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J Bacteriol. Apr 2007; 189(7): 2863–2872.
Published online Jan 19, 2007. doi:  10.1128/JB.01859-06
PMCID: PMC1855826

SOS Regulation of the Type III Secretion System of Enteropathogenic Escherichia coli[down-pointing small open triangle]

Abstract

Genomes of bacterial pathogens contain and coordinately regulate virulence-associated genes in order to cause disease. Enteropathogenic Escherichia coli (EPEC), a major cause of watery diarrhea in infants and a model gram-negative pathogen, expresses a type III secretion system (TTSS) that is encoded by the locus of enterocyte effacement (LEE) and is necessary for causing attaching and effacing intestinal lesions. Effector proteins encoded by the LEE and in cryptic prophage are injected into the host cell cytoplasm by the TTTS apparatus, ultimately leading to diarrhea. The LEE is comprised of multiple polycistronic operons, most of which are controlled by the global, positive regulator Ler. Here we demonstrated that the LEE2 and LEE3 operons also responded to SOS signaling and that this regulation was LexA dependent. As determined by a DNase I protection assay, purified LexA protein bound in vitro to a predicted SOS box located in the divergent, overlapping LEE2/LEE3 promoters. Expression of the lexA1 allele, encoding an uncleavable LexA protein in EPEC, resulted in reduced secretion, particularly in the absence of the Ler regulator. Finally, we obtained evidence that the cryptic phage-located nleA gene encoding an effector molecule is SOS regulated. Thus, we demonstrated, for the first time to our knowledge, that genes encoding components of a TTSS are regulated by the SOS response, and our data might explain how a subset of EPEC effector proteins, encoded in cryptic prophages, are coordinately regulated with the LEE-encoded TTSS necessary for their translocation into host cells.

Coordinate regulation of virulence determinants within a host is critical to a bacterial pathogen's ability to cause disease. Enteric pathogens perceive a number of environmental cues within the human intestinal tract, which signal the bacterium that it resides in the correct niche, allowing pathogenesis to commence.

Enteropathogenic Escherichia coli (EPEC), the causative agent of watery diarrhea primarily in infants in developing nations (42), also responds to a requisite set of environmental cues, which permits colonization of the small intestine. EPEC forms attaching and effacing (AE) intestinal lesions, mediated by a type III secretion system (TTSS), and more than 20 protein effector molecules are injected into the host cell cytoplasm (16), which leads to cytoskeletal rearrangement, intimate attachment to the host cell membrane, and pedestal formation. Tobe et al. recently demonstrated that in the locus of enterocyte effacement (LEE)-containing, related E. coli O157:H7 Sakai strain, 39 effector proteins are translocated into host cells, and many of these effectors are encoded in cryptic prophages (55). Similarly, the SopE effector protein of a few Salmonella enterica serovar Typhimurium strains is encoded in a temperate P2-like phage (37).

EPEC diarrhea results from a loss of absorptive tissue due to destruction of the microvilli during the formation of AE lesions, alteration of host cell signaling events that causes active ion secretion, increased intestinal permeability, loosening of tight junctions, and inflammation at the site of infection (for reviews, see references 7 and 23). Effector molecules translocated into the host cell by the TTSS apparatus include the LEE-encoded Tir, EspF, EspG, EspH, EspZ, and Map proteins, as well as NleA (also designated EspI), EspD, EspJ, EspG2, and Cif, which are encoded outside the LEE pathogenicity island (16). EspG2 is encoded in the EspC pathogenicity island (34), and interestingly, the NleA, NleD, EspJ, and Cif proteins are encoded in cryptic prophage (10, 19, 40).

Maximal secretion of EPEC effector molecules occurs during the early exponential phase of growth at 37°C at pH 7 and is influenced by medium conditions (24, 25, 48, 58). To date, however, we do not understand how the prophage-encoded effector molecules are coordinately regulated with the components of the LEE-encoded TTSS.

A number of regulatory proteins have been shown to control expression of the EPEC LEE (for a review, see http://www.ecosal.org/ecosal/toc/index.jsp); these proteins include the master regulator Ler, which is encoded in the LEE1 operon (13, 33). PerC, encoded in the E. coli attachment factor virulence plasmid, and integration host factor directly affect the expression of LEE1 and thus expression of Ler (3, 15, 33, 45). Fis and BipA have also been demonstrated to regulate LEE1 transcription and thus indirectly control the AE phenotype (17, 18). H-NS is an important regulator of LEE transcription at temperatures below that of the human host, directly silencing the LEE1, LEE2, and LEE3 operons (58) encoding the membrane-spanning components of the TTSS and the LEE5 operon (20) encoding Tir, its chaperone CesT, and the outer membrane protein intimin. The LEE is subject to quorum-sensing signaling (51, 53), and finally, the GrlRA (global regulator of LEE repressor and activator) proteins have been described as part of a complex regulatory loop controlling LEE gene expression, acting upstream of Ler (11).

