Genetic and physiological classification of periplasmic-leaky mutants of Salmonella typhimurium

Paradoxical Activation of a Type VI Secretion System Phospholipase Effector by Its Cognate Immunity Protein

Type VI secretion systems (T6SSs) deliver cytotoxic effector proteins into target bacteria and eukaryotic host cells. Antibacterial effectors are invariably encoded with cognate immunity proteins that protect the producing cell from self-intoxication. Here, we identify transposon insertions that disrupt the tli immunity gene of Enterobacter cloacae and induce autopermeabilization through unopposed activity of the Tle phospholipase effector. This hyperpermeability phenotype is T6SS dependent, indicating that the mutants are intoxicated by Tle delivered from neighboring sibling cells rather than by internally produced phospholipase. Unexpectedly, an in-frame deletion of tli does not induce hyperpermeability because Δtli null mutants fail to deploy active Tle. Instead, the most striking phenotypes are associated with disruption of the tli lipoprotein signal sequence, which prevents immunity protein localization to the periplasm. Immunoblotting reveals that most hyperpermeable mutants still produce Tli, presumably from alternative translation initiation codons downstream of the signal sequence. These observations suggest that cytosolic Tli is required for the activation and/or export of Tle. We show that Tle growth inhibition activity remains Tli dependent when phospholipase delivery into target bacteria is ensured through fusion to the VgrG β-spike protein. Together, these findings indicate that Tli has distinct functions, depending on its subcellular localization. Periplasmic Tli acts as a canonical immunity factor to neutralize incoming effector proteins, while a cytosolic pool of Tli is required to activate the phospholipase domain of Tle prior to T6SS-dependent export. IMPORTANCE Gram-negative bacteria use type VI secretion systems deliver toxic effector proteins directly into neighboring competitors. Secreting cells also produce specific immunity proteins that neutralize effector activities to prevent autointoxication. Here, we show the Tli immunity protein of Enterobacter cloacae has two distinct functions, depending on its subcellular localization. Periplasmic Tli acts as a canonical immunity factor to block Tle lipase effector activity, while cytoplasmic Tli is required to activate the lipase prior to export. These results indicate Tle interacts transiently with its cognate immunity protein to promote effector protein folding and/or packaging into the secretion apparatus.

Paradoxical activation of a type VI secretion system (T6SS) phospholipase effector by its cognate immunity protein

Type VI secretion systems (T6SS) deliver cytotoxic effector proteins into target bacteria and eukaryotic host cells. Antibacterial effectors are invariably encoded with cognate immunity proteins that protect the producing cell from self-intoxication. Here, we identify transposon insertions that disrupt the tli immunity gene of Enterobacter cloacae and induce auto-permeabilization through unopposed activity of the Tle phospholipase effector. This hyper-permeability phenotype is T6SS-dependent, indicating that the mutants are intoxicated by Tle delivered from neighboring sibling cells rather than by internally produced phospholipase. Unexpectedly, an in-frame deletion of tli does not induce hyper-permeability because Δ tli null mutants fail to deploy active Tle. Instead, the most striking phenotypes are associated with disruption of the tli lipoprotein signal sequence, which prevents immunity protein localization to the periplasm. Immunoblotting reveals that most hyper-permeable mutants still produce Tli, presumably from alternative translation initiation codons downstream of the signal sequence. These observations suggest that cytosolic Tli is required for the activation and/or export of Tle. We show that Tle growth inhibition activity remains Tli-dependent when phospholipase delivery into target bacteria is ensured through fusion to the VgrG β-spike protein. Together, these findings indicate that Tli has distinct functions depending on its subcellular localization. Periplasmic Tli acts as a canonical immunity factor to neutralize incoming effector proteins, while a cytosolic pool of Tli is required to activate the phospholipase domain of Tle prior to T6SS-dependent export.

