Homology of the gene coding for outer membrane lipoprotein within various Gram-negative bacteria

Bacterial lipoproteins; biogenesis, sorting and quality control

Bacterial lipoproteins are a subset of membrane proteins localized on either leaflet of the lipid bilayer. These proteins are anchored to membranes through their N-terminal lipid moiety attached to a conserved Cys. Since the protein moiety of most lipoproteins is hydrophilic, they are expected to play various roles in a hydrophilic environment outside the cytoplasmic membrane. Gram-negative bacteria such as Escherichia coli possess an outer membrane, to which most lipoproteins are sorted. The Lol pathway plays a central role in the sorting of lipoproteins to the outer membrane after lipoprotein precursors are processed to mature forms in the cytoplasmic membrane. Most lipoproteins are anchored to the inner leaflet of the outer membrane with their protein moiety in the periplasm. However, recent studies indicated that some lipoproteins further undergo topology change in the outer membrane, and play critical roles in the biogenesis and quality control of the outer membrane. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Aeromonas salmonicida possesses two genes encoding homologs of the major outer membrane protein, OmpA

Two homologs of the outer membrane protein OmpA were identified in Aeromonas salmonicida by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and amino-terminal sequence analyses. An A. salmonicida genomic DNA library was constructed by using lambda GEM-11 and recombinant phage carrying both genes ompAI and ompAII) selected by immunoscreening. A 5.0-kb BamHI fragment containing the two genes in tandem was subcloned in pBluescript and used for further subcloning and sequencing of the genes. The encoded proteins (Mr = 33,564 and 32,536 for mature OmpAI and OmpAII, respectively) had only 64% identity with each other and otherwise had the highest level of homology to OmpA proteins from the members of the family Enterobacteriaceae. Based on the Escherichia coli OmpA model, an eight-stranded amphipathic beta-barrel model for the membrane assembly of the N-terminal half of OmpAI and OmpAII was predicted. Most variation between the two proteins was localized to the predicted surface loops and periplasmic turns, while the transmembrane strands and C-terminals domains were highly conserved. Expression of ompAI and ompAII separately in E. coli indicated that both genes could be independently transcribed from their own promoters and that both gene products were assembled into the E. coli outer membrane. A survey of different Aeromonas spp. by PCR revealed that possession of two tandem ompA genes was widespread among this genus. This is the first report of any bacterial species possessing two genes for homologs of this major outer membrane protein.

Purification and immunological characterization of a major low-molecular-weight lipoprotein from Borrelia burgdorferi

Borrelia burgdorferi resembles gram-negative bacteria in having both cellular and outer membranes. We previously showed that a lipopolysaccharide (LPS)-like material could be extracted from B. burgdorferi with phenol-chloroform-petroleum ether (PCP). The PCP extract of B. burgdorferi exhibited biological activity in several in vitro assays (e.g., mitogenicity, pyrogenicity, and cytokine release). These activities suggested the presence of endotoxin. The PCP extract of B. burgdorferi, however, also contained a small amount of protein. Preliminary studies showed that monoclonal antibody prepared against this protein inhibited the mitogenic activity of the PCP extract toward murine spleen cells. The current study was therefore undertaken to characterize this protein and to establish methods for its separation from the LPS. The PCP-extracted protein consisted of a single, low-molecular-weight lipoprotein (apparent M(r), 10,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) (SDS-PAGE). By protein analysis, it accounted for 2% of the dry weight of defatted cells, thus making it a major constituent of the spirochete. It was purified from the LPS by initial extraction into 10% Triton X-100 followed by immunoaffinity chromatography in the presence of detergent. On removal of the LPS, the purified lipoprotein formed aggregates stable to SDS-PAGE which were detectable on Western blots (immunoblots) probed with either the monoclonal antibody or polyclonal antiserum. From a plot of the aggregate molecular weight versus aggregate size, a monomer molecular weight of 7,500 was obtained. Indirect immunofluorescence with the monoclonal antibody showed that the lipoprotein was exposed at the surface of the spirochete in only a small percentage of cells. The lipoprotein was present in several strains of B. burgdorferi but absent in other Borrelia spp., treponemes, and gram-negative human pathogens, indicating species specificity.

