MpeR regulates the mtr efflux locus in Neisseria gonorrhoeae and modulates antimicrobial resistance by an iron-responsive mechanism.

Previous studies have shown that the MpeR transcriptional regulator produced by Neisseria gonorrhoeae represses the expression of mtrF, which encodes a putative inner membrane protein (MtrF). MtrF works as an accessory protein with the Mtr efflux pump, helping gonococci to resist high levels of diverse hydrophobic antimicrobials. Regulation of mpeR has been reported to occur by an iron-dependent mechanism involving Fur (ferric uptake regulator). Collectively, these observations suggest the presence of an interconnected regulatory system in gonococci that modulates the expression of efflux pump protein-encoding genes in an iron-responsive manner. Herein, we describe this connection and report that levels of gonococcal resistance to a substrate of the mtrCDE-encoded efflux pump can be modulated by MpeR and the availability of free iron. Using microarray analysis, we found that the mtrR gene, which encodes a direct repressor (MtrR) of mtrCDE, is an MpeR-repressed determinant in the late logarithmic phase of growth when free iron levels would be reduced due to bacterial consumption. This repression was enhanced under conditions of iron limitation and resulted in increased expression of the mtrCDE efflux pump operon. Furthermore, as judged by DNA-binding analysis, MpeR-mediated repression of mtrR was direct. Collectively, our results indicate that both genetic and physiologic parameters (e.g., iron availability) can influence the expression of the mtr efflux system and modulate levels of gonococcal susceptibility to efflux pump substrates.

The iron-repressed, AraC-like regulator MpeR activates expression of fetA in Neisseria gonorrhoeae.

Neisseria gonorrhoeae is an obligate human pathogen that causes the common sexually transmitted infection gonorrhea. Gonococcal infections cause significant morbidity, particularly among women, as the organism ascends to the upper reproductive tract, resulting in pelvic inflammatory disease, ectopic pregnancy, and infertility. In the last few years, antibiotic resistance rates have risen dramatically, leading to severe restriction of treatment options for gonococcal disease. Gonococcal infections do not elicit protective immunity, nor is there an effective vaccine to prevent the disease. Thus, further understanding of the expression, function, and regulation of surface antigens could lead to better treatment and prevention modalities in the future. In the current study, we determined that an iron-repressed regulator, MpeR, interacted specifically with the DNA sequence upstream of fetA and activated FetA expression. Interestingly, MpeR was previously shown to regulate the expression of gonococcal antimicrobial efflux systems. We confirmed that the outer membrane transporter FetA allows gonococcal strain FA1090 to utilize the xenosiderophore ferric enterobactin as an iron source. However, we further demonstrated that FetA has an extended range of substrates that encompasses other catecholate xenosiderophores, including ferric salmochelin and the dimers and trimers of dihydroxybenzoylserine. We demonstrated that fetA is part of an iron-repressed, MpeR-activated operon which putatively encodes other iron transport proteins. This is the first study to describe a regulatory linkage between antimicrobial efflux and iron transport in N. gonorrhoeae. The regulatory nidus that links these systems, MpeR, is expressed exclusively by pathogenic neisseriae and is therefore expected to be an important virulence factor.