The AraC family transcription factor MarA activates ∼40 genes (the marA/soxS/rob regulon) of the Escherichia coli chromosome resulting in different levels of resistance to a wide array of antibiotics and to superoxides. Activation of marA/soxS/rob regulon promoters occurs in a well-defined order with respect to the level of MarA; however, the order of activation does not parallel the strength of MarA binding to promoter sequences. To understand this lack of correspondence, we developed a computational model of transcriptional activation in which a transcription factor either increases or decreases RNA polymerase binding, and either accelerates or retards post-binding events associated with transcription initiation. We used the model to analyze data characterizing MarA regulation of promoter activity. The model clearly explains the lack of correspondence between the order of activation and the MarA-DNA affinity and indicates that the order of activation can only be predicted using information about the strength of the full MarA-polymerase-DNA interaction. The analysis further suggests that MarA can activate without increasing polymerase binding and that activation can even involve a decrease in polymerase binding, which is opposite to the textbook model of activation by recruitment. These findings are consistent with published chromatin immunoprecipitation assays of interactions between polymerase and the E. coli chromosome. We find that activation involving decreased polymerase binding yields lower latency in gene regulation and therefore might confer a competitive advantage to cells. Our model yields insights into requirements for predicting the order of activation of a regulon and enables us to suggest that activation might involve a decrease in polymerase binding which we expect to be an important theme of gene regulation in E. coli and beyond.
When environmental conditions change, cell survival can depend on sudden production of proteins that are normally in low demand. Protein production is controlled by transcription factors which bind to DNA near genes and either increase or decrease RNA production. Many puzzles remain concerning the ways transcription factors do this. Recently we collected data relating the intracellular level of a single transcription factor, MarA, to the increase in expression of several genes related to antibiotic and superoxide resistance in Escherichia coli. These data indicated that target genes are turned on in a well-defined order with respect to the level of MarA, enabling cells to mount a response that is commensurate to the level of threat detected in the environment. Here we develop a computational model to yield insight into how MarA turns on its target genes. The modeling suggests that MarA can increase the frequency with which a transcript is made while decreasing the overall presence of the transcription machinery at the start of a gene. This mechanism is opposite to the textbook model of transcriptional activation; nevertheless it enables cells to respond quickly to environmental challenges and is likely of general importance for gene regulation in E. coli and beyond.
Transcriptional activation of the promoters of the mar/soxRS regulons by the sequence-related but independently inducible MarA and SoxS proteins renders Escherichia coli resistant to a broad spectrum of antibiotics and superoxide generators. Here, the effects of MarA and SoxS on transcription of the marRAB promoter itself were assayed in vitro by using a minimal transcription system and in vivo by assaying beta-galactosidase synthesized from marR::lacZ fusions. Purified MarA and MalE-SoxS proteins stimulated mar transcription about 6- and 15-fold, respectively, when the RNA polymerase/DNA ratio was 1. Purified MarA bound as a monomer to a 16-bp "marbox" located 69 to 54 nucleotides upstream of a putative RNA initiation site. Deletion of the marbox reduced MarA-mar binding 100-fold, abolished the stimulatory effects of MarA and SoxS on transcription in vitro, and reduced marR::lacZ synthesis about 4-fold in vivo. Deletion of upstream DNA adjoining the marbox reduced MarA binding efficiency 30-fold and transcriptional activation 2- to 3-fold, providing evidence for an accessory marbox. Although MarA and the mar operon repressor, MarR, bound to independent sites, they competed for promoter DNA in band shift experiments. Assays of marR::lacZ transcriptional fusions in marRAB deletion or soxRS deletion strains showed that the superoxide generator paraquat stimulates mar transcription via soxRS and that salicylate stimulates mar transcription both by antagonizing MarR and by a MarR-independent mechanism. Thus, transcription of the marRAB operon is autorepressed by MarR and autoactivated by MarA at a site that also can be activated by SoxS.
