Genetic analysis of Escherichia coli K-12 chromosomal mutants defective in expression of F-plasmid functions: identification of genes cpxA and cpxB

The role of the two-component systems Cpx and Arc in protein alterations upon gentamicin treatment in Escherichia coli

Background

The aminoglycoside antibiotic gentamicin was supposed to induce a crosstalk between the Cpx- and the Arc-two-component systems (TCS). Here, we investigated the physical interaction of the respective TCS components and compared the results with their respective gene expression and protein abundance. The findings were interpreted in relation to the global proteome profile upon gentamicin treatment.

Results

We observed specific interaction between CpxA and ArcA upon treatment with the aminoglycoside gentamicin using Membrane-Strep-tagged protein interaction experiments (mSPINE). This interaction was neither accompanied by detectable phosphorylation of ArcA nor by activation of the Arc system via CpxA. Furthermore, no changes in absolute amounts of the Cpx- and Arc-TCS could be determined with the sensitive single reaction monitoring (SRM) in presence of gentamicin. Nevertheless, upon applying shotgun mass spectrometry analysis after treatment with gentamicin, we observed a reduction of ArcA ~ P-dependent protein synthesis and a significant Cpx-dependent alteration in the global proteome profile of E. coli.

Conclusions

This study points to the importance of the Cpx-TCS within the complex regulatory network in the E. coli response to aminoglycoside-caused stress.

Electronic supplementary material

The online version of this article (10.1186/s12866-017-1100-9) contains supplementary material, which is available to authorized users.

Everything old is new again: an update on current research on the Cpx envelope stress response

The Cpx envelope stress response (ESR) has been linked to proteins that are integrated into and secreted across the inner membrane for several decades. Initial studies of the cpx locus linked it to alterations in the protein content of both the inner and outer membrane, together with changes in proton motive driven transport and conjugation. Since the mid 1990s, the predominant view of the Cpx envelope stress response has been that it serves to detect and respond to secreted, misfolded proteins in the periplasm. Recent studies in Escherichia coli and other Gram negative organisms highlight a role for the Cpx ESR in specifically responding to perturbations that occur at the inner membrane (IM). It is clear that Cpx adaptation involves a broad suite of changes that encompass many functions in addition to protein folding. Interestingly, recent studies have refocused attention on Cpx-regulated phenotypes that were initially published over 30years ago, including antibiotic resistance and transport across the IM. In this review I will focus on the insights and models that have arisen from recent studies and that may help explain some of the originally published Cpx phenotypes. Although the molecular nature of the inducing signal for the Cpx ESR remains enigmatic, recently solved structures of signaling proteins are yielding testable models concerning the molecular mechanisms behind signaling. The identification of connections between the Cpx ESR and other stress responses in the cell reveals a complex web of interactions that involves Cpx-regulated expression of other regulators as well as small proteins and sRNAs. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

The Escherichia coli Cpx envelope stress response regulates genes of diverse function that impact antibiotic resistance and membrane integrity

The Cpx envelope stress response mediates adaptation to stresses that cause envelope protein misfolding. Adaptation is partly conferred through increased expression of protein folding and degradation factors. The Cpx response also plays a conserved role in the regulation of virulence determinant expression and impacts antibiotic resistance. We sought to identify adaptive mechanisms that may be involved in these important functions by characterizing changes in the transcriptome of two different Escherichia coli strains when the Cpx response is induced. We show that, while there is considerable strain- and condition-specific variability in the Cpx response, the regulon is enriched for proteins and functions that are inner membrane associated under all conditions. Genes that were changed by Cpx pathway induction under all conditions were involved in a number of cellular functions and included several intergenic regions, suggesting that posttranscriptional regulation is important during Cpx-mediated adaptation. Some Cpx-regulated genes are centrally involved in energetics and play a role in antibiotic resistance. We show that a number of small, uncharacterized envelope proteins are Cpx regulated and at least two of these affect phenotypes associated with membrane integrity. Altogether, our work suggests new mechanisms of Cpx-mediated envelope stress adaptation and antibiotic resistance.

Evaluation of safety and protection efficacy on cpxR and lon deleted mutant of Salmonella Gallinarum as a live vaccine candidate for fowl typhoid

We evaluated a recently developed live fowl typhoid (FT) vaccine candidate, JOL916, the cpxR/lon mutant of Salmonella Gallinarum (SG), for safety and protection efficacy in 5-week-old layer chickens. Intramuscular vaccination with JOL916 revealed no or very few lesions in livers and spleens of the animals until the fourth week post-vaccination (wpv). This candidate clearly induced cellular immune responses in 5 of 5 chickens on the first and second wpv based on the peripheral lymphocyte proliferation assay. Systemic IgG responses were observed in 5 of 5 chickens from the first wpv and dramatic elevations were observed on the second and third wpv. Vaccination of chickens offered efficient protection against challenge by a wild-type SG; only slight anorexia and depression were temporarily observed after challenge in the vaccinated group while 100% mortality was observed in the positive control group. Body weight increases per day were slightly reduced between the 3rd and 6th day post challenge (dpc) compared to the negative control group; it was recovered from the 6th dpc. Collectively, these results demonstrate the safety and protective efficacy of JOL916 as a live vaccine for systemic FT.

