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Journal of Bacteriology, June 2000, p. 3467-3474, Vol. 182, No. 12
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Center for Adaptation Genetics and Drug Resistance and the Departments of Molecular Biology and Microbiology1 and Medicine,2 Tufts University School of Medicine, Boston, Massachusetts 02111
Received 28 December 1999/Accepted 31 March 2000
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ABSTRACT |
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Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
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In Escherichia coli, the MarA protein controls expression of multiple chromosomal genes affecting resistance to antibiotics and other environmental hazards. For a more-complete characterization of the mar regulon, duplicate macroarrays containing 4,290 open reading frames of the E. coli genome were hybridized to radiolabeled cDNA populations derived from mar-deleted and mar-expressing E. coli. Strains constitutively expressing MarA showed altered expression of more than 60 chromosomal genes: 76% showed increased expression and 24% showed decreased expression. Although some of the genes were already known to be MarA regulated, the majority were newly determined and belonged to a variety of functional groups. Some of the genes identified have been associated with iron transport and metabolism; other genes were previously known to be part of the soxRS regulon. Northern blot analysis of selected genes confirmed the results obtained with the macroarrays. The findings reveal that the mar locus mediates a global stress response involving one of the largest networks of genes described.
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INTRODUCTION |
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Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
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The chromosomal multiple-antibiotic-resistance (mar) locus, first described for Escherichia coli (22), is also present among other enteric bacteria (14). Molecular characterization of this locus has been performed in E. coli (11), Salmonella enterica serovar Typhimurium (51), and more recently Shigella flexneri (T. M. Barbosa and S. B. Levy, Abstr. 99th Gen. Meet. Am. Soc. Microbiol., abstr. A-42, p. 9, 1999). In all three genera, the locus consists of two divergently transcribed units, marC and marRAB, which are regulated from independent promoters (PmarI and PmarII, respectively) located in the marC-marR intergenic promoter/operator region. MarC has characteristics of a putative integral inner membrane protein whose function is unknown. marRAB specifies two regulatory proteins, MarR, the repressor of the operon, and MarA, a transcriptional activator. The function of MarB has not yet been defined. Increased expression of the marRAB operon results from mutations in marO or marR or from inactivation of MarR following exposure to different inducing agents, such as salicylate (1, 12). The resultant Mar phenotype includes resistance to structurally unrelated antibiotics (21, 43), organic solvents (6, 54), oxidative stress agents (4), and disinfectant products (40, 42).
The Mar phenotype is achieved through the differential expression of many chromosomal genes within the mar regulon. Regulation by MarA is achieved by its binding to a specific DNA sequence, "marbox," in the vicinity of the promoters of controlled genes (37) or by other mechanisms yet to be identified.
Considering the broad Mar phenotype, we hypothesized that MarA affected the expression of a much wider collection of genes than is currently known. Using E. coli Panorama gene macroarrays we identified a large number of genes differentially expressed by constitutive expression of MarA, whose products may be involved in the cell's response to different environmental stresses.
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MATERIALS AND METHODS |
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Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
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Bacterial strains, plasmids, and growth conditions.
E.
coli K-12 strain AG100 (21) was used for the PCR
amplification of specific DNA probes. This strain was originally
described (21, 22) as 
(gal-uvrB), but the
deletion was never characterized genotypically. Results obtained in the
present study and by PCR of genes located in that genomic segment
(galT and bioF) have, however, shown that this
region is not deleted in AG100 and its derivatives. E. coli
AG100Kan, a derivative of AG100 in which a 1.2-kb kanamycin resistance
cassette replaces the mar locus from within marC
to within marB (36), was used in the experiments described. pAS10 (48), derived from temperature-sensitive
pMAK705 (Chlr) (26), carries a 2.5-kb
PCR-amplified fragment containing the marCORAB sequence
bearing the marR5 mutation, which produces no MarR and thus
constitutively expresses MarA.
1.
RNA extraction. Total RNA from bacterial cultures in mid-logarithmic phase (A530 = 0.35 to 0.40) was isolated by a modification of the hot acidic phenol extraction method in accordance with the manufacturer's instructions (Sigma-Genosys Biotechnologies, Inc., The Woodlands, Tex.). Following ethanol precipitation the RNA pellet was resuspended in water and treated with DNase I (Life Technologies Inc., Gaithersburg, Md.). The absence of genomic DNA was confirmed by examining samples of the RNA in nondenaturing agarose gels and by performing PCR on DNase-treated RNA samples using primers known to target the genomic DNA. The RNA concentration was determined spectrophotometrically (47).