In Escherichia coli, Rob activates transcription of the SoxRS/MarA/Rob regulon. Previous work revealed that Rob resides in 3–4 immunostainable foci, that dipyridyl and bile salts are inducers of its activity, and that inducers bind to Rob’s C-terminal domain (CTD). We propose that sequestration inactivates Rob by blocking its access to the transcriptional machinery and that inducers activate Rob by mediating its dispersal, allowing interaction with RNA polymerase. To test “sequestration-dispersal” as a new mechanism for regulating the activity of transcriptional activators, we fused Rob’s CTD to SoxS and used indirect immunofluorescence microscopy to determine the effect of inducers on SoxS-Rob’s cellular localization. Unlike native SoxS, which is uniformly distributed throughout the cell, SoxS-Rob is sequestered without inducer, but is rapidly dispersed when cells are treated with inducer. In this manner, Rob’s CTD serves as an anti-sigma factor in regulating the co-sigma factor-like activity of SoxS when fused to it. Rob’s CTD also protects its N-terminus from Lon protease, since Lon’s normally rapid degradation of SoxS is blocked in the chimera. Accordingly, Rob’s CTD has novel regulatory properties that can be bestowed on another E. coli protein.
gene regulation; intracellular localization; immunofluorescence microscopy; anti-sigma factor; proteolysis
Escherichia coli K-12 strains are normally tolerant to n-hexane and susceptible to cyclohexane. Constitutive expression of marA of the multiple antibiotic resistance (mar) locus or of the soxS or robA gene product produced tolerance to cyclohexane. Inactivation of the mar locus or the robA locus, but not the soxRS locus, increased organic solvent susceptibility in the wild type and Mar mutants (to both n-hexane and cyclohexane). The organic solvent hypersusceptibility is a newly described phenotype for a robA-inactivated strain. Multicopy expression of mar, soxS, or robA induced cyclohexane tolerance in strains with a deleted or inactivated chromosomal mar, soxRS, or robA locus; thus, each transcriptional activator acts independently of the others. However, in a strain with 39 kb of chromosomal DNA, including the mar locus, deleted, only the multicopy complete mar locus, consisting of its two operons, produced cyclohexane tolerance. Deletion of acrAB from either wild-type E. coli K-12 or a Mar mutant resulted in loss of tolerance to both n-hexane and cyclohexane. Organic solvent tolerance mediated by mar, soxS, or robA was not restored in strains with acrAB deleted. These findings strongly suggest that active efflux specified by the acrAB locus is linked to intrinsic organic solvent tolerance and to tolerance mediated by the marA, soxS, or robA gene product in E. coli.
Expression of the marA or soxS genes is induced by exposure of Escherichia coli to salicylate or superoxides, respectively. This, in turn, enhances the expression of a common set of promoters (the mar/soxRS regulons), resulting in both multiple antibiotic and superoxide resistance. Since MarA protein is highly homologous to SoxS, and since a MalE-SoxS fusion protein has recently been shown to activate soxRS regulon transcription, the ability of MarA to activate transcription of these genes was tested. MarA was overexpressed as a histidine-tagged fusion protein, purified, cleaved with thrombin (leaving one N-terminal histidine residue), and renatured. Like MalE-SoxS, MarA (i) activated the transcription of zwf, fpr, fumC, micF, nfo, and sodA; (ii) required a 21-bp "soxbox" sequence to activate zwf transcription; and (iii) was "ambidextrous," i.e., required the C-terminal domain of the alpha subunit of RNA polymerase for activation of zwf but not fumC or micF. Thus, the mar and soxRS systems use activators with very similar specificities and mechanisms of action to respond to different environmental signals.
Three paralogous transcriptional activators MarA, SoxS, and Rob, activate >40 Escherichia coli promoters. To understand why MarA does not activate certain promoters as strongly as SoxS, we compared MarA, MarA mutants, and SoxS for their abilities to activate 16 promoters and to bind their cognate marbox binding sites. Replacement of the MarA glutamic acid residue 89 with alanine greatly increased the marbox binding and activation of many class I promoters. Like cells constitutive for SoxS, cells expressing the MarA with the E89A mutation were more resistant to superoxides than those harboring WT MarA. The activities of several other E89 substitutions ranked as follows: E89A > E89G > E89V > WT > E89D. Increased binding and activation occurred only at class I promoters when the 12th base of the promoter's marbox (a position at which there is no known interaction between the marbox and MarA) was not a T residue. Furthermore, WT MarA binding to a synthetic marbox in vitro was enhanced when the phosphate group between positions 12 and 13 was eliminated on one strand. The results demonstrate that relatively minor changes in a single amino acid side chain (e.g., alanine to valine or glutamic acid to aspartic acid) can strongly influence activity despite any evidence that the side chain is involved in positive interactions with either DNA or RNA polymerase. We present a model which attributes the differences in binding and activation to the interference between the β- and γ-carbons of the amino acid at position 89 and the phosphate group between positions 12 and 13.