Force-Generation by the Trans-Envelope Tol-Pal System

The Tol-Pal system spans the cell envelope of Gram-negative bacteria, transducing the potential energy of the proton motive force (PMF) into dissociation of the TolB-Pal complex at the outer membrane (OM), freeing the lipoprotein Pal to bind the cell wall. The primary physiological role of Tol-Pal is to maintain OM integrity during cell division through accumulation of Pal molecules at division septa. How the protein complex couples the PMF at the inner membrane into work at the OM is unknown. The effectiveness of this trans-envelope energy transduction system is underscored by the fact that bacteriocins and bacteriophages co-opt Tol-Pal as part of their import/infection mechanisms. Mechanistic understanding of this process has been hindered by a lack of structural data for the inner membrane TolQ-TolR stator, of its complexes with peptidoglycan (PG) and TolA, and of how these elements combined power events at the OM. Recent studies on the homologous stators of Ton and Mot provide a starting point for understanding how Tol-Pal works. Here, we combine ab initio protein modeling with previous structural data on sub-complexes of Tol-Pal as well as mutagenesis, crosslinking, co-conservation analysis and functional data. Through this composite pooling of in silico, in vitro, and in vivo data, we propose a mechanism for force generation in which PMF-driven rotary motion within the stator drives conformational transitions within a long TolA helical hairpin domain, enabling it to reach the TolB-Pal complex at the OM.

The multifarious roles of Tol-Pal in Gram-negative bacteria

In the 1960s several groups reported the isolation and preliminary genetic mapping of Escherichia coli strains tolerant towards the action of colicins. These pioneering studies kick-started two new fields in bacteriology; one centred on how bacteriocins like colicins exploit the Tol (or more commonly Tol-Pal) system to kill bacteria, the other on the physiological role of this cell envelope-spanning assembly. The following half century has seen significant advances in the first of these fields whereas the second has remained elusive, until recently. Here, we review work that begins to shed light on Tol-Pal function in Gram-negative bacteria. What emerges from these studies is that Tol-Pal is an energised system with fundamental, interlinked roles in cell division – coordinating the re-structuring of peptidoglycan at division sites and stabilising the connection between the outer membrane and underlying cell wall. This latter role is achieved by Tol-Pal exploiting the proton motive force to catalyse the accumulation of the outer membrane peptidoglycan associated lipoprotein Pal at division sites while simultaneously mobilising Pal molecules from around the cell. These studies begin to explain the diverse phenotypic outcomes of tol-pal mutations, point to other cell envelope roles Tol-Pal may have and raise many new questions.

Extracellular targeting of an active endoxylanase by a TolB negative mutant of Gluconobacter oxydans

Gluconobacter (G.) oxydans strains have great industrial potential due to their ability to incompletely oxidize a wide range of carbohydrates. But there is one major limitation preventing their full production potential. Hydrolysis of polysaccharides is not possible because extracellular hydrolases are not encoded in the genome of Gluconobacter species. Therefore, as a first step for the generation of exoenzyme producing G. oxydans, a leaky outer membrane mutant was created by deleting the TolB encoding gene gox1687. As a second step the xynA gene encoding an endo-1,4-β-xylanase from Bacillus subtilis was expressed in G. oxydans ΔtolB. More than 70 % of the total XynA activity (0.91 mmol h−1 l culture−1) was detected in the culture supernatant of the TolB mutant and only 10 % of endoxylanase activity was observed in the supernatant of G. oxydans xynA. These results showed that a G. oxydans strain with an increased substrate spectrum that is able to use the renewable polysaccharide xylan as a substrate to produce the prebiotic compounds xylobiose and xylooligosaccharides was generated. This is the first report about the combination of the process of incomplete oxidation with the degradation of renewable organic materials from plants for the production of value-added products.

Different Cellular Origins and Functions of Extracellular Proteins from Escherichia coli O157:H7 and O104:H4 as Determined by Comparative Proteomic Analysis

Extracellular proteins play important roles in bacterial interactions with the environmental matrices. In this study, we examined the extracellular proteins from Escherichia coli O157:H7 and O104:H4 by tandem mass spectrometry. We identified 500 and 859 proteins from the growth media of E. coli O157:H7 and O104:H4, respectively, including 371 proteins common to both strains. Among proteins that were considered specific to E. coli O157:H7 or present at higher relative abundances in O157:H7 medium, most (57 of 65) had secretion signal sequences in their encoding genes. Noticeably, the proteins included locus of enterocyte effacement (LEE) virulence factors, proteins required for peptidyl-lipoprotein accumulation, and proteins involved in iron scavenging. In contrast, a much smaller proportion of proteins (37 of 150) that were considered specific to O104:H4 or presented at higher relative abundances in O104:H4 medium had signals targeting them for secretion. These proteins included Shiga toxin 2 subunit B and O104:H4 signature proteins, including AAF/1 major fimbrial subunit and serine protease autotransporters. Most of the abundant proteins from the growth medium of E. coli O104:H4 were annotated as having functions in the cytoplasm. We provide evidence that the extensive presence of cytoplasmic proteins in E. coli O104:H4 growth medium was due to biological processes independent of cell lysis, indicating alternative mechanisms for this potent pathogen releasing cytoplasmic contents into the growth milieu, which could play a role in interaction with the environmental matrices, such as pathogenesis and biofilm formation.

