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Journal of Bacteriology, July 2007, p. 4569-4577, Vol. 189, No. 13
0021-9193/07/$08.00+0     doi:10.1128/JB.00286-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Differential Regulation of ponA and pilMNOPQ Expression by the MtrR Transcriptional Regulatory Protein in Neisseria gonorrhoeae

Jason P. Folster,^1,2 Vijaya Dhulipala,^1,2 Robert A. Nicholas,³ and William M. Shafer^1,2*

Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, Georgia 30322,¹ Laboratories of Bacterial Pathogenesis, VA Medical Center, Decatur, Georgia 30033,² Department of Pharmacology, University of North Carolina-Chapel Hill School of Medicine, Chapel Hill, North Carolina³

Received 22 February 2007/ Accepted 24 April 2007

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Neisseria gonorrhoeae utilizes the mtrCDE-encoded efflux pump system to resist not only host-derived, hydrophobic antimicrobials that bathe mucosal surfaces, which likely aids in its ability to colonize and infect numerous sites within the human host, but also antibiotics that have been used clinically to treat infections. Recently, overexpression of the MtrC-MtrD-MtrE efflux pump was shown to be critically involved in the capacity of gonococci to develop chromosomally mediated resistance to penicillin G, which for over 40 years was used to treat gonococcal infections. Mutations in either the promoter or the coding sequence of the mtrR gene, which encodes a repressor of the efflux pump operon, decrease gonococcal susceptibility to penicillin. We now describe the capacity of MtrR to directly or indirectly influence the expression of two other loci that are involved in gonococcal susceptibility to penicillin: ponA, which encodes penicillin-binding protein 1 (PBP 1), and the pilMNOPQ operon, which encodes components of the type IV pilus secretion system, with PilQ acting as a channel for entry for penicillin. We determined that MtrR increases the expression of ponA directly or indirectly, resulting in increased levels of PBP 1, while repressing the expression of the divergently transcribed pilM gene, the first gene in the pilMNOPQ operon. Taken together with other studies, the results presented herein indicate that transcriptional regulation of gonococcal genes by MtrR is centrally involved in determining levels of gonococcal susceptibility to penicillin and provides a framework for understanding how resistance developed over the years.

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The strict human pathogen Neisseria gonorrhoeae continues to be a public health problem due to its frequency of infection (estimated at over 60 million cases per year), its ability to evade the immune system, and its increasing resistance to antibiotics (2). Antibiotic resistance is especially worrisome in underdeveloped countries, where effective treatment often requires the use of more expensive drugs that are not always available. The relatively inexpensive antibiotic penicillin G was used clinically for over 40 years until 1985, when it became evident that there was an unacceptably high prevalence of clinically resistant strains. These strains often did not produce ß-lactamase (9) but instead had a number of chromosomal mutations that affected penicillin entry, target recognition, or efflux (5, 11, 24, 27, 36, 38). The report (10) of one such resistant strain in the United States prompted the removal of penicillin from the CDC-recommended treatment regimen.

Mutations in at least five chromosomal genes are required for high-level chromosomally mediated penicillin resistance in gonococci. These genes encode mutated forms of penicillin-binding protein 1 (PBP 1) (27) and PBP 2 (36), the major outer membrane protein PorB_IB (24), the type IV pilus secretin protein PilQ (44), and the transcriptional repressor MtrR (14, 25), which negatively regulates the expression of the mtrCDE-encoded efflux pump. Accumulating evidence (25, 38) has revealed the importance of overexpression of the MtrC-MtrD-MtrE efflux pump, which has the capacity to recognize and export structurally diverse antimicrobials (14), in gonococcal penicillin resistance due to mtrR mutations. The 23-kDa MtrR protein, a member of the TetR family of transcriptional repressors with a helix-turn-helix motif near its N terminus (25), binds in a specific manner to the mtrCDE promoter (20) through two homodimers that recognize two pseudo-direct repeats within the mtrCDE promoter (16). MtrR may have a more global regulatory property in that it has been shown to negatively regulate the expression of two additional genes: mtrF, which encodes an inner membrane accessory protein required for maximal efflux of antimicrobials by the MtrC-MtrD-MtrE efflux pump in mtrR mutants (39, 40), and farR, which encodes the transcriptional repressor of the farAB-encoded efflux pump (18, 19). Moreover, earlier work by Dougherty et al. (5) suggested that introduction of a then-undefined mtrR mutation into a penicillin-susceptible recipient strain resulted in a decreased level of penicillin binding to PBP 1 (encoded by ponA) as determined by [³H]penicillin G binding. Because changes in the expression of the mtrCDE efflux pump would not be expected to have an effect on [³H]penicillin G binding to isolated membranes used in the experiments described by Dougherty et al. (5), we hypothesized that the mtrR mutation might modulate the expression of ponA, in addition to its primary role of regulating the expression of the mtrCDE efflux pump. Interestingly, ponA is adjacent to, but transcriptionally divergent from, the pilMNOPQ operon. This operon encodes components of the type IV pilus secretion system (6, 7), and recent studies (3, 44) have implicated the multimeric PilQ secretin in antibiotic (including penicillin) permeation in gonococci.

