MpeR Regulates the mtr Efflux Locus in Neisseria gonorrhoeae and Modulates Antimicrobial Resistance by an Iron-Responsive Mechanism

Abstract

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.

Control of RNA Stability by NrrF, an Iron-Regulated Small RNA in Neisseria gonorrhoeae

“…Under high-Fe conditions (Fig. 3A), Fur directly represses both MpeR (58) and NrrF (24). This relieves the MpeR-mediated repression of mtrF, which is nevertheless still transcribed at a low level, because this locus is repressed by MtrR under high-Fe conditions; MtrR repression is itself relieved indirectly by the Furmediated repression of MpeR.…”

“…MtrF has been proposed either to shuttle antimicrobials to the pump for export or to help stabilize one or more components of the efflux pump (56,57). MtrF and NGO1024 appear to be linked in a regulatory sense, in that the transcripts for these proteins are controlled directly or indirectly by at least four regulatory elements, including NrrF, Fur, and two additional transcriptional repressors, MtrR and MpeR (57,58). The relationships between these regulatory elements are summarized in Fig.…”

Characterization of the Neisseria gonorrhoeae Iron and Fur Regulatory Network

“…regulatory proteins (58,59). Targets of these proteins include genes involved in antimicrobial resistance (MtrR) (60), and thus these genes are indirectly regulated by Fur.…”

Antimicrobial Resistance in Neisseria gonorrhoeae in the 21st Century: Past, Evolution, and Future

“…2). In addition, expression of mtrCDE is linked to the availability of free iron (285). Such iron-mediated regulation of mtrCDE involves the capacity of the MpeR regulator to transcriptionally repress mtrR, encoding the master repressor of mtrCDE, as well as mtrF (286) (Fig.…”