IMPORTANCE

In this study, we compared the extracellular proteins from two of the most prominent foodborne pathogenic E. coli organisms that have caused severe outbreaks in the United States and in Europe. E. coli O157:H7 is a well-studied Shiga toxigenic foodborne pathogen of the enterohemorrhagic pathotype that has caused numerous outbreaks associated with various contaminated foods worldwide. E. coli O104:H4 is a newly emerged Shiga toxigenic foodborne pathogen of the enteroaggregative pathotype that gained notoriety for causing one of the most deadly foodborne outbreaks in Europe in 2011. Comparison of proteins in the growth medium revealed significant differences in the compositions of the extracellular proteins for these two pathogens. These differences may provide valuable information regarding the cellular responses of these pathogens to their environment, including cell survival and pathogenesis.

A genome-wide screen for bacterial envelope biogenesis mutants identifies a novel factor involved in cell wall precursor metabolism

The cell envelope of Gram-negative bacteria is a formidable barrier that is difficult for antimicrobial drugs to penetrate. Thus, the list of treatments effective against these organisms is small and with the rise of new resistance mechanisms is shrinking rapidly. New therapies to treat Gram-negative bacterial infections are therefore sorely needed. This goal will be greatly aided by a detailed mechanistic understanding of envelope assembly. Although excellent progress in the identification of essential envelope biogenesis systems has been made in recent years, many aspects of the process remain to be elucidated. We therefore developed a simple, quantitative, and high-throughput assay for mutants with envelope biogenesis defects and used it to screen an ordered single-gene deletion library of Escherichia coli. The screen was robust and correctly identified numerous mutants known to be involved in envelope assembly. Importantly, the screen also implicated 102 genes of unknown function as encoding factors that likely impact envelope biogenesis. As a proof of principle, one of these factors, ElyC (YcbC), was characterized further and shown to play a critical role in the metabolism of the essential lipid carrier used for the biogenesis of cell wall and other bacterial surface polysaccharides. Further analysis of the function of ElyC and other hits identified in our screen is likely to uncover a wealth of new information about the biogenesis of the Gram-negative envelope and the vulnerabilities in the system suitable for drug targeting. Moreover, the screening assay described here should be readily adaptable to other organisms to study the biogenesis of different envelope architectures.

Bacteria are surrounded by complex structures called cell envelopes that play an essential role in maintaining cellular integrity. Organisms classified as Gram-negative have especially complicated envelopes that consist of two membranes with a tough cell wall exoskeleton sandwiched between them. This envelope architecture is extremely proficient at preventing drug molecules from entering the cell. Gram-negative bacteria are therefore intrinsically resistant to many antibiotics, limiting the therapeutic options for treating infections caused by these organisms. To reveal new weaknesses in the Gram-negative envelope for drug targeting, we developed a quantitative, high-throughput assay for mutants with envelope biogenesis defects and used it to screen an ordered single-gene deletion library of the model Gram-negative bacterium Escherichia coli. Importantly, the screen implicated 102 genes of previously unknown function as encoding factors that likely participate in envelope biogenesis. As a proof of principle, one of these factors, ElyC (YcbC), was characterized further and shown to play a critical role in the metabolism of the essential lipid carrier used for cell wall synthesis. Further study of ElyC function and that of other factors identified in our screen is likely to reveal novel ways to disrupt the envelope assembly process for therapeutic purposes.