Physiology, pathogenicity and immunogenicity of lon and/or cpxR deleted mutants of Salmonella Gallinarum as vaccine candidates for fowl typhoid

To construct a novel live vaccine candidate for fowl typhoid (FT) caused by Salmonella Gallinarum (SG), the lon and cpxR genes that are related to host-pathogen interaction were deleted from a wild type SG using the allelic exchange method. The mutants were grown normally, as was the wild type. The biochemical properties of the mutants remained very similar to those of the wild-type, while JOL914 (Δlon) and JOL916 (ΔlonΔcpxR) were mucoid. Extracellular polysaccharide increased 30.6-, 1.3-, and 46.2-fold in JOL914, JOL915 (ΔcpxR), and JOL916, respectively. Dot-blot analysis demonstrated significant increases of FimA expression at 6.77-, 2.33-, and 3.90-fold for JOL914, JOL915, and JOL916, respectively. Internalizations of JOL914, JOL915, and JOL916, in chicken abdominal macrophages, were increased at 4.65-, 0.50-, and 2.72-fold, respectively. Virulences of JOL914, JOL915 and JOL916, analyzed by LD₅₀ using 1-week-old chickens, were attenuated approximately at 10¹-, 10¹-, and > 10³-fold, respectively. The oral inoculations of 2 × 10⁷ cfu of the wild type, JOL914, JOL915 and JOL916 caused 55.6, 16.7, 22.2, and 0.0% mortality, respectively. Significantly moderate gross lesions of the liver and spleen were observed in the JOL916 group compared to the other groups. An induced immune response and significant peripheral mononuclear proliferation reaction were observed in the JOL916 group. At the protection against the wild type challenge, JOL916 offered 100% protection. Thus, the results of this study suggest that JOL916 among the mutants studied represented the safest and most effective live vaccine candidate against FT.

Envelope Stress Responses

The gram-negative bacterial envelope is a complex extracytoplasmic compartment responsible for numerous cellular processes. Among its most important functions is its service as the protective layer separating the cytoplasmic space from the ever-changing external environment. To adapt to the diverse conditions encountered both in the environment and within the mammalian host, Escherichia coli and Salmonella species have evolved six independent envelope stress response systems . This review reviews the sE response, the CpxAR and BaeSR two-component systems (TCS) , the phage shock protein response, and the Rcs phosphorelay system. These five signal transduction pathways represent the most studied of the six known stress responses. The signal for adhesion to abiotic surfaces enters the pathway through the novel outer membrane lipoprotein NlpE, and activation on entry into the exponential phase of growth occurs independently of CpxA . Adhesion could disrupt NlpE causing unfolding of its unstable N-terminal domain, leading to activation of the Cpx response. The most recent class of genes added to the Cpx regulon includes those involved in copper homeostasis. Two separate microarray experiments revealed that exposure of E. coli cells to high levels of external copper leads to upregulation of several Cpx regulon members. The BaeSR TCS has also been shown to mediate drug resistance in Salmonella. Similar to E. coli, the Bae pathway of Salmonella enterica mediates resistance to oxacillin, novobiocin, deoxycholate, β-lactams, and indole.

Epigenetic gene regulation in the bacterial world

Like many eukaryotes, bacteria make widespread use of postreplicative DNA methylation for the epigenetic control of DNA-protein interactions. Unlike eukaryotes, however, bacteria use DNA adenine methylation (rather than DNA cytosine methylation) as an epigenetic signal. DNA adenine methylation plays roles in the virulence of diverse pathogens of humans and livestock animals, including pathogenic Escherichia coli, Salmonella, Vibrio, Yersinia, Haemophilus, and Brucella. In Alphaproteobacteria, methylation of adenine at GANTC sites by the CcrM methylase regulates the cell cycle and couples gene transcription to DNA replication. In Gammaproteobacteria, adenine methylation at GATC sites by the Dam methylase provides signals for DNA replication, chromosome segregation, mismatch repair, packaging of bacteriophage genomes, transposase activity, and regulation of gene expression. Transcriptional repression by Dam methylation appears to be more common than transcriptional activation. Certain promoters are active only during the hemimethylation interval that follows DNA replication; repression is restored when the newly synthesized DNA strand is methylated. In the E. coli genome, however, methylation of specific GATC sites can be blocked by cognate DNA binding proteins. Blockage of GATC methylation beyond cell division permits transmission of DNA methylation patterns to daughter cells and can give rise to distinct epigenetic states, each propagated by a positive feedback loop. Switching between alternative DNA methylation patterns can split clonal bacterial populations into epigenetic lineages in a manner reminiscent of eukaryotic cell differentiation. Inheritance of self-propagating DNA methylation patterns governs phase variation in the E. coli pap operon, the agn43 gene, and other loci encoding virulence-related cell surface functions.