Nucleotide sequence of the ospAB operon of a Borrelia burgdorferi strain expressing OspA but not OspB

The nucleotide sequence of a 1.6-kb clone containing the gene for outer surface protein A (OspA) of a German strain (GO2) of Borrelia burgdorferi was determined. The deduced amino acid sequence showed a homology of 82% to the OspA molecules from three other B. burgdorferi strains. The best-conserved region was recognized at the 36-amino-terminal amino acids of OspA. OspB could not be identified in the strain investigated, probably because the nucleotide sequence of the ospAB operon prevented expression of the OspB gene.

Nucleotide sequence of the Pseudomonas fluorescens signal peptidase II gene (lsp) and flanking genes

The lsp gene encoding prolipoprotein signal peptidase (signal peptidase II) is organized into an operon consisting of ileS and three open reading frames, designated genes x, orf149, and orf316 in both Escherichia coli and Enterobacter aerogenes. A plasmid, pBROC128, containing a 5.8-kb fragment of Pseudomonas fluorescens DNA was found to confer pseudomonic acid resistance on E. coli host cells and to contain the structural gene of ileS from P. fluorescens. In addition, E. coli strains carrying pBROC128 exhibited increased globomycin resistance. This indicated that the P. fluorescens lsp gene was present on the plasmid. The nucleotide sequences of the P. fluorescens lsp gene and of its flanking regions were determined. Comparison of the nucleotide sequences of the lsp genes in E. coli and P. fluorescens revealed two highly conserved domains in this enzyme. Furthermore, the five genes which constitute an operon in E. coli and Enterobacter aerogenes were found in P. fluorescens in the same order as in the first two species.

Cloning, sequencing, expression, and site-directed mutagenesis of the gene from Clostridium perfringens encoding pyruvoyl-dependent histidine decarboxylase

The DNA encoding pyruvoyl-dependent histidine decarboxylase (HisDCase) of Clostridium perfringens was cloned, sequenced, and overexpressed in Escherichia coli. The gene encodes a single polypeptide of 320 amino acids, Mr 35,526, demonstrating that clostridial HisDCase, which has an (alpha beta)6 structure, is synthesized as a precursor (proHisDCase, pi 6). No pi subunits of proHisDCase were observed in crude or purified preparations of the cloned HisDCase; they appear to undergo rapid cleavage in vivo to the alpha (Mr 24,887) and beta (Mr 10,526) subunits characteristic of this HisDCase. This cleavage occurs between Ser-96 and Ser-97; Ser-97 gives rise to the catalytically essential pyruvoyl group blocking the N-termini of the alpha subunits of the active enzyme. When Ser-97 was converted to an alanyl residue by site-specific mutagenesis, the expressed, inactive protein (pi' 6) contained a single peptide species (pi', Mr 35,510) that was not cleaved either in vivo or in vitro. These results support previous conclusions that activation of the wild-type clostridial proenzyme occurs via nonhydrolytic serinolysis. Although clostridial HisDCase has only a 47% sequence similarity to HisDCase from Lactobacillus 30a, all of the residues known to be important for substrate binding and catalytic action of the Lactobacillus HisDCase are conserved in the C. perfringens enzyme. While the encoded N-terminal Met of clostridial HisDCase is removed by E. coli, the cloned enzyme retains a 10-residue presequence (NKNLEANRNR) not present in the mature enzyme isolated from C. perfringens.

