Baseline expression of resistance
nodulation division (RND) efflux pumps is controlled by a locally encoded TetR family
repressor, and levels of the AraC family transcription factor, which can relieve
TetR-mediated repression are kept low by repression from the multiple antibiotic
resistance protein (MarR) family repressor. b | Mutations in the marR family repressor
gene cause the repressor to lose the ability to inhibit AraC family activators. Increased
expression of the AraC activator confers increased expression of the RND efflux pump
AcrB and the periplasmic adaptor AcrA. c | Mutation in the TetR family repressor gene
reduces the repressor binding upstream of acrA and acrB, leading to increased
transcription of acrAB. d | Antibiotics bind to MarR and cause conformational changes
that prevent repression of the AraC activator. Increased expression of the AraC activator
leads to increased transcription of acrA and acrB. Activated pathways are indicated by
solid arrows and inhibited pathways are indicated by dashed arrows.