Molecular Characterization of the PmrA Regulon

The two-component system PmrA/PmrB of Salmonella enterica controls expression of several loci including those mediating modifications in the lipopolysaccharide that result in polymyxin resistance. To gain insight in the regulation of polymyxin resistance, we mapped the transcription start sites of the PmrA-regulated genes pmrC, pmrG, pbgPE, and ugd and identified a conserved sequence in the promoter region of the first three genes. His-tagged PmrA protein could gel shift DNA fragments containing the promoters of the pmrC, pmrG, and pbgPE genes but not the udg promoter. DNase I footprinting analysis of the pmrC, pmrG, and pbgPE promoters indicate that phosphorylated as well as unphosphorylated PmrA bind to a 16-base pair imperfect inverted repeat sequence (5′-TTAAKTTCTTAAKGTT-3′), which is found 40, 80, and 38 nucleotides upstream from the transcription start sites of the pmrC, pmrG, and pbgPE genes, respectively. Our data suggest that a PmrA dimer activates transcription of the divergent pmrG and pbgPE promoters by binding to a single site in the pmrG-pbgPE intergenic region and that the ugd gene is regulated by the PmrA/PmrB system only indirectly.