Monoclonal antibodies against three different enterobacterial outer membrane proteins. Characterization, cross-reactivity, and binding to bacteria

BALB/c mice were immunized with whole-cells of Escherichia coli 055:B5 or Proteus mirabilis NCTC 60 to produce broadly cross-reacting monoclonal antibodies (MAbs) against outer membrane (OM) proteins. A total of 10 anti-OM MAbs of the IgG class were selected. These included 5 MAbs against the heat-modifiable (Hm) protein, 3 against the peptidoglycan-associated lipoprotein (PALp), and 2 against Braun's lipoprotein (BLp). Based on competition ELISA, the MAbs defined 2 Hm protein binding sites (Hm I and Hm II), 2 PALp sites (PALp I and PALp II), and one BLp site (BLp I). The MAbs showed broad cross-reactivity against 74 strains of 10 different genera of the Enterobacteriaceae. Non-cross-reacting enteric bacilli occurred only among bacteria of the genera Salmonella, Proteus, and Providentia. The results revealed that Proteus and Providentia strains differed from other enteric bacilli with regard to BLp synthesis or specificity. A panel of 30 non-enteric Gram-negative bacteria did not cross-react. Testing of MAb binding to bacteria showed that a part of the BLp I, PALp I, and PALp II sites was immunoaccessible in intact homologous bacteria, and that the Hm I and Hm II epitopes were inaccessible. The MAbs should facilitate studies of structure and immunobiological function of enterobacterial OM proteins and should have a potential as immunodiagnostic reagents.

The construction and characterization of Neisseria gonorrhoeae lacking protein III in its outer membrane

Protein III (PIII) is a highly conserved, antigenically stable gonococcal outer membrane protein that is closely associated with the major outer membrane protein, protein I (PI). We have previously reported the cloning of the PIII gene. This gene was inserted into the Eco RI site of the runaway plasmid pMOB45. The beta-lactamase (beta la) Bam HI restriction fragment from the gonococcal plasmid pFA3 was inserted at the Xba I site in the PIII gene. The plasmid construct was Hae III methylated and the PIII/beta la insert was excised with Eco RI and used to transform gonococcal strain F62. One beta la+, ampicillin-resistant transformant was isolated and designated 2D. A Western blot of 2D whole cell lysate was probed with affinity-purified polyclonal PIII antisera. No PIII reactivity was detected. Southern blot analysis was performed on F62 and 2D chromosomal DNA that were cut with Eco RI or Cla I. A PIII DNA probe hybridized with fragments 2.2 kb larger in strain 2D than strain F62. This corresponds to the size of the beta la insert. A beta la-specific probe hybridized with the same 2D restriction fragments as above, but did not react with any F62 fragments, confirming that homologous recombination had occurred. There were minimal phenotypic changes between 2D and its parent strain, F62. Chromosomal DNA from 2D was able to transform gonococcal strains F62, UU1, and Pgh 3-2, rendering these PIII-. 2D and other PIII- transformants can now be used to study the role of PIII in gonococcal physiology, metabolism, membrane structure, and pathogenesis. Moreover, we now have organisms from which we can purify gonococcal proteins without PIII contamination.

Cloning, characterization, and effects of overexpression of the Escherichia coli rnd gene encoding RNase D

RNase D is a 3'-exoribonuclease whose in vitro specificity has suggested that it is involved in the processing of tRNA precursors. Its in vivo role has remained unclear, however, because mutant cells devoid of the enzyme display no defect in growth or tRNA processing. To learn more about the structure and function of RNase D, we cloned the Escherichia coli rnd gene, which is thought to code for this enzyme. The rnd gene was isolated from a cosmid library based on elevated RNase D activity and was subcloned as a 1.4-kilobase-pair fragment in pUC18. Maxicell analysis of the cloned fragment revealed that a single protein of approximately 40 kilodaltons, which is the size of RNase D, was synthesized. The rnd gene is present as a single copy on the E. coli chromosome and is totally absent in a deletion mutant. Cells that harbored the cloned rnd gene displayed RNase D activity that was elevated as much as 20-fold over that of the wild type. As growth of the culture progressed, however, RNase D specific activity declined dramatically, together with a similar decrease in plasmid copy number. In contrast, no decrease in copy number was observed with an inactive rnd gene. Placement of the rnd gene downstream from the lac promoter led to inducible RNase D overexpression and concomitantly slowed cell growth. These findings support the idea that rnd is the structural gene for RNase D and indicate that elevated RNase D activity is deleterious to E. coli.