Preparation of labeled cDNA and hybridization to the arrays.
Labeled cDNA was prepared using the E. coli cDNA-labeling
primers (Sigma-Genosys) by following the manufacturer's instructions. The primers were annealed to 1 µg of total RNA in the presence of 333 µM dATP, dCTP, and dTTP and reverse transcriptase buffer in a final
volume of 25 µl at 90°C for 2 min. The mixture was cooled to
42°C, and 50 U of avian myeloblastosis virus reverse transcriptase
(Boehringer Mannheim, Indianapolis, Ind.) and 20 µCi of
[
-33P]dGTP (2,000 Ci/mmol) (New England Nuclear) were
added. Incubation was at 42°C for 2 h 30 min. The unincorporated
nucleotides were removed using a NucTrap probe purification column
(Stratagene, La Jolla, Calif.).
Description and quantification of the arrays. The Panorama E. coli gene arrays (Sigma-Genosys) contain 4,290 PCR-amplified open reading frames (ORFs) of the E. coli K-12 (MG1655) genome (8), spotted in duplicate (see Tao et al. [52] for a more-detailed description of the arrays).
Quantification of the hybridizing signals in the phosphorimager file was carried out by Sigma-Genosys using the Array Vision&Trade software (Imaging Research, Inc.). The relative pixel values for the duplicate spots of each gene were averaged and normalized by expressing the averaged spot signal as a percentage of the signal from the averaged pixel values of the genomic DNA spots in the respective field where each gene was printed (Fig. 1). The ratio between these values in samples from cells expressing or lacking MarA represented the fold change in gene expression. Background values were determined for each field in each array by averaging the pixel values of the empty spaces located in the same secondary grid as the genomic DNA (Fig. 1). Genes whose averaged pixel values were close to background (less than a twofold difference from background values) in both experimental and control samples were not considered here. Genes identified by computer analysis as being differentially regulated by constitutive expression of MarA (greater than or equal to a twofold change in at least one experiment and with the same regulation trend, i.e., up-regulated or down-regulated, in the other) were confirmed by visual analysis of autoradiograms of the arrays in three independent experiments. Only those genes that satisfied both criteria were considered to be affected by MarA.Northern blot analysis. Duplicate samples of DNase I-treated total RNA (5 to 10 µg) were separated on 1 to 1.2% denaturing formaldehyde-agarose gels, and RNA was transferred to nylon membranes (Hybond-N; Amersham Life Science Inc., Arlington Heights, Ill.) using established capillary blotting methods (47). DNA probes were amplified by PCR from E. coli AG100 chromosomal DNA using the appropriate PCR primer pairs (Sigma-Genosys), according to the supplier specifications. Labeling of DNA probes with [32P]dCTP (New England Nuclear) using the room temperature stable (RTS) RadPrime DNA-labeling system (Life Technologies) was carried out according to the manufacturer's instructions. Hybridizations were performed at 65°C, and RNA membranes were washed at 65°C for 15-min intervals, four times in 2× SSC (1× SSC is 0.15 M NaCl plus 0.015 M sodium citrate) buffer-0.1% SDS and two to four times in 0.1× SSC buffer-0.1% SDS. Hybridizing bands were visualized as described above.
DNA manipulations. Genomic and plasmid DNA was purified from E. coli strains using the QIAamp tissue kit and the QIAprep spin Miniprep kit (Qiagen), respectively, by following the manufacturer's instructions.
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RESULTS AND DISCUSSION |
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Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
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Identification of genes affected by constitutive MarA expression. DNA macroarrays, which contain most of the genomic ORFs of E. coli (8), allowed studies of expression of the complete genome in the presence or absence of MarA. E. coli AG100Kan strain (36) bearing only plasmid pMAK705 represented the control, i.e., a strain deficient in mar expression. Experimental strain AG100Kan(pAS10), containing the pMAK705-derived plasmid pAS10, expresses MarA constitutively (48) and showed the expected increase (~4- to 20-fold) in resistance to multiple antibiotics (data not shown).
33P-labeled cDNAs prepared from RNA extracted from mar-deleted and mar-expressing strains were hybridized to paired macroarrays, and phosphorimager files and autoradiograms were obtained (Fig. 1). Previously ~15 genes were known to be regulated by MarA (2). The gene macroarrays identified a total of 62 genes responsive to constitutive expression of MarA in logarithmic phase: 47 induced and 15 repressed (Table 1). The differential regulation of the genes listed in Table 1 was confirmed visually in all three experiments.
