The regulation of porin expression in Escherichia coli: effect of turgor stress

The OmpF and OmpC porins are major outer membrane proteins of Escherichia coli. Their expression is affected by medium osmolarity such that OmpF is normally produced at low osmolarity and OmpC at high osmolarity. Potassium ion accumulation is a major means by which cells maintain their internal osmolarity in high osmolarity medium in the absence of organic osmolytes such as glycine-betaine. Starvation for potassium causes cells to become turgor stressed. The effect of turgor stress and potassium ion concentration on OmpF and OmpC expression was examined. It was found that ompF gene expression was switched off by turgor stress but there was no concomitant increase in OmpC. Instead, ompC expression responded to the accumulation of potassium ions by the cell in high osmolarity medium.     

Heterologous expression of Escherichia coli porin genes in Salmonella typhi Ty2: regulation by medium osmolarity, temperature and oxygen availability

Abstract Electrophoretic analysis of outer membrane proteins showed that Salmonella typhi OmpC expression is not reciprocally regulated relative to OmpF as described for Escherichia coli and S. typhimurium . When bacteria were grown in minimal media, both OmpC and OmpF were repressed as the osmolarity increased. However, in Luria broth, expression of OmpC was slightly induced by osmolarity up to 0.3 osmM. Plasmids bearing E. coli ompC-lacZ or ompF-lacZ gene fusions were studied for their expression in S. typhi and E. coli . Under anaerobic growth conditions, expression of ompC-lacZ in S. typhi was maximal at 0.16 osmM, while in E. coli expression was maximal at 0.7 osmM. ompF-lacZ expression was similarly repressed by medium osmolarity and anaerobiosis in both species. In contrast, a drastic difference in the regulation of OmpF by temperature was observed; at 37 °C ompF-lacZ expression was repressed in E. coli . while in S. typhi it was induced.     

Relaxed mutants of Escherichia coli RNA polymerase

When Escherichia coli cells are treated with either polymixin or gramicidin at concentrations that block protein and RNA synthesis, they accumulate a significant amount of guanosine tetraphosphate ppGpp. Such accumulation occurs in stringent (relA+) as well as in relaxed (relA) strains and no guanosine pentaphosphate pppGpp is then detected within the cells. These observations suggest that polypeptide antibiotics elicit ppGpp formation through a mechanism different from the stringent control system triggered by amino acid starvation of bacteria. Experiments based on tetracycline action indicate, moreover, that the accumulation of ppGpp under polymixin or gramicidin treatment is connected with a strong restriction of the degradation rate of this nucleotide.     

Protein localization in Escherichia coli K-12: an analysis of ompC-lacZ gene fusions

Abstract Two conditionally expressed lacZU131 gene fusions were constructed in vivo to the ompC gene which encodes a major outer membrane protein in Escherichia coli . The resulting hybrid molecules contained approximately 25% and 50% of the mature OmpC protein fused to the LacZ. Export analysis showed that under nonoverproducing conditions essentially all synthesized OmpC-LacZ hybrid protein was effectively processed in vivo unless the signal peptide cleavage was inhibited by ethanol addition. Also, the hybrid proteins were highly accessible to solid phase iodination of whole cells under conditions where cytoplasmic proteins remained unlabelled. Thus, hybrids containing large portions of the OmpC protein were clearly recognized by the cellular export machinery, and probably all synthesized hybrid protein was partially translocated through the cytoplasmic membrane.     

Construction of a series of ompC-ompF chimeric genes by in vivo homologous recombination in Escherichia coli and characterization of their translational products

Summary OmpC and OmpF are major outer membrane proteins and although they are homologous proteins, they function differently in several respects. As an approach to elucidate the submolecular structures that determine their differences, we have constructed a series of ompC-ompF chimeric genes by in vivo homologous recombination between these two genes, which are adjacent on a plasmid. The recombination sites in the chimeric genes were localized by means of restriction endonuclease analysis and nucleotide sequence determination. Most of the chimeric gene products were accumulated in the outer membrane. One of the chimeric gene products, with a fusion site in a central region between the OmpC and OmpF proteins, was normally expressed but not accumulated in the outer membrane. The trimeric structures of some of the chimeric gene products appeared to be extremely unstable in a SDS solution. From these results, domains contributing to the formation of specific structures in which the OmpC and OmpF proteins differ were identified. Bacterial cells possessing the chimeric gene products were also investigated as to their sensitivity to phages that require either OmpC or OmpF as a receptor component. With the aid of the chimeric gene products, the immunogenic determinants for three anti-OmpC monoclonal antibodies were found to be localized at different portions of the OmpC polypeptide: the N-terminal, central and C-terminal portions, respectively.     