Isolation and characterization of OmpC porin mutants with altered pore properties

The LamB protein is normally required for the uptake of maltodextrins. Starting with a LamB- OmpF- strain, we have isolated mutants that will grow on maltodextrins. The mutation conferring the Dex+ phenotype in the majority of these mutants has been mapped to the ompC locus. These mutants, unlike LamB- OmpF- strains, grew on maltotriose and maltotetraose, but not on maltopentaose, and showed a significantly higher rate of [14C]maltose uptake than the parent strain did. In addition, these mutants showed increased sensitivity to certain beta-lactam antibiotics and sodium dodecyl sulfate, but did not exhibit an increase in sensitivity to other antibiotics and detergents. The nucleotide sequence of these mutants has been determined. In all cases, residue 74 (arginine) of the mature OmpC protein was affected. The results suggest that this region of the OmpC protein is involved in the pore domain and that the alterations lead to an increased pore size.

Deletion analysis of sucrose metabolic genes from a Salmonella plasmid cloned in Escherichia coli K12

The sucrose operon from pUR400, a 78-kbp conjugative Salmonella plasmid, was cloned in Escherichia coli K12. The operon was located in a 5.7-kbp SalI restriction fragment and was subcloned, in each of two possible orientations, using the expression vector pUC18. The insert DNA was restriction mapped and duplicate restriction sites in the insert and in the polylinker of the vector were used to create various deletions promoter distal in the operon sequence. Additional deletions were made with the restriction exonuclease Bal31. Cells containing hybrid plasmids with specified deletions lacked the ability to transport sucrose or were constitutive for hydrolase and/or uptake activities. The scrA (enzyme IIScr) and scrR (regulatory) genes resided within 2900-bp SmaI-SalI DNA fragment and were assigned the order scrB, scrA, scrR. An amplified sucrose-inducible gene product, Mr 68,000, was detected only in the membrane fraction from recombinant cells that contained plasmid with the intact operon sequence. This protein represented 11% of the total membrane protein and was resistant to extraction with 0.5 M sodium chloride, 2% Triton X-100, and 0.5% sodium deoxycholate. The protein did not appear to be the product of either the scrA, scrB, or scrR gene and may therefore represent a previously unidentified membrane-bound sucrose protein. A new gene, scrC, is proposed. In addition, the cloned 5.7-kbp SalI and 2.5-kbp SmaI-SalI DNA fragments failed to hybridize to chromosomal DNA from Bacillus subtilis, Streptococcus lactis, Streptococcus mutans, and Lactobacillus acidophilus as well as to DNA from a sucrose plasmid from Salmonella tennessee. However, the probes showed weak homology with a 20-kbp EcoRI restriction fragment from Klebsiella pneumoniae.

Cloning and expression of a common Legionella outer membrane antigen in Escherichia coli

A genomic library of Legionella pneumophila was constructed by inserting L. pneumophila knoxville-1 strain (LPK-1) chromosome fragments into cosmid vector pHC79. Screening of the library with antibodies directed against a major outer membrane protein/lipopolysaccharide complex from LPK-1 resulted in the identification of six clones that reacted with the antiserum. Western blot analysis indicated that a 19,000 dalton (19 kDa) component was the reactive antigen in all of the clones. Western blot analysis of outer membranes from L. pneumophila serogroups and other Legionella species revealed that the cloned 19 kDa antigen was common to all serogroups and all but one of the five other Legionella species examined. One of the 19 kDa expressing clones was used as an immunoabsorbent to recover antibody to the 19 kDa antigen thus confirming the surface localization of this L. pneumophila antigen in E. coli.