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Confirmation of previously identified MarA-regulated genes. The differential expression of most of the genes previously identified as part of the mar regulon, e.g., inaA, sodA, ompF, zwf, and fumC (4, 25, 30, 46) was confirmed (Table 1). A major role in the Mar phenotype is played by the efflux system acrAB, which acts by pumping toxic compounds out of the cell (42, 44, 54). An increase in the expression of the acrA gene of the acrAB operon was also observed (Table 1); however, the expression values for acrB were not above background. This kind of finding is not fully understood but could arise from differential processing of the polycistronic transcript and/or by differences in transcript stability.
Previous studies suggest coordinate activation of TolC and the AcrAB efflux pump in the development of the Mar phenotype (3, 19). Changes in the expression of outer membrane proteins (e.g., increased OmpX expression and decreased OmpF and LamB expression) in E. coli marR mutants and wild-type strains overexpressing MarA have also been reported (3, 13). Down-regulation of ompF translation is controlled by micF, a regulatory antisense RNA known to be activated by MarA, which binds to the 5' untranslated region of the former gene mRNA, blocking translation (16). We confirm some of these reports and show for the first time that MarA expression increases the transcription of both tolC and ompX (Table 1). Although we observed a decrease in the levels of ompF, we found no evidence for a similar decrease in lamB expression, suggesting that LamB may not be the underproduced protein identified in the earlier study (3) or that regulation may be posttranscriptional. The micF gene is not spotted on the arrays (which contain only genes coding for ORFs), and therefore we were unable to confirm activation of this gene by MarA. Nevertheless, under this assumption and given the observed down-regulation of ompF, the results indicate that micF is also involved in the destabilization of the ompF mRNA as suggested by others (13, 16). Transcription of the previously identified mlr1 (b1451) and mlr2 (b0603) genes (48) was increased in the mar-expressing strain in two experiments but appeared to be unaffected in a third experiment, so these genes were not included in Table 1. Expression of the slp gene, previously described as repressed by MarA (48), was so low that any mar-mediated change would have been difficult to detect. This observation may reflect the fact that our experiments were performed on cells in mid-logarithmic phase while slp is a stationary-phase-inducible gene (48).Relationship between soxRS and mar regulons. SoxS is the activator of the soxRS regulon (17), which mediates a cellular response to oxidative stress and, like MarA, is a member of the XylS/AraC family of transcriptional activators (20). Many oxidative stress genes, which are known to respond to SoxS, are also responsive to MarA (30, 41). Conversely, SoxS is able to confer a Mar phenotype via activation of genes that are under the control of MarA (4, 25). Genes known to be regulated directly or indirectly by both the MarA and SoxS regulators include zwf, fpr, fumC, micF, nfo, inaA, sodA, and acrAB (4, 25, 30, 46, 54). We confirmed the positive regulation of zwf, fumC, acrA, inaA, and sodA by MarA and also the down-regulation of ompF. However, although binding of MarA to nfo and fpr was shown in cell-free studies (30), no significant change in expression of these two genes was detected using the experimental conditions employed here.
Our findings revealed further overlap between the mar and soxRS regulons. The levels of aconitase (acnA) and GTP cyclohydrolase II (ribA) genes and that of the major oxygen-insensitive nitroreductase gene (nfsA/mdaA), previously known to be under the control of soxRS (15, 31, 33), were increased in mar-expressing strains (Table 1). While NfsA was shown to be the major isoenzyme affected by paraquat (33), the oxygen-sensitive NADPH nitroreductase B gene, nfnB (also designated nfsB), was shown to be slightly induced. We found that nfnB, like nfsA, is under the positive control of MarA (Table 1). nfsA was initially designated mdaA (modulator of drug activity), as one of two genes associated with bacterial resistance to tumoricidal compounds (10). The other gene, designated mdaB, was also found to be affected by MarA (Table 1). Information about mdaB is very limited, and its function remains unknown. Our findings provide suggestive evidence for a putative physiological role in protection against environmental stresses. The exact mechanisms for the overlapping regulation by MarA and SoxS are still poorly understood. Multiple-antibiotic resistance encoded by the soxRS locus appeared partly dependent on an intact mar locus; strains overexpressing SoxS showed increased levels of marRAB transcription (41). On the other hand, other work showed that regulation of some genes by mar and by soxRS can occur through independent pathways, e.g., inaA (46). An effect of mar on soxRS has not been detected, and we observed no up-regulation of soxS expression by MarA. Therefore, MarA appears to operate independently of SoxS. A recent report suggests promoter discrimination by the two transcriptional activators dependent on differential binding to the marboxes of the involved genes (38). Rob, a MarA and SoxS homologue, is also able to bind to promoters of genes belonging to the mar regulon, and overexpression of this protein leads to multiple-antibiotic resistance and organic solvent tolerance in E. coli (5, 29). We found no substantial change in the expression of rob due to MarA.Effect of constitutive expression of MarA on operons and cotranscribed units. Some of the genes affected by constitutive expression of MarA were clustered in discrete regions, as part of documented or predicted operons (Fig. 2). Interestingly, we observed considerable variability in the levels of expression of different genes from the same operon, and therefore only some of these genes were eligible for listing in Table 1. For example, the fold increase in expression of the three genes in the tryptophanase operon (tnaLAB; 83.8 min) was 1.7 for tnaL and 8.1 for tnaA (averaged values), while tnaB was unclear; it gave background values in one experiment but was clearly up-regulated in the other two experiments.