Multiple antibiotic resistance in Escherichia coli can be mediated by induction of the SoxS or MarA protein, triggered by oxygen radicals (in the soxRS regulon) or certain antibiotics (in the marRAB regulon), respectively. These small proteins (SoxS, 107 residues; MarA, 127 residues) are homologous to the C terminus of the XylS-AraC family of proteins and are more closely related to a approximately 100-residue segment in the N terminus of Rob protein, which binds the right arm of the replication origin, oriC. We investigated whether the SoxS-MarA homology in Rob might extend to the regulation of some of the same inducible genes. Overexpression of Rob indeed conferred multiple antibiotic resistance similar to that known for SoxS and MarA (against chloramphenicol, tetracycline, nalidixic acid, and puromycin), as well as resistance to the superoxide-generating compound phenazine methosulfate. The Rob-induced antibiotic resistance depended only partially on the micF antisense RNA that down-regulates the OmpF outer membrane porin to limit antibiotic uptake. Similar antibiotic resistance was conferred by expression of a Rob fragment containing only the N-terminal 123 residues that constitute the SoxS-MarA homology. Both intact Rob and the N-terminal fragment activated expression of stress genes (inaA, fumC, sodA) but with a pattern distinct from that found for SoxS and MarA. Purified Rob protein bound a DNA fragment containing the micF promoter (50% bound at approximately 10(-9) M Rob) as strongly as it did oriC, and it bound more weakly to DNA containing the sodA, nfo, or zwf promoter (50% bound at 10(-8) to 10(-7) M). Rob formed multiple DNA-protein complexes with these fragments, as seen previously for SoxS. These data point to a DNA-binding gene activator module used in different protein contexts.
The paralogous transcriptional activators, MarA, SoxS and Rob, activate a common set of promoters, the marA/soxS/rob regulon of Escherichia coli, by binding a cognate site (marbox) upstream of each promoter. The extent of activation varies from one promoter to another and is only poorly correlated with the in vitro affinity of the activator for the specific marbox. Here, we examine the dependence of promoter activation on the level of activator in vivo by manipulating the steady-state concentrations of MarA and SoxS in Lon protease mutants and measuring promoter activation using lacZ transcriptional fusions. We found that: (i) the MarA concentrations needed for half-maximal stimulation varied by at least 19-fold among the 10 promoters tested; (ii) most marboxes were not saturated when there were 24,000 molecules of MarA per cell; (iii) the correlation between MarA concentration needed for half-maximal promoter activity in vivo with marbox binding affinity in vitro was poor and (iv) the two activators differed in their promoter activation profiles. The marRAB and sodA promoters could both be saturated by MarA and SoxS in vivo. However, saturation by MarA resulted in greater marRAB and lesser sodA transcription than did saturation by SoxS implying that the two activators interact with RNAP in different ways at the different promoters. Thus, the concentration and nature of activator determines which regulon promoters are activated and the extent of their activation.
gene regulation; AraC protein family; stress response
Activation of Escherichia coli oxidative stress regulon genes (sodA, zwf, fumC, nfo, etc.) is mediated by a two-stage regulatory system: the redox-sensitive SoxR protein transcriptionally activates the soxS gene, whose product then stimulates transcription of the regulon genes. Previous experiments showed that limited 3' truncation of soxR gene causes constitutive soxRS expression. DNA sequence analysis of the soxR genes from the soxRS-constitutive strains isolated originally (Greenberg et al. (1990) Proc. Natl. Acad. Sci. USA 87, 6181-6185) revealed that three alleles encode amino acid substitutions or a chain termination clustered near the C-terminus of SoxR. Two other single-amino-acid substitutions in constitutive alleles mapped to the helix-turn-helix motif and to a region of unknown function in the center of the polypeptide, respectively. No constitutive mutation was found within the region encoding the cysteines of the SoxR FeS center, in the soxR or soxS promoters, or in the soxS structural gene. Since an in-frame deletion of just nine SoxR residues (136-144; full-length SoxR = 154 residues) gave rise to a powerful constitutive allele, it appears that a small segment of the SoxR C-terminus maintains the protein in the inactive state. Conservely, an intact C-terminus is evidently not required for gene activation by SoxR.