Since mutations in ponA and pilQ are necessary for gonococci to express high-level chromosomally mediated resistance to penicillin (27, 32) and such strains also bear mutations in mtrR, we investigated whether MtrR also regulates these genes. We now report that MtrR can positively regulate the expression of ponA while repressing pilMNOPQ transcription and propose that this DNA-binding protein functions in modulating gonococcal susceptibility to penicillin by controlling the expression of multiple genes. (A preliminary account of these findings was presented at the 15th International Pathogenic Neisseria Conference [10 to 15 September 2006] in Cairns, Australia.)

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Bacterial strains and culture conditions. The bacterial strains used in this study are listed in Table 1. Escherichia coli strain TOP10 (Invitrogen, Carlsbad, CA) and E. coli DH5 mcr (29) were used in all cloning experiments. E. coli strains were grown in Luria-Bertani (LB) broth or on LB agar plates at 37°C. N. gonorrhoeae strain FA19 was used as the primary gonococcal strain (21, 35). N. gonorrhoeae strains were grown on gonococcal medium base (GCB) agar (Difco Laboratories, Detroit, MI) containing glucose and iron supplements at 37°C under 3.8% (vol/vol) CO₂ as described previously (30). All chemicals were purchased from Sigma Biochemical (St. Louis, MO).

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TABLE 1. Bacterial strains and plasmids used in this study

Construction of strain FA19 mtrR GC3mtrR. In order to complement the mtrR deletion in FA19 mtrR, the mtrR gene and promoter region were amplified by PCR (pGC35'mtrR, 5'-GGTTAATTAACGCCTTAGAAGCATAAAAAGC-3'; 3'mtrR, 5'-GGGTTTAAACTTATTTCCGGCGCAGGCAG-3') from wild-type strain FA19, which produces a functional MtrR repressor (20). The resulting DNA fragment was inserted into the PmeI and PacI sites of pGCC3 (22) (kindly provided by A. Jerse and H. Seifert) to produce pGC3mtrR, and the correct orientation and nucleotide sequence were confirmed by DNA sequencing. pGC3mtrR was digested with ClaI, and the fragment containing the gonococcal lctP gene, mtrR, ermC (an erythromycin resistance cassette), and aspC was purified and used to transform gonococcal strain FA19 mtrR. Transformations were performed as previously described (12). Transformants were selected on GCB containing 1 µg/ml of erythromycin.

Construction of the ponA-lacZ and pilM-lacZ fusions in gonococci. Translational lacZ fusions were constructed as previously described (34). In brief, the promoter sequence of ponA was amplified by PCR from strain FA19 using primers 5'PponA (5'-GGGGATCCTTCCAATTGAATTTGGTTTAAACT-3') and 3'PponA (5'-GGGGATCCCGAATCATAGCTGAATAATAATTTAC-3'). The promoter sequence of pilM was amplified by PCR from strain FA19 using primers 5'PpilM (5'-ATGGATCCAACGGCATTTTAGGCTGGTAA-3') and 3'PpilM (5'-ATGGATCCCGGCGCATGATGAAAGTTCCTG-3'). The resulting DNA fragments were inserted into the BamHI site of pLES94 (33), and the recombinant plasmids were introduced into E. coli DH5 mcr by transformation. Correct insertion and orientation were confirmed by PCR analysis and DNA sequencing. The plasmids were used to transform strains FA19, FA19 mtrR, and FA19 mtrR GC3mtrR to allow insertion into the chromosomal proAB locus. Transformants were selected on GCB agar containing 1 µg/ml of chloramphenicol.