Differential expression of genes within MarA-regulated operons could arise as a result of other factors besides regulation of transcriptional initiation, e.g., differences in mRNA stability or the presence of regulatory secondary structures in the intercistronic regions of the operon. For example, the
-methylgalactoside (mgl) transport
operon is composed of three ORFs, mglBAC.
Northern analysis showed the presence of two transcripts, a
polycistronic mglBAC mRNA and a smaller transcript which
corresponds to the first gene in the operon, mglB
(28). This finding may result from 3'-to-5' nuclease
degradation of the larger mRNA and from protection of the smaller
transcript by a repetitive extragenic palindrome sequence located at
its 3' end (28). In agreement, our findings showed the
smaller transcript at a much higher level than the larger one (Fig.
3).
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Relationship between the mar regulon and iron. Some of the genes affected by MarA expression are associated with iron, e.g., hemB, fumC, fecA, acnA, and sodA. Some of the encoded proteins contain iron-sulfur clusters, which play a major role in sensing O2 and iron and in regulatory functions (7). Iron is an essential element for the bacterial cell, and iron acquisition from the host is important in bacterial pathogenesis (34). However, iron can also be harmful, as it catalyzes the production of hydroxyl ions via the Fenton reaction, which may damage all cellular components and even lead to cell death (56). Some genes known to be regulated by Fur (ferric uptake regulator) are also responsive to SoxS, MarA, and other regulators, e.g., acnA and sodA (15, 50). This coregulation would allow the cell to deal with the iron-associated oxidative stress and provides a suggested role for mar in bacterial pathogenesis.
Northern blot analysis of selected genes. The altered expression of 10 genes newly identified by the macroarrays was confirmed by Northern blot analysis, which showed changes in the expression of mono- or polycistronic transcripts (Fig. 3). The magnitude of these changes, not unexpectedly, differed somewhat from that obtained for the macroarrays. Regulation of gshB, mdaA, and aceE genes involved alteration in the levels of multiple transcripts as expected based on reported or predicted involvement of these genes in polycistronic elements (8).
Conclusions. The transcriptional activator MarA may control the expression of genes directly or indirectly. It could activate intermediate activator or inhibitor regulatory proteins, which then could up- or down-regulate the expression of other genes in the regulon. An example is the MarA regulation of ompF. MarA activates micF, an antisense RNA which negatively affects the translation of ompF, leading to decreased outer membrane porin OmpF (13). Furthermore, transcriptional activators can act also as repressor proteins, depending on the position of the regulator binding site at the exclusive zone of repression (23).