The Cpx system of Escherichia coli, a strategic signaling pathway for confronting adverse conditions and for settling biofilm communities?

Amongst the thirty or so two-component systems known in Escherichia coli, the Cpx system has been described as being a stress response system the main function of which is to respond to damage to the cell envelope via activation of proteases and folding catalysts. Nevertheless, the size of the Cpx regulon (several dozens of target genes) and the diversity of the physiological functions associated with it (resistance to hostile conditions, mobility, adherence factors, metabolism, etc.) indicate that the role of Cpx in cell physiology is undoubtedly more complex. The range of cellular functions affected by activation of the Cpx pathway corresponds quite closely to the description of the physiological state of cells grown in biofilms. We suggest that Cpx is a strategic signaling pathway for facing adverse conditions and for settling biofilm communities. Current knowledge of the regulatory mechanisms of the CpxR response (transcriptional and post-transcriptional) and the interactions between CpxR and the other bacterial regulatory systems are presented.

Periplasmic stress and ECF sigma factors

Envelope stress responses play important physiological roles in a variety of processes, including protein folding, cell wall biosynthesis, and pathogenesis. Many of these responses are controlled by extracytoplasmic function (ECF) sigma factors that respond to external signals by means of a membrane-localized anti-sigma factor. One of the best-characterized, ECF-regulated responses is the sigma(E) envelope stress response of Escherichia coli. The sigma(E) pathway ensures proper assembly of outer-membrane proteins (OMP) by controlling expression of genes involved in OMP folding and degradation in response to envelope stresses that disrupt these processes. Prevailing evidence suggests that, in E. coli, a second envelope stress response controlled by the Cpx two-component system ensures proper pilus assembly. The sensor kinase CpxA recognizes misfolded periplasmic proteins, such as those generated during pilus assembly, and transduces this signal to the response regulator CpxR through conserved phosphotransfer reactions. Phosphorylated CpxR activates transcription of periplasmic factors necessary for pilus assembly.

Cpx two-component signal transduction in Escherichia coli: excessive CpxR-P levels underlie CpxA* phenotypes

In Escherichia coli, the CpxA-CpxR two-component signal transduction system and the sigma(E) and sigma(32) response pathways jointly regulate gene expression in adaptation to adverse conditions. These include envelope protein distress, heat shock, oxidative stress, high pH, and entry into stationary phase. Certain mutant versions of the CpxA sensor protein (CpxA* proteins) exhibit an elevated ratio of kinase to phosphatase activity on CpxR, the cognate response regulator. As a result, CpxA* strains display numerous phenotypes, many of which cannot be easily related to currently known functions of the CpxA-CpxR pathway. It is unclear whether CpxA* phenotypes are caused solely by hyperphosphorylation of CpxR. We here report that all of the tested CpxA* phenotypes depend on elevated levels of CpxR-P and not on cross-signalling of CpxA* to noncognate response regulators.

The CpxRA signal transduction system of Escherichia coli: growth-related autoactivation and control of unanticipated target operons

In Escherichia coli, the CpxRA two-component signal transduction system senses and responds to aggregated and misfolded proteins in the bacterial envelope. We show that CpxR-P (the phosphorylated form of the cognate response regulator) activates cpxRA expression in conjunction with RpoS, suggesting an involvement of the Cpx system in stationary-phase survival. Engagement of the CpxRA system in functions beyond protein management is indicated by several putative targets identified after a genomic screening for the CpxR-P recognition consensus sequence. Direct negative control of the newly identified targets motABcheAW (specifying motility and chemotaxis) and tsr (encoding the serine chemoreceptor) by CpxR-P was shown by electrophoretic mobility shift analysis and Northern hybridization. The results suggest that the CpxRA system plays a core role in an extensive stress response network in which the coordination of protein turnover and energy conservation may be the unifying element.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

The response to extracytoplasmic stress in Escherichia coli is controlled by partially overlapping pathways

The activity of the alternate sigma-factor sigmaE of Escherichia coli is induced by several stressors that lead to the extracytoplasmic accumulation of misfolded or unfolded protein. The sigmaE regulon contains several genes, including that encoding the periplasmic protease DegP, whose products are thought to be required for maintaining the integrity of the cell envelope because cells lacking sigmaE are sensitive to elevated temperature and hydrophobic agents. Selection of multicopy suppressors of the temperature-sensitive phenotype of cells lacking sigmaE revealed that overexpression of the lipoprotein NlpE restored high temperature growth to these cells. Overexpression of NlpE has been shown previously to induce DegP synthesis by activating the Cpx two-component signal transduction pathway, and suppression of the temperature-sensitive phenotype by NlpE was found to be dependent on the Cpx proteins. In addition, a constitutively active form of the CpxA sensor/kinase also fully suppressed the temperature-sensitive defect of cells lacking sigmaE. DegP was found to be necessary, but not sufficient, for suppression. Activation of the Cpx pathway has also been shown to alleviate the toxicity of several LamB mutant proteins. Together, these results reveal the existence of two partially overlapping regulatory systems involved in the response to extracytoplasmic stress in E. coli.

