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.
mexCD-oprJ is an envelope stress-inducible multidrug efflux operon of Pseudomonas aeruginosa. A gene encoding a homologue of the NfxB repressor of this operon, PA4596, occurs downstream of oprJ and was proposed as a second repressor of this efflux operon. Inactivation of this gene had no impact on mexCD-oprJ expression in cells not exposed to envelope stress although its loss under envelope stress conditions yielded a > 10-fold increase in mexCD-oprJ expression. Consistent with PA4596 functioning as a mexCD-oprJ repressor, the purified protein was able to bind to a DNA fragment carrying the mexCD-oprJ promoter region. Expression of PA4596 was induced under conditions of envelope stress dependent on the AlgU envelope stress sigma factor, consistent with PA4596 operating under envelope stress conditions where it possibly serves to moderate envelope stress-inducible mexCD-oprJ expression. nfxB mutants showed elevated PA4596 expression and purified NfxB bound to DNA encompassing the PA4596 upstream region, an indication that NfxB functions as a repressor of PA4596 expression. Elimination of PA4596 in P. aeruginosa lacking nfxB and hyperexpressing mexCD-oprJ had no additional impact on mexCD-oprJ expression, regardless of the presence of envelope stress, suggesting that PA4596 repressor activity may be dependent on NfxB. This envelope stress-regulated repressor of mexCD-oprJ has been renamed esrC.
Exposure of P. aeruginosa to the aminoglycoside (AG) paromomycin (PAR) induced expression of the PA3720-armR locus and the mexAB-oprM multidrug efflux operon that AmgR controls, although PAR induction of mexAB-oprM was independent of armR. Multiple AGs promoted mexAB-oprM expression and this was lost in the absence of the amgRS locus encoding an aminoglycoside-activated envelope stress-responsive 2-component system (TCS). Purified AmgR bound to the mexAB-oprM promoter region consistent with this response regulator directly regulating expression of the efflux operon. The thiol-active reagent, diamide, which, like AGs, promotes protein aggregation and cytoplasmic membrane damage also promoted AmgRS-dependent mexAB-oprM expression, a clear indication that the MexAB-OprM efflux system is recruited in response to membrane perturbation and/or circumstances that lead to this. Despite the AG and diamide induction of mexAB-oprM, however, MexAB-OprM does not appear to contribute to resistance to these agents.
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