Preparation of cell extracts and ß-Gal assays. The strains containing lacZ translational fusions were grown overnight on GCB agar plates containing 1 µg/ml of chloramphenicol. Cells were scraped, washed once with phosphate-buffered saline (pH 7.4), and resuspended in lysis buffer (0.25 mM Tris, pH 8.0). Cells were broken by repeated freeze-thaw cycles. The cell debris was removed by centrifugation at 15,000 x g for 8 min at 4°C. ß-Galactosidase (ß-Gal) assays were performed as previously described (34).

EMSAs. Electrophoretic mobility shift assays (EMSAs) using purified MtrR fused to maltose-binding protein (MBP) were performed as previously described (20). All probes were amplified by PCR from FA19 chromosomal DNA. In brief, the intergenic region of ponA-pilM was amplified by PCR using 3'PponA and 3'PpilM, and the promoter sequence of mtrC was PCR amplified using 5'PmtrC (5'-CGTTTCGGGTCGGTTTGACG-3') and 3'PmtrC (5'-GCTTTGATACCCGAATGTTCG-3'). The overlapping probes used in the MtrR-binding site study were amplified by PCR using the following primer pairs: 5'ponA225 (5'-GCAACCAGACCCACTCCA-3') and 3'ponA75 (5'-TTGAAACCGTGCTTTGTAG-3'), 5'ponA75 (5'-TGTGCAAAGAACAAGGAATCC-3') and 3'pilM-75 (5'- ATTGAGTCCCGAAGATTTTTTA-3'), 5'ponA50 (5'-CGGATACCGAAACGGTTAC-3') and 3'pilM-100 (5'-TATCGATGCCGATTGCCGC-3'), and 5'ponA25 (5'-TACAAATAAAGCAGGAACTTTCA-3') and 3'pilM-125 (5'-ACCATTTTGATGGAATGCTGG-3'). The resulting PCR products were end labeled with [-³²P]dATP using T4 polynucleotide kinase (New England Biolabs, Beverly, MA). The labeled DNA fragments (10 ng) were incubated with purified MBP-MtrR in 30 µl of reaction buffer [10 nM Tris-HCl (pH 7.5), 0.5 mM dithiothreitol, 0.5 EDTA, 4% (vol/vol) glycerol, 1 mM MgCl₂, 50 mM NaCl, poly(dI-dC) (0.5 µg/ml), salmon sperm (200 ng/ml)] at 4°C for 20 min. Samples were subjected to electrophoresis on a 6% (wt/vol) polyacrylamide gel at 4°C, followed by autoradiography. Densitometry was performed using Scion Image (v. alpha 4.0.3.2; Frederick, MD).

Mutagenesis of the MtrR-binding site. PCR mutagenesis was performed using the overlapping primers 5'mutMBS (5'-CGGTTACTCAAGTGCACCATAAAGCAGG-3') and 3'mutMBS (5'-CCTGCTTTATGGTGCACTTGAGTAACCG-3'), each containing the 8-bp transversion mutation of the MtrR-binding site. First, two fragments were amplified by PCR from FA19 chromosomal DNA using the primer sets 3'PponA/3'mutMBS and 3'PpilM/5'mutMBS. The resultant fragments were gel purified using a QIAquick purification kit (QIAGEN Inc., Valencia, CA), and these fragments then served as both primers and templates for a second PCR. After 8 reaction cycles, primers 3'PponA and 3'PponA were added to the PCR and amplification continued for an additional 25 cycles. The resulting DNA fragment containing the mutation was purified and served as the template for the last PCR, using primers 5'PponA and 3'PponA. The resulting DNA fragment was inserted into the BamHI site of pLES94, resulting in the ponA-lacZ^¥ construct. The recombinant plasmid was introduced into DH5 mcr by transformation. Correct insertion and orientation were confirmed by PCR analysis, and DNA sequencing analysis confirmed the mutation of the MtrR-binding site. The plasmid was used to transform strains FA19 and FA19 mtrR to allow insertion into the chromosomal proAB gene. Transformants were selected on GCB agar containing 1 µg/ml of chloramphenicol.