The Cpx two-component signal transduction pathway is activated in Escherichia coli mutant strains lacking phosphatidylethanolamine

The CpxA-CpxR two-component signal transduction pathway of Escherichia coli was studied in a mutant (pss-93) lacking phosphatidylethanolamine (PE). Several properties of this mutant are comparable to phenotypes of cpxA point mutants, indicating that this two-component pathway is activated in PE-deficient cells. In contrast to point mutants, cpx operon null mutants have a wild-type phenotype. By use of this information, a cpx operon null allele was introduced into a pss-93 mutant. Certain altered properties of PE-deficient mutants, which were consistent with activation of the Cpx pathway, returned to the wild-type phenotype, namely, active accumulation of proline and thiomethyl-beta-D-galactopyranoside was partially restored to wild-type levels, increased resistance to amikacin returned to wild-type sensitivity, and high levels of degP expression returned to repressed wild-type levels. Elevated levels of acetyl phosphate and nlpE gene product can result in activation of the Cpx pathway. However, inactivation of the nlpE gene or mutations eliminating the ability to make acetyl phosphate did not alter the high level of degP expression in pss-93 mutants. We propose that the lack of PE results in an alteration in cell envelope structure or physical properties, leading to direct activation of the Cpx pathway.

Involvement of cpxA, a sensor of a two-component regulatory system, in the pH-dependent regulation of expression of Shigella sonnei virF gene

In Shigella species, IpaBCD proteins encoded on the virulence plasmid direct the entry of this bacterium into host epithelial cells. Expression of the ipaBCD genes is under the control of several environmental conditions, such as temperature and osmolarity. Extracellular pH also controlled the the expression of the genes, and this regulation occurred mainly at the step of expression of virF, a plasmid-encoded positive regulator of ipaBCD. The expression of virF was activated at high pH (pH 7.4) and repressed at low pH (pH 6.0). We isolated a Tn10 transposon mutant in Escherichia coli K-12 which altered this regulation at the transcriptional level. The Tn10 in the mutant inserted within a reading frame of the cpxA gene, whose product belongs to a family of sensor proteins of two-component signal transduction systems. Complementation analysis showed that cpxA was involved in the pH-dependent regulation of virF gene expression. A gene homologous to cpxA was conserved in Shigella spp. as well as in E. coli. These results may indicate that CpxA senses directly or indirectly a change in extracellular pH and influences the expression of virF in E. coli and Shigella spp.

Accumulation of the F plasmid TraJ protein in cpx mutants of Escherichia coli

We report here studies of the cellular control of F plasmid TraJ protein levels, focusing on the effects of chromosomal cpx mutations. The principal conclusion from our results is that the cpx mutations impair accumulation of the TraJ protein, thereby reducing tra gene expression. We measured TraJ activity in vivo by expression of a traY'-'lacZ fusion gene and TraJ protein by immuno-overlay blot. In strains with normal TraJ levels, traY expression and donor-related functions were reduced in cells carrying any of four cpxA mutations. In the strain background used to isolate cpx mutants, these reductions were especially evident in cells grown to high density, when traY expression and donor activity both increased in cpx+ cells. In each of the four cpxA mutants tested, TraJ levels were lower than in the otherwise isogenic cpxA+ strain. In cells grown to high density, the differences ranged from 4-fold in the cpxA6 strain to > 10-fold in the cpxA2, cpxA5, and cpxA9 strains. The cpxA2 mutation had little or no effect on traY expression or on donor-related functions when TraJ was present in excess of its limiting level in F' or Hfr cells or on a mutant traY promoter whose expression in vivo was independent of TraJ.

Arc and Sfr functions of the Escherichia coli K-12 arcA gene product are genetically and physiologically separable

The Escherichia coli arcA gene product regulates chromosomal gene expression in response to deprivation of oxygen (Arc function; Arc stands for aerobic respiration control) and is required for expression of the F plasmid DNA transfer (tra) genes (Sfr function; Sfr stands for sex factor regulation). Using appropriate lacZ fusions, we have examined the relationship between these two genetic regulatory functions. Arc function in vivo was measured by anaerobic repression of a chromosomal sdh-lacZ operon fusion (sdh stands for succinate dehydrogenase). Sfr function was measured by activation of a plasmid traY-lacZ gene fusion. An eight-codon insertion near the 5' terminus of arcA, designated arcA1, abolished Arc function, as previously reported by S. Iuchi and E.C.C. Lin (Proc. Natl. Acad. Sci. USA 85:1888-1892, 1988), but left Sfr function largely (greater than or equal to 60%) intact. Similarly, the arcB1 mutation, which depressed sdh expression and is thought to act by abolishing the signal input that elicits ArcA function, had little effect (less than or equal to 20%) on the Sfr function of the arcA+ gene product. Conversely, a valine-to-methionine mutation at codon 203 (the sfrA5 allele) essentially abolished Sfr activity without detectably altering Arc activity. These data indicate that Sfr and Arc functions are separately expressed and regulated properties of the same protein.