Brucella outer membrane lipoprotein shares antigenic determinants with Escherichia coli Braun lipoprotein and is exposed on the cell surface

In an enzyme-linked immunosorbent assay (ELISA), purified Brucella abortus and Escherichia coli peptidoglycan-linked lipoproteins gave a strong cross-reaction with sera from rabbits hyperimmunized with the heterologous lipoprotein. When smooth E. coli cells were used as ELISA antigens, the immunological cross-reaction was not observed unless the cells were treated to remove lipopolysaccharide and other outer membrane components. In contrast, intact cells from smooth strains of B. abortus and Brucella melitensis bound anti-lipoprotein immunoglobulin G, and the controls performed by ELISA showed that this reaction was not due to antibodies to the lipopolysaccharide, group 3 outer membrane proteins, or porins. Electron microscopy of cells labeled with antilipoprotein serum and protein A-colloidal gold showed specific labeling of smooth cells from both B. abortus and B. melitensis, even though unspecific labeling by nonimmune serum was observed with rough B. abortus. These results confirm the close similarity between E. coli and Brucella peptidoglycan-linked lipoproteins and show that, in contrast to E. coli, the lipoprotein of B. abortus and B. melitensis is partially exposed on the surface of smooth cells.

Cloning of the structural genes of three H8 antigens and of protein III of Neisseria gonorrhoeae

A bank of gonococcal DNA was constructed in the lambda gt11 expression vector. immunological screening of the bank resulted in the isolation of a clone that contains the structural gene of protein III. In addition, several clones reactive with mAbs specific for the H8 antigen were isolated. DNA hybridization studies revealed that these H8-reactive clones were derived from three different gonococcal genes. When the products produced by these clones were used to absorb antibodies from a rabbit antiserum, and the eluted antibodies were used in immunological studies, it could be shown that the parent gonococcus expressed the product of two of these H8 genes, and in strain R10, these had Mr of approximately 19,700 21,200 respectively. The larger form has not been recognized hitherto because the epitope reactive with the H8 mAb may be masked in this product.

Identification, immunochemical characterization, and purification of a major lipoprotein antigen associated with the inner (cytoplasmic) membrane of Escherichia coli

A major antigenic constituent of the inner membrane of Escherichia coli ML308-225 was identified as a 28.5-kilodalton lipoprotein containing covalently bound glycerol and palmitate. This lipoprotein corresponded to antigen 47 in the crossed immunoelectrophoresis profile of membrane vesicles (P. Owen and H.R. Kaback, Proc. Natl. Acad. Sci. USA 75:3148-3152, 1978) and to new lipoprotein 4 described for E. coli B by Ichihara et al. (S. Ichihara, H. Hussain, and S. Mizushima, J. Biol. Chem. 256:3125-3129, 1980). Experiments involving isopycnic centrifugation of spheroplast envelopes indicated that antigen 47 was enriched in cytoplasmic membrane subfractions of low density. The protein did not manifest an obvious association with peptidoglycan of the types displayed by the bound form of the Braun (Lpp) lipoprotein, the 21-kilodalton peptidoglycan-associated lipoprotein, or the ompF/C gene products. Antibodies specific for antigen 47 were used to demonstrate that the molecule was immunologically distinct from both the Braun lipoprotein and the peptidoglycan-associated lipoprotein of E. coli. Antigens of similar molecular mass to and cross-reacting with antigen 47 were present in the envelopes of eight type species of the Enterobacteriaceae. A protocol for the purification of antigen 47, based upon its solubility in a chloroform-methanol-water mixture, was developed.

Cloning part of the region encoding biosynthetic enzymes for surface antigen (O-antigen) of Salmonella typhimurium

The rfb gene cluster of Salmonella typhimurium encodes the enzymes required for the biosynthesis of the O-Antigen. A part of it has been cloned in plasmid vectors pBR322 and pUC9 using an adjacent, previously cloned, part of the his operon (Barnes 1981) as a molecular probe for the first clone. A detailed restriction enzyme map of 7.57 kb of rfb DNA is presented and the approximate locations of two of the genes, rfbK and rfbM have been defined.