Monoclonal antibodies against the FhuA (TonA) protein of the Escherichia coli outer membrane

Abstract Outer membranes of Escherichia coli K-12 were used to isolate hybridoma cell lines that produce monoclonal antibodies against the FhuA (TonA) protein. Two monoclonal antibodies were obtained from independent immunization and fusion experiments. The antibodies belonged to the subclass IgG₁ and κ, and IgG_2b and κ, respectively. The latter antibody was purified by affinity chromatography on protein A-Sepharose. The culture supernatants of the hybridoma cell lines and the isolated antibody inhibited adsorption of the phages T5 and T1 to E. coli cells while binding of phage ø80, which also uses the FhuA protein as a receptor, remained unaffected. The specificity of the antibodies to the FhuA protein was supported by the prevention of killing of cells by colicin M and by the lack of inhibition of colicin B and of phage BG23. Transport of iron(III) as ferrichrome complex was not inhibited by the isolated antibody. However, partial competition with the adsorption of the phages T2, TuIb and T6 was observed which may indicate an organization of certain functional phage receptors into clusters.     

Uptake of the Herbicidal Glyphosate by Escherichia coli K-12

The uptake of the aminoacid biosynthesis inhibitor, used as the broad-spectrum herbicide ingredient, glyphosate (N-[phosphonomethyl]-glycine) was investigated in E. coli as a model to study mechanisms of cell resistance to antimetabolites as drugs and pesticides. Unlike the glyphosate-degrading Arthrobacter sp. strain for which the first successful measurement of glyphosate uptake and its inhibition by orthophosphate was reported [15], E. coli K-12 cannot take up this inhibitor either in the presence of orthophosphate, or after a prolonged starvation for it. However, cells made competent after an overnight cold CaCl₂ exposure followed by dimethyl sulfoxide (DMSO) treatment could take up this compound (K _m for glyphosate uptake, 274 M). Neither amino acids, belonging to a single transport system, nor orthophosphate gave essential inhibition of glyphosate uptake by these cells.     

Survival of nonspecific porin-deficient mutants of Escherichia coli in black sea water

AIMS: The aim of this study was to investigate the links between survival of Escherichia coli in sea water microcosms in the laboratory and the presence of porins in the outer membrane. The E. coli strains studied were a wild-type strain and a series of outer membrane protein (omp) mutants. METHODS AND RESULTS: Bacteria were suspended in natural or filtered-autoclaved sea water microcosms and numbers determined over an incubation period by plate count and by count of cells capable of respiration. CONCLUSIONS: The type of omp mutation has a significant impact in bacterial survival. The double OmpC-OmpF mutant and the OmpR mutant (which was incapable of synthesizing OmpC and OmpF) survived poorly compared with single omp mutants and the wild-type strain. This suggests that these proteins are important in determining the entry of E. coli into the survival mode. The EnvZ mutant, which lacks the protein by which the cell senses some changes in the environment, survived as well as the wild-type strain when compared by plate counts and by respiring cell count. The loss of the EnvZ protein has no effect on survival but it could prevent the organism sensing the changes in the environment through which entry into the survival state is triggered. SIGNIFICANCE AND IMPACT OF THE STUDY: This work is another piece in the puzzle as to how bacteria survive stress conditions.     

Uropathogenic Escherichia coli (UPEC) strains may carry virulence properties of diarrhoeagenic E. coli

To analyze whether Escherichia coli strains that cause urinary tract infections (UPEC) share virulence characteristics with the diarrheagenic E. coli (DEC) pathotypes and to recognize their genetic diversity, 225 UPEC strains were examined for the presence of various properties of DEC and UPEC (type of interaction with HeLa cells, serogroups and presence of 30 virulence genes). No correlation between adherence patterns and serogroups was observed. Forty-five serogroups were found, but 64% of the strains belonged to one of the 12 serogroups (O1, O2, O4, O6, O7, O14, O15, O18, O21, O25, O75, and O175) and carried UPEC virulence genes (pap, hly, aer, sfa, cnf). The DEC genes found were: aap, aatA, aggC, agg3C, aggR, astA, eae, ehly, iha, irp2, lpfA(O113), pet, pic, pilS, and shf. Sixteen strains presented aggregative adherence and/or the aatA sequence, which are characteristics of enteroaggregative E. coli (EAEC), one of the DEC pathotypes. In summary, certain UPEC strains may carry DEC virulence properties, mostly associated to the EAEC pathotype. This finding raises the possibility that at least some faecal EAEC strains might represent potential uropathogens. Alternatively, certain UPEC strains may have acquired EAEC properties, becoming a potential cause of diarrhoea.     

Molecular analysis of mutant ompR genes exhibiting different phenotypes as to osmoregulation of the ompF and ompC genes of Escherichia coli

Summary Expression of the ompF and ompC genes coding for major outer membrane proteins is osmoregulated by solutes, such as sucrose and NaCl, in the growth medium. The OmpR protein, a positive regulator of these genes, is involved in the osmoregulation (Dairi et al. 1985; Nara et al. 1984). In the present work, five mutant ompR genes exhibiting different phenotypes of osmoregulation were cloned and sequenced. Three of them, ompR1, ompR2 and ompR20, were previously isolated mutants. The others, ompR3 and ompR4, were isolated in the present work. The ompR1 mutation resulted in the deletion of 19 amino acids near the C-terminus of the OmpR protein. The ompR3 and ompR4 mutations resulted in Arg¹⁵ to Cys and Arg⁷¹ to Thr conversions, respectively, at the N-terminal portion, whereas the ompR20 and ompR2 mutations resulted in Arg¹⁵⁰ to Cys and Val²⁰⁷ to Met conversions, respectively, at the C-terminal portion. Based on these results, the structure and function of the OmpR protein are discussed in relation to the mechanism of osmoregulation.Abbreviations Tc^r tetracycline resistance - Sm^r streptomycin resistance - Cm^r chloramphenicol resistance - Km^r kanamycin resistance - SDS sodium dodecyl sulphate     

Characterization of serotypes and outer membrane protein profiles in enteroaggregative Escherichia coli strains.