Regulation of the F plasmid traY promoter in Escherichia coli K12 as a function of sequence context

TraJ and SfrA are, respectively, plasmid and host (Escherichia coli)-encoded proteins normally required for F plasmid traY promoter function. Beginning with plasmids in which a traY-lacZ fusion gene, designated phi (traY'-'lacZ)hyb, and lacY are expressed from the F plasmid traY promoter, we isolated mutants in which lac gene expression was SfrA or TraJ-independent. A total of 45 of 50 SfrA-independent isolates obtained after 2-aminopurine mutagenesis proved to have chromosomal mutations, whereas four out of four isolates obtained without mutagenesis had plasmid mutations. All of 17 isolates selected for TraJ-independent expression after mutagenesis had plasmid mutations. By restriction endonuclease digestions, 25 of 26 SfrA-independent and TraJ-independent plasmid mutations were insertions. Four of the former and three of the latter were examined further. By sequence analysis, all seven proved to be IS1 or IS2 insertions defining five insertion sites between base-pairs -49 and -82 with respect to the major traY transcription initiation site. In two cases, the same insertion allele was obtained from the two selection schemes. All three of the mutants selected for TraJ-independent gene expression manifested SfrA-independent expression as well, and levels of beta-galactosidase in different plasmid mutant strains lacking TraJ and SfrA were indistinguishable. By primer extension analysis, transcription initiation sites for traY mRNA synthesis were unaltered by the mutations. Replacing the tra sequence upstream from base-pair -78, without genetic selection, increased beta-galactosidase activity in the absence of TraJ and SfrA greater than tenfold. Activity increased two- to threefold more in a traJ+ sfrA mutant strain, and fivefold more in a traJ+ sfrA+ strain. Activity was unaltered in an sfrA+ strain without TraJ. By primer extension analysis, the traY promoter was utilized under all conditions. The data indicate that regulation of traY promoter activity is strongly dependent on sequence context.

The Cpx proteins of Escherichia coli K-12: evidence that cpxA, ecfB, ssd, and eup mutations all identify the same gene

An existing cpxA(Ts) mutant was resistant to amikacin at levels that inhibited completely the growth of a cpxA+ and a cpxA deletion strain and failed to grow as efficiently on exogenous proline. These properties are similar to those of mutants altered in a gene mapped to the cpxA locus and variously designated as ecfB, ssd, and eup. The amikacin resistance phenotype of the cpxA mutant was inseparable by recombination from the cpxA mutant phenotype (inability to grow at 41 degrees C without exogenous isoleucine and valine) and was recessive to the cpxA+ allele of a recombinant plasmid. Using methods that ensured independent mutations in the cpxA region of the chromosome, we isolated six new amikacin-resistant mutants following nitrosoguanidine mutagenesis. Three-factor crosses mapped the mutations to the cpxA locus. When transferred by P1 transduction to a cpxB11 Hfr strain, each of the mutations conferred the Tra- and Ilv- phenotypes characteristic of earlier cpxA mutants. Two of the new mutations led to a significantly impaired ability to utilize exogenous proline, and four led to partial resistance to colicin A. Two of the new cpxA alleles were recessive to the cpxA+ allele, and four were dominant, albeit to different degrees. On the basis of these data, we argue that cpxA, ecfB, eup, and ssd are all the same gene. We discuss the cellular function of the cpxA gene product in that light.

Structure of genes narL and narX of the nar (nitrate reductase) locus in Escherichia coli K-12

narL and narX mediate nitrate induction of nitrate reductase synthesis and nitrate repression of fumarate reductase synthesis. We report here the nucleotide sequences of narL and narX. The deduced protein sequences aid in defining distinct subclasses of regulators and sensors in the family of two-component regulatory proteins.

Insertion proQ220::Tn5 alters regulation of proline porter II, a transporter of proline and glycine betaine in Escherichia coli

Mutation pro-220::Tn5, which increases the resistance of Escherichia coli to 3,4-dehydroproline (M. E. Stalmach, S. Grothe, and J. M. Wood, J. Bacteriol. 156:481-486, 1983), is not linked to putP, proP, or proU. It was located at 40.4 min on the E. coli chromosomal linkage map, by conjugational and transductional mapping, and is now denoted proQ220::Tn5. Proline porter II was not detectable when proQ220::Tn5 proP+ bacteria were cultivated under optimal conditions or with nutritional stress (amino acid limitation). Toxic proline analog sensitivity and proline porter II activity were partially restored to proQ220::Tn5 proP+ bacteria, but not to a proQ220::Tn5 proP219 strain, by a hyperosmotic shift and by growth under osmotic stress. Elevated expression of a proP::lacZ gene fusion, for bacteria grown under osmotic stress, was not influenced by the proQ220::Tn5 insertion. We propose that the proQ locus encodes a positive regulatory element which elevates proline porter II activity.