We only report genes whose expression trends were consistent in three experiments. It is therefore likely that the size of the mar regulon is underestimated. Some of the genes containing putative marboxes in their promoter regions (37) were not shown to be part of the mar regulon under the conditions used here. Moreover, a large number of genes were expressed at background level or responded to MarA expression with small changes that were below the threshold applied in this study and therefore were not included. Under a different set of experimental conditions, such as examining cells in a different stage of the growth phase or grown in different media, it is possible that the magnitude of these changes would increase or that new genes would be affected. Certainly small and transient changes in gene expression could have important implications in the cell's response to external stresses. Observed differences in global expression analysis between experiments have been seen and extensively addressed by other authors (45, 52). The authors observed that, among other factors, the signal intensities of some genes were significantly different between experiments when different batches of RNA were used. We hope to have addressed this problem in part by performing the study in triplicate; two experiments were quantified, and the MarA-affected genes were judged visually in all three. Changes detected by the gene array method should also be confirmed by other available molecular and biochemical techniques, such as Northern blot analysis (as was done for selected genes) and promoter fusion studies, using cells with a different genetic background. E. coli is a natural inhabitant of different ecosystems and hosts. In order to successfully survive in such diverse conditions, this bacterium has presumably developed regulatory loci which control adaptational responses to the different environmental stresses to which it is exposed (e.g., fluctuations in temperature and pH, oxidative stress and oxygen limitation, antibiotics, and starvation). Regulatory systems, such as SoxRS, OxyR, Mar, SOS, and Fur, share the capacity to produce a global response by activating or repressing multiple genes in the bacterial chromosome. While some genes are members only of one regulon, others can be regulated by different transcriptional factors. Some members of the mar regulon are known to be directly or indirectly controlled by other transcriptional regulators. For example, acnA expression can be activated by cyclic AMP receptor protein (CRP), FruR, Fur, and SoxRS and repressed by ArcA and FNR (15); aldA is repressed by the ArcA system and induced by an inducer-regulator complex and by CRP (32); ndh can be repressed by FNR and integration host factor (IHF) and activated by Arr (24); sodA is regulated not only by SoxS and MarA but also by FNR, ArcAB, IHF, and Fur (50); and fumC is regulated by SoxS, MarA, ArcAB and
s (50). Both MarA and MarR have
been identified as members of the heat shock stimulon (45).
Additionally, pflB and guaB, which are
repressed in response to heat shock, are also part of the mar regulon. A fine-tuning of the cross talk between
these global regulators and the genes which they control may
provide the cell with the required machinery to enhance its fitness in
the new environments which it encounters.
E. coli global responses involving multiple genes include
the heat shock stimulon (119 genes) (45), the Fnr modulon
with over 70 genes (35), the SOS regulon with over 20 unlinked genes (49), and the soxRS and
oxyR regulons, which comprise approximately 15 and 12 genes,
respectively (17, 50, 56). Use of two-dimensional gel
electrophoresis also identified 16 proteins as being induced upon cold
shock of the E. coli cell, although only 12 of them have
been identified (53). Despite the caveat that some of the changes observed may result from constitutive rather than induced expression of MarA, it seems reasonable to conclude that mar
is one of the largest E. coli regulons known to date.
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ACKNOWLEDGMENTS |
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This work was supported by NIH grant GM51661.
We thank Laura M. McMurry and Michael N. Alekshun for helpful comments in the preparation of the manuscript.
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FOOTNOTES |
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* Corresponding author. Mailing address: Center for Adaptation Genetics and Drug Resistance, Tufts University School of Medicine, 136 Harrison Ave., Boston, MA 02111. Phone: (617) 636-6764. Fax: (617) 636-0458. E-mail: slevy{at}opal.tufts.edu.
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REFERENCES |
|---|
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Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
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| 1. |
Alekshun, M. A., and S. B. Levy.
1999.
Alteration of the repression activity of MarR, the negative regulator of the Escherichia coli marRAB locus, by multiple chemicals in vitro.
J. Bacteriol.
181:4669-4672 |
| 2. | Alekshun, M. N., and S. B. Levy. 1997. Regulation of chromosomally mediated multiple antibiotic resistance: the mar regulon. Antimicrob. Agents Chemother. 41:2067-2075[Medline]. |
| 3. |
Aono, R.,
N. Tsukagoshi, and M. Yamamoto.
1998.
Involvement of outer membrane protein TolC, a possible member of the mar-sox regulon, in maintenance and improvement of organic solvent tolerance of Escherichia coli K-12.
J. Bacteriol.
180:938-944 |
| 4. |
Ariza, R. R.,
S. P. Cohen,
N. Bachhawat,
S. B. Levy, and B. Demple.
1994.
Repressor mutations in the marRAB operon that activate oxidative stress genes and multiple antibiotic resistance in Escherichia coli.
J. Bacteriol.
176:143-148 |
| 5. |
Ariza, R. R.,
Z. Li,
N. Ringstad, and B. Demple.
1995.
Activation of multiple antibiotic resistance and binding of stress-inducible promoters by Escherichia coli Rob protein.
