Abstract
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.
Highlights
To survive in different environments, bacteria modulate expression of their genes often using two-component signal transduction systems [1, 2]
That identical transcription start sites were detected when expression of PmrA-activated genes was induced by growth in low magnesium or by mild acidification suggests that these signals act by modifying the activity of the PmrA protein and argues against a model in which different promoters are used in response to different signals
The PmrA Protein Binds to the Promoter Regions of the pmrC, pmrG, and pbgPE Genes—To examine the ability of the PmrA protein to bind the promoter regions of PmrA-activated genes, we first constructed a derivative of the pmrA gene encoding a protein with six histidine residues at its amino terminus, and we showed that a plasmid encoding this pmrA derivative could restore transcription of the pmrC, pbgPE, pmrG, and ugd genes in a pmrA null mutant
Summary
To survive in different environments, bacteria modulate expression of their genes often using two-component signal transduction systems [1, 2] These systems typically consist of a sensor histidine kinase and a response regulator. The pmrA and pmrB genes of Salmonella enterica serovar Typhimurium encode products with sequence similarity to DNA binding response regulators and autophosphorylatable histidine kinases, respectively [4]. Transcription of PmrA-activated genes is induced in response to mild acidic conditions [9] or during growth in a low extracellular magnesium media in a process that requires the PhoP/PhoQ two-component system [10, 11] (Fig. 1). Our experiments define two classes of PmrA-regulated genes as follows: those that are directly regulated by the PmrA protein (pmrCAB, pmrG, and pbgPE), and those that are regulated indirectly by the PmrA protein (ugd)
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