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In  Enterobacteriaceae , the transcriptional regulator AmpR, a member of the LysR family, regulates the expression of a chromosomal β-lactamase AmpC. The regulatory repertoire of AmpR is broader in  Pseudomonas aeruginosa , an opportunistic pathogen responsible for numerous acute and chronic infections including cystic fibrosis. In addition to regulating  ampC ,  P. aeruginosa  AmpR regulates the sigma factor AlgT/U and production of some quorum sensing (QS)-regulated virulence factors. In order to better understand the  ampR  regulon, we compared the transcriptional profile generated using DNA microarrays of the prototypic  P. aeruginosa  PAO1 strain with its isogenic  ampR  deletion mutant, PAOΔ ampR . Transcriptome analysis demonstrates that the AmpR regulon is much more extensive than previously thought, with the deletion of  ampR  influencing the differential expression of over 500 genes. In addition to regulating resistance to β-lactam antibiotics via AmpC, AmpR also regulates non-β-lactam antibiotic resistance by modulating the MexEF-OprN efflux pump. Other virulence mechanisms including biofilm formation and QS-regulated acute virulence factors are AmpR-regulated. Real-time PCR and phenotypic assays confirmed the microarray data. Further, using a  Caenorhabditis elegans  model, we demonstrate that a functional AmpR is required for  P. aeruginosa  pathogenicity. AmpR, a member of the core genome, also regulates genes in the regions of genome plasticity that are acquired by horizontal gene transfer. Further, we show differential regulation of other transcriptional regulators and sigma factors by AmpR, accounting for the extensive AmpR regulon. The data demonstrates that AmpR functions as a global regulator in  P. aeruginosa  and is a positive regulator of acute virulence while negatively regulating biofilm formation, a chronic infection phenotype. Unraveling this complex regulatory circuit will provide a better understanding of the bacterial response to antibiotics and how the organism coordinately regulates a myriad of virulence factors in response to antibiotic exposure.

The Regulatory Repertoire of Pseudomonas aeruginosa AmpC ß-Lactamase Regulator AmpR Includes Virulence Genes