Recent reports have implicated the SOS regulon in the control of virulence-associated phenotypes in both gram-negative and gram-positive pathogens. In E. coli β-lactam antibiotics induce the SOS response and mitigate the lethal effects of drugs on the bacteria (35). Additionally, resistance to the DNA gyrase inhibitor ciprofloxacin in E. coli requires a specific mutation that is mediated by cleavage of the SOS repressor LexA, inducing associated expression of error-prone polymerases (5). Ciprofloxacin induces Staphylococcus aureus fibronectin binding in a RecA-LexA-dependent manner (1), and in E. coli serotype O157:H7 prophage-encoded Stx2 production is enhanced by the presence of DNA-damaging antibiotics (54, 62). In Vibrio cholerae, LexA and the phage-encoded protein RstR act together to control expression of the CTXΦ phage promoter PrstA (32, 46). In this study we demonstrate that expression of the TTSS of EPEC is regulated by the SOS response in a LexA-dependent manner, and below we discuss the implications of this finding for EPEC pathogenesis, the evolution of pathogenic E. coli bacteria, and the clinical treatment of bacterial infections.

MATERIALS AND METHODS

Bacterial strains, plasmids, and phage.

Bacterial strains, plasmids, and bacteriophages used in this study are listed in Table Table1.1. Strains were grown aerobically at 37°C in Luria-Bertani (LB) medium supplemented with the appropriate antibiotics at the following final concentrations: ampicillin, 100 μg/ml; kanamycin, 50 μg/ml; chloramphenicol, 30 μg/ml; and tetracycline, 15 μg/ml.

TABLE 1.
Bacterial strains, plasmids, and phages used in this study

Generation of regulatory fragments.

Promoter fragments for transcriptional studies were generated by PCR by standard protocols (21) utilizing Taq polymerase (Sigma), oligonucleotide primers listed in Table Table22 (Invitrogen), and genomic DNA isolated from E. coli strain MG1655 using a QIAGEN genomic DNA kit (QIAGEN).

TABLE 2.
Oligonucleotides used in this study

The recA gene was amplified by PCR using the 5′-recA and 3′-recA primers (Table (Table2).2). Amplified DNA fragments were gel isolated using a Qiaquick gel extraction kit (QIAGEN) and were subsequently cloned with a TOPO TA cloning kit (Invitrogen) used according to the manufacturer's instructions. Regulatory fragments were verified by restriction mapping and DNA sequence analysis.

Construction of a chromosomally encoded, single-copy recA transcriptional fusion.

The recA transcriptional reporter gene fusion was constructed by excising the recA regulatory fragment (positions −97 to 827) from the TOPO TA plasmid vector with restriction endonuclease EcoRI and was cloned upstream of the promoterless lacZ gene in pRS551 previously cleaved with EcoRI and dephosphorylated with shrimp alkaline phosphatase (Roche), creating plasmid pKH281. Electrocompetent DH5α was transformed by using a Bio-Rad Micropulser electroporation apparatus and selecting for ampicillin resistance. Plasmid DNA was isolated using an UltraClean mini plasmid prep kit (Mo Bio) according the manufacturer's instructions and was subjected to restriction enzyme digestion and DNA sequence analysis for verification of the specific and recA-lacZ transcriptional fusions.

In order to isolate the chromosomally encoded, single-copy recA-lacZ transcriptional fusion, the plasmid was allowed to recombine with λRS45 (50) in E. coli strain MC4100. Specialized transducing lysates were used to transduce MC4100 to kanamycin resistance. The single-copy, chromosomally encoded recA fusion was isolated by screening kanamycin-resistant transductants for ampicillin sensitivity using established protocols (33, 50).

Enzymatic assays.

β-Galactosidase assays were performed as described by Miller (36), except that saturated overnight cultures were diluted 50-fold and allowed to grow to the exponential phase (optical density at 600 nm [OD600], ~0.5). Individual cultures were subsequently diluted to obtain OD600 of ~0.1 to begin the time course. These cultures were then divided in two, and mitomycin C was added to one of the cultures at a final concentration of 1 μg/ml at time zero. Aliquots were removed after 30 and 60 min of growth and kept on ice until β-galactosidase assays were performed. The data presented below are expressed in Miller units and are the means of at least two independent experiments performed in triplicate.

Generalized P1vir transduction.

Strains containing recA1 or lexA1 alleles were constructed by P1vir transduction (36). P1vir lysates were generated from donor strains that contained Tn10 insertions, which encoded tetracycline resistance, linked to mutant recA1 or lexA1 alleles, which conferred UV sensitivity (6, 39). To isolate a Tn10 insertion linked to the lexA1 allele, we transduced strain DM803 to tetracycline resistance by using a P1vir lysate from strain TL229 that contained Tn10 linked to lexA, zjb-729::Tn10. By selecting for tetracycline resistance and screening for UV sensitivity strain JLM803 was isolated. (Strains were determined to be UV sensitive if, when cross-streaked on LB agar, they died when they were exposed to 30 s of UV light from a germicidal STERILAMP [Westinghouse].) Once strains containing Tn10 insertions linked to lexA1 (JLM803) or recA1 (USL10) were isolated, we infected the appropriate recipient strains with P1vir lysates generated from strains JLM803 and USL10, selecting for tetracycline resistance. Transductants that had inherited the Tn10 and mutant lexA1 or recA1 alleles exhibited tetracycline resistance and UV sensitivity.

