Identification of the promoters directing in vivo expression of hemolysin genes in Proteus vulgaris and Escherichia coli

Activation of regulator ArcA in the presence of hypochlorite in Salmonella enterica serovar Typhimurium

Oxidative stress is the main mechanism behind efficient disinfectants, causing damage in bacterial macromolecules. Importantly, bacteria activate resistance mechanisms in response to damage generated by oxidative stress. Strategies allowing pathogens to survive oxidative stress are highly conserved among microorganisms. Many of these strategies entail genomic responses triggered by signals transduced through Two Component Systems (TCS). Recently, we demonstrated that the TCS ArcAB (specifically ArcA) participates in bacterial responses to hypochlorite, regulating the uptake of this toxic compound and being involved in resistance and survival inside neutrophils, where hypochlorous acid abounds. Here, we demonstrated that ArcA is required in the response to oxidative stress generated by hypochlorite, independent of its cognate sensor ArcB or the Asp54 of ArcA, the only phosphorylable residue in ArcA, which is required to function as a gene regulator. Our results suggest that ArcA could have additional functions to respond to oxidative stress, independent of its regulatory activity, which might require interaction with other unknown relevant proteins.

The Transcription Factor ArcA Modulates Salmonella's Metabolism in Response to Neutrophil Hypochlorous Acid-Mediated Stress

Salmonella Typhimurium, a bacterial pathogen with high metabolic plasticity, can adapt to different environmental conditions; these traits enhance its virulence by enabling bacterial survival. Neutrophils play important roles in the innate immune response, including the production of microbicidal reactive oxygen species (ROS). In addition, the myeloperoxidase in neutrophils catalyzes the formation of hypochlorous acid (HOCl), a highly toxic molecule that reacts with essential biomolecules, causing oxidative damage including lipid peroxidation and protein carbonylation. The bacterial response regulator ArcA regulates adaptive responses to oxygen levels and influences the survival of Salmonella inside phagocytic cells. Here, we demonstrate by whole transcriptomic analyses that ArcA regulates genes related to various metabolic pathways, enabling bacterial survival during HOCl-stress in vitro. Also, inside neutrophils, ArcA controls the transcription of several metabolic pathways by downregulating the expression of genes related to fatty acid degradation, lysine degradation, and arginine, proline, pyruvate, and propanoate metabolism. ArcA also upregulates genes encoding components of the oxidative pathway. These results underscore the importance of ArcA in ATP generation inside the neutrophil phagosome and its participation in bacterial metabolic adaptations during HOCl stress.

The ArcAB two-component regulatory system promotes resistance to reactive oxygen species and systemic infection by Salmonella Typhimurium

Salmonella enterica Serovar Typhimurium (S. Typhimurium) is an intracellular bacterium that overcomes host immune system barriers for successful infection. The bacterium colonizes the proximal small intestine, penetrates the epithelial layer, and is engulfed by macrophages and neutrophils. Intracellularly, S. Typhimurium encounters highly toxic reactive oxygen species including hydrogen peroxide and hypochlorous acid. The molecular mechanisms of Salmonella resistance to intracellular oxidative stress is not completely understood. The ArcAB two-component system is a global regulatory system that responds to oxygen. In this work, we show that the ArcA response regulator participates in Salmonella adaptation to changing oxygen levels and is also involved in promoting intracellular survival in macrophages and neutrophils, enabling S. Typhimurium to successfully establish a systemic infection.

MgaSpn and H-NS: Two Unrelated Global Regulators with Similar DNA-Binding Properties

Global regulators play an essential role in the adaptation of bacterial cells to specific niches. Bacterial pathogens thriving in the tissues and organs of their eukaryotic hosts are a well-studied example. Some of the proteins that recognize local DNA structures rather than specific nucleotide sequences act as global modulators in many bacteria, both Gram-negative and -positive. To this class of regulators belong the H-NS-like proteins, mainly identified in γ-Proteobacteria, and the MgaSpn-like proteins identified in Firmicutes. H-NS and MgaSpn from Escherichia coli and Streptococcus pneumoniae, respectively, neither have sequence similarity nor share structural domains. Nevertheless, they display common features in their interaction with DNA, namely: (i) they bind to DNA in a non-sequence-specific manner, (ii) they have a preference for intrinsically curved DNA regions, and (iii) they are able to form multimeric complexes on linear DNA. Using DNA fragments from the hemolysin operon regulatory region of the E. coli plasmid pHly152, we show in this work that MgaSpn is able to recognize particular regions on extended H-NS binding sites. Such regions are either located at or flanked by regions of potential bendability. Moreover, we show that the regulatory region of the pneumococcal P1623B promoter, which is recognized by MgaSpn, contains DNA motifs that are recognized by H-NS. These motifs are adjacent to regions of potential bendability. Our results suggest that both regulatory proteins recognize similar structural characteristics of DNA.

