Molecular basis of the magnesium deprivation response in Salmonella typhimurium: identification of PhoP-regulated genes

Abstract

The PhoP-PhoQ two-component system is essential for virulence in Salmonella typhimurium. This system controls expression of some 40 different proteins, yet most PhoP-regulated genes remain unknown. To identify PhoP-regulated genes, we isolated a library of 50,000 independent lac gene transcriptional fusion strains and investigated whether production of ␤-galactosidase was regulated by PhoP. We recovered 47 lac gene fusions that were activated and 7 that were repressed when PhoP was expressed. Analysis of 40 such fusions defined some 30 loci, including mgtA and mgtCB, which encode two of the three Mg 2؉ uptake systems of S. typhimurium; ugd, encoding UDP-glucose dehydrogenase; phoP, indicative that the phoPQ operon is autoregulated; and an open reading frame encoding a protein with sequence similarity to VanX, a dipeptidase required for resistance to vancomycin. Transcription of PhoP-activated genes was regulated by the levels of Mg 2؉ in a PhoP-dependent manner. Strains with mutations in phoP or phoQ were defective for growth in low-Mg 2؉ media. The mgtA and mgtCB mutants reached lower optical densities than the wild-type strain in low-Mg 2؉ liquid media but displayed normal growth on low-Mg 2؉ solid media. Six PhoP-activated genes were identified as essential to form colonies on low-Mg 2؉ solid media. Cumulatively, our experiments establish that the PhoP-PhoQ system governs the adaptation to magnesium-limiting environments.

Closing the loop: The PmrA/PmrB two-component system negatively controls expression of its posttranscriptional activator PmrD

“…In contrast, the negative feedback loop of the PmrA protein on pmrD transcription occurs at a much later step. This makes sense because inhibiting the first step in the pathway (i.e., PhoQ activation) would compromise expression of the large number of genes that encompass the PhoP regulon (34,35). Furthermore, because the PmrA͞PmrB system can be activated independently of pmrD by mild acid pH or Fe 3ϩ (2,11,13), continuous PmrD production under such conditions might interfere with turning off expression of PmrAactivated genes when the environment changes to noninducing conditions.…”

Mg 2+ regulates transcription of mgtA in Salmonella Typhimurium via translation of proline codons during synthesis of the MgtL peptide

“…Transcription of mgtA and the mgtCBR operon is dependent on the PhoQP two-component system, in which the inner membrane protein PhoQ carries out phosphorylation and dephosphorylation of the DNA-binding transcriptional regulator PhoP in response to periplasmic stimuli (4). The PhoQP system regulates directly or indirectly the transcription of 5% of the genes of Salmonella and Escherichia coli, including the phoPQ operon itself and genes involved in virulence, membrane composition, antimicrobial peptide resistance, and acid stress resistance (5).…”

The PhoP/PhoQ two-component system stabilizes the alternative sigma factor RpoS in Salmonella enterica

“…The Salmonella PhoP͞PhoQ system governs the adaptation to low Mg 2ϩ environments (22)(23)(24), intramacrophage survival, and virulence in mice (25)(26)(27). The sensor protein PhoQ responds to low Mg 2ϩ by promoting phosphorylation of the response regulator PhoP (28)(29)(30)(31), which binds to its target promoters to stimulate transcription of PhoP-activated genes (24,32).…”

“…The sensor protein PhoQ responds to low Mg 2ϩ by promoting phosphorylation of the response regulator PhoP (28)(29)(30)(31), which binds to its target promoters to stimulate transcription of PhoP-activated genes (24,32). The Salmonella PhoP͞PhoQ system controls expression of Ϸ3% of the genome (33).…”