The Escherichia coli tolC encodes a major outer membrane protein with multiple functions in export (e. g., diverse xenobiotics, hemolysin) and as an attachment site for phage and colicins. tolC is regulated in part by MarA, SoxS and Rob, three paralogous transcriptional activators which bind a sequence called the marbox and which activate multiple antibiotic and superoxide resistance functions. Two previously identified tolC promoters, p1 and p2, are not regulated by MarA, SoxS or Rob but p2 is activated by EvgAS and PhoPQ which also regulate other functions. Using transcriptional fusions and primer extension assays, we show here that tolC has two additional strong overlapping promoters, p3 and p4, which are downstream of p1, p2 and the marbox and are activated by MarA, SoxS and Rob. p3 and p4 are configured so that a single marbox suffices to activate transcription from both promoters. At the p3 promoter, the marbox is separated by 20 bp from the −10 hexamer for RNA polymerase but at the p4 promoter, the same marbox is separated by 30 bp from the −10 hexamer. The multiple tolC promoters may allow the cell to respond to diverse environments by coordinating tolC transcription with other appropriate functions.
gene regulation; outer membrane protein; transcriptional start sites; efflux pumps; antibiotic resistance
Bacteria possess multiple mechanisms to survive exposure to various chemical stresses and antimicrobial compounds. In the enteric bacterium Escherichia coli, three homologous transcription factors—MarA, SoxS, and Rob—play a central role in coordinating this response. Three separate systems are known to regulate the expression and activities of MarA, SoxS, and Rob. However, a number of studies have shown that the three do not function in isolation but rather are coregulated through transcriptional cross talk. In this work, we systematically investigated the extent of transcriptional cross talk in the mar-sox-rob regulon. While the three transcription factors were found to have the potential to regulate each other's expression when ectopically expressed, the only significant interactions observed under physiological conditions were between mar and rob systems. MarA, SoxS, and Rob all activate the marRAB promoter, more so when they are induced by their respective inducers: salicylate, paraquat, and decanoate. None of the three proteins affects the soxS promoter, though unexpectedly, it was mildly repressed by decanoate by an unknown mechanism. SoxS is the only one of the three proteins to repress the rob promoter. Surprisingly, salicylate somewhat activates transcription of rob, while decanoate represses it a bit. Rob, in turn, activates not only its downstream promoters in response to salicylate but also the marRAB promoter. These results demonstrate that the mar and rob systems function together in response to salicylate.
Multiple factors control the expression of the outer membrane porins OmpF and OmpC in Escherichia coli. In this work, we investigated the role of the mar-sox-rob regulon in regulating outer membrane porin expression in response to salicylate. We provide both genetic and physiological evidence that MarA and Rob can independently activate micF transcription in response to salicylate, leading to reduced OmpF expression. MarA was also found to repress OmpF expression through a MicF-independent pathway. In the case of OmpC, we found that its transcription was moderately increased in response to salicylate. However, this increase was independent of MarA and Rob. Finally, we found that the reduction in OmpF expression in a tolC mutant is due primarily to Rob. Collectively, this work further clarifies the coordinated role of MarA and Rob in regulating the expression of the outer membrane porins.
In Escherichia coli K-12, transcription of zwf, the gene for glucose 6-phosphate dehydrogenase, is subject to growth rate-dependent regulation and is activated by SoxS in response to superoxide stress. To define genetically the site of SoxS activation, we undertook a detailed deletion analysis of the zwf promoter region. Using specifically targeted 5' and 3' deletions of zwf sequences, we localized the SoxS activation site to a 21-bp region upstream of the zwf promoter. This minimal "soxbox" was able to confer paraquat inducibility when placed upstream of a normally unresponsive gnd-lacZ protein fusion. In addition, we used these findings as the basis for resecting unnecessary sequences from the region upstream of the promoters of two other SoxS-regulated genes, sodA and nfo. Like the zwf soxbox, the regions required for SoxS activation of sodA and nfo appear to lie just upstream or overlap the -35 hexamers of the corresponding promoters. Importantly, the sequence boundaries established here by deletion analysis agree with the primary SoxS recognition sites of zwf, sodA, and nfo that we previously identified in vitro by gel mobility shift and DNase I protection assays with a purified MalE-SoxS fusion protein.