RNA dot blot procedure. Total RNA was isolated from bacterial cultures grown in LB medium with and without mitomycin C for 60 min during exponential growth. Strains E2348/69 and MC4100 were inoculated into 2 ml of LB medium, grown overnight at 37°C with aeration, and diluted 50-fold in LB medium the next morning. Cultures were grown at 37°C with shaking until the A600 was between 0.5 and 1.0. These cultures were diluted to obtain an A600 of ~0.1 in LB medium with and without 1 μg/ml of mitomycin C. After 60 min of growth, cells were harvested by centrifugation and stored at −80°C. Total RNA was isolated from each culture by using Trizol (Invitrogen) according to the manufacturer's instructions.

Total RNA was then treated with 10 U of DNase I (Sigma) at 37°C for 30 min in buffer containing 10 mM Tris (pH 8.0), 2.5 mM MgCl2, and 0.5 mM CaCl2. The reaction was quenched by addition of EDTA to a final concentration of 5 mM and heating to 75°C for 10 min. RNA integrity and the absence of contaminating DNA were analyzed by agarose gel electrophoresis, followed by staining with ethidium bromide. Clear, sharp 23S and 16S RNA species were observed, and no detectable genomic DNA was present. The concentration of RNA was determined spectrophotometrically by determining the absorbance at 260 nm.

Total RNA (10 μg/dot) in transfer buffer (2% formaldehyde, 2.5× SSC [1× SSC is 0.15 M NaCl plus 0.015 M sodium citrate]) was heated at 75°C for 15 min, vacuum blotted onto a Hybond-N membrane (GE Healthcare), and dried completely at 80°C for 2 h. The blots were then rinsed with 2× SSC and incubated with prehybridization solution (5× SSC, 5× Denhardt's reagent, 0.5% sodium dodecyl sulfate [SDS], 100 μg/ml salmon sperm DNA [Invitrogen]) at 65°C with agitation for more than 2 h. The blots were then probed individually with ~100 ng of radioactively labeled DNA fragments.

Probe fragments were generated by PCR utilizing the oligonucleotide primers listed in Table Table22 and were subsequently cloned by TOPO TA cloning (Invitrogen) performed according to the manufacturer's instructions. The LEE2, LEE3, LEE5, and nleA probe fragments were generated from E2348/69 genomic DNA, and the recA fragment was generated from MG1655 genomic DNA. All plasmids used for probes were subjected to DNA sequence analysis to verify their nucleotide sequences. Each probe fragment of DNA was excised from the plasmid, gel isolated (Qiaquick gel extraction kit), and radioactively labeled with [α-32P]dATP (NEBlot kit) according to the manufacturer's instructions. Unincorporated nucleotides were removed with a spin column (Probe Quant G-50; GE Healthcare), and the DNA was denatured at 100°C for 5 min and chilled on ice. The probes were then added individually to each prehybridization solution and membrane and incubated at 65°C with agitation overnight to allow hybridization. The hybridization solution was removed, and the membranes were washed twice in 2× SSC-0.1% SDS at 65°C for 15 min, twice in 1× SSC-0.1% SDS at 65°C for 15 min, and three times in 0.1× SSC-0.1% SDS at 65°C for 30 min. The membranes were then wrapped in Saran Wrap, mounted on Kodak X-OMAT LS X-ray film, and exposed for various amounts of time at room temperature. After satisfactory images were obtained, the membranes were cut into 1-cm squares containing each RNA dot and subjected to liquid scintillation analysis to determine specific RNA levels (Packard TRICARB 2900 TR liquid scintillation analyzer). The cpm for each RNA dot under each condition were normalized to the 16S rRNA cpm, and the relative increase (expressed as a percentage) in the transcript level was calculated for each operon or gene by comparing the signal in the presence of the DNA-damaging agent mitomycin C and the signal in the absence of mitomycin C. The results for two independent RNA isolations performed in triplicate were subjected to statistical analysis (Student's paired t test), and a P value of <0.05 was considered significant.

Cloning and expression of the lexA1 allele.