J. Bacteriol.
177:1655-1661 |
| 6. | Asako, H., K. Nakajima, K. Kobayashi, M. Kobayashi, and R. Aono. 1997. Organic solvent tolerance and antibiotic resistance increased by overexpression of marA in Escherichia coli. Appl. Environ. Microbiol. 63:1428-1433[Abstract]. |
| 7. | Beinert, H., and P. J. Kiley. 1999. Fe-S proteins in sensing and regulatory functions. Curr. Opin. Chem. Biol. 3:152-157[CrossRef][Medline]. |
| 8. |
Blattner, F. R.,
G. Plunkett III,
C. A. Bloch,
N. T. Perna,
V. Burland,
M. Riley,
J. Collado-Vides,
J. D. Glasner,
C. K. Rode,
G. F. Mayhew,
J. Gregor,
N. W. Davis,
H. A. Kirkpatrick,
M. A. Goeden,
D. J. Rose,
B. Mau, and Y. Shao.
1997.
The complete genome sequence of Escherichia coli K-12.
Science
277:1453-1462 |
| 9. | Chater, K. F., and H. Nikaido. 1999. Cell regulation. Maintaining integrity and efficiency in microbial cells. Curr. Opin. Microbiol. 2:121-125. |
| 10. |
Chatterjee, P. K., and N. L. Sternberg.
1995.
A general genetic approach in Escherichia coli for determining the mechanism(s) of action of tumoricidal agents: application to DMP 840, a tumoricidal agent.
Proc. Natl. Acad. Sci. USA
92:8950-8954 |
| 11. |
Cohen, S. P.,
H. Hachler, and S. B. Levy.
1993.
Genetic and functional analysis of the multiple antibiotic resistance (mar) locus in Escherichia coli.
J. Bacteriol.
175:1484-1492 |
| 12. |
Cohen, S. P.,
S. B. Levy,
J. Foulds, and J. L. Rosner.
1993.
Salycilate induction of antibiotic resistance in Escherichia coli: activation of the mar operon and a mar-independent pathway.
J. Bacteriol.
175:7856-7862 |
| 13. |
Cohen, S. P.,
L. M. McMurry, and S. B. Levy.
1988.
marA locus causes decreased overexpression of OmpF porin in multiple-antibiotic-resistance (Mar) mutants of Escherichia coli.
J. Bacteriol.
170:5416-5422 |
| 14. | Cohen, S. P., W. Yan, and S. B. Levy. 1993. A multidrug resistance regulatory chromosomal locus is widespread among enteric bacteria. J. Infect. Dis. 168:484-488[Medline]. |
| 15. | Cunningham, L., M. J. Gruer, and J. R. Guest. 1997. Transcriptional regulation of the aconitase genes (acnA and acnB) of Escherichia coli. Microbiology 143:3795-3805[Abstract]. |
| 16. | Delihas, N. 1997. Antisense micF RNA and 5'-UTR of the target ompF RNA: phylogenetic conservation of primary and secondary structures. Nucleic Acids Symp. Ser. 36:33-35. |
| 17. |
Demple, B.
1996.
Redox signaling and gene control in the Escherichia coli soxRS oxidative stress regulon a review.
Gene
179:53-57[CrossRef][Medline].
|
| 18. | Ferguson, G. P. 1999. Protective mechanisms against toxic electrophiles in Escherichia coli. Trends Microbiol. 7:242-247[CrossRef][Medline]. |
| 19. |
Fralick, J. A.
1996.
Evidence that TolC is required for functioning of the Mar/AcrAB efflux pump of Escherichia coli.
J. Bacteriol.
178:5803-5805 |
| 20. | Gallegos, M.-T., R. Schleif, A. Bairoch, K. Hofmann, and J. L. Ramos. 1997. AraC/XylS family of transcriptional regulators. Microbiol. Mol. Biol. Rev. 61:393-410[Abstract]. |
| 21. |
George, A. M., and S. B. Levy.
1983.
Amplifiable resistance to tetracycline, chloramphenicol, and other antibiotics in Escherichia coli: involvement of a non-plasmid-determined efflux of tetracycline.
J. Bacteriol.
155:531-540 |
| 22. |
George, A. M., and S. B. Levy.
1983.
Gene in the major cotransduction gap of the Escherichia coli K-12 linkage map required for the expression of chromosomal resistance to tetracycline and other antibiotics.