The Type 1 secretion pathway - the hemolysin system and beyond

Type 1 secretion systems (T1SS) are wide-spread among Gram-negative bacteria. An important example is the secretion of the hemolytic toxin HlyA from uropathogenic strains. Secretion is achieved in a single step directly from the cytosol to the extracellular space. The translocation machinery is composed of three indispensable membrane proteins, two in the inner membrane, and the third in the outer membrane. The inner membrane proteins belong to the ABC transporter and membrane fusion protein families (MFPs), respectively, while the outer membrane component is a porin-like protein. Assembly of the three proteins is triggered by accumulation of the transport substrate (HlyA) in the cytoplasm, to form a continuous channel from the inner membrane, bridging the periplasm and finally to the exterior. Interestingly, the majority of substrates of T1SS contain all the information necessary for targeting the polypeptide to the translocation channel - a specific sequence at the extreme C-terminus. Here, we summarize our current knowledge of regulation, channel assembly, translocation of substrates, and in the case of the HlyA toxin, its interaction with host membranes. We try to provide a complete picture of structure function of the components of the translocation channel and their interaction with the substrate. Although we will place the emphasis on the paradigm of Type 1 secretion systems, the hemolysin A secretion machinery from E. coli, we also cover as completely as possible current knowledge of other examples of these fascinating translocation systems. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Complicated catheter-associated urinary tract infections due to Escherichia coli and Proteus mirabilis

Catheter-associated urinary tract infections (CAUTIs) represent the most common type of nosocomial infection and are a major health concern due to the complications and frequent recurrence. These infections are often caused by Escherichia coli and Proteus mirabilis. Gram-negative bacterial species that cause CAUTIs express a number of virulence factors associated with adhesion, motility, biofilm formation, immunoavoidance, and nutrient acquisition as well as factors that cause damage to the host. These infections can be reduced by limiting catheter usage and ensuring that health care professionals correctly use closed-system Foley catheters. A number of novel approaches such as condom and suprapubic catheters, intermittent catheterization, new surfaces, catheters with antimicrobial agents, and probiotics have thus far met with limited success. While the diagnosis of symptomatic versus asymptomatic CAUTIs may be a contentious issue, it is generally agreed that once a catheterized patient is believed to have a symptomatic urinary tract infection, the catheter is removed if possible due to the high rate of relapse. Research focusing on the pathogenesis of CAUTIs will lead to a better understanding of the disease process and will subsequently lead to the development of new diagnosis, prevention, and treatment options.

Characterization of the dapA-nlpB genetic locus involved in regulation of swarming motility, cell envelope architecture, hemolysin production, and cell attachment ability in Serratia marcescens

Swarming migration of Serratia marcescens requires both flagellar motility and cellular differentiation and is a population-density-dependent behavior. While the flhDC and quorum-sensing systems have been characterized as important factors regulating S. marcescens swarming, the underlying molecular mechanisms are currently far from being understood. Serratia swarming is thermoregulated and is characterized by continuous surface migration on rich swarming agar surfaces at 30 degrees C but not at 37 degrees C. To further elucidate the mechanisms, identification of specific and conserved regulators that govern the initiation of swarming is essential. We performed transposon mutagenesis to screen for S. marcescens strain CH-1 mutants that swarmed at 37 degrees C. Analysis of a "precocious-swarming" mutant revealed that the defect in a conserved dapA(Sm)-nlpB(Sm) genetic locus which is closely related to the synthesis of bacterial cell wall peptidoglycan is responsible for the aberrant swarming phenotype. Further complementation and gene knockout studies showed that nlpB(Sm), which encodes a membrane lipoprotein, NlpB(Sm), but not dapA(Sm), is specifically involved in swarming regulation. On the other hand, dapA(Sm) but not nlpB(Sm) is responsible for the determination of cell envelope architecture, regulation of hemolysin production, and cellular attachment capability. While the nlpB(Sm) mutant showed similar cytotoxicity to its parent strain, the dapA(Sm) mutant significantly increased in cytotoxicity. We present evidence that DapA(Sm) is involved in the determination of cell-envelope-associated phenotypes and that NlpB(Sm) is involved in the regulation of swarming motility.

Temperature- and H-NS-dependent regulation of a plasmid-encoded virulence operon expressing Escherichia coli hemolysin

Proteins H-NS and Hha form a nucleoprotein complex that modulates expression of the thermoregulated hly operon of Escherichia coli. We have been able to identify two H-NS binding sites in the hly regulatory region. One of them partially overlaps the promoter region (site II), and the other is located about 2 kbp upstream (site I). In contrast, Hha protein did not show any preference for specific sequences. In vitro, temperature influences the affinity of H-NS for a DNA fragment containing both binding sites and H-NS-mediated repression of hly operon transcription. Deletion analysis of the hly regulatory region confirms the relevance of site I for thermoregulation of this operon. We present a model to explain the temperature-modulated repression of the hly operon, based on the experiments reported here and other, preexisting data.

Swarming-coupled expression of the Proteus mirabilis hpmBA haemolysin operon

The HpmA haemolysin toxin of Proteus mirabilis is encoded by the hpmBA locus and its production is upregulated co-ordinately with the synthesis and assembly of flagella during differentiation into hyperflagellated swarm cells. Primer extension identified a sigma(70) promoter upstream of hpmB that was upregulated during swarming. Northern blotting indicated that this promoter region was also required for concomitant transcription of the immediately distal hpmA gene, and that the unstable hpmBA transcript generated a stable hpmA mRNA and an unstable hpmB mRNA. Transcriptional luxAB fusions to the DNA regions 5' of the hpmB and hpmA genes confirmed that hpmB sigma(70) promoter activity increased in swarm cells, and that there was no independent hpmA promoter. Increased transcription of the hpmBA operon in swarm cells was dependent upon a 125 bp sequence 5' of the sigma(70) promoter -35 hexamer. This sequence spans multiple putative binding sites for the leucine-responsive regulatory protein (Lrp), and band-shift assays with purified Lrp confirmed the presence of at least two such sites. The influence on hpmBA expression of the key swarming positive regulators FlhD(2)C(2) (encoded by the flagellar master operon), Lrp, and the membrane-located upregulator of the master operon, UmoB, was examined. Overexpression of each of these regulators moderately increased hpmBA transcription in wild-type P. mirabilis, and the hpmBA operon was not expressed in any of the flhDC, lrp or umoB mutants. Expression in the mutants was not recovered by cross-complementation, i.e. by overexpression of FlhD(2)C(2), Lrp or UmoB. Expression of the zapA protease virulence gene, which like hpmBA is also upregulated in swarm cells, did not require Lrp, but like flhDC it was upregulated by UmoB. The results indicate intersecting pathways of control linking virulence gene expression and swarm cell differentiation.

