is still in contact with its receptor and import machinery when it interacts with the inner membrane. From these competition experiments, we conclude that each Escherichia coli cell contains approximately 400 and 1,000 colicin A receptors and translocation sites, respectively.

Rescue by vitamin B12 of Escherichia coli cells treated with colicins A and E allows measurement of the kinetics of colicin binding on BtuB

Sensitivity of Escherichia coli bacteria to colicins A and E1 was significantly increased by overproduction of the BtuB receptor protein. The amount of vitamin B12 needed before colicins A and E1 treatment to protect cells against killing was found to be a function of the number of BtuB molecules present at the cell surface. Cells treated by colicins A and E were rescued from killing by addition of vitamin B12 shortly after colicin treatment. The rate of reversal by vitamin B12 may correspond to the kinetics of irreversible binding to BtuB of the various colicins.

Surface exclusion genes traS and traT of the F sex factor of Escherichia coli K-12. Determination of the nucleotide sequence and promoter and terminator activities

The DNA encoding the surface exclusion genes traS and traT of the F sex factor of Escherichia coli K-12 has been sequenced and the biological activity of the various terminators and promoters determined. The data show that traS encodes a 16,861 Mr protein with no apparent signal sequence, as expected for its cytoplasmic membrane location. The protein is extremely hydrophobic. traS has its own promoter and a weak terminator region follows the gene. After the traS termination loop there is a small intergenic region before the traT promoter. The traT gene encodes a 25,932 Mr precursor for the 23,709 Mr mature protein. The amino-terminal signal peptide is 21 amino acid residues, consistent with it being an outer membrane lipoprotein. A very strong termination loop follows the gene and adjacent to this a further loop can be predicted from the sequence. These secondary structures would be expected to enhance the stability of the mRNA in the presence of 3' specific ribonucleases accounting for the apparent long half-life of the messenger. The amino acid sequence of the mature product of traT of F differs from that of R100 by only a single amino acid substitution (Gly for Ala at position 119), whereas that of pED208 (Folac) differs at 40 positions. traT lies in a region of heteroduplex homology between F and R100, and the nucleotide sequence confirms this and demonstrates that this homology breaks down immediately preceding and following the coding region. Sequence analysis shows that this is also so for pED208. Thus the entire traS of F, R100 and pED208 are very different at the DNA level. An open reading frame, preceded by a typical promoter sequence and a weak and poorly located Shine-Dalgarno sequence, follows traT and corresponds to the start of traD. Alone, this promoter appears to be inactive.

Export of protein in Escherichia coli: a novel mutation in ompC affects expression of other major outer membrane proteins

A mutation within the ompC structural gene of Escherichia coli K-12 which affects expression of outer membrane proteins was characterized. The mutation consisted of a 6-base-pair deletion near the 3' end of the gene which removed the amino acids Val-300 and Gly-301 of the mature coding sequence but otherwise left the reading frame intact. The deletion occurred within a region highly conserved among the porins. No protein product was detected from a single copy of the mutant gene. The mutation caused a trans-dominant decrease in the expression of a wild-type ompC allele. The mutation caused a similar decrease in the amounts of OmpA, OmpF, LamB, and Lc proteins, yet it did not appear to affect the minor outer membrane proteins. It had no significant effect on transcription from either ompF or ompC promoters as measured with lacZ operon fusions. The effects of the mutation on other proteins were completely eliminated when the signal sequence was disrupted so that the mutant protein no longer interacted with the secretion machinery of the cell but instead accumulated as precursor in the cytoplasm. A model is proposed involving the translocation of proteins to the outer membrane and the importance of protein conformation in this process.

DNA sequence of the Escherichia coli tonB gene

The nucleotide sequence of a cloned section of the Escherichia coli chromosome containing the tonB gene has been determined. Transcription initiation and termination sites for tonB RNA have been determined by S1 nuclease mapping. The tonB promoter and terminator resemble other E. coli promoters and terminators; the sequence of the tonB terminator region suggests that it may function bidirectionally. The DNA sequence specifies an open translation reading frame between the 5' and 3' RNA termini whose location is consistent with the position of previously isolated tonB::IS1 mutations. The DNA sequence predicts a proline-rich protein with a calculated size of 26.1-26.6 kilodaltons (239-244 amino acids), depending on which of three potential initiation codons is utilized. The predicted NH2 terminus of tonB protein resembles the proteolytically cleaved signal sequences of E. coli periplasmic and outer membrane proteins; the overall hydrophilic character of the protein sequence suggests that the bulk of the tonB protein is not embedded within the inner or outer membrane. A significant discrepancy exists between the calculated size of tonB protein and the apparent size of 36 kilodaltons determined by sodium dodecyl sulfate/polyacrylamide gel electrophoresis.

Siderophore production by Enterobacter cloacae and a common receptor protein for the uptake of aerobactin and cloacin DF13

Iron-starved cultures of Enterobacter cloacae produced two siderophores, identified as enterochelin and aerobactin. The aerobactin was excreted in larger amounts than was enterochelin, and it was synthesized preferentially in the late logarithmic and stationary growth phases under iron-deficient conditions. Enterochelin was synthesized by cultures in the logarithmic phase of growth and preferentially in medium with 1 microM ferric chloride. Both siderophores appeared to be excreted immediately after their synthesis, since no intracellular aerobactin or enterochelin could be detected. The killing activity of the bacteriocin cloacin DF13 was inhibited by aerobactin. It was shown that aerobactin and cloacin DF13 bound to the same receptor sites located in the outer membrane. The synthesis of these receptor sites was induced by iron limitation. We conclude that the receptor for the uptake of aerobactin also functions as receptor for cloacin DF13.