soxR and soxS are adjacent genes that govern a superoxide response regulon. Previous studies revealed that induction of the regulon is accompanied by increased transcription of soxS, which can activate the target genes. Therefore, induction may occur in two stages: the soxR-dependent activation of soxS, followed by the soxS-dependent induction of other genes. However, the requirement for soxR was unproven because the only existing soxR mutations either were of the regulon-constitutive type or also involved soxS. Therefore, we produced an insertion mutation that was shown by complementation to inactivate only soxR. In confirmation of the two-stage model, soxR was required for the induction by paraquat of the target genes studied (nfo, zwf, and sodA), for paraquat resistance, and for the 47- to 76-fold induction of soxS-lacZ gene fusions. Paraquat did not affect the expression of soxR-lacZ gene fusions. In a soxRS deletion mutant, the regulon was constitutively activated by a runaway soxS+ plasmid. However, a lower-copy-number plasmid failed to activate nfo, zwf, or sodA but did increase the paraquat resistance of a soxRS mutant. Therefore, there is a differential response of the regulon genes to soxS overproduction. A soxR regulon-constitutive mutation was suppressed by a soxR+ plasmid, suggesting a competition between native and activated forms of SoxR. It is proposed that to enhance the sensitivity of the response, the cell minimizes such potential competition by manufacturing only a small amount of this sensor protein, thereby necessitating signal amplification via induction of soxS.
Escherichia coli SoxS activates transcription of the genes of the soxRS regulon, which provide the cell's defense against oxidative stress. In response to this stress, SoxS is synthesized de novo. Because the DNA binding site of SoxS is highly degenerate, SoxS efficiently activates transcription by the mechanism of prerecruitment. In prerecruitment, newly synthesized SoxS first forms binary complexes with RNA polymerase. These complexes then scan the chromosome for class I and class II SoxS-dependent promoters, using the specific DNA-recognition properties of SoxS and σ70 to distinguish SoxS-dependent promoters from the vast excess of sequence-equivalent soxboxes that do not reside in promoters. Previously, we determined that SoxS interacts with RNA polymerase in two ways, by making protein-protein interactions with the DNA-binding determinant of the α subunit and by interacting with σ70 region 4 (σ70 R4) both “on-DNA” and “off-DNA”. Here, we address the question of how SoxS and σ70 R4 co-exist at class II promoters, where the binding site for SoxS either partially or completely overlaps the -35 region of the promoter, which is usually bound by σ70 R4. To do so, we created a tri-alanine scanning library that covers all of σ70 R4. We determined that interactions between σ70 R4 and the DNA in the promoter's −35 region are required for activation of class I promoters, where the binding site lies upstream of the −35 hexamer, but they are not required at class II promoters. In contrast, specific three-amino acid stretches are required for activation of class I (lac) and class II (galP1) cyclic AMP receptor protein-dependent promoters. We conclude from these data that SoxS and σ70 R4 interact with each other in a novel way at class II SoxS-dependent promoters such that the two proteins do not accommodate one another in the −35 region but instead SoxS binding there occludes the binding of σ70 R4.
SoxRS regulon; genetic epistasis; protein-DNA interactions; pre-recruitment
Efflux pumps function to rid bacteria of xenobiotics, including antibiotics, bile salts, and organic solvents. TolC, which forms an outer membrane channel, is an essential component of several efflux pumps in Escherichia coli. We asked whether TolC has a role during growth in the absence of xenobiotics. Because tolC transcription is activated by three paralogous activators, MarA, SoxS, and Rob, we examined the regulation of these activators in tolC mutants. Using transcriptional fusions, we detected significant upregulation of marRAB and soxS transcription and Rob protein activity in tolC mutants. Three mechanisms could be distinguished: (i) activation of marRAB transcription was independent of marRAB, soxR, and rob functions; (ii) activation of soxS transcription required SoxR, a sensor of oxidants; and (iii) Rob protein was activated posttranscriptionally. This mechanism is similar to the mechanisms of upregulation of marRAB, soxS, and Rob by treatment with certain phenolics, superoxides, and bile salts, respectively. The transcription of other marA/soxS/rob regulon promoters, including tolC itself, was also elevated in tolC mutants. We propose that TolC is involved in the efflux of certain cellular metabolites, not only xenobiotics. As these metabolites accumulate during growth, they trigger the upregulation of MarA, SoxS, and Rob, which in turn upregulate tolC and help rid the bacteria of these metabolites, thereby restoring homeostasis.