Membrane interaction of Escherichia coli hemolysin: flotation and insertion-dependent labeling by phospholipid vesicles

The 1,024-amino-acid acylated hemolysin of Escherichia coli subverts host cell functions and causes cell lysis. Both activities require insertion of the toxin into target mammalian cell membranes. To identify directly the principal toxin sequences dictating membrane binding and insertion, we assayed the lipid bilayer interaction of native protoxin, stably active toxin, and recombinant peptides. Binding was assessed by flotation of protein-liposome mixtures through density gradients, and insertion was assessed by labeling with a photoactivatable probe incorporated into the target lipid bilayer. Both the active acylated hemolysin and the inactive unacylated protoxin were able to bind and also insert. Ca(2+) binding, which is required for toxin activity, did not influence the in vitro interaction with liposomes. Three overlapping large peptides were expressed separately. A C-terminal peptide including residues 601 to 1024 did not interact in either assay. An internal peptide spanning residues 496 to 831, including the two acylation sites, bound to phospholipid vesicles and showed a low level of insertion-dependent labeling. In vitro acylation had no effect on the bilayer interaction of either this peptide or the full-length protoxin. An N-terminal peptide comprising residues 1 to 520 also bound to phospholipid vesicles and showed strong insertion-dependent labeling, ca. 5- to 25-fold that of the internal peptide. Generation of five smaller peptides from the N-terminal region identified the principal determinant of lipid insertion as the hydrophobic sequence encompassing residues 177 to 411, which is conserved among hemolysin-related toxins.

In vivo proteolytic degradation of the Escherichia coli acyltransferase HlyC

Escherichia coli hemolysin (HlyA) is the prototype toxin of a major family of exoproteins produced by Gram-negative bacteria known as "repeats in toxins." Only fatty acid-acylated HlyA molecules at residues Lys564 and Lys690 are able to damage the target cell membrane. Fatty acylation of pro-HlyA is dependent on the co-synthesized acyltransferase HlyC and the acylated form of acyl-carrier protein. By using a collection of hlyA and hlyC mutant strains, the processing of HlyC was investigated. HlyC was not detected by Western blot in an E. coli strain encoding hlyC and hlyA, but it was present in a strain encoding only hlyC. The hlyC mRNA pattern, however, was similar in both strains indicating that the turnover of HlyC does not occur at the transcriptional level. HlyC was detected in Western blots of cell lysates from an E. coli strain encoding HlyC and a HlyA derivative where both acylation sites were substituted. Similar results were obtained when HlyC was expressed in a hlyA mutant strain lacking part of a putative HlyC binding domain, indicating that this particular HlyA region affects HlyC stability. We did not detect HlyC in cell lysates from hlyC mutants with different abilities to acylate pro-HlyA, suggesting that the degradation of HlyC is not related to the HlyA acylation process. The protease systems ClpAP, ClpXP, and FtsH were found to be responsible for the HlyA-dependent processing of HlyC.

An ordered reaction mechanism for bacterial toxin acylation by the specialized acyltransferase HlyC: formation of a ternary complex with acylACP and protoxin substrates

The 110 kDa haemolysin protoxin (proHlyA) is activated in the Escherichia coli cytosol by acyl carrier protein-dependent fatty acylation of two internal lysine residues, directed by the co-synthesized protein HlyC. Using an in vitro maturation reaction containing purified protoxin peptides and acylACP, we show unambiguously that HlyC possesses an apparently unique acyltransferase activity fully described by Michaelis-Menten analysis. The Vmax of HlyC at saturating levels of both substrates was approximately 115 nmol acyl group min-1 mg-1 with KMacylACP of 260 nM and KMproHlyA of 27 nM, kinetic parameters sufficient to explain why in vivo HlyC is required at a concentration equimolar to proHlyA. HlyC bound the fatty acyl group from acylACP to generate an acylated HlyC intermediate that was depleted in the presence of proHlyA, but enriched in the presence of proHlyA derivatives lacking acylation target sites. HlyC was also able to bind in vivo 4'-phosphopantetheine. Substitution of conserved amino acids that could act as putative covalent attachment sites did not prevent binding of the fatty acyl or 4'-phosphopantetheine groups. These data and substrate variation analyses suggest that the unique acylation reaction does not involve covalent attachment of fatty acid to the acyltransferase, but rather that it proceeds via a sequential ordered Bi-Bi reaction mechanism, requiring the formation of a non-covalent ternary acylACP-HlyC-proHlyA complex.