Outer Membrane Vesicles

Outer membrane vesicles (blebs) are produced by Escherichia coli, Salmonella, and all other gram-negative bacteria both in vitro and in vivo. Most of the research in the field has focused on the properties of vesicles derived from pathogenic bacteria and their interactions with eukaryotic cells. These data indicate that vesicles are able to contribute to pathogenesis. Thus, it appears that pathogenic gram-negative bacteria have co-opted vesicles for the dissemination of virulence determinants. However, the role of vesicle production by nonpathogenic bacteria is less obvious. This section reviews the data demonstrating the mechanistic and physiological basis of outer membrane vesicle production by bacteria. Vesiculation can be seen as a mechanism for cells to react to conditions in the surrounding environment by carrying away unnecessary components and allowing rapid modification of the outer membrane composition. In addition, vesicles can transmit biological activities distant from the originating cell. Vesicles could act to bind and deplete host immune factors at the site of infection that would otherwise attack the bacteria. Vesicles in the area surrounding the cell may also provide the cell protection inside a human or animal host. The concept of vesicles as virulence factors has received considerable attention, and they are likely to play a significant role in the pathogenesis of gram-negative bacteria. By analysis of their composition, mechanism of formation, regulation, and physiological function, progress is being made in understanding the ubiquitous nature of outer membrane vesicles produced by gram-negative bacteria.

Escherichia coli strains blocked in Tat-dependent protein export exhibit pleiotropic defects in the cell envelope

The Tat system is a recently discovered protein export pathway that serves to translocate folded proteins, often containing redox cofactors, across the bacterial cytoplasmic membrane. Here we report that tat strains are associated with a mutant cell septation phenotype, where chains of up to 10 cells are evident. Mutant strains are also hypersensitive to hydrophobic drugs and to lysis by lysozyme in the absence of EDTA, and they leak periplasmic enzymes, characteristics that are consistent with an outer membrane defect. Both phenotypes are similar to those displayed by strains carrying point mutations in the lpxC (envA) gene. The phenotype was not replicated by mutations affecting synthesis and/or activity of all known or predicted Tat substrates.

The envA permeability/cell division gene of Escherichia coli encodes the second enzyme of lipid A biosynthesis. UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase

The envA gene of Escherichia coli has been shown previously to be essential for cell viability (Beall, B. and Lutkenhaus, J. (1987) J. Bacteriol. 169, 5408-5415), yet it encodes a protein of unknown function. Extracts of strains harboring the mutant envA1 allele display 3.5-18-fold reductions in UDP-3-O-acyl-N-acetylglucosamine deacetylase specific activity. The deacetylase is the second enzymatic step of lipid A biosynthesis. The structural gene coding for the deacetylase has not been assigned. In order to determine if the envA gene encodes the deacetylase, envA was cloned into an isopropyl-1-thio-beta-D-galactopyranoside-inducible T7-based expression system. Upon induction, a protein of the size of envA was highly overproduced, as judged by SDS-PAGE. Direct deacetylase assays of cell lysates revealed a concomitant approximately 5,000-fold overproduction of activity. Assays of the purified, overproduced EnvA protein demonstrated a further approximately 5-fold increase in specific activity. N-terminal amino acid sequencing of the purified protein showed that the first 20 amino acids matched the predicted envA nucleotide sequence. Contaminating species were present at less than 1% of the level of the EnvA protein. Thus, envA is the structural gene for UDP-3-O-acyl-GlcNAc deacetylase. Based on its function in lipid A biosynthesis, we propose the new designation lpxC for this gene.

Fts insertional mutant of Salmonella typhimurium

A temperature-sensitive filamentation (fts) Salmonella typhimurium mutant was isolated after transposon mutagenesis with mini-Tn 10dTc. The mutant was unable to form colonies after 20 h incubation at 37 degrees C on LB agar. Colonies appeared, however, after longer incubation at the restrictive temperature. Filamentation affected only part of the bacterial population. Rapid mapping using Mu dP22 hybrid phages revealed that the mutation, ftsD220, lies within minutes 68.5 and 73.6 on the genetic map. Further analysis revealed that the ftsD220 mapped at min 73 and that it is linked to cysG (6%) and to aroB (39%). Complementation tests suggested that the ftsD220 mutation is not homologous to a Escherichia coli ftsH mutation.

Leakage of periplasmic enzymes from envA1 strains of Escherichia coli

Previous work ascribed antibiotic hypersensitivity of the envA1 mutant to lowered lipopolysaccharide levels and exposure of the lipid bilayer. In the detailed characterization of the EnvA permeability phenotype presented here, the envA1 mutation was shown to confer leakage of the periplasmic enzymes beta-lactamase and RNase I. Leakage was observed in three different genetic backgrounds, including the original envA1 strain and its parent. In contrast, no detectable leakage of the cytoplasmic enzyme beta-galactosidase was observed. Sensitivity of envA1 strains to a range of antibiotics not previously reported was tested, and lipophilicity (partition coefficient) of a number of antibiotics was determined. On the basis of observations of periplasmic leakage and sensitivity to large hydrophilic antibiotics and lysozyme, part of the permeability phenotype of the envA1 mutant is proposed to be due to transient rupture and resealing of the EDTA-sensitive outer membrane layer. In this regard, the EnvA permeability phenotype falls into a general class of permeability/leaky mutants of both Escherichia coli and Salmonella typhimurium.