Western blotting of PBP 1. Western blotting of PBP 1 was done essentially as described previously (28). Briefly, cultures of FA19, FA19 mtrR, and FA19 mtrR GC3mtrR were grown overnight on GC plates, and cells were swabbed into GC broth containing 10 mM MgCl₂ and diluted to an 0.18 optical density at 600 nm. One-milliliter aliquots of each sample were pelleted, the cell pellets were dissolved in 100 µl of 1x sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis buffer and boiled, and aliquots were separated on an 8% polyacrylamide-SDS gel. Alternatively, the cells were lysed, the particulate fractions were isolated, and equal levels of protein were electrophoresed as described above. The proteins were transferred to polyvinylidene difluoride membranes in 10 mM CAPS [3-(cyclohexylamino)-1-propanesulfonic acid], pH 11, 5% methanol, 0.005% SDS for 4 h at 200 mA on a semidry blotting apparatus (Owl Scientific, Portsmouth, NH). The blot was incubated with a 1/5,000 dilution of rabbit PBP 1 antisera (28) followed by horseradish peroxidase-conjugated goat anti-rabbit antibody, and the protein bands were visualized with Pierce SuperSignal West Pico chemiluminescence reagent (Rockford, IL). Films were imaged on a Bio-Rad Fluor-S system and quantified with Bio-Rad QuantityOne software (Bio-Rad, Hercules, CA).

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We examined the capacity of MtrR to regulate the expression of the ponA-pilMNOPQ locus (Fig. 1) in the gonococcal chromosome to determine if this DNA-binding protein can control the expression of proteins involved in penicillin resistance in addition to the mtrCDE-encoded efflux pump. We chose to analyze this region of the gonococcal chromosome because levels of PBP 1 may be affected by mtrR mutations (5), the multimeric PilQ has been implicated in penicillin permeation, and a mutation in pilQ (pilQ2, previously termed penC) is necessary for high-level chromosomally mediated penicillin resistance in stepwise transformants of FA19 (27).

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FIG. 1. (A) Genetic organization of the divergently transcribed ponA and pilMNOPQ genes. (B) DNA sequence of the 150-bp intergenic region. Repeat I (upper strand) and repeat II (lower strand, opposite direction) are shown underlined and bolded, indicating the location of the MtrR-binding site.

Expression of ponA is activated by MtrR. To monitor the regulation of ponA expression by MtrR, a translational reporter fusion system was employed. For this purpose, a ponA promoter-lacZ fusion was constructed and transformed into the isogenic strains FA19 and FA19 mtrR, which resulted in a single copy of the ponA promoter fused translationally to lacZ within the proAB chromosomal locus. ß-Gal activity in cell lysates from these strains indicated that the expression of ponA decreased about twofold in strain FA19 mtrR strain compared to that in its otherwise isogenic wild-type parental strain, FA19 (Fig. 2A). To confirm that these results were due to deletion of mtrR and not to a polar effect, FA19 mtrR ponA-lacZ was complemented with the wild-type mtrR gene from strain FA19, which was inserted at a secondary site within the gonococcal chromosome (between the lctP and aspC genes) to create FA19 mtrR GC3mtrR ponA-lacZ (22). Using this strain, we found that complementation of FA19 mtrR ponA-lacZ with the mtrR gene restored ponA expression to a level similar but slightly higher than that observed in FA19 ponA-lacZ (Fig. 2A).

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FIG. 2. (A) Regulation of ponA expression by MtrR. Shown are the ß-Gal activities per mg of total protein in cell extracts of reporter strains FA19 ponA-lacZ, FA19 mtrR ponA-lacZ, and FA19 mtrR GC3mtrR ponA-lacZ. The figure represents one experiment of three replicates; each replicate was performed in triplicate. Error bars represent 1 standard deviation. The differences between strains FA19 ponA-lacZ and FA19 mtrR ponA-lacZ as well as between strains FA19 mtrR ponA-lacZ and FA19 mtrR GC3mtrR ponA-lacZ were significant (P < 0.0001). (B) Total membranes from strains FA19, FA19 mtrR, and FA19 mtrR GC3mtrR were prepared, and PBP 1 levels were determined by Western blotting using rabbit anti-PBP 1 antisera. Equal amounts of protein (3 µg) were loaded in each lane. Western blotting of whole-cell lysates from these same strains gave similar results (data not presented). (C) Comparison of the MtrR-dependent regulation of ponA gene expression and PBP 1 protein levels determined by densitometry. Densitometry was performed to determine the relative ratios of PBP 1 in strains FA19, FA19 mtrR, and FA19 mtrR GC3mtrR. These values were plotted on the same graph as that shown in panel A, with the densitometry levels for FA19 PBP 1 set to the same values as the ß-Gal activity levels for FA19 ponA-lacZ for purposes of comparison. Light gray bars represent gene expression, while dark gray bars represent protein levels.