Cationic antimicrobial peptides (CAMPs), a component of the mammalian immune system, protect the host from bacterial infections. The roles of the Escherichia coli transcriptional regulators MarA, SoxS and Rob in susceptibility to these peptides were examined. Overexpression of marA, either in an antibiotic-resistant marR mutant or from a plasmid, decreased bacterial susceptibility to CAMPs. Overexpression of the soxS gene from a plasmid, which decreased susceptibility to antibiotics, unexpectedly caused no decrease in CAMP susceptibility; instead it produced increased susceptibility to different CAMPs. Deletion or overexpression of rob had little effect on CAMP susceptibility. The marRAB operon was upregulated when E. coli was incubated in sublethal amounts of CAMPs polymyxin B, LL-37 or human β-defensin-1; however, this upregulation required Rob. Deletion of acrAB increased bacterial susceptibility to polymyxin B, LL-37 and human β-defensin-1 peptides. Deletion of tolC yielded an even greater increase in susceptibility to these peptides and also led to increased susceptibility to human α-defensin-2. Inhibition of cellular proton-motive force increased peptide susceptibility for wild-type and acrAB deletion strains; however, it decreased susceptibility of tolC mutants. These findings demonstrate that CAMPs are both inducers of marA-mediated drug resistance through interaction with Rob and also substrates for efflux in E. coli. The three related transcriptional regulators show different effects on bacterial cell susceptibility to CAMPs.
Genomic studies with bacteria have identified redox-responsive genes without known roles in counteracting oxidative damage. Previous transcriptional profiling showed that expression of one such gene, yggX, was activated by superoxide stress in Escherichia coli. Here we show that this activation could be mimicked by artificial expression of the regulatory protein SoxS. Northern analysis confirmed the transcriptional activation of yggX by oxidative stress or SoxS expression but not in response to the related MarA or Rob proteins. Northern analysis showed that mltC, which codes for a peptidoglycan hydrolase and is positioned immediately downstream of yggX, was also regulated by oxidative stress or ectopic expression of SoxS. Purified SoxS protein bound to the predicted yggX promoter region, between positions 223 and 163 upstream from the yggX translational start site. Within this region, a 20-bp sequence was found to be necessary for oxidative stress-mediated activation of yggX transcription. A yggX deletion strain was hypersensitive to the redox-cycling agent paraquat, and a plasmid expressing YggX complemented the sensitivity of the deletion strain. Under exposure to paraquat, the yggX deletion strain showed a deficiency in aconitase activity compared to the isogenic wild-type strain, while expression of YggX from a multicopy plasmid increased the aconitase levels above those of the wild-type strain. These results demonstrate the direct regulation of the yggX gene by the redox-sensing SoxRS system and provide further evidence for the involvement of yggX in protection of iron-sulfur proteins against oxidative damage.
Escherichia coli and Salmonella enterica serovar Typhimurium have evolved genetic systems, such as the soxR/S and marA regulons, to detoxify reactive oxygen species, like superoxide, which are formed as by-products of metabolism. Superoxide also serves as a microbicidal effector mechanism of the host's phagocytes. Here, we investigate whether regulatory genes other than soxR/S and marA are active in response to oxidative stress in Salmonella and may function as virulence determinants. We identified a bacterial gene, which was designated ramA (342 bp) and mapped at 13.1 min on the Salmonella chromosome, that, when overexpressed on a plasmid in E. coli or Salmonella, confers a pleiotropic phenotype characterized by increased resistance to the redox-cycling agent menadione and to multiple unrelated antibiotics. The ramA gene is present in Salmonella serovars but is absent in E. coli. The gene product displays 37 to 52% homology to the transcriptional activators soxR/S and marA and 80 to 100% identity to a multidrug resistance gene in Klebsiella pneumoniae and Salmonella enterica serovar Paratyphi A. Although a ramA soxR/S double null mutant is highly susceptible to intracellular superoxide generated by menadione and displays decreased Mn-superoxide dismutase activity, intracellular survival of this mutant within macrophage-like RAW 264.7 cells and in vivo replication in the spleens in Ityr mice are not affected. We concluded that despite its role in the protective response of the bacteria to oxidative stress in vitro, the newly identified ramA gene, together with soxR/S, does not play a role in initial replication of Salmonella in the organs of mice.