The lexA1 allele encoding the uncleavable LexA1 protein was amplified by PCR using the lexA-specific primers 5′-lexA new and 3′-lexA new. These primers amplify a minimal fragment encoding a promoterless open reading frame corresponding to the lexA gene from E. coli strain MG1655, including the ribosome binding sequence. Genomic DNA from strain JLM281A (Table (Table1),1), which contains the mutant uncleavable lexA1 allele that was verified to confer a non-SOS-inducible phenotype in this work, was used as the template DNA for PCR amplification. A single fragment was observed, gel isolated, and cloned directly into pBAD33 using the XbaI/HindIII sites engineered into the primer sequences, creating plasmid pKH296. The pBAD33 plasmid contained the arabinose-inducible PBAD promoter, which drove transcription of the cloned open reading frame in the presence of arabinose. Using DNA sequence analysis, this plasmid was verified to contain the G-to-A transition mutation that resulted in a G80D amino acid substitution that rendered the protein encoded by the lexA1 allele uncleavable (28).

Analysis of TTSS-dependent protein secretion.

EPEC strains were inoculated into 2 ml of LB medium containing chloramphenicol for plasmid pBAD selection and 0.2% glucose to inhibit expression of the lexA1 allele cloned in pKH296. The cultures were incubated overnight at 37°C with shaking. The cultures were then diluted 50-fold into Dulbecco modified Eagle medium (DMEM) containing 50 mM HEPES buffer (pH 7.4), chloramphenicol, and 0.1% arabinose to induce expression of the lexA1 allele. The cultures were incubated at 37°C with shaking until the optical density at 600 nm was between 0.8 and 1.0. Bacteria were removed from the medium by centrifugation at 12,000 × g for 30 min. The equivalent of 4.5 OD600 units of medium was removed with a serological pipette, and proteins were precipitated as described previously (24). Protein pellets were resuspended in Laemmli SDS-polyacrylamide gel electrophoresis loading buffer and resolved on a 15% SDS-polyacrylamide gel. Proteins were visualized by overnight staining with 0.25% Coomassie blue R-250 in 40% methanol and 10% acetic acid. Gels were destained by repeated washing in 45% methanol and 10% acetic acid.

DNase I protection assays.

Fragments suitable for DNase I protection assays were PCR amplified from genomic DNA. The recA fragment (positions −97 to 62) was amplified using the 5′rec-foot and 3′rec-foot primers and genomic DNA isolated from strain MG1655. The 159-bp fragment was gel isolated, cleaved with EcoRI and BamHI, and ligated into pBluescript II KS cut with the same restriction endonucleases, creating pKH286. The LEE2 (positions −83 to 75)/LEE3 (positions −94 to 64) and LEE5 (positions −75 to 88) DNA fragments were PCR amplified from genomic DNA isolated from EPEC strain E2348/69. Primers 5′LEE3 and 3′LEE3 were used to amplify the 158-bp fragment that encoded the overlapping LEE2/LEE3 promoters, including the putative LexA binding site. Primers Tir5 and TIR+88 were used to amplify the 163-bp fragment encoding the LEE5 promoter and putative LexA binding site. The fragments were gel isolated and cloned using a TOPO TA cloning kit (Invitrogen). The LEE2/LEE3 and LEE5 promoter fragments were then excised using EcoRI and BamHI and cloned into pBluescript II KS, creating pKH284 and pDG588, respectively.

Specific binding of regulatory sequences by purified LexA protein in vitro was demonstrated by DNase I protection assays. DNA fragments containing the LEE2/LEE3, LEE5, and recA promoters were asymmetrically labeled by cleavage of pKH284, pDG588, or pKH286 with NotI. The resulting 5′ overhangs were filled in using the Klenow fragment of DNA polymerase I (NEB), [α-32P]dCTP (General Electric Healthcare), and dGTP (NEB) at room temperature for 5 min. The polymerase was then heat inactivated at 70°C for 15 min. The labeled fragment was then released from the vector by cleavage with EcoRI and gel isolated. LexA binding reactions were performed by incubating 70,000 cpm of probe DNA, 40 μg/ml nonspecific DNA (pBluescript II KS), and various amounts of purified LexA protein in binding buffer (20 mM Tris HCl [pH 7.4], 10% sucrose, 1 mM dithiothreitol, 0.1 mM EDTA, 1.5 mM CaCl2, 2.5 mM MgCl2, 0.1% bovine serum albumin, 50 mM NaCl) (30) at room temperature for 10 min. After LexA binding, 40 mU of DNase I (Sigma) was added, and the preparations were incubated at room temperature for 2 min. The reactions were terminated by addition of stop solution (200 mM NaCl, 2 mM EDTA, 1% SDS). The reaction mixtures were then extracted once with phenol and once with chloroform, followed by ethanol precipitation (2 volumes of 100% ethanol, 0.1 volume of 7.5 M ammonium acetate [pH 7.5], 1 μl of glycogen [Invitrogen]). The pellets were resuspended in 2.5 μl water and 2.5 μl Sequenase stop solution (USB), denatured at 75°C for 2 min immediately prior to loading, and separated in a denaturing 6% polyacrylamide-Tris-borate-EDTA gel. The gel was then transferred to Whatman paper, dried at 80°C for 1 h, and exposed to Kodak X-Omat LS film. Sequencing reaction mixtures containing each plasmid and the BluescriptNotI primer were loaded adjacent to the DNase I reaction mixtures to determine the exact positions of protected DNA sequences.

