Mg2+ sensing by the Mg2+ sensor PhoQ of Salmonella enterica

J Mol Biol. 2003 Jan 24;325(4):795-807. doi: 10.1016/s0022-2836(02)01268-8.


The PhoP/PhoQ two-component regulatory system governs the adaptation to low Mg(2+) environments and virulence in several Gram-negative species. During growth in low Mg(2+), the sensor PhoQ modifies the activity of the response regulator PhoP promoting gene transcription, whereas growth in high Mg(2+) represses transcription of PhoP-activated genes. The PhoQ protein harbors a periplasmic domain of 146 amino acid residues that binds Mg(2+) in vitro and is required for Mg(2+)-mediated repression in vivo. Here, we identify periplasmic mutants of the Salmonella PhoQ protein that allow transcription of PhoP-activated genes even under high Mg(2+) concentrations. When expressed in a strain harboring a PhoP variant that is phosphorylated from acetyl phosphate, some of the mutants failed to repress PhoP-promoted transcription in high Mg(2+), whereas others displayed a wild-type ability to do so. Mutant PhoQ proteins that allowed expression of PhoP-activated genes in high Mg(2+) displayed a pattern of iron-mediated cleavage in vitro that was different from that displayed by wild-type PhoQ, indicative of altered Mg(2+) binding. A PhoQ protein with the conserved histidine residue (H277) substituted by alanine could not promote transcription of PhoP-activated genes in low Mg(2+) but could turn off expression in response to high Mg(2+). Our studies demonstrate that residues G93, W97, H120 and T156 are required for a wild-type response to Mg(2+), and suggest that Mg(2+) binding to the periplasmic domain regulates several activities in the PhoQ protein.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Amino Acid Substitution
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Base Sequence
  • Binding Sites
  • Conserved Sequence
  • DNA, Bacterial / genetics
  • Magnesium / metabolism*
  • Molecular Sequence Data
  • Mutagenesis, Site-Directed
  • Phosphorylation
  • Protein Structure, Tertiary
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Salmonella enterica / genetics
  • Salmonella enterica / growth & development
  • Salmonella enterica / metabolism*
  • Sequence Homology, Amino Acid
  • Transcription, Genetic


  • Bacterial Proteins
  • DNA, Bacterial
  • PhoQ protein, Bacteria
  • Recombinant Proteins
  • Magnesium