Bacteriophage receptor area of outer membrane protein OmpA of Escherichia coli K-12

A number of T-even-like bacteriophages use the outer membrane protein OmpA of Escherichia coli as a receptor. We had previously analyzed a series of ompA mutants which are resistant to such phages and which still produce the OmpA protein (R. Morona, M. Klose, and U. Henning, J. Bacteriol. 159:570-578, 1984). Mutational alterations were found near or at residues 70, 110 and 154. Based on these and other results a model was proposed showing the amino-terminal half of the 325-residue protein crossing the outer membrane repeatedly and being cell surface exposed near residues 25, 70, 110, and 154. We characterized, by DNA sequence analysis, an additional 14 independently isolated phage-resistant ompA mutants which still synthesize the protein. Six of the mutants had alterations identical to the ones described before. The other eight mutants possessed seven new alterations: Ile-24----Asn, Gly-28----Val, deletion of Glu-68, Gly-70----Cys, Ser-108----Phe, Ser-108----Pro, and Gly-154----Asp (two isolates). Only the latter alteration resulted in a conjugation-deficient phenotype. The substitutions at Ile-24 and Gly-28 confirmed the expectation that this area of the protein also participates in its phage receptor region. It is unlikely that still other such sites of the protein are involved in the binding of phage, and it appears that the phage receptor area of the protein has now been characterized completely.

Evolution of the lipoprotein gene in the enterobacteriaceae. Cloning and DNA sequence of the lpp gene from Proteus mirabilis

We cloned the lipoprotein gene from Proteus mirabilis and determined its DNA sequence. Comparison with the lpp genes from Escherichia coli, Serratia marcescens, Erwinia amylovora and Morganella morganii revealed several unique features of the evolution of the lpp gene in the Enterobacteriaceae and enabled us to establish phylogenetic relationships between these bacteria.

Presence of DNA, encoding parts of bacteriophage tail fiber genes, in the chromosome of Escherichia coli K-12

The classical T-even bacteriophages recognize host cells with their long tail fibers. Gene products 35, 36, and 37 constitute the distal moiety of these fibers. The free ends of the tail fibers, which are formed by the CO2H terminus of gene product 37, possess the host range determinants. It was found that 4 out of 10 different strains of Escherichia coli K-12 contained regions of chromosomal DNA which hybridized with a probe consisting of genes 35, 36, and 37 of the T-even phage K3. From one strain this homologous DNA, which was associated with an EcoRI fragment of about 5 kilobases, was cloned into plasmid pUC8. Two independently recovered hybrid plasmids had undergone a peculiar rearrangement which resulted in the loss of about 3 kilobases of cloned DNA and a duplication of both the vector and the remaining chromosomal DNA. The mechanisms causing this duplication-deletion may be related to that of transposases. The cloned DNA was capable of recombination with phage T4 gene 36 and a phage T2 gene 37 amber mutant. DNA sequencing revealed the existence of regions of identity between the cloned DNA and genes 36 and 37 of phage T2. In addition, after growth of a derivative of phage K3 on a strain harboring T2 DNA, it was found that this phage contained the same parts of the T2 tail fiber genes which had been recovered from the bacterial chromosome. There appears to be little doubt that the phage had picked up this DNA from the host. The possibility is considered that a repertoire of parts of genes 36 and 37 of various T-even-type phages is present in their hosts, allowing the former to change their host ranges.

Antigenic polymorphism of the LamB protein among members of the family Enterobacteriaceae

In this study we demonstrate that most members of the family Enterobacteriaceae possess a maltose-inducible outer membrane protein homologous to the LamB protein of Escherichia coli K-12. These proteins react with polyclonal antibodies raised against the LamB protein of E. coli K-12. We compared the antigenic structure of the LamB protein in members of the family Enterobacteriaceae with six monoclonal antibodies raised against the LamB protein of E. coli K-12. Four of them reacted with epitopes located at the outer face of the membrane, and two reacted with epitopes located at the inner face of the membrane. A great degree of variability was observed for the external epitopes. Even in a single species, such as E. coli, an important polymorphism was present. In contrast, the internal epitopes were more conserved.