Based on these results, we next examined whether protein levels of PBP 1 were decreased due to the mtrR deletion mutation. Accordingly, total membrane fractions were prepared from strains FA19, FA19 mtrR, and FA19 mtrR GC3mtrR, and PBP 1 levels were determined by Western blotting using rabbit anti-PBP 1 antisera (28) (Fig. 2B). Densitometry was performed to determine the ratio of PBP 1 levels in FA19 relative to those in FA19 mtrR and FA19 mtrR GC3mtrR. These values were then plotted on the same graph as the gene expression (ß-Gal activity) data, with the densitometry levels adjusted such that the value observed in FA19 was equal to that determined for gene expression in strain FA19 (Fig. 2C). These data show that PBP 1 levels were decreased in strain FA19 mtrR to nearly the same extent as that determined in the gene expression assay and recovered to wild-type levels in the complemented strain FA19 mtrR GC3mtrR. This experiment was repeated two other times with whole-cell lysates of the three strains, and results similar to those with membranes were obtained (data not shown). Taken together, these results suggested that MtrR directly or indirectly stimulates ponA expression, resulting in increased levels of PBP 1.

Expression of pilM is repressed by MtrR. pilM is the first gene in the gonococcal pilMNOPQ operon (Fig. 1), and transcription of this operon, including pilQ, appears to be driven by a promoter upstream of pilM. The ponA-pilM intergenic region is only 150 bp, and the predicted –10 and –35 consensus sites for these loci appear to overlap. Based on this gene organization, we tested whether MtrR regulates pilM expression as was observed for the divergently transcribed ponA gene. For this purpose, a pilM promoter-lacZ fusion was constructed and transformed into FA19, FA19 mtrR, and FA19 mtrR GC3mtrR, which resulted in a single copy of the promoter of pilM fused translationally to lacZ within the proAB locus of the gonococcal chromosome. Expression of pilM increased greater than twofold in strain FA19 mtrR versus that in the parental strain, FA19, indicating that MtrR represses the expression of pilM (Fig. 3). Complementation of mtrR in strain FA19 mtrR GC3mtrR restored the repression of pilM to wild-type levels.

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FIG. 3. Regulation of pilM expression by MtrR. ß-Gal activities per mg of total protein in cell extracts of reporter strains FA19 pilM-lacZ, FA19 mtrR pilM-lacZ, and FA19 mtrR GC3mtrR pilM-lacZ are shown. The figure represents one experiment of three replicates; each replicate was performed in triplicate. Error bars represent 1 standard deviation. The P value (Student's t test) between strains FA19 pilM-lacZ and FA19 mtrR pilM-lacZ was 0.0024, and that between strains FA19 mtrR pilM-lacZ and FA19 mtrR GC3mtrR pilM-lacZ was 0.0017.

MtrR directly binds to the ponA-pilM promoter region. Because MtrR appeared to increase ponA expression (Fig. 2A) while repressing pilM expression (Fig. 3), we examined whether MtrR could bind in a specific manner to the ponA-pilM promoter region. MtrR has been shown previously to bind specifically to the mtrR-mtrC intervening region (20), and this interaction served as a positive control in the DNA-binding experiments described below. Initial DNA-binding experiments using EMSA showed that MtrR bound to both the mtrR-mtrC and the ponA-pilM promoter regions in a concentration-dependent manner (data not shown). To verify the specificity of this interaction, a competitive EMSA was performed using specific unlabeled PCR-derived probes from the ponA-pilM and mtrR-mtrC promoter regions and a nonspecific probe derived from an internal region of the mtrC gene. We observed that increasing amounts of the unlabeled ponA-pilM probe competed with MtrR binding to the labeled ponA-pilM region (Fig. 4), whereas the unlabeled, nonspecific probe was nearly fivefold less inhibitory, as determined by densitometric analysis. It is important to note that whereas 100x unlabeled ponA-pilM promoter probe decreased MtrR binding to the labeled ponA-pilM probe, 100x unlabeled mtrR-mtrC promoter probe completely abolished MtrR binding, suggesting that MtrR binds to the mtrC-mtrR promoter region with much higher affinity than to the ponA-pilM promoter region.

