Pentachlorophenol Induction of the Pseudomonas aeruginosa mexAB-oprM Efflux Operon: Involvement of Repressors NalC and MexR and the Antirepressor ArmR

Abstract

Pentachlorophenol (PCP) induced expression of the NalC repressor-regulated PA3720- armR operon and the MexR repressor-controlled mexAB-oprM multidrug efflux operon of Pseudomonas aeruginosa . PCP's induction of PA3720- armR resulted from its direct modulation of NalC, the repressor's binding to PA3720- armR promoter-containing DNA as seen in electromobility shift assays (EMSAs) being obviated in the presence of this agent. The NalC binding site was localized to an inverted repeat (IR) sequence upstream of PA3720- armR and overlapping a promoter region whose transcription start site was mapped. While modulation of MexR by the ArmR anti-repressor explains the upregulation of mexAB-oprM in nalC mutants hyperexpressing PA3720- armR , the induction of mexAB-oprM expression by PCP is not wholly explainable by PCP induction of PA3720- armR and subsequent ArmR modulation of MexR, inasmuch as armR deletion mutants still showed PCP-inducible mexAB-oprM expression. PCP failed, however, to induce mexAB-oprM in a mexR deletion strain, indicating that MexR was required for this, although PCP did not modulate MexR binding to mexAB-oprM promoter-containing DNA in vitro . One possibility is that MexR responds to PCP-generated in vivo effector molecules in controlling mexAB-oprM expression in response to PCP. PCP is an unlikely effector and substrate for NalC and MexAB-OprM - its impact on NalC binding to the PA3720- armR promoter DNA occurred only at high µM levels - suggesting that it mimics an intended phenolic effector/substrate(s). In this regard, plants are an abundant source of phenolic antimicrobial compounds and, so, MexAB-OprM may function to protect P. aeruginosa from plant antimicrobials that it encounters in nature.

Mutational Analysis of the Multiple-Antibiotic Resistance Regulator MarR Reveals a Ligand Binding Pocket at the Interface between the Dimerization and DNA Binding Domains

“…MarR homologues control pathways that are critical to bacterial physiology such as stress responses (41,42), virulence (43), metabolism (10), and multiple-drug resistance (23,44,45). The activity of a subset of these proteins can be modified through the formation of a disulfide bond (19,41,46).…”

The Challenge of Efflux-Mediated Antibiotic Resistance in Gram-Negative Bacteria

“…Recent data, however, showed that pentachlorophenol stimulates MexABOprM production, surprisingly, in an ArmR-independent (although MexR-dependent) manner (809). Possibly, the regulatory pathway here involves in vivo-generated catabolite effectors mimicking more specific antimicrobial phenolic compounds than pentachlorophenol that P. aeruginosa may encounter in its natural environment (809). The physiological conditions under which ArmR ultimately activates mexAB-oprM thus remain unclear and require further studies.…”

“…However, chemostat experiments with strain PAO1 showed that various chlorinated phenols, including the environmental contaminant pentachlorophenol, can induce the expression of the PA3720-PA3719 and mexAB-oprM operons through reversible, noncovalent binding to the NalC protein (807,808). Recent data, however, showed that pentachlorophenol stimulates MexABOprM production, surprisingly, in an ArmR-independent (although MexR-dependent) manner (809). Possibly, the regulatory pathway here involves in vivo-generated catabolite effectors mimicking more specific antimicrobial phenolic compounds than pentachlorophenol that P. aeruginosa may encounter in its natural environment (809).…”

Stress responses as determinants of antimicrobial resistance in Pseudomonas aeruginosa: multidrug efflux and more

“…Expression of mexAB-oprM is also induced by chlorinated phenols like pentachlorophenol (Ghosh et al 2011;Muller et al 2007;Starr et al 2012), an uncoupler of oxidative phosphorylation that may also promote oxidative stress that impacts the redox-sensitive MexR repressor. Pentachlorophenol has, for example, been shown to dramatically increase O 2 flux in P. aeruginosa, generating an oxidative stress that could impact MexR activity (McLamore et al 2010).…”