The Escherichia coli soxRS regulon activates oxidative stress and antibiotic resistance genes in two transcriptional stages. SoxR protein becomes activated in cells exposed to excess superoxide or nitric oxide and then stimulates transcription of the soxS gene, whose product in turn activates>/=10 regulon promoters. Purified SoxR protein is a homodimer containing a pair of [2Fe-2S] centers essential for soxS transcription in vitro . The [2Fe-2S] centers are thought to be anchored by a C-terminal cluster of four cysteine residues in SoxR. Here we analyze mutant SoxR derivatives with individual cysteines replaced by alanine residues (Cys-->Ala). The mutant proteins in cell-free extracts bound the soxS promoter with wild-type affinity, but upon purification lacked Fe or detectable transcriptional activity for soxS in vitro . Electron paramagnetic resonance measurements in vivo indicated that the Cys-->Ala proteins lacked the [2Fe-2S] centers seen for wild-type SoxR. The Cys-->Ala mutant proteins failed to activate soxS expression in vivo in response to paraquat, a superoxide- generating agent. However, when expressed to approximately 5% of the cell protein, the Cys-->Ala derivatives increased basal soxS transcription 2-4-fold. Overexpression of the Cys119-->Ala mutant protein strongly interfered with soxS activation by wild-type SoxR in response to paraquat. These studies demonstrate the essential role of the [2Fe-2S] centers for SoxR activation in vivo ; the data may also indicate oxidant-independent mechanisms of transcriptional activation by SoxR.
Paralogous transcriptional regulators MarA, Rob, and SoxS act individually and together to control expression of more than 80 Escherichia coli genes. Deletion of marA, rob, and soxS from an E. coli clinical isolate prevents persistence beyond 2 days postinfection in a mouse model of pyelonephritis. We used microarray analysis to identify 242 genes differentially expressed between the triple deletion mutant and its parent strain at 2 days postinfection in the kidney. One of these, znuC of the zinc transport system ZnuACB, displayed decreased expression in the triple mutant compared to that in the parental strain, and deletion of znuC from the parental strain reduced persistence. The marA rob soxS triple deletion mutant was less viable in vitro under limited-Zn and Zn-depleted conditions, while disruption of znuC caused a reduction in the growth rates for the parental and triple mutant strains to equally low levels under limited-Zn or Zn-depleted conditions. Complementation of the triple mutant with soxS, but not marA or rob, restored the parental growth rate in Zn-depleted medium, while deletion of only soxS from the parental strain led to low growth in Zn-depleted medium. Both results suggested that SoxS is a major regulator responsible for growth under Zn-depleted conditions. Gel shift experiments failed to show direct binding of SoxS to the znuCB promoter, thus suggesting indirect control of znuCB expression by SoxS. While SoxS expression in the triple mutant fully restored persistence, increased expression of znuACB via a plasmid in this mutant only partially restored wild-type levels of persistence in the kidney. This work implicates SoxS control of znuCB expression as a key factor in persistence of E. coli in murine pyelonephritis.
The soxRS regulon is a cornerstone of the adaptive defense systems of Escherichia coli against oxidative stress. Unexpectedly, activation of this regulon also enhances bacterial resistance to multiple antibiotics that seem unrelated to oxygen radicals. We previously correlated this multiple antibiotic resistance with a reduced rate of synthesis of the OmpF outer membrane porin that does not affect the OmpC or OmpA porins. Studies presented here, with operon and gene fusions of ompF to lacZ, show that the soxRS-dependent repression of OmpF is achieved posttranscriptionally. We also show posttranscriptional repression of OmpF mediated by the soxQ1 mutation, which maps to the marA locus. These repressions are dependent on the micF gene, which encodes a small RNA partially complementary to the 5' end of the ompF message. Northern (RNA) blotting experiments show that micF transcription is strongly inducible by the superoxide-generating agent paraquat in a manner that depends completely on the soxRS locus. The soxR-constitutive and soxQ1 mutations elevate the expression of micF in the absence of redox stress. However, the antibiotic resistance mediated by a soxR-constitutive mutation was only partially reversed upon deletion of micF. The soxRS regulon therefore includes other components that contribute to general antibiotic resistance, although the relation of this phenotype to oxidative stress remains to be established.