Novel genes that upregulate the Proteus mirabilis flhDC master operon controlling flagellar biogenesis and swarming

By screening for restoration of multicellular migration in a non-swarming but motile Proteus mirabilis mutant lacking the FIgN facilitator of flagella assembly, we identified four distinct genes that, in trans and multicopy, increased flagella production and cell length. Each of the genes upregulated expression of the flhDC master operon that controls flagellar biogenesis, cell division and swarming, not only in the mutant but also in the wild type. The genes were named umoA, umoB, umoC and umoD. Disruption of each of the wild-type chromosomal umo genes caused corresponding reductions in swarming and cell elongation, which correlated with decreased expression of the flhDC operon. The umoA, umoB, umoC and umoD genes are not closely linked, and only umoB is part of an operon. The sequences of the calculated gene products, UmoA (20.6 kDa), UmoB (78.0 kDa), UmoC (15.2 kDa) and UmoD (19.2 kDa), contain putative N-terminal secretion signals and predict a location in the cell membranes or periplasm. UmoB and UmoD have sequence similarity to the Escherichia coli uncharacterized open reading frames YrfF and YcfJ respectively; UmoA and UmoC have no known homologues. The umoB and umoC gene transcripts were present at very low levels, but umoA and umoD expression was similar to that of flhDC and increased in parallel with flhDC expression during differentiation into elongated hyperflagellated swarm cells. Like flhDC, umoA and umoD expression was subject to negative feedback in aflagellar assembly mutant lacking the FlhA inner membrane component of the export machinery. Assays of umo gene expression and cross-complementation indicated that the umo genes do not act in sequence within a pathway to upregulate flhDC, but revealed that umoA and umoD are reciprocally upregulated by FlhDC. Our findings strengthen the picture of the flhDC master operon as a major assimilatory checkpoint in Proteus mirabilis and other Gram-negative bacteria and expand the view of a complex regulatory network coupled to flagellar biogenesis.

Negative feedback from a Proteus class II flagellum export defect to the flhDC master operon controlling cell division and flagellum assembly

The Proteus mirabilis flagellum class I flhDC operon was isolated, and its transcript was shown to originate from a sigma70 promoter 244 bp 5' of flhD and 29 bp 3' of a putative cyclic AMP receptor protein-binding site. Expression of this regulatory master operon increased strongly as cells differentiated into elongated hyperflagellated swarm filaments, and cell populations artificially overexpressing flhDC migrated sooner and faster. A class II flhA transposon mutant was reduced in flagellum class III gene expression, as would be expected from the FlgM anti-sigma28 accumulation demonstrated in Salmonella typhimurium, but was unexpectedly also reduced in cell elongation. Here, we show that levels of flhDC transcript were ca. 10-fold lower in this flagellum export mutant, indicating that in cells defective in flagellum assembly, there is additional negative feedback via flhDC. In support of this view, artificial overexpression of flhDC in the flhA mutant restored elongation but not class III flagellum gene transcription.

Potential virulence factors of Proteus bacilli

The object of this review is the genus Proteus, which contains bacteria considered now to belong to the opportunistic pathogens. Widely distributed in nature (in soil, water, and sewage), Proteus species play a significant ecological role. When present in the niches of higher macroorganisms, these species are able to evoke pathological events in different regions of the human body. The invaders (Proteus mirabilis, P. vulgaris, and P. penneri) have numerous factors including fimbriae, flagella, outer membrane proteins, lipopolysaccharide, capsule antigen, urease, immunoglobulin A proteases, hemolysins, amino acid deaminases, and, finally, the most characteristic attribute of Proteus, swarming growth, enabling them to colonize and survive in higher organisms. All these features and factors are described and commented on in detail. The questions important for future investigation of these facultatively pathogenic microorganisms are also discussed.

RfaH enhances elongation of Escherichia coli hlyCABD mRNA

Escherichia coli hlyCABD operons encode the polypeptide component (Hly A) of an extracellular cytolytic toxin, as well as proteins required for its acylation (HlyC) and sec-independent secretion (HlyBD). Previous reports suggested that the E. coli protein RfaH is required for wild-type hemolysin expression, either by positively activating hly transcript initiation (M. J. A. Bailey, V. Koronakis, T. Schmoll, and C. Hughes, Mol. Microbiol. 6:1003-1012, 1992) or by promoting proper insertion of hemolysin export machinery in the E. coli outer membrane (C. Wandersman and S. Letoffe, Mol. Microbiol. 7:141-150, 1993). RfaH is also required for wild-type levels of mRNA transcribed from promoter-distal genes in the rfaQ-K, traY-Z, and rplK-rpoC gene clusters, suggesting that RfaH is a transcriptional antiterminator. We tested these models by analyzing the effects of rfaH mutations on hlyCABD mRNA synthesis and decay, HlyA protein levels, and hemolytic activity. The model system included a uropathogenic strain of E. coli harboring hlyCABD on the chromosome and E. coli K-12 transformed with the hlyCABD operon on a recombinant plasmid. Our results suggest that RfaH enhances hlyCABD transcript elongation, consistent with the model of RfaH involvement in transcriptional antitermination in E. coli. We also demonstrated that RfaH increases toxin efficacy. Modulation of hemolysin activity may be an indirect effect of RfaH-dependent E. coli outer membrane chemotype, which is consistent with the model of lipopolysaccharide involvement in hemolytic activity.

Evidence for post-transcriptional regulation of the synthesis of the Escherichia coli HlyB haemolysin translocator and production of polyclonal anti-HlyB antibody

Extensive attempts were made to overexpress the Escherichia coli haemolysin translocator protein HlyB, and HlyB fragments, utilising high copy number plasmids or hlyB expressed from strong promoters including lambda PR, ptrp and the T7 promoter. Analysis of both cytoplasmic and membrane fractions failed to detect any overexpression of the protein, although all the constructs showed biological activity and there was no evidence of HlyB-induced toxicity. In some constructs, the effect of removing a stem-loop structure, immediately upstream of the start codon and implicated in rho-independent termination of transcription, was tested but this did not lead to over-expression. Nevertheless, analysis of hlyB specific mRNA synthesis revealed that some constructs showed at least a 50-fold increase in mRNA levels, indicating that expression of HlyB may be limited at the translational level. When HlyB was expressed as a hybrid, downstream of LacZ, extremely high level overproduction was then detected in total cell extracts. When the expression of HlyB or HlyB fragments expressed from a T7 promoter was examined, the C-terminal ATPase domain was dramatically overexpressed but the production of fragments encompassing the N-terminal membrane domain, was reduced at least 1000-fold. These results indicate that mRNA structures corresponding to the membrane domain of HlyB greatly limit the post-transcriptional expression of HlyB. When such structures are deleted, or disrupted when part of a larger mRNA, HlyB or the HlyB ATPase domain can be overproduced in milligram quantities and this has facilitated the production of high titre antibodies to HlyB.