Signal strains that can detect certain DNA replication and membrane mutants of Escherichia coli: isolation of a new ssb allele, ssb-3

Mutations in several dna genes of Escherichia coli, when introduced into a strain with a lac fusion in the SOS gene sulA, resulted in formation of blue colonies on plates containing 5-bromo-4-chloro-3-indolyl-beta-D-galactoside (X-Gal). Unexpectedly, several lines of evidence indicated that the blue colony color was not primarily due to induction of the SOS system but rather was due to a membrane defect, along with the replication defect, making the cell X-Gal extrasensitive (phenotypically Xgx), possibly because of enhanced permeability to X-Gal or leakage of beta-galactosidase. (i) In most cases, beta-galactosidase specific activity increased only two- to threefold. (ii) Mutations conferring tolerance to colicin E1 resulted in blue colony color with no increase in beta-galactosidase specific activity. (iii) Mutations in either the dnaA, dnaB, dnaC, dnaE, dnaG, or ssb gene, when introduced into a strain containing a bioA::lac fusion, produced a blue colony color without an increase in beta-galactosidase synthesis. These lac fusion strains can serve as signal strains to detect dna mutations as well as membrane mutations. By localized mutagenesis of the 92-min region of the chromosome of the sulA::lac signal strain and picking blue colonies, we isolated a novel ssb allele that confers the same extreme UV sensitivity as a delta recA allele, which is a considerably greater sensitivity than that conferred by the two well-studied ssb alleles, ssb-1 and ssb-113. The technique also yielded dnaB mutants; fortuitously, uvrA mutants were also found.

Accumulation of a murein-membrane attachment site fraction when cell division is blocked in lkyD and cha mutants of Salmonella typhimurium and Escherichia coli

Membrane fractionation studies were performed on Salmonella typhimurium lkyD(Ts) and E. coli cha(Ts) mutants that appeared to be blocked at a late stage of the cell division cycle. In both cases growth of the mutant strains at nonpermissive temperatures was associated with accumulation of a characteristic cell envelope fraction (fraction OML) that contained inner membrane, murein, and outer membrane components. The isolated fraction corresponded in composition and bouyant density to a fraction from wild-type strains that had previously been suggested (M. H. Bayer, G. P. Costello, and M. E. Bayer, J. Bacteriol. 149:758-767, 1982; K. Ishidate, E. S. Creeger, J. Zrike, S. Deb, B. Glauner, T. J. MacAlister, and L. I. Rothfield, J. Biol. Chem. 261:428-443, 1986) to contain adhesion sites between inner membrane, murein, and outer membrane. The accumulation of OML in LkyD- and Cha- cells was prevented by treatments that blocked DNA synthesis. The effects of interference with DNA synthesis did not appear to involve the SOS response.

Biochemical and morphological properties of membranes of unsaturated fatty acid auxotrophs of Salmonella typhimurium: effects of fluorinated myristic acids

In order to investigate the utility of the fluorine-19 nucleus as a spectroscopic probe, a fluorinated analog of myristic acid has been incorporated into the membrane lipids of an unsaturated fatty acid auxotroph of Salmonella typhimurium. It is capable of supporting limited growth at temperatures above 37 degrees C. Freeze-fracture electron microscopic examinations of the membrane ultrastructure show a temperature and fatty acid supplement-dependent segregation of intramembranous protein particles into distinct patches in the auxotrophic membrane leaving intramembranous protein-denuded areas. The occurrence of these patches seems to be related to the phase separation of membrane lipids. Corresponding changes in the transport and accumulation of methyl thio-beta-D-galactopyranoside and tetracycline are observed. However, transport of histidine does not appear to be dependent on the physical state of the membrane lipids. The auxotroph shows differences in growth and morphological characteristics from those of the wild type. Functions of both inner and outer membranes are shown to be affected as a response to the fatty acid chain composition of the lipids.