Twenty-four Escherichia coli strains mainly isolated from children with diarrhea in São Paulo, and showing characteristics of enteroaggregative E. coli (EAEC), were characterized by serotyping and outer membrane protein (OMP) profiles. The relationship between these characteristics was evaluated, as well as the usefulness of OMP profiles in the clonal analysis of EAEC strains. All strains presented aggregative adherence to HeLa cells and were classified in two groups based on their interaction with the EAEC DNA probe. A diversity of serotypes and OMP profiles was observed in both groups studied. Although no significant correlation between serotypes and OMP profiles was observed, unique OMP profiles were identified in 80% of the probe-positive strains which were distributed in only 4 OMP profiles. This result may indicate the presence of a few clones in the probe-positive group. On the other hand, probe-negative strains seem to constitute a more diverse group. In general, the observed heterogeneity in serotypes and OMP profiles described in the present study suggest a great genetic diversity in EAEC isolates of either the same or different serotypes and in strains presenting the same EAEC markers identified in our community.     

Cold-sensitive ompB mutants affecting expression of ompC in Escherichia coli K12

The regulatory locus ompB, consisting of 2 genes, ompR and envZ, is required for the expression of ompC and ompF genes encoding the major outer membrane porin proteins OmpC and OmpF in Escherichia coli K12. We utilized localized mutagenesis to isolate cold-sensitive mutants in the ompB operon. The isolated mutants exhibited a cold-sensitive OmpC⁻ phenotype, but remained OmpF⁺. Furthermore, ompC expression was still regulated by medium osmolarity. The cold-sensitive OmpC⁻ phenotype was complemented by plasmids carrying the wild-type ompB operon, but not by plasmids containing either envZ or ompR genes alone. This suggests that the mutations are in the ompB promotor. We show that the mutations can be used to control expression vectors based on the ompC promotor.     

PhoE protein pores in the outer membrane of Escherichia coli K-12 not only have a preference for Pi and Pi-containing solutes but are general anion-preferring channels

Previous studies on the question of whether the PhoE protein pore has a preference for P_i and P_i-containing solutes only or whether it constitutes a general anion-preferring channel, have not given an unequivocal answer either because the presence of the phosphate binding protein was not ascertained or because only arsenate was tested as a non P_i-containing control solute. Permeability properties of PhoE, OmpF and OmpC protein pores for negatively charged solutes were measured in vivo in the presence of phosphate-binding protein. It appeared that the PhoE protein pore is the most efficient channel for the three tested solutes phosphate, succinate and sulphate. Conditions were established to measure the frequency of ethyl methane sulphonate induced mutations as a function of the presence of pore proteins. These results indicate that PhoE protein also forms the most efficient channel for ethyl methane sulphonate. We conclude that the preference of the PhoE protein pore is not restricted to P_i and P_i-containing solutes but also concerns several other negatively charged solutes.     

Underexpression of porin protein OmpF in agar-entrapped, sessile-like Escherichia coli

The electrophoretic patterns of the outer membrane proteins of agar-entrapped Escherichia coli cells were found to be different from those of free organisms. In particular, the porin protein OmpF was underexpressed in immobilized bacteria, that displayed enhanced resistance to latamoxef.     

Construction of hypervesiculation Escherichia coli strains and application for secretory protein production

Outer membrane vesicles (OMVs) are extracellular vesicles released from the surface of Gram-negative bacteria, including Escherichia coli. Several gene-deficient mutants relating to envelope stress (nlpI and degP) and phospholipid accumulation in the outer leaflet of the outer membrane (mlaA and mlaE) increase OMV production. This study examined the combinatorial deletion of these genes in E. coli and its effect on OMV production. The nlpI and mlaE double-gene-knockout mutant (ΔmlaEΔnlpI) showed the highest OMV production. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis-based quantitative analysis showed that OMV production by strain ΔmlaEΔnlpI was ~30 times that by the wild-type (WT). In addition, to evaluate the protein secretion capacity of OMVs, a green fluorescent protein (GFP) fused with outer membrane protein W (OmpW) was expressed in OMVs. Western blot analysis showed that GFP secretion through OMVs reached 3.3 mg/L in the culture medium of strain ΔmlaEΔnlpI/gfp, 500 times that for the WT. Our approach using OMVs for extracellular protein secretion in E. coli is an entirely new concept compared with existing secretion systems.