Rob is a member of the Sox/Mar subfamily of AraC/XylS-type transcriptional regulators implicated in bacterial multidrug, heavy metal, superoxide, and organic solvent resistance phenotypes. We demonstrate that, in Salmonella enterica, Rob overexpression upregulates the transcription of mgtA, which codes for the MgtA Mg2+ transporter. mgtA was previously characterized as a member of the Mg2+-modulated PhoPQ regulon. Here we demonstrate that Rob (but not its paralog protein SoxS or MarA) is able to induce mgtA transcription in a PhoP-independent fashion by binding to a conserved Mar/Sox/Rob motif localized downstream of the PhoP-box and overlapping the PhoP-dependent transcriptional start site. We found that Rob-induced mgtA expression confers low-level cyclohexane resistance on Salmonella. Because mgtA intactness is required for Rob-induced cyclohexane resistance, provided the AcrAB multidrug efflux pump can be expressed, we postulate that MgtA is involved in the AcrAB-mediated cyclohexane detoxification mechanism promoted by Rob in Salmonella.
Transcription of the multiple antibiotic resistance marRAB operon increases when one of the sequence-related activators, MarA, SoxS, or Rob, binds to the "marbox" centered at -61.5 relative to the transcriptional start site. Previous deletion analyses showed that an adjacent upstream "accessory region" was needed to augment the marbox-dependent activation. To analyze the roles of the marbox and accessory regions on mar transcription, thirteen promoters, each with a different 5-bp transversion of the -96 to -32 sequence, were synthesized, fused to lacZ, and assayed for beta-galactosidase production in single-copy lysogens with appropriate genotypes. The accessory region is shown here to be a binding site for Fis centered at -81 and to bind Fis, a small DNA-binding and -bending protein, with a Kd of approximately 5 nM. The binding of MarA to the marbox and that of Fis to its site were independent of each other. MarA, SoxS, and Rob each activated the mar promoter 1.5-to 2-fold when it had a wild-type marbox but Fis was absent. In the presence of MarA, SoxS, or Rob, Fis further enhanced the activity of the promoter twofold provided the promoter was also capable of binding Fis. However, in the absence of MarA, SoxS, or Rob or in the absence of a wild-type marbox, Fis nonspecifically lowered the activity of the mar promoter about 25% whether or not a wild-type Fis site was present. Thus, Fis acts as an accessory transcriptional activator at the mar promoter.
Escherichia coli responds to the redox stress imposed by superoxide-generating agents such as paraquat by activating the synthesis of as many as 80 polypeptides. Expression of a key group of these inducible proteins is controlled at the transcriptional level by the soxRS locus (the soxRS regulon). A two-stage control system was hypothesized for soxRS, in which an intracellular redox signal would trigger the SoxR protein as a transcriptional activator of the soxS gene and the resulting increased levels of SoxS protein would activate transcription of the various soxRS regulon genes (B. Demple and C.F. Amábile Cuevas, Cell 67:837-839, 1990). We have constructed operon fusions of the E. coli lac genes to the soxS promoter to monitor soxS transcription. Expression from the soxS promoter is strongly inducible by paraquat in a manner strictly dependent on a functional soxR gene. Several other superoxide-generating agents also trigger soxR(+)-dependent soxS expression, and the inductions by paraquat and phenazine methosulfate were dependent on the presence of oxygen. Numerous other oxidative stress agents (H2O2, gamma rays, heat shock, etc.) failed to induce soxS, while aerobic growth of superoxide dismutase-deficient bacteria triggered soxR-dependent soxS expression. These results indicate a specific redox signal for soxS induction. A direct role for SoxR protein in the activation of the soxS gene is indicated by band-shift and DNase I footprinting experiments that demonstrate specific binding of the SoxR protein in cell extracts to the soxS promoter. The mode of SoxR binding to DNA appears to be similar to that of its homolog MerR in that the SoxR footprint spans the -10 to -35 region of the soxS promoter.