J. Bacteriol.
155:541-548 |
| 23. | Gralla, J. D., and J. Collado-Vides. 1996. Organization and function of transcriptional regulatory elements, p. 1232-1245. In F. C. Neidhardt, R. Curtiss III, J. L. Ingraham, E. C. C. Lin, K. B. Low, B. Magasanik, W. S. Reznikoff, M. Riley, M. Schaechter, and H. E. Umbarger (ed.), Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed. American Society for Microbiology, Washington, D.C. |
| 24. | Green, J., M. F. Anjum, and J. R. Guest. 1997. Regulation of the ndh gene of Escherichia coli by integration host factor and a novel regulator, Arr. Microbiology 143:2865-2875[Abstract]. |
| 25. |
Greenberg, J. T.,
J. H. Chou,
P. A. Monach, and B. Demple.
1991.
Activation of oxidative stress genes by mutations at the soxQ/cfxB1/marA locus of Escherichia coli.
J. Bacteriol.
173:4433-4439 |
| 26. |
Hamilton, C. M.,
M. Aldea,
B. K. Washburn,
P. Babitzke, and S. R. Kushner.
1989.
New method for generating deletions and gene replacements in Escherichia coli.
J. Bacteriol.
171:4617-4622 |
| 27. | Hidalgo, E., and B. Demple. 1995. Adaptive response to oxidative stress: the soxRS and oxyR regulons, p. 433-450. In E. C. C. Lin, and A. S. Lynch (ed.), Regulation of gene expression in Escherichia coli. R. G. Landes Company, Austin, Tex. |
| 28. | Hogg, R. W., C. Voelker, and I. von Carlowitz. 1991. Nucleotide sequence and analysis of the mgl operon of Escherichia coli K12. Mol. Gen. Genet. 229:453-459[CrossRef][Medline]. |
| 29. |
Jair, K.-W.,
X. Yu,
K. Skarstad,
B. Thony,
N. Fujita,
A. Ishihama, and R. E. Wolf, Jr.
1996.
Transcriptional activation of promoters of the superoxide and multiple antibiotic resistance regulons by Rob, a binding protein of the Escherichia coli origin of chromosomal replication.
J. Bacteriol.
178:2507-2513 |
| 30. |
Jair, K.-W.,
R. G. Martin,
J. L. Rosner,
N. Fujita,
A. Ishihama, and R. E. Wolf, Jr.
1995.
Purification and regulatory properties of MarA protein, a transcriptional activator of Escherichia coli multiple antibiotic resistance and superoxide resistance promoters.
J. Bacteriol.
177:7100-7104 |
| 31. | Koh, Y. S., W.-H. Chung, J.-H. Lee, and J.-H. Roe. 1999. The reversed SoxS-binding site upstream of the ribA promoter in Escherichia coli. Mol. Gen. Genet. 261:374-380[CrossRef][Medline]. |
| 32. | Limon, A., E. Hidalgo, and J. Aguilar. 1997. The aldA gene of Escherichia coli is under the control of at least three transcriptional regulators. Microbiology 143:2085-2095[Abstract]. |
| 33. |
Liochev, S. I.,
A. Hausladen, and I. Fridovich.
1999.
Nitroreductase A is regulated as a member of the soxRS regulon of Escherichia coli.
Proc. Natl. Acad. Sci. USA
96:3537-3539 |
| 34. |
Litwin, C. M., and S. B. Calderwood.
1993.
Role of iron in regulation of virulence genes.
Clin. Microbiol. Rev.
6:137-149 |
| 35. | Lynch, A. S., and E. C. C. Lin. 1996. Responses to molecular oxygen, p. 1526-1538. In F. C. Neidhardt, R. Curtiss III, J. L. Ingraham, E. C. C. Lin, K. B. Low, B. Magasanik, W. S. Reznikoff, M. Riley, M. Schaechter, and H. E. Umbarger (ed.), Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed. American Society for Microbiology, Washington, D.C. |
| 36. | Manneewannakul, K., and S. B. Levy. 1996. Identification of mar mutants among quinolone-resistant clinical isolates of Escherichia coli. Antimicrob. Agents Chemother. 40:1695-1698[Abstract]. |
| 37. | Martin, R. G., W. K. Gillette, S. Rhee, and J. L. Rosner. 1999. Structural requirements for marbox function in transcriptional activation of mar/sox/rob regulon promoters in Escherichia coli: sequence, orientation and spatial relationship to the core promoter. Mol. Microbiol. 34:431-441[CrossRef][Medline]. |
| 38. | Martin, R. G., W. K. Gillette, and J. L. Rosner. 2000. Promoter discrimination by the related transcriptional activators MarA and SoxS: differential regulation by differential binding. Mol. Microbiol. 35:623-634[CrossRef][Medline]. |
| 39. |
Martin, R. G.,
P. S. Nyantakyi, and J. L. Rosner.
1995.