Gene fusions to the ptsM/pel locus of Escherichia coli

We have constructed gene fusions between ptsM/pel and lacZ. These fusions affect both phenotypes assigned to the ptsM/pel locus (at 40 min), namely, no growth on mannose or glucosamine and inhibition of the penetration of bacteriophage lambda DNA, as well as that of other lambdoid phages such as Hy-2. Since the lacZ gene fusions are insertion mutations that abolish target gene function by disrupting the linear contiguity of the gene, it would appear that ptsM and pel are either the same gene or two genes within the same operon. Several size classes of these ptsM/pel-lacZ fusions have been isolated and the corresponding hybrid proteins are associated with the cytoplasmic membrane of Escherichia coli. This is consistent with the proposal that ptsM/pel codes for Enzyme II of the phosphotransferase transport system (PTS) specific for mannose, glucosamine, fructose and glucose. However, we have also identified Tn10 insertion mutations that confer a Man- phenotype but have no effect on the Pel phenotype. Complementation analysis indicates that the Tn10 insertions and the lacZ gene fusions are in different genes. Both of these genes are involved in mannose uptake. This suggests that the locus at 40 min can be subdivided into two genes whose products are required for mannose uptake and that only one of these is involved in the penetration of lambda DNA.

Identification of five new essential genes involved in the synthesis of a secreted protein in Escherichia coli

To define additional components of the export machinery of Escherichia coli, I have isolated extragenic suppressors of a mutant [secA(Ts)] that is temperature sensitive for growth and secretion at 37 degrees C. Suppressors that restored growth at 37 degrees C, but that rendered the cell cold sensitive for growth at 28 degrees C, were obtained. The suppressor mutations fall into at least seven loci, two of which (prlA and secC) have been previously implicated in protein secretion. The five remaining loci (ssaD, ssaE, ssaF, ssaG, and ssaH) have been mapped by P1 transduction and appear to define new genes in E. coli. All of the suppressor mutations allow both enhanced growth and protein secretion of the secA(Ts) mutant at 37 degrees C, but not 42 degrees C, indicating a continued requirement for SecA protein. Strains carrying solely the cold-sensitive mutations show reduced levels of certain periplasmic proteins when grown at low temperatures. In at least one case, that of maltose-binding protein, this defect is at the level of synthesis of the protein. Since mutants in any of seven genes as well as secA amber mutants halt or reduce the synthesis of an exported protein, it appears that E. coli may possess a general and complex mechanism for coupling protein synthesis and secretion.

Identification of the Escherichia coli K-12 cpxA locus as a single gene: construction and analysis of biologically-active cpxA gene fusions

In the accompanying communication we showed that a 2 kb EcoRI-BamHI restriction fragment from the pfkA-rha interval of the Escherichia coli K-12 chromosome fully complemented a chromosomal cpxA mutation when the fragment was cloned in pBR325. The same fragment cloned in pBR322 lacked any complementing activity. We show here that minicells containing the pBR325 derivative (pRA310) synthesized a 33 kDa polypeptide, designated phi 33, that was not synthesized in minicells containing the pBR322 derivative (pRA311) or either of the parent plasmids. Synthesis of the phi 33 polypeptide did not occur in minicells containing Tn5 insertion alleles of pRA310 that inactivated its cpxA complementing activity. These insertions mapped within the vector cat (chloramphenicol acetyltransferase gene) sequence immediately adjacent to the EcoRI site of pRA310 and within the 700-800 bp of the cloned EcoRI-BamHI fragment immediately adjacent to the EcoRI site. Tn5 insertions located within the fragment but closer to the BamHI terminus affected neither the cpxA complementing activity of pRA310 nor synthesis of the phi 33 polypeptide in minicells. Plasmid pRA311 could be converted to a plasmid with cpxA complementing activity by cloning into its EcoRI site a restriction fragment containing a hybrid trp-lacUV5 promoter, the lacZ ribosome binding site, and the first eight lacZ codons.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical and genetic structure of the glpK-cpxA interval of the Escherichia coli K-12 chromosome