A single recombinant plasmid expressing two major outer surface proteins of the Lyme disease spirochete

A gene bank of DNA from the Lyme disease spirochete was constructed in the plasmid pBR322. Plasmid pTRH32, a recombinant that in Escherichia coli expresses the two major outer surface proteins of the Lyme disease spirochete, was identified. One of the recombinant products, designated OspA, represents a surface protein that appears to be common to all Lyme disease spirochetes, whereas the other recombinant product, designated OspB, represents a more variable surface protein. This recombinant plasmid provides a foundation for future studies on the epidemiology and pathogenesis of Lyme disease as well as on the genetic organization of the etiologic agent.

Molecular characterisation of the Stc- mutation of Escherichia coli K-12

The previously described Stc- (suppressor of TolC) mutation modifies the phenotype of tolC mutants from OmpF- to OmpF+. Restriction mapping of chromosomal DNA from Stc+ and Stc- strains was performed to investigate the nature of the mutation which was shown to be a deletion, upstream of the ompC gene. DNA from the region of the deletion was cloned into pUC18 and a 650-bp PstI-EcoRI fragment was sequenced. The deletion started 49 bp upstream of the AUG start codon of the ompC gene, thus removing part of the ompC promoter and the whole of the micF gene. We suggest that the deletion of micF gives rise to the Stc- phenotype since the effect of micF expression is assumed to reduce ompF expression, and the Stc- phenotype involves increase in ompF expression.

Porin from bacterial and mitochondrial outer membranes

The outer membrane of gram-negative bacteria acts as a molecular filter with defined exclusion limit for hydrophilic substances. The exclusion limit is dependent on the type of bacteria and has for enteric bacteria like Escherichia coli and Salmonella typhimurium a value between 600 and 800 Daltons, whereas molecules with molecular weights up to 6000 can penetrate the outer membrane of Pseudomonas aeruginosa. The molecular sieving properties result from the presence of a class of major proteins called porins which form trimers of identical subunits in the outer membrane. The porin trimers most likely contain only one large but well-defined pore with a diameter between 1.2 and 2 nm. Mitochondria are presumably descendents of gram-negative bacteria. The outer membrane of mitochondria contains in agreement with this hypothesis large pores which are permeable for hydrophilic substances with molecular weights up to 6000. The mitochondrial porins are processed by the cell and have molecular weights around 30,000 Daltons. There exists some evidence that the pore is controlled by electric fields and metabolic processes.

Escherichia coli K-12 outer membrane protein (OmpA) as a bacteriophage receptor: analysis of mutant genes expressing altered proteins

The outer membrane protein OmpA of Escherichia coli K-12 serves as a receptor for a number of T-even-like phages. We have isolated a series of ompA mutants which are resistant to such phages but which still produce the OmpA protein. None of the mutants was able to either irreversibly or reversibly bind the phage with which they had been selected. Also, the OmpA protein is required for the action of colicins K and L and for the stabilization of mating aggregates in conjugation. Conjugal proficiency was unaltered in all cases. Various degrees of colicin resistance was found; however, the resistance pattern did not correlate with the phage resistance pattern. DNA sequence analyses revealed that, in the mutants, the 325-residue OmpA protein had suffered the following alterations: Gly-65----Asp, Gly-65----Arg, Glu-68----Gly, Glu-68----Lys (two isolates), Gly-70----Asp (four isolates), Gly-70----Val, Ala-Asp-Thr-Lys-107----Ala-Lys (caused by a 6-base-pair deletion), Val-110----Asp, and Gly-154----Ser. These mutants exhibited a complex pattern of resistance-sensitivity to 14 different OmpA-specific phages, suggesting that they recognize different areas of the protein. In addition to the three clusters of mutational alterations around residues 68, 110, and 154, a site around residue 25 has been predicted to be involved in conjugation and in binding of a phage and a bacteriocin (R. Freudl, and S. T. Cole, Eur. J. Biochem, 134:497-502, 1983; G. Braun and S. T. Cole, Mol. Gen. Genet, in press). These four areas are regularly spaced, being about 40 residues apart from each other. A model is suggested in which the OmpA polypeptide repeatedly traverses the outer membrane in cross-beta structure, exposing the four areas to the outside.