Cloning, expression, and characterization of the lon gene of Erwinia amylovora: evidence for a heat shock response

The gene encoding the Lon protease of Erwinia amylovora has been cloned by complementation of an Escherichia coli lon mutant. Analysis of the determined nucleotide sequence of the lon gene revealed extensive homology to the nucleotide sequences of cloned lon genes from E. coli, Myxococcus xanthus, and Bacillus brevis. The predicted amino acid sequence of the E. amylovora Lon protease was 94, 59, and 54% identical to the predicted amino acid sequences of the Lon proteases of E. coli, M. xanthus, and B. brevis, respectively. The -10 and -35 promoter regions of the cloned lon gene had extensive homology to the respective consensus sequences of E. coli heat shock promoters. Promoter mapping of the lon gene located the start site 7 bases downstream of the -10 region. Cloning of the lon promoter upstream of a cat reporter gene demonstrated that expression of the E. amylovora lon gene was inducible by a heat shock. This is the first demonstration of a heat shock-regulated gene in E. amylovora. Site-directed mutagenesis of the -10 region of the lon promoter confirmed that the heat shock expression of the E. amylovora lon gene may be mediated by a sigma 32-like factor. Insertional inactivation of the E. amylovora chromosomal lon gene confirmed that the lon gene was not essential for either vegetative growth or infection of apple seedlings. E. amylovora lon mutants had increased sensitivity to UV irradiation and elevated levels of extracellular polysaccharide, suggesting comparable roles for the Lon proteases in both E. amylovora and E. coli.

Requirement for FlhA in flagella assembly and swarm-cell differentiation by Proteus mirabilis

Swarming by Proteus mirabilis is characterized by cycles of rapid population migration across surfaces, following differentiation of typical rods into long, aseptate swarm cells that overexpress flagella and virulence factors, particularly haemolysin. A non-swarming Tn5phoA mutant was unable to synthesize flagella, to fully elongate or to induce high levels of the toxin. The mutation lay within a 2091 bp gene encoding a homologue of the Escherichia coli FlhA belonging to a family of proteins that are required for assembly of flagella or virulence proteins and that are suggested to act either directly in membrane translocation and/or in regulating synthesis of the export apparatus. In trans expression of multicopy flhA restored cell elongation and migration and generated differentiation-specific hyperexpression of flagellin and toxin genes to levels above those seen in the wild-type strain. Transcription of flhA was strongly induced during differentiation, from its own putative sigma 28 promoter. The results suggest a mechanistic coupling of flagella assembly and swarm-cell differentiation.

Regulation of Actinobacillus actinomycetemcomitans leukotoxin expression: analysis of the promoter regions of leukotoxic and minimally leukotoxic strains

The leukotoxin of Actinobacillus actinomycetemcomitans has been implicated as a virulence determinant in various human infections and is encoded by a multigene operon consisting of four known genes, designated ltxC, ltxA, ltxB, and ltxD. The ltx operon appears to be present in all A. actinomycetemcomitans strains, but levels of toxin expression vary greatly among strains. Thus, to gain a better understanding of the expression and regulation of the ltx operon, we have analyzed the ltx promoters of a highly toxic (JP2) and a minimally toxic (652) strain of A. actinomycetemcomitans. The nucleotide sequence of the JP2 ltx promoter contains -10 and -35 elements situated 350 bases upstream of ltxC, and primer extension of JP2 RNA confirmed that they are functional in vivo. However, a second primer extension product of 40 bases was present, and analysis of a series of truncated JP2 promoters fused to lacZ suggested that the region immediately upstream of ltxC also promotes transcription in Escherichia coli. These results suggest that two promoters may direct ltx expression in JP2. In addition, a small open reading frame capable of encoding a peptide of 78 amino acids was identified upstream of ltxC. Northern blots showed that this open reading frame is transcribed as part of a 4.2-kb mRNA, a transcript not previously identified as being derived from the ltx operon. In contrast, strain 652 expresses low steady-state levels of ltx mRNA, and its intact ltx promoter was inefficient in transcribing lacZ in E. coli. The nucleotide sequence of the 652 promoter is similar to that of the JP2 promoter but contains a region of 530 bp that is not present in JP2. Of 15 additional strains of A. actinomycetemcomitans that were analyzed, 13 contained promoters resembling the 652 sequence and 2 possessed JP2-like promoters. Both strains possessing the JP2-like promoter expressed 10- to 20-fold-higher levels of leukotoxin than did the strains possessing promoters resembling the 652 promoter. These results suggest that high levels of leukotoxin expression may correlate with the presence of the JP2-like promoter.