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FIG. 4. Competitive EMSA using purified MBP-MtrR fusion protein. The radiolabeled probe migrating at the bottom of the gel is the 150-bp intergenic region of ponA-pilM. SP, specific cold probe (ponA-pilM intergenic region); NSP, nonspecific cold probe (internal region of mtrC); PC, positive-control cold probe (mtrR-mtrC intergenic region). The lane without MtrR is designated by 0 and shows the electrophoretic migration of the labeled probe alone. The arrow indicates the protein/DNA complex. At a 100-fold level of unlabeled probe, SP was nearly 5-fold more effective than the NSP in competing with the labeled probe for MtrR-binding.

Identification of the MtrR-binding site. The results from the EMSA experiments suggested that MtrR has the capacity to bind within the ponA-pilM intergenic region. Therefore, we sought to identify the location of the MtrR-binding site within this 150-bp region. Initial attempts to do this by DNase I footprinting gave inconsistent results (data not presented), perhaps due to the lower affinity of MtrR for this site than for the mtrCDE promoter region. Accordingly, a series of 150-bp overlapping probes (Fig. 5) were used in EMSAs to delimit the region required for MtrR binding. In the absence of MtrR, each labeled probe migrated as two bands, likely due to differences in the secondary structure of the DNA; the migration position of the MtrR-DNA complexes is designated by the arrow in Fig. 5. MtrR failed to bind to the probe (from bp 225 to bp 75) encompassing part of the ponA coding sequence and the first 75 bp of the intergenic region (Fig. 5). However, MtrR did bind to the probe (from bp 75 to bp –75) encompassing the last 75 bp of the intergenic region and part of pilM, suggesting that the MtrR-binding site is located within the last 75 bp of the ponA-pilM intergenic region. Within this 75-bp fragment, MtrR bound to the probe from bp 50 to bp –100 but was unable to bind to the probe from bp 25 to bp –125. Taken together, these results suggested that a 25-bp region located between 50 and 25 bp upstream of the pilM start codon contains either part of or the entire MtrR-binding site.

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FIG. 5. EMSAs performed on the 150-bp overlapping radiolabeled probes within the ponA-pilM intergenic region using purified MBP-MtrR protein. The diagram at top indicates the location of each probe. Increasing amounts (0, 2, 4, and 8 µg) of MBP-MtrR protein were added to each binding reaction. The lane without MtrR is designated by 0 and shows the electrophoretic migration of the labeled probes alone. The arrow indicates the protein/DNA complex.

MtrR binds to the mtrR-mtrC intergenic region (20), and a 15-bp binding site containing two pseudo-direct repeats (underlined; 5'-CCGTGCAATCGTGTA-3') was recently shown to be necessary and sufficient for MtrR binding within the mtrCDE promoter (16). Based on homology to these repeats, a possible MtrR-binding site containing two pseudorepeats within the required 25-bp sequence of the ponA-pilM region was identified (5'-TTGTACA-3' and 5'-CTGTACA-3' [bold, underlined sequences in Fig. 1]). One of these repeats overlapped the putative –10 promoter hexamer sequence used for transcription of ponA that was previously predicted by Ropp and Nicholas (28). To determine if these repeats were required for MtrR binding, pseudorepeat I was mutated from 5'-TTGTACA-3' to 5'-GGTGCAC-3' and pseudorepeat II was mutated from 5'-CTGTACA-3' to 5'-AGTGCAC-3'. The impact of these mutations on MtrR binding to the intergenic region was then determined by EMSA. As opposed to its binding to the wild-type target DNA (Fig. 6), MtrR failed to bind the mutated target DNA, indicating that at least one (and likely both) of these two sites is required for its binding to the transcriptional regulator.

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FIG. 6. EMSA performed on a wild-type or mutated ponA-pilM promoter region using purified MBP-MtrR protein. Probe ponA-pilM contains the wild-type MtrR-binding site, while the ponA-pilM¥ probe contains the mutated MtrR-binding site. Increasing amounts (0, 2, 4, and 8 µg) of MBP-MtrR protein were added to each binding reaction. The lane without MtrR is designated by 0 and shows the electrophoretic migration of the labeled probes alone. The arrow indicates the protein/DNA complex.