RESULTS

Putative LexA binding sites identified in the LEE2, LEE3, and LEE5 promoters.

We identified a putative LexA binding site in the overlapping EPEC LEE2/LEE3 promoters extending from position −4 to position −23 and from position −13 to position 7 in relation to the start of transcription for the LEE2 and LEE3 promoters (33), respectively (Table (Table33 and Fig. Fig.1A).1A). Similarly, we identified a putative LexA binding site in the LEE5 promoter, extending from position −20 to position −1 in relation to the start of transcription (12) (Table (Table33 and Fig. Fig.1B).1B). These sequences contained the nearly invariant CTG motif of the canonical LexA recognition sequence (9, 61) and were located directly within the LEE2, LEE3, and LEE5 promoters. In comparison, the canonical LexA binding site in recA spans from position −30 to position −11 in relation to the recA start of transcription (30) (Fig. (Fig.1).1). Therefore, based on the position of the putative LexA binding sites in relation to the LEE2/LEE3 and LEE5 promoters, we predicted that these loci would be responsive to DNA damage-associated signaling in a RecA-LexA-dependent manner.

FIG. 1.
Putative and established LexA binding sites in the LEE2/LEE3, LEE5, and recA promoters. (A) The putative LexA binding site (61) in the LEE2 promoter spans from position −4 to position −23, corresponding to positions −13 to 7 in ...
TABLE 3.
LexA binding sites

LEE promoters responsive to DNA damage-associated signaling in a RecA-dependent manner.

To test our hypothesis, we determined β-galactosidase activities derived from single-copy LEE-lacZ fusions constructed in the E. coli K-12 laboratory strain MC4100 in the presence and absence of the DNA-damaging agent mitomycin C. Mitomycin C induces interstrand DNA cross-links (56, 57), causing double-strand breaks, which induces the SOS response (47, 61). Because genes of the SOS regulon are induced at various times after DNA damage occurs, we monitored β-galactosidase activities at 30 and 60 min after the addition of mitomycin C. At 60 min, mitomycin C caused ~4-fold-increased expression from the LEE2-lacZ fusion (expression increased from 50 to 213 Miller units) (Fig. (Fig.2A).2A). Similarly, mitomycin C caused ~3-fold- and ~4-fold-increased expression from the LEE3-lacZ and LEE5-lacZ fusions, respectively (expression increased from 66 to 181 Miller units and from 45 to 181 Miller units, respectively) (Fig. 2B and C). We did not observe mitomycin C-dependent increased expression in the LEE2-lacZ, LEE3-lacZ, and LEE5-lacZ fusion strains containing the mutant recA1 allele (Fig. 2A to C), as mutations in recA prevented the bacteria from inducing the SOS response (61).

FIG. 2.
DNA damage-associated responses of single-copy LEE-lacZ fusions occur in a RecA-LexA-dependent manner in E. coli K-12-derived strains. β-Galactosidase activities derived from LEE2-lacZ (A), LEE3-lacZ (B), LEE5-lacZ (C), and recA-lacZ (D) fusions ...

As a positive control, we constructed a recA-lacZ single-copy fusion in strain MC4100, and mitomycin C caused expression to increase from 288 to 1,435 Miller units and from 275 to 2,740 Miller units at 30 and 60 min, respectively (Fig. (Fig.2D).2D). The ~10-fold increase in activity observed with the recA transcriptional fusion at 60 min was consistent with previously reported results (9, 44). As a negative control, we showed that transcriptional activity derived from a lac-lacZ single-copy fusion whose construction was identical to that of all other fusions in strain MC4100 increased less than twofold in the presence of mitomycin C. This fusion contained the lacZYA promoter, as well as the lacI gene encoding the Lac repressor, and exhibited >100-fold induction in the presence of the inducer isopropyl-β-d-thiogalactopyranoside (IPTG) (data not shown). We also observed that expression from single-copy LEE1-lacZ and LEE4-lacZ fusions increased less than twofold in the presence of mitomycin C, and we concluded that unlike LEE2, LEE3, and LEE5, the lacZYA, LEE1 and LEE4 operons did not respond to DNA damage-associated signaling in the K-12-derived strains (data not shown).

SOS regulation of LEE operons is dependent on functional LexA protein.