New Salmonella typhimurium mutants with altered outer membrane permeability

We describe three new classes of Salmonella typhimurium mutants with increased sensitivity to hydrophobic agents. In contrast to many previously described mutants, the phage sensitivity pattern of these mutants did not give any indication of defective lipopolysaccharide. Furthermore, they had no detectable changes in their phospholipid or outer membrane protein composition, and their growth rate and cell morphology were normal. Class B mutants were nearly as sensitive to novobiocin, fusidic acid, erythromycin, rifampin, and clindamycin as are deep rough (heptoseless) mutants; in addition they were sensitive to methicillin, penicillin (to which heptoseless mutants are resistant), gentian violet, and anionic and cationic detergents. Class A and C mutants had less sensitive, but characteristic phenotypes. None of the three classes were sensitive to serum bactericidal action. The class B mutation mapped between map positions 7 and 11 on the S. typhimurium chromosome, and the class C mutation mapped between positions 5 and 7. The map position for the class A mutation remained undefined, but it was separate from the class B and C mutations and, like those, did not correspond to any gene loci known to participate in the synthesis of major outer membrane constituents.

Lysis of nitrofurantoin-resistant strain of Vibrio el tor

Both nitrofurantoin-sensitive and nitrofurantoin-resistant strains of Vibrio el tor were found to lyze in the presence of Tris-EDTA at alkaline pH. The rate of lysis was appreciably enhanced by lysozyme. The amounts of intracellular components, viz. proteins and carbohydrates, released from the nitrofurantoin-sensitive strain by Tris-EDTA treatment, were significantly lower than those from the nitrofurantoin-resistant strain. Differences in periplasmic proteins released from Tris-EDTA treated cells of nitrofurantoin-resistant and -sensitive strains were revealed by gel electrophoresis.

Bacterial growth and division: genes, structures, forces, and clocks

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Heterogeneity of antibiotic resistance in mucoid isolates of Pseudomonas aeruginosa obtained from cystic fibrosis patients: role of outer membrane proteins

Mucoid Pseudomonas aeruginosa strains isolated from cystic fibrosis patients are very heterogeneous and include a class which is hypersusceptible to carbenicillin (minimum inhibitory concentration, less than or equal to 1 microgram/ml). Hypersusceptible mucoid P. aeruginosa isolates were found in 12 of 22 cystic fibrosis patients examined. In cystic fibrosis patients having both resistant and hypersusceptible mucoid strains, 24 of 54 mucoid colonies obtained from a sputum sample were found to belong to the hypersusceptible class. In most instances, hypersusceptible and resistant strains isolated from the same sputum sample were indistinguishable, aside from their antibiotic susceptibilities, by classical methods. A particular pair of mucoid isolates (one hypersusceptible and one resistant) was chosen for further study. The hypersusceptibility was not limited to carbenicillin but was found to extend to other penicillins, tetracycline, and trimethoprim but not to the aminoglycosides gentamicin and tobramycin. The hypersusceptibility of the mucoid strain was found to be unrelated to amount or ability to synthesize alginate. The hypersusceptible strain was found to have two additional outer membrane proteins (32,000 and 25,000 daltons) as compared with the resistant strain. The 32,000-dalton protein, termed protein N1, was found to be correlated to the hypersusceptibility phenotype, as all spontaneous mutants of the hypersusceptible mucoid strain which were capable of growing in the presence of 50 microgram of carbenicillin per ml had lost the 32,000-dalton outer membrane protein. The possible origins of the hypersusceptibility phenotype and the implications of the heterogeneity of mucoid P. aeruginosa in the pathogenesis of P. aeruginosa are discussed.

Genetic and biochemical characterization of periplasmic-leaky mutants of Escherichia coli K-12

Periplasmic-leaky mutants of Escherichia coli K-12 were isolated after nitrosoguanidine-induced mutagenesis. They released periplasmic enzymes into the extracellular medium. Excretion of alkaline phosphatase, which started immediately in the early exponential phase of growth, could reach up to 90% of the total enzyme production in the stationary phase. Leaky mutants were sensitive to ethylenediaminetetraacetic acid, cholic acid, and the antibiotics rifampin, chloramphenicol, mitomycin C, and ampicillin. Furthermore, they were resistant to colicin E1 and partially resistant to phage TuLa. Their genetic characterization showed that the lky mutations mapped between the suc and gal markers, near or in the tolPAB locus. A biochemical analysis of cell envelope components showed that periplasmic-leaky mutants contained reduced amounts of major outer membrane protein OmpF and increased amounts of a 16,000-dalton outer membrane protein.