Mutations at the cpxA locus of Escherichia coli K-12 affect cellular processes that are not otherwise related. We have now determined the physical and genetic structure of the E. coli chromosome in the region of cpxA (87.5 min). Our results indicate that cpxA is a single gene. Previous studies showed cpxA to be linked to tpiA. We therefore isolated two tpiA+ recombinant plasmids, pRA200 and pRA300, from EcoRI and BamHI digests of F'133, respectively. By genetic complementation or enzyme overproduction, the 9.5 kb EcoRI fragment in pRA200 was shown to include glpK, tpiA and cdh. The 13.6 kb BamHI fragment of pRA300 lacks glpK, but includes tpiA, pfkA and cpxA. Neither fragment complemented a deletion of the rha operon. These data indicate the chromosomal gene order: 87 min-rha-cpxA-pfkA-cdh-tpiA-glpK-88 min. The EcoRI and BamHI fragments overlap in an interval corresponding to about 8.2 kb of DNA. The total region of the E. coli K12 chromosome covered by the two fragments is about 15 kb. A terminal 2 kb EcoRI-BamHI fragment from pRA300 complemented the chromosomal cpxA2[Ts] allele with respect to isoleucine and valine synthesis, RNA bacteriophage sensitivity and surface exclusion in Hfr strains, and envelope protein composition. Complementation occurred when the fragment was subcloned in pBR325 but not when it was subcloned in pBR322, suggesting that the 2 kb fragment lacks expression sequences that are supplied by cat (chloramphenicol acetyltransferase gene) expression sequences of pBR325. The cpxA locus on the E. coli chromosome was established with respect to two chromosomal Tn10 insertions by a combination of genetic and physical analyses. The locus established by those analyses was consistent with the location of the 2 kb EcoRI-BamHI fragment in the physical map of the region. Physical analyses of (rha-pfkA) and (rha-tpiA) deletion strains showed that they lack cpxA and surrounding genes. Since these strains were viable, cpxA is not essential under all growth conditions.

Regulation of expression of Escherichia coli pilus K99

An immunoassay demonstrated that the assembled K99 pilus on the surface of Escherichia coli grown in minimal medium appeared during the logarithmic phase of growth, but the synthesis of K99 subunits, as measured by nonequilibrium two-dimensional gel electrophoresis, occurred throughout the life cycle of the cell. Contrary to other reports, the addition of glucose to the growth medium did not affect K99 pilus assembly or subunit synthesis, although in a K99+ adenyl cyclase (cya) mutant, subunit synthesis was reduced. There was no reduction in the amount of assembled K99 on the cell surface of the cya mutant compared with the wild-type parent. The addition of L-alanine to minimal medium repressed K99 synthesis. However, if L-threonine or L-isoleucine was also included in the growth medium, the effect of L-alanine was reduced. Chloramphenicol caused a complete inhibition of K99 subunit synthesis, but assembly proceeded normally. Growth at 18 degrees C inhibited both subunit synthesis and pilus assembly. Approximately 92% of all cellular K99 was associated with the outer membrane, and 4% was associated with the inner membrane. No K99 was detected in the cytoplasm.

Cyclic AMP and its receptor protein are required for expression of transfer genes of conjugative plasmid F in Escherichia coli

A number of cya and crp mutants of Escherichia coli HfrH were analyzed for several Tra functions of the F plasmid. The mutants were observed to be deficient in conjugal donor ability, absorption of phages MS2 and Q beta and surface exclusion. These defects were suppressed in cya mutants grown with cAMP supplementation. A cAMP concentration of 3 X 10(-4) M produced maximal suppression of donor ability defect in a cya strain. cAMP did not suppress the Tra- phenotype of crp mutants. Latent periods of MS2 were shorter in cya and crp bacteria. Phage T7 development appeared similar in wild type, cya, and crp cells. It is concluded that tra genes of F plasmid are expressed only to a small extent in cya and crp mutants and that cAMP and its receptor protein are required for the normal expression of tra genes.

Synthesis of outer membrane proteins in cpxA cpxB mutants of Escherichia coli K-12

Two major proteins, the murein lipoprotein and the OmpF matrix porin, are deficient in the outer membrane of cpxA cpxB mutants of Escherichia coli K-12. We present evidence that the cpx mutations prevent or retard the translocation of these proteins to the outer membrane. The mutations had no effect on the rate of lipoprotein synthesis. Mutant cells labeled for 5 min with radioactive arginine accumulated as much lipoprotein as otherwise isogenic cpxA+ cpxB+ cells. This lipoprotein accumulated as such; no material synthesized in mutant cells and reactive with antilipoprotein antibodies had the electrophoretic mobility of prolipoprotein. Hence, the initial stages of prolipoprotein insertion into the inner membrane leading to its cleavage to lipoprotein appeared normal. However, after a long labeling interval, mutant cells were deficient in free lipoprotein and lacked lipoprotein covalently bound to peptidoglycan, suggesting that little if any of the lipoprotein synthesized in mutant cells reaches the outer membrane. Immunoreactive OmpF protein could also be detected in extracts of mutant cells labeled for 5 min, but the amount that accumulated was severalfold less in mutant cells than in cpxA+ cpxB+ cells. Analysis of beta-galactosidase synthesis from ompF-lacZ fusion genes showed this difference to be the result of a reduced rate of ompF transcription in mutant cells. Even so, little or none of the ompF protein synthesized in mutant cells was incorporated into the outer membrane.