An MtrR-binding site mutation deregulates ponA expression. Since mutation of the MtrR-binding site resulted in the inability of MtrR to bind to the ponA-pilM intergenic region, we next tested whether these mutations would prevent MtrR-dependent regulation of these genes. The above-mentioned mutations were introduced into the pPponA-lacZ plasmid, resulting in plasmid construct pPponA-lacZ^¥. This construct was then transformed into strains FA19 and FA19 mtrR, and ponA expression was assessed by quantifying ß-Gal activity. As previously observed, the expression of ponA from the wild-type promoter was decreased in FA19 mtrR compared to that in its parental strain, FA19 (Fig. 7). However, there was no significant difference in ponA expression observed in the FA19 ponA-lacZ^¥ and FA19 mtrR ponA-lacZ^¥ strains, with the ponA promoter containing the mutation (Fig. 7). Therefore, mutation of the MtrR-binding site within the ponA promoter results in the loss of MtrR-dependent regulation of ponA. Interestingly, the mutation of the MtrR-binding site should have also disrupted the predicted –10 region of the ponA promoter. However, our data clearly demonstrate that ponA expression was not abolished by this mutation, as expression in both FA19 ponA-lacZ^¥ and FA19 mtrR ponA-lacZ^¥ was at least as great as that observed for FA19 mtrR ponA-lacZ (Fig. 7), suggesting that this was not the location of the –10 region. Taken together, these results are consistent with MtrR serving as an enhancer of ponA expression.

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FIG. 7. Mutation of the MtrR-binding sites within the ponA-pilMNOPQ intergenic region disrupts MtrR-dependent regulation of ponA expression. Shown are the ß-Gal activities per mg of total protein in cell extracts of reporter strains FA19 ponA-lacZ, FA19 mtrR ponA-lacZ, FA19 ponA-lacZ¥, and FA19 mtrR ponA-lacZ¥. The figure represents one experiment of three replicates; each replicate was performed in triplicate. Error bars represent 1 standard deviation. The P value (Student's t test) between strains FA19 ponA-lacZ and FA19 mtrR ponA-lacZ was <0.0001. The difference in activity determined for FA19 ponA-lacZ¥ and FA19 mtrR ponA-lacZ¥ was not significant.

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The loss of penicillin as an effective antibiotic in the treatment of gonorrhea resulted not from the acquisition of the ß-lactamase plasmid (still rarely found in gonococci) that was reported in the 1970s (9) but rather from the sequential accumulation over a 40-year time period of mutations in the mtrR, porB_1B, and ponA genes as well as the creation and horizontal transfer of mosaic penA genes (24, 27, 36, 44). Each of these resistance determinants, which are transferred from a resistant strain to a susceptible strain in a specific order (27, 36, 40, 44), incrementally increases in resistance until treatment failure occurs (MIC 2 µg/ml). Mutations in ponA and penA decrease penicillin susceptibility in gonococci by decreasing the rate of penicillin acylation of PBPs 1 and 2, respectively, while mtrR mutations result in overexpression of the MtrC-MtrD-MtrE efflux pump. Although overexpression of the efflux pump on its own results in only a small increase in resistance to penicillin, mtrR mutations are required for gonococci to exhibit high-level chromosomally mediated resistance to penicillin (25, 38). The penB resistance determinant, which encodes PorB_IB variants with multiple amino acid changes in the putative loop 3 constriction loop, was originally thought to reduce antibiotic entry by altering permeation of antibiotics through the porin channel, but this has recently been brought into question (17). Finally, recent evidence indicates that mutations in pilQ also affect antibiotic entry into gonococci (3).

Because of the ability of MtrR to control the expression of mtrCDE (14) and the impact of mtrR mutations on pump levels and penicillin resistance (38), we asked if this DNA-binding protein might also regulate other genes important for penicillin resistance. The evidence presented herein strongly suggests that the transcriptionally divergent ponA-pilMNOPQ gene cluster is subject to MtrR transcriptional control. Specifically, ponA expression appears to be increased in the presence of MtrR while pilMNOPQ expression is subject to MtrR repression at a level similar to that observed for mtrCDE (14, 15). EMSA and mutagenesis studies strongly suggest that the pseudorepeats TTGTACA and CTGTACA, located within the ponA-pilM intergenic region, form the MtrR-binding site. Interestingly, these sites are on opposite strands of the DNA, a feature not previously observed for the TetR family of transcriptional regulators but perhaps necessary for control of the divergently transcribed ponA-pilMNOPQ region.