Promoter exchange between ompF and ompC, genes for osmoregulated major outer membrane proteins of Escherichia coli K-12

Expression of the ompF and ompC genes coding for major outer membrane proteins OmpF and OmpC is regulated in opposite directions by medium osmolarity. Chimera genes were constructed by a reciprocal exchange of the promoter-signal sequence region between the two genes. The chimera gene construction was designed so that the proteins synthesized by these genes were essentially the same as the OmpC and OmpF proteins. Studies with the chimera genes demonstrated that the osmoregulation of the OmpF-OmpC synthesis was promoter dependent. They also showed that cells grew normally even when the osmoregulation took place in opposite directions. The effects of the ompR2 and envZ mutations, which suppress ompC and ompF expression, respectively, also became reversed. The reduced expression was still subject to the promoter-controlled osmoregulation. Based on these observations, the mechanism of regulation of the ompF-ompC gene expression and its physiological importance are discussed.

Isolation of a major cell envelope protein from Fusobacterium nucleatum

A major, heat-modifiable cell envelope protein was identified in Fusobacterium nucleatum FDC 364 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This protein, designated HM-1, had apparent molecular weights of 38,500 and 50,000 when heated in sodium dodecyl sulfate at 50 and 100 degrees C, respectively. Whole cells were labeled with 125I, and the results suggested that the HM-1 protein may be exposed on the bacterial surface. The HM-1 protein was isolated in association with the peptidoglycan by extraction of whole cells or cell envelopes with 2% sodium dodecyl sulfate at 55 degrees C. Heating the peptidoglycan-HM-1 protein complex in the detergent at 100 degrees C resulted in the quantitative release of the protein. Isoelectric focusing experiments and amino acid analysis revealed that the HM-1 protein had a basic character and was moderately hydrophilic. Various strains of F. nucleatum as well as three oral fusiform isolates contained a serologically related protein. The abundance and location of the HM-1 protein in F. nucleatum suggest that it has the potential to participate in cell surface-related interactions of this bacterium.

Cloning and expression of Legionella pneumophila antigens in Escherichia coli

To isolate and characterize Legionella pneumophila antigens, we constructed a genomic library of L. pneumophila serogroup 1 (strain 130b). L, pneumophila DNA fragments (2.5 to 7.5 megadaltons) obtained by partial digestion with Sau 3A endonuclease and size fractionation on a sucrose density gradient were inserted into the dephosphorylated BamHI site of vector pBR322; CaCl2-treated Escherichia coli cells of strain HB101 were transformed with hybrid plasmids. To detect expression of antigens, 2,559 ampicillin-resistant transformants were transferred to nitrocellulose paper, lysed in situ, and screened by enzyme immunoassay (EIA) with E. coli-absorbed rabbit anti-L. pneumophila sera. A total of 77 (3%) of the colonies were reactive by EIA; 31 (1.2%) were strongly reactive, and 6 were strongly reactive by EIA without colony lysis. Analysis of 29 stable, strongly reactive clones by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electroblotting showed antigenic bands in 18 clones by EIA with E. coli-absorbed antisera. Absorption of antisera with heat- and Formalin-killed L. pneumophila antigen eliminated or diminished the reactivity of the antigenic bands in representative clones. These studies confirm that several L. pneumophila antigens can be cloned and expressed in E. coli.

Conservation of capR (lon) DNA of Escherichia coli K-12 between distantly related species

Mutations in the capR gene of Escherichia coli K-12 are responsible for a wide variety of phenotypic changes, including defects in cell division. Since this gene plays a critical role in cell division, it might be evolutionarily conserved. Of the DNAs examined by Southern analysis, capR probe sequences were found not only in other enterics but also in Caulobacter crescentus CB13 and the distantly related archebacterium Halobacterium halobium R1.

Expression of Treponema pallidum antigens in Escherichia coli K-12

A colony bank of recombinant plasmids harboring Treponema pallidum DNA inserts has been established in Escherichia coli K-12. By using an in situ immunoassay, we identified four E. coli clones that expressed T. pallidum antigens. Thus, recombinant DNA technology may provide powerful new tools for studying the pathogenesis of T. pallidum infection.