Based on our observation that increased LEE transcription in response to DNA damage was dependent on a functional RecA protein, we predicted that this signaling was also dependent on a functional LexA protein. We therefore transduced the lexA1 allele, which encodes a mutant LexA protein unable to undergo autocatalytic cleavage in the presence of the coprotease RecA and thus renders the cell unable to induce the SOS response (39), into the LEE-lacZ fusion strains and monitored the β-galactosidase activity in response to DNA damage. Reporter gene assays in the presence of the recA1 and lexA1 alleles were performed with derivatives of K-12 strain MC4100 because the wild-type EPEC pathogenic strain E2348/69 is recalcitrant to such genetic manipulations; i.e., generalized transduction to move the recA1 or lexA1 allele is not possible using wild-type EPEC. As predicted, addition of mitomycin C did not result in increased activity from the LEE2-lacZ, LEE3-lacZ, and LEE5-lacZ fusions in the presence of the lexA1 allele (Fig. 2A to C). Transduction of the lexA1 allele into the recA-lacZ strain also eliminated the ability of this fusion to respond to the DNA damage signaling induced by addition of mitomycin C (Fig. (Fig.2D).2D). Therefore, we concluded that the EPEC LEE2, LEE3, and LEE5 operons were regulated by the SOS response in K-12-derived strains in a RecA-LexA-dependent manner.

SOS regulation of the LEE in EPEC.

To demonstrate SOS regulation of the LEE operons in wild-type EPEC, we performed an RNA dot blot analysis. Briefly, whole-cell RNA was isolated from strain E2348/69 in the exponential phase in the absence and in the presence of mitomycin C at 60 min, conditions which were identical to the conditions used for monitoring β-galactosidase activities resulting from the LEE-lacZ single-copy fusions in the K-12-derived strains. Ten-microgram aliquots of RNA were dotted onto a nylon membrane and hybridized with radiolabeled DNA probes generated by PCR using the oligonucleotide primers shown in Table Table2.2. Dots were cut from the membrane and subjected to scintillation counting to quantify changes in transcription in response to DNA damage. Consistent with the results for the K-12-derived strains, transcription of LEE2 and LEE3 increased 60 min after the addition of mitomycin C (127% [P < 0.001] and 136% [P = 0.038], respectively) (Table (Table4).4). Surprisingly, we did not observe a significant increase in LEE5 transcription in EPEC in response to DNA damage (108%; P = 0.128). However, predictably, we observed an increase in transcription of the cryptic prophage-located, effector-encoding nleA gene (164%; P = 0.016) (Table (Table4).4). Transcription of the positive control gene, recA, also increased in the presence of mitomycin C (503%; P < 0.001). The negative control lacZ gene, the LEE1 and LEE4 operons, and the cryptic prophage-located effector molecule-encoding nleD and espJ genes did not exhibit significantly increased transcription in response to DNA damage (data not shown).

TABLE 4.
Mitomycin C increased transcription of LEE2, LEE3, and nleA in EPEC

To further demonstrate SOS regulation of the EPEC LEE, we monitored protein secretion in the presence of the lexA1 allele, which has a dominant negative phenotype in E. coli (39). Because EPEC is intractable to genetic manipulation by transducing phage, we amplified the lexA1 allele from strain DM803 and cloned this gene under control of the arabinose-inducible PBAD promoter in plasmid pBAD33 (the resulting plasmid was designated pKH296), and we transformed the plasmid into EPEC strains E2348/69, CVD452 with escN (the gene encoding the ATPase of the TTSS [14]) deleted, and SE796 with the global regulator ler gene deleted (33). EPEC strains containing the pKH296 plasmid and the pBAD33 empty vector control were grown in HEPES (pH 7.4)-buffered DMEM, a tissue culture medium that maximizes secretion, and harvested in the late exponential phase, and secreted proteins were precipitated and then separated by polyacrylamide gel electrophoresis. As expected, we observed no secretion of proteins from strain CVD452 with escN deleted in the presence of either pBAD33 or pKH296 (Fig. (Fig.3,3, lanes 1 and 2). For wild-type EPEC strain E2348/69 we observed modest decreases in the quantities of some secreted proteins, most notably EspB/D in the presence of the lexA1 allele (Fig. (Fig.3,3, lanes 3 and 4). Because Ler is a key regulator of EPEC virulence, we also monitored protein secretion in strain SE796 containing a ler deletion, which dampened expression of the LEE and resulted in a better understanding of the contribution of the SOS response control of TTSS function. In strain SE796, expression of the uncleavable LexA protein eliminated secretion of all proteins except one protein comigrating with the 66-kDa molecular mass marker, compared to the pBAD33 plasmid control (Fig. (Fig.3,3, lanes 5 and 6). These results were consistent with the alterations in transcriptional activities of LEE operons observed in both K-12 and wild-type EPEC strains. We therefore concluded that the LexA protein and the SOS response regulate LEE transcription and secretion of proteins by the EPEC TTSS.

FIG. 3.
LexA-dependent secretion by the EPEC TTSS. Strains were grown to an A600 of ~0.8 to 1.0 in DMEM buffered with 50 mM HEPES (pH 7.4) in the presence of 0.1% arabinose to induce lexA1 expression. Bacterial cultures were centrifuged, supernatants ...

Purified LexA protein binds in vitro to the predicted binding site located in the EPEC LEE2/LEE3 promoter region.