Regulation of the multiple antibiotic resistance (mar) regulon by marORA sequences in Escherichia coli.
J. Bacteriol.
177:4176-4178 |
| 40. | McMurry, L. M., M. Oethinger, and S. B. Levy. 1998. Overexpression of marA, soxS or acrAB produces resistance to triclosan in Escherichia coli. FEMS Microbiol. Lett. 166:305-309[CrossRef][Medline]. |
| 41. |
Miller, P. F.,
L. F. Gambino,
M. C. Sulavik, and S. J. Gracheck.
1994.
Genetic relationship between soxRS and mar loci in promoting multiple antibiotic resistance in Escherichia coli.
Antimicrob. Agents Chemother.
38:1773-1779 |
| 42. | Moken, M. C., L. M. McMurry, and S. B. Levy. 1997. Selection of multiple-antibiotic-resistant (Mar) mutants of Escherichia coli by using the disinfectant pine oil: roles of the mar and acrAB loci. Antimicrob. Agents Chemother. 41:2770-2772[Abstract]. |
| 43. |
Oethinger, M.,
I. Podglajen,
W. V. Kern, and S. B. Levy.
1998.
Overexpression of the marA or soxS regulatory gene in clinical topoisomerase mutants of Escherichia coli.
Antimicrob. Agents Chemother.
42:2089-2094 |
| 44. |
Okusu, H.,
D. Ma, and H. Nikaido.
1996.
AcrAB efflux pump plays a major role in the antibiotic resistance phenotype of Escherichia coli multiple-antibiotic-resistance phenotype (Mar) mutants.
J. Bacteriol.
178:306-308 |
| 45. |
Richmond, C. S.,
J. D. Glasner,
R. Mau,
H. Jin, and F. R. Blattner.
1999.
Genome-wide expression profiling in Escherichia coli K-12.
Nucleic Acids Res.
27:3821-3835 |
| 46. |
Rosner, J. L., and J. L. Slonczewski.
1994.
Dual regulation of inaA by the multiple antibiotic resistance (Mar) and superoxide (SoxRS) stress response systems of Escherichia coli.
J. Bacteriol.
176:6262-6269 |
| 47. | Sambrook, J., E. F. Fritsch, and T. Maniatis. 1989. Molecular cloning: a laboratory manual, 2nd ed. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. |
| 48. |
Seoane, A. S., and S. B. Levy.
1995.
Identification of new genes regulated by the marRAB operon in Escherichia coli.
J. Bacteriol.
177:530-535 |
| 49. |
Smith, B. T., and G. C. Walker.
1998.
Mutagenesis and more: umuDC and the Escherichia coli SOS response.
Genetics
148:1599-1610 |
| 50. | Storz, G., and J. A. Imlay. 1999. Oxidative stress. Curr. Opin. Microbiol. 2:188-194[CrossRef][Medline]. |
| 51. |
Sulavick, M. C.,
M. Dazer, and P. F. Miller.
1997.
The Salmonella typhimurium mar locus: molecular and genetic analysis and assessment of its requirement for virulence.
J. Bacteriol.
179:1857-1866 |
| 52. |
Tao, H.,
C. Bausch,
C. Richmond,
F. R. Blattner, and T. Conway.
1999.
Functional genomics: expression analysis of Escherichia coli growing on minimal and rich media.
J. Bacteriol.
181:6425-6440 |
| 53. | Thieringer, H. A., P. G. Jones, and M. Inouye. 1998. Cold shock and adaptation. Bioessays. 20:49-57[CrossRef][Medline]. |
| 54. |
White, D. G.,
J. D. Goldman,
B. Demple, and S. B. Levy.
1997.
Role of the acrAB locus in organic solvent tolerance mediated by expression of marA, soxS, or robA in Escherichia coli.
J. Bacteriol.
179:6122-6126 |
| 55. |
Zenno, S.,
H. Koike,
A. N. Kumar,
R. Jayarman,
M. Tanokura, and K. Saigo.
1996.
Biochemical characterization of NfsA, the Escherichia coli major nitroreductase exhibiting a high amino acid sequence homology to Frp, a Vibrio harveyi flavin oxidoreductase.
J. Bacteriol.
178:4508-4514 |
| 56. |
Zheng, M.,
B. Doan,
T. D. Schneider, and G. Storz.
1999.
OxyR and SoxRS regulation of fur.
J. Bacteriol.
181:4639-4643 |
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