Cell differentiation of Proteus mirabilis is initiated by glutamine, a specific chemoattractant for swarming cells

Swarming by Proteus mirabilis involves differentiation of typical short vegetative rods into filamentous hyperflagellated swarm cells which undergo cycles of rapid and co-ordinated population migration across surfaces and exhibit high levels of virulence gene expression. By supplementing a minimal growth medium (MGM) unable to support swarming migration we identified a single amino acid, glutamine, as sufficient to signal initiation of cell differentiation and migration. Bacteria isolated from the migrating edge of colonies grown for 8 h with glutamine as the only amino acid were filamentous and synthesized the characteristic high levels of flagellin and haemolysin. In contrast, addition of the other 19 common amino acids (excluding glutamine) individually or in combination did not initiate differentiation even after 24 h, cells remaining typical vegetative rods with basal levels of haemolysin and flagellin. The glutamine analogue gamma-glutamyl hydroxamate (GH) inhibited swarming but not growth of P. mirabilis on glutamine MGM and transposon mutants defective in glutamine uptake retained their response to glutamine signalling and its inhibition by GH, suggesting that differentiation signalling by glutamine may be transduced independently of the cellular glutamine transport system. Levels of mRNA transcribed from the haemolysin (hpmA) and flagellin (fliC) genes were low in vegetative cells grown on MGM without glutamine or with glutamine and GH, but were specifically increased c. 40-fold during glutamine-dependent differentiation. In liquid glutamine-MGM cultures, differentiation to filamentous hyper-flagellated hyper-haemolytic swarm cells occurred early in the exponential phase of growth, and increased concomitantly with the concentration of glutamine from a 0.1 mM threshold up to 10 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Co-ordinate expression of virulence genes during swarm-cell differentiation and population migration of Proteus mirabilis

The uropathogenic Gram-negative bacterium Proteus mirabilis exhibits a form of multicellular behaviour termed swarming, which involves cyclical differentiation of typical vegetative cells into filamentous, multinucleate, hyperflagellate swarm cells capable of rapid and co-ordinated population migration across surfaces. We observed that differentiation into swarm cells was accompanied by substantial increases in the activities of intracellular urease and extracellular haemolysin and metalloprotease, which are believed to be central to the pathogenicity of P. mirabilis. In addition, the ability of P. mirabilis to invade human urothelial cells in vitro was primarily a characteristic of differentiated swarm cells, not vegetative cells. These virulence factor activities fell back as the cells underwent cyclical reversion to the vegetative form (consolidation), in parallel with the diagnostic modulation of flagellin levels on the cell surface. Control cellular alkaline phosphatase activities did not increase during differentiation or consolidation. Non-flagellated, nonmotile transposon insertion mutants were unable to invade urothelial cells and they generated only low-level activities of haemolysin, urease and protease (0-10% of wild type). Motile mutants unable to differentiate into swarm cells were comparably reduced in their haemolytic, ureolytic and invasive phenotypes and generated threefold less protease activity. Mutants that were able to form swarm cells but exhibited various aberrant patterns of swarming migration produced wild-type activities of haemolysin, urease and protease, but their ability to enter urothelial cells was three- to 10-fold lower.(ABSTRACT TRUNCATED AT 250 WORDS)

Escherichia coli HlyT protein, a transcriptional activator of haemolysin synthesis and secretion, is encoded by the rfaH (sfrB) locus required for expression of sex factor and lipopolysaccharide genes

Synthesis and secretion of the 110kDa haemolysin toxin of Escherichia coli and other pathogenic Gram-negative bacteria are governed by the four genes of the hly operon. We have identified, by transposon mutagenesis, an E. coli cellular locus, hlyT, required for the synthesis and secretion of haemolysin encoded in trans by intact hly operons carrying the hly upstream regulatory region. Mutation of the hlyT locus specifically reduced the level of hlyA structural gene transcript 20-100-fold and thus markedly lowered both intracellular and extracellular levels of the HlyA protein. Genetic and structural analysis of the hlyT locus mapped it at co-ordinate 3680 kbp (minute 87) on the chromosome adjacent to the fadBA operon, and identified it specifically as the rfaH (sfrB) locus which is required for transcription of the genes encoding synthesis of the sex pilus and also the lipopolysaccharide core for attachment of the O-antigen of E. coli and Salmonella. Expression of the hly operon in the E. coli hlyT mutant was restored in trans by both the hlyT and rfaH genes, suggesting that the rfaH gene is an important activator of regulon structures that are central to the fertility and virulence of these pathogenic bacteria. DNA sequencing of the hlyT locus identifies the HlyT/RfaH transcriptional activator as a protein of 162 amino acids (Mr 18325) which shows no identity to characterized transcription factors.

Transcription of the Vibrio cholerae haemolysin gene, hlyA, and cloning of a positive regulatory locus, hlyU

Transcription of the Vibrio cholerae hlyA gene, which encodes a cytotoxic haemolysin, has been investigated. The hlyA transcript initiates 430 nucleotides (nt) upstream of the translational start site. hlyA-cat transcriptional fusion constructs were active in V. cholerae but not in Escherichia coli. An hlyA-cat fusion was used to select, from a V. cholerae O17 plasmid library, a clone that could activate the hlyA promoter in E. coli. This regulatory locus has been designated hlyU. hlyU appears to be distinct from the previously described hlyR locus.

The mdoA locus of Escherichia coli consists of an operon under osmotic control

In Escherichia coli, the 5 kb mdoA locus is involved in the osmotically controlled biosynthesis of periplasmic membrane-derived oligosaccharides (MDOs). The structure of this locus was analysed by in vitro cassette insertion, transposon mutagenesis, and gene-fusion analysis. A 'neo' cassette, derived from the neomycin phosphotransferase II region of transposon Tn5, was inserted into mdoA, borne by a multicopy plasmid. This plasmid was shown to complement two previously described mdoA mutations, depending on the orientation of the exogenous gene. Thus, the gene altered by these mutations could be expressed under the control of the exogenous promoter. Moreover, the 'neo' cassette inactivated another, uncharacterized, mdo gene, because when this insertion was transferred into the chromosome MDO synthesis was abolished. The existence of a second gene was confirmed by complementation analysis with a collection of Tn1000 insertions into mdoA. Two groups were defined, and the two genes are organized into an operon (mdoGH). This conclusion was reached because Tn1000 insertions in the first gene displayed a polar effect on the expression of the second gene. An active gene fusion was obtained on a multicopy plasmid between the beginning of mdoH and lacZ. The hybrid beta-galactosidase activity followed the same osmotically controlled response as that described for of MDO synthesis. This regulation was unaffected by the presence, or absence, of MDOs in the periplasm. Finally, the amount of mdoA-specific mRNAs, determined by dot blot hybridization, decreased when the osmolarity of the growth medium increased.