Mutations in genes cpxA and cpxB alter the protein composition of Escherichia coli inner and outer membranes

Mutations in chromosomal genes cpxA and cpxB altered the protein composition of the inner and outer bacterial membranes. Electrophoretic analyses of membrane proteins from isogenic strains differing only at their cpx loci and of spontaneous cpxA+ revertants of a cpxA cpxB double mutant showed that the alterations define a pattern that is uniquely attributable to the cpx mutations. Two major outer membrane proteins, the OmpF matrix porin and the murein lipoprotein, were deficient or absent from the outer membrane of mutant cells, whereas the quantities of two other major outer membrane proteins, the OmpC matrix porin and the OmpA protein, were not significantly altered. The cpx mutations did not generally alter the functional or chemical properties of the cell envelope. In the electron microscope, mutant cells appeared ovoid, but individual cells showed no surface irregularities to suggest gross defects in the cell envelope. These observations suggest that the primary effect of the mutations is to alter selectively the synthesis or translocation of certain envelope proteins.

Mutations in genes cpxA and cpxB of Escherichia coli K-12 cause a defect in acetohydroxyacid synthase I function in vivo

Mutations in Escherichia coli genes cpxA and cpxB together cause a temperature-sensitive defect in isoleucine and valine syntheses that is related specifically to acetohydroxyacid synthase I. This enzyme catalyzes the first pair of homologous reactions required for the synthesis of these two amino acids. At both permissive and nonpermissive temperatures, mutant cells containing ilvB (the structural gene for acetohydroxyacid synthase I) cloned in a derivative of plasmid pBR322 synthesized comparable amounts of ilvB mRNA and contained several times the enzyme activity normally required to sustain exponential growth, yet these cells remained temperature sensitive for growth in the absence of isoleucine and valine. These observations suggest that the primary effect of the cpx mutations is to block enzyme function in vivo. The enzyme was unstable in mutant cells at growth temperatures above 37 degrees C, but this instability appeared to be a secondary effect on the cpx mutations.

New acetohydroxy acid synthase activity from mutational activation of a cryptic gene in Escherichia coli K-12

A mutation in an allele identified as ilvJ662 causes the expression of acetohydroxy acid synthase activity that is resistant to feedback inhibition by L-valine. the ilvJ662 allele was transduced as an unselected marker into a strain, CU1126 (ilvB, ilvHI), deficient in acetohydroxy acid synthase activity. The ilvJ662 allele appears to code for a new acetohydroxy acid synthase activity (acetohydroxy acid synthase IV), with physical, kinetic, and physiological properties distinct from the other three isozymes. The catalytic function of acetohydroxy acid synthase IV is highly stable at 37 degrees C in the presence or absence of ethylene glycol. However, sensitivity to feedback inhibition by valine is rapidly lost at 37 degrees C, but this property is somewhat stabilized by ethylene glycol. The rate of synthesis of acetohydroxy acid synthase IV is uniquely repressed by either leucine or isoleucine. These results suggest that the ilvJ+ allele is cryptic for acetohydroxy acid synthase IV, an isozyme distinct from the other acetohydroxy acid synthases.

Gene cpxA is a new addition to the linkage map of Escherichia coli K-12

The cpxA gene of E. coli K-12 lies between genes glpK and tpi, closely linked to the latter at 87.8 min on the linkage map. Since no other gene has been mapped in this interval, cpxA is a new addition to the linkage map.

Another gene affecting sexual expression of Escherichia coli

We have examined the relationship between two chromosomal mutations of Escherichia coli K-12, fexA (0 min) and fexB (85 min), in regulating expression of the F sex factor. Together, fexA and fexB exert a pleiotropic effect on the expression of the F tra genes. F pilus synthesis, conjugal donor activity, and surface exclusion activity are all inhibited in the fexA fexB mutant. Either fex mutation alone is cryptic.

Mutations in genes cpxA and cpxB of Escherichia coli K-12 cause a defect in isoleucine and valine syntheses

Mutations in two chromosomal genes of Escherichia coli, cpxA and cpxB, produced a temperature-sensitive growth defect that was remedied specifically by the addition of isoleucine and valine to the minimal medium. This auxotrophy was manifested only when the medium contained exogenous leucine, suggesting that mutant cells fail to elaborate active acetohydroxy acid synthase, isozyme I. In the presence of leucine, this enzyme was required to catalyze the first reaction common to the biosynthesis of isoleucine and valine. Measurements of enzyme activity in crude extracts showed that mutant cells were seven- to eightfold deficient in active isozyme I when the cells were grown in the presence of leucine. When grown in the absence of leucine, mutant cells contained more acetohydroxy acid synthase activity. We attribute this activity to isozyme III, the product of the ilvHI genes, which are derepressed in the absence of exogenous leucine. The cpxA and cpxB mutations appear to affect the production of active isozyme I, rather than its activity, since (i) neither the cpxA nor the cpxB gene mapped near the structural gene for isozyme I (ilvB), (ii) the growth of mutant cells shifted from the permissive (34 degrees C) to the nonpermissive (41 degrees C) temperature did not immediately cease, but declined gradually over a period corresponding to several normal generation times, and (iii) the enzyme from mutant cells grown at 34 degrees C was as stable at 41 degrees C as the enzyme from cpx+ cells.