ExpA: a conditional mutation affecting the expression of a group of exported proteins in Escherichia coli K-12

A mutant of Escherichia coli K-12 was isolated as conditionally deficient in the expression of two exported proteins simultaneously (i.e. two acid phosphatases). The mutant was found to be thermosensitive on minimal medium at 37 degrees C and above, but grew normally on rich media at these temperatures. The mutation, named expA and located at 22 min on the recalibrated linkage map, depressed the levels of six periplasmic enzymatic activities in bacteria grown at 37 degrees C. At least ten proteins were greatly reduced in the periplasm under these conditions. The mutation also affected some outer membrane proteins, among which were the ompF protein and a protein which may be protein III, but had little effect on cytoplasmic membrane proteins. The gel patterns of the soluble cytoplasmic proteins were not modified except for one major protein of MW 47,000. The activities of beta-galactosidase and of aspartate transcarbamylase were unmodified. After growth at 30 degrees C no difference was observed between expA and expA+ isogenic strains. The results are discussed with respect to the mechanism of protein export.

Escherichia coli pleiotropic mutant that reduces amounts of several periplasmic and outer membrane proteins

We have isolated a mutant of Escherichia coli K-12 that is reduced from 6- to 10-fold in the amount of alkaline phosphatase found in the periplasmic space. The reduced synthesis is not due to effects at the level of transcription regulation of the phoA gene, the structural gene for the enzyme. In addition, the mutation (termed perA) responsible for this phenotype results in reduced amounts of possibly six or more other periplasmic proteins and at least three outer membrane proteins. One of the outer membrane proteins affected is protein IA (D. L. Diedrich, A. O. Summers, and C. A. Schnaitman, J. Bacteriol. 131:598-607, 1977). Although other possibilities exist, one explanation for the phenotype of the perA mutation is that it affects the cell's secretory apparatus.

Positive selection of mutants with cell envelope defects of a Salmonells typhimurium strain hypersensitive to the products of genes hisF and hisH

Strain SB564 and its derivative DA78 are hypersensitive to the inhibitory action of the proteins coded for by genes hisF and hisH on cell division. Transduction of hisO1243, a regulatory mutation that elicits a very high level of expression of the histidine operon, into these strains resulted in the production of long filamentous cells carrying large "balloons" and in growth failure. Forty-one hisO1242 derivatives that escaped inhibition were isolated. These strains showed a large variety of alterations, many of which were related to the cell envelope. The more-frequent alterations included: changes in cell shape, increased sensitivity to one or more of several drugs (deoxycholate, cycloserine, penicillin, novobiocin, acridine orange), increased autolytic activity in alkaline buffer, anomalous fermentation of maltose on eosin--methylene blue plates, and temperature-conditional cell division. The alterations are produced, in some of the strains, by pleiotropic mutations in gene envB (Antón, Mol, Gen. Genet. 160:277--286, 1978) or envD (Antón and Orce, Mol. Gen. Genet. 144:97--105, 1976). Strains affected in divC, divD, and rodA loci have also been identified. Genetic analysis has shown that several strains carry more than one envelope mutation. It is assumed that envelope mutations are positively selected because they somehow alleviate the particularly severe inhibition of cell division caused, in strains SB564 and DA78, by the excessive synthesis of hisF and hisH gene products.

Constitutive expression of the iron-enterochelin and ferrichrome uptake systems in a mutant strain of Salmonella typhimurium

Two high-affinity iron uptake systems are known in Salmonella typhimurium, one utilizing iron-enterochelin and the other utilizing ferrichrome. It has been shown previously that expression of several elements of the iron-enterochelin uptake system are regulated by the iron content of the medium, with growth in high-iron medium resulting in repression of enzymes of enterochelin synthesis and degradation and of the ability of whole cells to take up iron-enterochelin. In this study we describe a mutant strain in which growth in high-iron medium was associated with constitutive expression of: (i) iron-enterochelin uptake by whole cells; (ii) ferrichrome uptake by whole cells; (iii) synthesis of enterochelin; (iv) intracellular degradation of iron-enterochelin; and (v) synthesis of three major outer membrane proteins (OM1, OM2, and OM3). In contrast, in the wild-type strain these properties were expressed only after growth in iron-deficient medium. It is proposed that the mutation affects a gene responsible for regulating expression of the structural genes for the components of the high-affinity iron uptake systems. The term fur, for iron (Fe) uptake regulation, is suggested for this new class of mutant.