Virulence factors in Escherichia coli urinary tract infection

Uropathogenic strains of Escherichia coli are characterized by the expression of distinctive bacterial properties, products, or structures referred to as virulence factors because they help the organism overcome host defenses and colonize or invade the urinary tract. Virulence factors of recognized importance in the pathogenesis of urinary tract infection (UTI) include adhesins (P fimbriae, certain other mannose-resistant adhesins, and type 1 fimbriae), the aerobactin system, hemolysin, K capsule, and resistance to serum killing. This review summarizes the virtual explosion of information regarding the epidemiology, biochemistry, mechanisms of action, and genetic basis of these urovirulence factors that has occurred in the past decade and identifies areas in need of further study. Virulence factor expression is more common among certain genetically related groups of E. coli which constitute virulent clones within the larger E. coli population. In general, the more virulence factors a strain expresses, the more severe an infection it is able to cause. Certain virulence factors specifically favor the development of pyelonephritis, others favor cystitis, and others favor asymptomatic bacteriuria. The currently defined virulence factors clearly contribute to the virulence of wild-type strains but are usually insufficient in themselves to transform an avirulent organism into a pathogen, demonstrating that other as-yet-undefined virulence properties await discovery. Virulence factor testing is a useful epidemiological and research tool but as yet has no defined clinical role. Immunological and biochemical anti-virulence factor interventions are effective in animal models of UTI and hold promise for the prevention of UTI in humans.

The HpmA hemolysin is more common than HlyA among Proteus isolates

Two different hemolysins, HpmA and HlyA, have been reported in Proteus spp. To study the distribution of these hemolysins among Proteus strains, isolates from various infections and normal feces were screened for hemolysin production. All 63 Proteus mirabilis strains and 23 of the 24 Proteus vulgaris strains produced a calcium-independent hemolytic activity detectable in cell-free supernatants. The calcium-independent activity was due to HpmA; this activity correlated with the presence of hpmA sequences and the production of an extracellular 166-kilodalton (kDa) protein that reacted with anti-HpmA antiserum. HpmA- mutants, constructed by deletion of the central portion of the hpmA gene, did not produce the 166-kDa protein and were no longer hemolytic when compared with their respective parent strains. Among the 87 P. mirabilis and P. vulgaris isolates examined, calcium-dependent hemolytic activity was produced by only two P. vulgaris strains. These strains produced a 110-kDa protein which comigrated with the Escherichia coli hemolysin (HlyA) antibodies in immunoblots. These studies show that Proteus spp. produce two distinct extracellular hemolysins, with nearly all strains producing the calcium-independent hemolysin, HpmA, but only an occasional P. vulgaris isolate producing HlyA.

Cytotoxic activity of the Proteus hemolysin HpmA

We previously showed that hpmA is the hemolysin determinant most commonly found among Proteus isolates. To assess the potential contribution of HpmA to virulence, we first characterized the toxic activities of this hemolysin. Hemolytic activity was present in total cell cultures and cell-free supernatants of Proteus clinical isolates as well as Escherichia coli containing cloned hpm genes. HpmA also possesses cytotoxic activity which was detected by a chromium release assay against a variety of target cell lines (Daudi, Raji, T24, U937, and Vero). Analysis of the dose response of bacterial cells against both T24 cells and erythrocytes showed that E. coli containing cloned hpm genes was 30-fold more cytotoxic than Proteus mirabilis BA6163. Also, 10(5)-fold more bacterial cells were needed to lyse T24 cells than to lyse erythrocytes. HpmA- mutants of two Proteus strains in which the central portion of hpmA was deleted were constructed. These HpmA- mutants, which have lost the hemolytic and cytotoxic activities exhibited by their respective parent strains, demonstrate that HpmA is needed for both of these activities. In an ascending model of murine urinary tract infection, the hpmA mutant strain WPM111 behaved no differently from its parent strain, BA6163, with respect to either the level of kidney colonization or histopathological changes in the kidney. However, WPM111 had a sixfold higher 50% lethal dose than BA6163 when injected intravenously into C3H mice.

HlyB-dependent secretion of hemolysin by uropathogenic Escherichia coli requires conserved sequences flanking the chromosomal hly determinant

The synthesis and secretion of hemolysin (HlyA) by Escherichia coli are governed by four contiguous genes (hlyCABD) that are closely conserved on plasmids and, among human pathogenic strains, on the chromosome. We have previously shown that in plasmid pHly152 the coexpressed synthesis and export functions are uncoupled by intraoperon transcription termination, which is in turn alleviated by antitermination dictated in cis by a region upstream of the hly operon. In this study we describe an analogous region of ca. 1,100 base pairs flanking the chromosomal hly determinant of the uropathogenic strain E. coli 2001. This region had no significant effect on intracellular levels of hemolysin but activated strongly, both in cis and in trans, the specific hlyB-hlyD-dependent hemolysin secretion function. The secretion-activating region increased the transcription of the secretion gene hlyB, but the transcription effect was not as pronounced as that seen in the pHly152 determinant and was not evident when the region was present in trans to the hemolysin genes, suggesting that, in addition to transcriptional activation, the region may possibly exert a secondary posttranscriptional influence. Southern hybridizations with the 1,100-base pairs secretion-activating sequence showed low identity to plasmid pHly152 and no identity with total DNA from nonhemolytic uropathogenic E. coli or hemolytic isolates of Proteus vulgaris, Proteus mirabilis, and Morganella morganii. In contrast, hybridization to total DNA from hemolytic E. coli isolates belonging to different serotypes showed strong conservation of the activating sequence, indicating that it is an integral component of the chromosomal hly determinant that is widespread among uropathogenic E. coli.