Outer membrane-dependent transport systems in Escherichia coli: effect of repression or cessation of colicin receptor synthesis on colicin receptor activities

Proteins in the outer membrane of gram-negative bacteria serve as general porins or as receptors for specific nutrient transport systems. Many of these proteins are also used as receptors initiating the processes of colicin or phage binding and uptake. The functional activities of several outer membrane proteins in Escherichia coli K-12 were followed after cessation or repression of their synthesis. Cessation of receptor synthesis was accomplished with a thermolabile suppressor activity acting on amber mutations in btuB (encoding the receptor for vitamin B(12), the E colicins, and phage BF23) and in fepA (encoding the receptor for ferric enterochelin and colicins B and D). After cessation of receptor synthesis, cells rapidly became insensitive to the colicins using that receptor. Treatment with spectinomycin or rifampin blocked appearance of insensitive cells and even increased susceptibility to colicin E1. Insensitivity to phage BF23 appeared only after a lag of about one division time, and the receptors remained functional for B(12) uptake throughout. Therefore, possession of receptor is insufficient for colicin sensitivity, and some interaction of receptor with subsequent uptake components is indicated. Another example of physiological alteration of colicin sensitivity is the protection against many of the tonB-dependent colicins afforded by provision of iron-supplying siderophores. The rate of acquisition of this nonspecific protection was found to be consistent with the repression of receptor synthesis, rather than through direct and immediate effects on the tonB product or other components of colicin uptake or action.

Bypass of receptor-mediated resistance to colicin E3 in Escherichia coli K-12

Colicin E3 was found to kill, under conditions of osmotic shock, cells lacking a functional outer membrane receptor (bfe). Under such conditions, component A of the colicin, carrying endonucleolytic activity, also killed bfe cells, whereas fragment T2, obtained by tryptic digestion of the colicin and also active endonucleolytically, was inactive. Tolerance to the colicin caused by defects in the outer membrane could be overcome by osmotic shock, whereas tolerance probably caused by an altered plasma membrane could not.

Outer membrane proteins of Escherichia coli. VI. Protein alteration in bacteriophage-resistant mutants

Protein 1 was shown to be the receptor for phage PA-2 by the observations that the purified protein inactivates the phage, mutants lacking the protein are resistant to the phage, and mutants selected for PA-2 resistance have altered protein. Protein 1 appears as two bands (1a and 1b) on high-resolution polyacrylamide gels. The most abundant classes of mutants (ParI and ParII) selected for PA-2 resistance were found to lack band 1b. The mutations responsible for the ParI and ParII phenotypes were mapped at a locus termed par, which is near nalA on the Escherichia coli chromosome. The cyanogen bromide peptides of proteins 1a and 1b are similar, suggesting that these bands represent modified forms of the same polypeptide. Strains carrying the tolF mutation produce only band 1b. When a par tolF double mutant was constructed, this strain produced only band 1a. These results suggest that genes at the par and tolF loci are involved in modification of protein 1, or regulation of such modification, and are not structural genes for protein 1.

Functional stability of the bfe and tonB gene products in Escherichia coli

The expression of several functional properties of the products of the bfe and tonB genes in Escherichia coli was measured after the specific termination of the synthesis of the products of these genes. This was accomplished by the use of a temperature-sensitive amber suppressor mutation, which allowed control, by manipulation of the growth temperature, of the level of product formed from suppressible mutant alleles of the bfe or tonB gene. The bfe product is an outer membrane receptor protein for vitamin B12, the E-colicins, and bacteriophage BF23. The identity of the tonB product is unknown, but it is necessary for a subsequent step of uptake of vitamin B12, iron chelates, all of the group B colicins, and bacteriophages T1 and phi 80. Results from a different experimental system had shown that the termination of expression of the bfe locus was rapidly followed by loss of sensitivity to colicins E2 and E3 and, subsequently, to bacteriophage BF23. This was confirmed with this experimental system. Receptors that were no longer functional for colicin or phage uptake remained fully effective for B12 uptake, showing that receptors are stable on the cell surface. This supports previous contentions for the presence of different functional states for colicin receptors. The functional properties of the tonB product, measured by B12 uptake or sensitivity to the group B colicin D, were unstable, declining extensively after cessation of its synthesis.

Relation of cell growth and colicin tolerance to vitamin B12 uptake in Escherichia coli

The uptake of vitamin B12 was measured in cells of Escherichia coli whose growth had been inhibited by any of a variety of treatments. In all cases, the secondary, energy-dependent phase of B12 uptake was depressed in proportion to the decrease in growth rate, but uptake was constant in cells growing logarithmically at different rates. The depression of B12 uptake activity was independent of the site of cell metabolism affected by the inhibitor or by its effect on cell viability, and was both more rapid and of greater degree than the effects on the uptake of any of the six amino acids tested. The decline was not affected by inhibitors of either cell division or proteolysis and was manifested without any apparent decrease in the surface B12 binding activity. Transport activity was rapidly regained upon reversal of the inhibition of protein synthesis. Prompted by this response, the uptake of B12 was contrasted to the apparent uptake of the E colicins, which share the same outer membrane receptor. Sensitivity to colicin E1, measured by its inhibition of proline uptake, was not affected by growth inhibition by antibiotic treatment. Finally, there was no specific depression of B12 uptake in cells rendered colicin tolerant either by mutation or as a consequence of phage f1 infection.