Based on the genetic and phenotypic evidence presented above, we predicted that purified LexA protein would bind to EPEC LEE regulatory DNA in vitro. We therefore performed DNase I protection assays using purified LexA protein, which is identical in the K-12 MG1655 and EPEC E2348/69 strains (2; http://www.sanger.ac.uk/Projects/Escherichia_Shigella/), and LEE2/LEE3 and LEE5 regulatory fragments containing the putative LexA binding sites (Fig. (Fig.11 and Table Table3).3). As a positive control, we also performed the DNase I protection assay using a DNA fragment containing the demonstrated LexA binding site of the recA promoter (30). For the recA promoter-containing DNA fragment purified LexA protein protected positions −2 to −30 from DNase I digestion, results which were nearly identical to results described previously (Fig. (Fig.4C)4C) (30). We observed a band corresponding to hypersensitivity to DNase I digestion at positions −32 and −5 in relation to the recA start of transcription, near the 5′ and 3′ points of LexA binding, respectively, on the coding DNA strand (Fig. (Fig.1C1C and and4C4C).

FIG. 4.
DNase I protection assays to establish purified LexA protein binding in LEE and recA regulatory DNA fragments in vitro. (A) The region of protection from DNase I cleavage, demonstrating LexA binding at the LEE2/LEE3 promoters, is indicated by a vertical ...

Purified LexA protein bound the LEE2/LEE3 regulatory fragment from position −2 to position −25 in relation to the LEE2 start of transcription (Fig. (Fig.1A1A and and4A).4A). This site corresponds to positions −15 to +9 in relation to the overlapping LEE3 promoter (Fig. (Fig.1A1A and and4A).4A). We observed regions of DNA that were hypersensitive to DNase I cleavage at positions −28, +1, +2, and +3 of the LEE2 promoter, which we interpreted as indicating the extent of LexA binding, closely corresponding to the predicted LexA binding site (Fig. (Fig.1A1A and and4A4A and Table Table3).3). LexA protein bound discretely in the −10 hexamers of both LEE2 and LEE3 promoters, even at a relatively high protein concentration, 2 μM (Fig. (Fig.4A).4A). Although the LEE5 operon was regulated by the SOS response in a RecA-LexA-dependent manner in the K-12-derived lacZ fusion strain in vivo (Fig. (Fig.2C),2C), multiple DNase I protection assays demonstrated that purified LexA protein did not bind specifically to the LEE5 promoter region in vitro (Fig. (Fig.4B).4B). This result, however, was consistent with the lack of SOS regulation of the LEE5 operon in EPEC as determined by dot blot analysis (Table (Table4).4). Because purified LexA protein bound specifically to the predicted LEE2/LEE3 binding site in vitro, we concluded that the LexA protein directly represses EPEC LEE2/LEE3 transcription.

DISCUSSION

In this report we present genetic, biochemical, and phenotypic evidence that the SOS response regulates expression of LEE genes of EPEC, which encode components of a TTSS. The DNA-damaging agent mitomycin C caused increased transcription of single-copy LEE2-lacZ, LEE3-lacZ, and LEE5-lacZ fusions in a K-12-derived strain, and this activity was eliminated in the presence of the recA1 allele, which prevented the bacterium from inducing the SOS response (Fig. (Fig.2)2) (6, 61). Additionally, increased transcriptional activity of these operons was abrogated in the presence of the lexA1 allele, encoding an uncleavable LexA protein (39, 52). In wild-type EPEC strain E2348/69 we demonstrated increased transcriptional activity of the LEE2 and LEE3 operons by RNA dot blot analysis (Table (Table4),4), consistent with results obtained for the K-12-derived strains.

Using a DNase I protection assay, we found that purified LexA protein specifically bound a LEE2/LEE3 regulatory fragment. We observed that LexA binding protected the LEE2/LEE3 overlapping, divergent promoter fragment under conditions identical to those where LexA protected a recA promoter fragment from DNase I digestion (Fig. (Fig.4).4). We therefore concluded that LexA directly regulates expression of the LEE2/LEE3 operons in EPEC. Binding of LexA at the LEE2/LEE3 site is predicted to prevent transcription by occlusion of RNA polymerase binding, consistent with the demonstrated mechanism of LexA repression. Single operator sites have also been observed at the divergent uvrA/ssb, ybiA/dinG, and umuCD/hylE promoters, and LexA is predicted to inhibit transcription of both operons in each case (9).

Consistent with the absence of increased LEE5 transcription in response to DNA damage in EPEC as determined by RNA dot blot analysis (Table (Table4),4), we did not observe binding of the LexA protein to a LEE5 regulatory fragment in vitro (Fig. (Fig.4).4). The most likely explanation for observing a LEE5 response to DNA damage in the K-12 strain but not in the wild-type pathogen is that an EPEC-specific repressor dampens transcription of LEE5 and does not allow a response to the DNA damage signal under the conditions tested. We hypothesized that without an EPEC-specific repressor in the K-12 strain, SOS regulation of the LEE5 operon is demonstrable. Evidence that a non-H-NS, negative regulator acts at the LEE5 operon in EPEC has been reported previously (49, 58). In addition, we predicted that if the LEE5 operon of EPEC were subject to SOS regulation, the regulation would be indirect, because the purified LexA protein did not bind to the LEE5 regulatory DNA in vitro (Fig. (Fig.44).