Haemolysin secretion from E coli

Haemolysin (HlyA) secretion from E coli is directed by a specific C-terminal targeting signal, located within the last 27-50 amino acids, with quite novel characteristics. The HlyA molecule is secreted directly to the medium without a periplasmic intermediate or detectable proteolytic processing. The C-terminal domain of HlyA can also be used to promote the secretion of several other E coli and mammalian proteins. HlyD and HlyB are essential for translocation of HlyA to the medium and we propose that these proteins form a transenvelope complex which initially binds the HlyA signal followed by transport of HlyA to the medium. HlyB is a member of a family of membrane proteins engaged in ATP dependent secretion mechanisms conserved in many organisms including man (P-glycoprotein and the CF protein). In this review we discuss the structure, function and regulation of the secretion mechanism.

Isolation of the Actinobacillus pleuropneumoniae haemolysin gene and the activation and secretion of the prohaemolysin by the HlyC, HlyB and HlyD proteins of Escherichia coli

The gene encoding the c. 105 kD secreted haemolysin protein of the porcine pathogen Actinobacillus pleuropneumoniae serotype 1 has been isolated by screening a lambda gt11 expression library in Escherichia coli with antiserum raised against the wild-type protein. A derivative recombinant DNA pJFF702 expressed the hlylA haemolysin gene from the pUC19 lac promoter but the resulting haemolysin I protein remained within the E. coli cell and was haemolytically inactive. Export of the intracellular A. pleuropneumoniae prohaemolysin out into the medium was achieved by the presence in trans of the E. coli haemolysin secretion genes hlyB and hlyD, and high levels of intracellular haemolytic activity were attained similarly by the E. coli post-translational haemolysin activator gene, hlyC. Southern hybridization of A. pleuropneumoniae parental DNA nevertheless indicated only a low degree of nucleotide sequence identity to the haemolysin structural and secretion genes hlyA and hlyB of E. coli. The data show that despite substantial nucleotide sequence divergence the A. pleuropneumoniae serotype 1 haemolysin determinant is closely related to that which is dispersed throughout other Gram-negative human and animal pathogens.

Regulation of expression of the Pasteurella haemolytica leukotoxin determinant

The Pasteurella haemolytica leukotoxin determinant is composed of four contiguous genes encoded on the same DNA strand and denoted lktCABD, in the order of their genetic organization. To gain a better understanding of the expression and regulation of the leukotoxin, the transcripts and promoters of the lkt determinant were mapped. Northern (RNA) blot analysis revealed two sets of transcripts. One set was 3.7 and 3.4 kilobases long, encoded lktCA, and comprised approximately 90% of the transcripts, whereas the other set was 7.4 and 7.1 kilobases long and encoded lktCABD. Two promoters were present, and each had features similar to the Escherichia coli consensus promoter sequences. Both promoters were located upstream from lktC; they were separated by 258 base pairs, as mapped by primer extension analysis. These results suggest a mechanism of expression similar to that of the related E. coli hemolysin. Transcription initiated upstream from lktC at either promoter and continued through lktC and lktA to a rho-independent transcriptional termination signal in the lktA-lktB intercistronic region. This signal attenuated expression by terminating 90% of transcription to generate the 3.7- and 3.4-kilobase lktCA transcripts. The remaining readthrough transcription generated full-length 7.4- and 7.1-kilobase lktCABD transcripts. Expression of the leukotoxin was greatly reduced by growth at 30 degrees C, pH 6.5, and Fe2+ limitation. These conditions also modulated the expression of a number of other secreted proteins, which suggests that all of these secreted proteins are controlled by the same regulatory mechanism.

Isolation and analysis of the C-terminal signal directing export of Escherichia coli hemolysin protein across both bacterial membranes

We have studied the C-terminal signal which directs the complete export of the 1024-amino-acid hemolysin protein (HlyA) of Escherichia coli across both bacterial membranes into the surrounding medium. Isolation and sequencing of homologous hlyA genes from the related bacteria Proteus vulgaris and Morganella morganii revealed high primary sequence divergence in the three HlyA C-termini and highlighted within the extreme terminal 53 amino acids the conservation of three contiguous sequences, a potential 18-amino-acid amphiphilic alpha-helix, a cluster of charged residues, and a weakly hydrophobic terminal sequence rich in hydroxylated residues. Fusion of the C-terminal 53 amino acid sequence to non-exported truncated Hly A directed wild-type export but export was radically reduced following independent disruption or progressive truncation of the three C-terminal features by in-frame deletion and the introduction of translation stop codons within the 3' hlyA sequence. The data indicate that the HlyA C-terminal export signal comprises multiple components and suggest possible analogies with the mitochondrial import signal. Hemolysis assays and immunoblotting confirmed the intracellular accumulation of non-exported HlyA proteins and supported the view that export proceeds without a periplasmic intermediate. Comparison of cytoplasmic and extracellular forms of an independently exported extreme C-terminal 194 residue peptide showed that the signal was not removed during export.