Increasing evidence indicates that MtrR plays a central role in modulating levels of gonococcal susceptibility to antimicrobial agents (15), and there is strong evidence to suggest that it performs this function primarily by modulating levels of the MtrC-MtrD-MtrE efflux pump. We have also determined that MtrR regulates the expression of mtrF (11), which encodes a cytoplasmic membrane protein needed for high-level hemagglutinin resistance mediated by the MtrC-MtrD-MtrE pump, as well as farR, which encodes the transcriptional repressor of the farAB-encoded efflux pump that exports long-chain fatty acids (18). The results presented herein implicate MtrR as a regulator of ponA and pilMNOPQ expression and lend further support to the notion that MtrR has more global regulatory properties than previously appreciated (15, 37). A recurring theme of the MtrR-regulated genes identified thus far is that all appear to be involved in resistance to host-derived antimicrobials or classical antibiotics. With respect to PilQ, a point mutation in pilQ (pilQ1) was previously shown to result in the decreased resistance of gonococci to a number of antimicrobials through a change in the structure of the PilQ outer membrane pore that enhanced the entry of antimicrobials (3). A second pilQ point mutation, termed penC (pilQ2) (44), appeared to cause a defect in PilQ multimerization and a subsequent loss of antibiotic entry. Moreover, this mutation was found to be essential for the phenotypic expression of high-level penicillin resistance in laboratory transformation experiments. Interestingly, the increase in resistance due to pilQ2 was observed only in strains containing the penA, mtrR, and penB resistance determinants, and together with ponA, these five determinants were capable of conferring high-level penicillin resistance to the same level as that found in clinical isolates.

In contrast to that of other MtrR-regulated genes, ponA gene expression appears to be transcriptionally enhanced by MtrR, a finding that was somewhat surprising as members of the TetR family are usually repressors. However, one other TetR member, DhaS from Lactococcus lacti, has been shown to be an activator of gene expression (4). While the physiological relevance of mtrCDE regulation by MtrR and the impact of mtrR mutations in gonococci have been documented (38), the benefit of ponA and pilMNOPQ regulation by this transcriptional regulator is less clear. At first glance, it would seem to be disadvantageous in vivo for gonococci to differentially regulate ponA and pilMNOPQ, as such changes in gene expression that occur in mtrR mutants would be expected to increase antibiotic uptake due to increased expression of pilQ and decrease peptidoglycan structure or bacterial growth rate due to decreased expression of ponA. However, since the type IV pilus has been shown to be required for motility, DNA uptake, virulence, and biofilm formation (26, 37, 41, 42), increased expression of the pilus apparatus genes may give gonococci with mtrR mutations a survival advantage, along with increased MtrC-MtrD-MtrE pump levels, during host infections. We propose that MtrR activation of ponA either directly or indirectly provides gonococci a mechanism to resist the lethal action of the host environment, perhaps due to changes in peptidoglycan structure, which makes the gonococci less susceptible to damaging agents. In Neisseria meningitidis, alterations in PBP 2 have been shown to result in compositional modifications in peptidoglycan structure (1). Thus, host antimicrobials that damage the cell envelope integrity may have reduced activities against mtrR mutants, which were shown previously (13) to have altered peptidoglycan cross-linking and lytic behavior. More-in-depth interpretations of the biological significance of the data presented herein will require additional experimentation. However, in support of our hypothesis that these changes in gene expression are not disadvantageous in vivo are reports that gonococci with mtrR mutations are often isolated from patients with rectal or urogenital infections (8, 23, 31, 43). Moreover, recent experiments showed that mutations in mtrR can enhance gonococcal fitness in a murine vaginal infection model (D. M. Warner, J. P. Folster, W. M. Shafer, and A. Jerse, unpublished results). Hence, we propose that under certain circumstances and infections, MtrR regulation of gene expression is an important determinant of gonococcal survival in vivo. A thorough understanding of the MtrR regulon in gonococci should therefore provide important insights regarding antibiotic resistance and pathogenesis as well as the link between these two phenotypes.

 
We thank L. Pucko for help in manuscript preparation and submission. We also are grateful to R. Brennan for his help in identifying the MtrR-binding site near the ponA gene.

This study was supported by NIH grants AI-21150 (W. M. S.) and AI-36901 (R. A. N.) and funds from the Veterans Affairs Medical Research Services (W.M.S.). W. M. S. was supported by a Senior Research Career Scientist Award from the Veterans Affairs Medical Research Service.

 
* Corresponding author. Mailing address: Research Service (VAMC), Room 5A181, 1670 Clairmont Road, Decatur, GA 30033. Phone: (404) 728-7688. Fax: (404) 329-2210. E-mail: wshafer{at}emory.edu

Published ahead of print on 4 May 2007.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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Journal of Bacteriology, July 2007, p. 4569-4577, Vol. 189, No. 13
0021-9193/07/$08.00+0     doi:10.1128/JB.00286-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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