SOS induction of the LEE operons was slower than SOS induction of the recA gene in response to DNA damage. The recA gene was induced 5-fold and 10-fold 30 and 60 min after the addition of mitomycin C, respectively, while the LEE2 and LEE3 operons required 60 min of exposure to the DNA-damaging agent before induction was observed (Fig. (Fig.2).2). We attributed this difference in induction kinetics to the relative strength of induction; the LEE2 and LEE3 operons were induced 3- to 4-fold, while the recA gene was induced 10-fold at 60 min.

DNA damage signals that induce the SOS response and ultimately phage gene expression play an important role in bacterial pathogenesis. Prophages are central to pathogenesis in E. coli serotype O157:H7, a pathogen closely related to EPEC, and V. cholerae, among other bacteria. The Stx2 toxin of serotype O157:H7 is encoded in the 933W λ-like prophage and is transcribed from the PR′ promoter (43, 60). Agents that cause DNA damage and thus phage induction increase Stx2 biosynthesis and release in this pathogen (54, 62). The PrstA promoter of the cholera toxin-encoding CTXΦ prophage is regulated by the SOS response and is dependent upon LexA cleavage for induction (46). We demonstrate here that expression of the TTSS of EPEC is regulated by the same mechanism, the SOS response, and we found that TTSS-dependent protein secretion was reduced in the presence of the lexA1 allele encoding an uncleavable LexA protein (Fig. (Fig.33).

Several effector molecules secreted by the TTSS of EPEC strains are encoded in defective prophages, including Cif, NleA, NleD, and EspJ (16), and the genes encoding these proteins are likely to be coordinately regulated with the TTSS, which is necessary for their secretion so that they can play a role in pathogenesis. The CP-933P prophage-encoded NleA protein colocalizes with the Golgi apparatus in the host cell and is not required for AE lesion formation, but it does play an important, although not fully understood, role in the Citrobacter rodentium-mouse infection model (19, 40). We provide evidence in this report that transcription of the prophage-encoded nleA gene is regulated by the SOS response (Table (Table4).4). For bacteriophage λ, activation of the coprotease RecA leads to autocatalytic cleavage and release of the λ CI repressor (29, 41), expression of phage genes, and thus phage induction. Some phages in fact use LexA as the repressor to prevent induction (46). Activation of the SOS response upon DNA damage, leading to autocatalytic cleavage of the LexA protein and/or phage repressors by the RecA coprotease bound to single-stranded DNA, might explain how a subset of the cryptic prophage-encoded EPEC effector molecules and associated phenotypes are coordinately regulated with the LEE-encoded TTSS within a host.

Multiple mechanisms most likely induce the SOS response of bacterial pathogens in the human gastrointestinal tract, and during infection the organisms are subjected to tremendous selective pressure. For example, membrane-membrane interactions cause DNA damage and activation of repair mechanisms in Neisseria meningitidis (38). Neutrophils of the human innate immune system produce agents, such as H2O2, that can cause DNA damage in the invading bacterium (59). Antibiotics, such as β-lactams, that damage the bacterial cell wall also induce the SOS response (35). EPEC inhibited expression of inducible nitric oxide synthase, which produces the antibacterial agent nitric oxide (NO), in human intestinal epithelial cells in culture (31). This activity was dependent on attachment to and delivery of effector molecules in the host cell, and these findings provide further evidence that EPEC perceives and responds to DNA damage signals in the human host. We now know that the SOS response regulates expression of at least one of the phage-encoded effector molecules and its cognate TTSS, a structure necessary for EPEC and many other gram-negative bacteria to cause disease. This regulation links functions fundamental to bacterial survival in the host, including SOS control of stalled DNA replication forks and expression of error-prone polymerases that lead to increased mutation frequencies, with expression of a structure fundamental to EPEC pathogenesis, the TTSS. These findings underscore the need to diversify our therapeutic arsenal directed against EPEC and related pathogens.

Acknowledgments

We thank John Little for generously providing purified LexA protein and Susan Gottesman and Justin Courcelle for helpful critical comments on the manuscript. We acknowledge Amy Cheng Vollmer, Bianca Colonna, and Tim Larson for providing strains.

This work was supported by NIH AREA grant R15 AI047802-02 awarded to J.L.M., in part by Howard Hughes Medical Institute and James F. & Marion L. Miller Foundation grants awarded to Reed College, and by an American Society for Microbiology undergraduate research fellowship awarded to D.C.G, including funds for attending the ASM General Meeting.

Footnotes

[down-pointing small open triangle]Published ahead of print on 19 January 2007.

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