Role of murein lipoprotein in morphogenesis of the bacterial division septum: phenotypic similarity of lkyD and lpo mutants

Phenotypes were compared in two different classes of mutants with defects in murein-lipoprotein (lkyD mutants of Salmonella typhimurium and an lpo mutant of Escherichia coli). Both mutations are associated with the same triad of phenotypic abnormalities, consisting of defective formation of the division septum, leakage of periplasmic proteins during growth, and increased sensitivity to several unrelated external toxic agents. The abnormality in septum formation consists of a defect in invagination of the outer membrane during formation of the nascent septum. The results suggest that formation of the murein-lipoprotein link plays an important role in differentiation of the division septum and perhaps also in maintaining the normal barrier function of the outer membrane.

Basis for the observed fluctuation of carboxypeptidase II activity during the cell cycle in BUG 6, a temperature-sensitive division mutant of Escherichia coli

Diaminopimelyl-d-alanyl carboxypeptidase (carboxypeptidase II) is most active at the time of division, whether measured in toluene-treated cells of Escherichia coli K-12 strain D11-1, fractionated by size, or in toluene-treated cells of the temperature-sensitive division mutant, BUG 6 (B. D. Beck and J. T. Park, 1976). The present investigation has now shown that, under conditions that permit division, the increased carboxypeptidase II activity in toluenetreated cells of BUG 6 is probably not due to protein synthesis. Although dividing cells are more permeable than nondividing cells, permeability differences are not sufficient to account for the changes in carboxypeptidase II activity. Thus, in the toluene-treated nondividing cells, carboxypeptidase II is present, but its activity is masked, which suggests the presence of an inhibitor. Another striking difference between nondividing and dividing cells is that carboxypeptidase II is much more readily released from dividing cells by both tris(hydroxymethyl)aminomethane-ethylenediaminetetraacetic acid and toluene treatment. Carboxypeptidase II was partially purified and found to be an 86,000-molecular-weight protein consisting of two 43,000-molecular-weight polypeptides. Tris(hydroxymethyl)aminomethane-ethylenediaminetetraacetic acid treatment of nondividing cells releases less than 10% of the carboxypeptidase II and other periplasmic proteins that are releasable from dividing cells.

Release of outer membrane fragments from normally growing Escherichia coli

A complex containing lipopolysaccharides, phospholipids and proteine separated from the medium by gelfiltration on Sephadex G-200 or by centrifugation. Electron microscopy revealed that this material is released as vesicles and membrane fragements. To determine the origin of these fragments, they were compared to outer and cytoplasmic membranes with respect to keto-deoxyoctulosonic acid, phospholipid, and protein content, phospholipid composition, fatty acid composition, protein distribution on sodium dodecyl sulfate-polyacrylamide gels, buoyant density, and content of several membrane marker enzymes. The results of this comparison indicate that the membrane fragments found in the culture supernatant of normally growing Escherichia coli consist of practically unmodified outer membrane. Possible mechanisms as to the cause of the release of outer membrane fragments, and its relationship to cell-division, are discussed.

Genetic and physiological regulation of intrinsic proteins of the outer membrane of Salmonella typhimurium

Four major outer membrane polypeptides, accounting for approximately 20% of the total protein of the outer membrane of Salmonella typhimurium, were induced by growth in minimal medium. The polypeptides were tightly bound membrane components. Physiological and genetic evidence indicates that the four polypeptides fall in two separate regulation groups. Synthesis of one of these groups was coordinately regulated by the concentration of iron in the medium, and a mutant strain has been identified in which there is constitutive synthesis of this group of major outer membrane proteins.

Morphogenesis of the bacterial division septum: a new class of septation-defective mutants

A new class of mutants of Salmonella typhimurium (lkyD mutants) are described. The mutants are defective in morphogenesis of the division septum, and are characterized by a failure of the outer membrane to invaginate despite normal ingrowth of the cytoplasmic membrane and murein layers of the growing septum. The cell envelopes of the mutants show a significant decrease in the bound form of murein-lipoprotein and a corresponding increase in the free form of the lipoprotein. This suggests that the morphogenic defect may result from a defect in formation of covalent bonds between the free lipoprotein of the outer membrane and the murein of the nascent septum.