Mg2+ as an Extracellular Signal: Environmental Regulation of Salmonella Virulence

Abstract

Ions are not traditionally thought to act as first messengers in signal transduction cascades. However, while searching for genes regulated by the PhoP/PhoQ virulence regulatory system of Salmonella typhimurium, we recovered two loci whose expression is controlled by the concentration of Mg2+. To determine whether Mg2+ is the signal modulating the whole PhoP/PhoQ system, we evaluated the gene expression pattern of six PhoP-activated genes. Growth in physiological concentrations of divalent cations repressed transcription of PhoP-activated genes and rendered wild-type Salmonella phenotypically PhoP-. Mg2+ changed the conformation of the periplasmic domain of PhoQ, identifying this protein as a Mg2+ sensor. A mutation in the sensing domain of PhoQ altered the set point for Mg2+ and rendered Salmonella avirulent.

Isolation and Molecular Characterization of the Locked-on Mutant of Mg2+Sensor PhoQ inEscherichia coli

“…13) In the case of PhoQ sensor kinase, it has been shown that Mg 2þ also bound to the periplasmic sensory domain and transmitted inhibitory signals to the cytoplasmic catalytic domain. 5,6) Based on this model, we investigated membrane-bound PhoQ autophosphorylation as previously described. 14) E. coli BL21(DE3)/pMHK028 (the phoQ expression strain) was grown at 37 C in an LB medium and induced with IPTG.…”

“…10) The known periplasmic mutants of PhoQ still responded to Mg 2þ to some extent, and no mutation was found to become fully constitutive. 5,6,10) We describe in this study the isolation and phenotypic characterization of 10 mutants with single-amino-acid substitutions in the periplasmic region (PhoQT47, T48, N68, D90, D149, D150, D151, D152 and D179) in Escherichia coli, finding that PhoQD179L(A) does not respond at all to either Mg 2þ or Ca 2þ .…”

“…3,4) Such a regulatory effect of Mg 2þ resulted from conformational change in the periplasmic domain of the PhoQ protein, which features two transmembrane regions, a cytoplasmic tail and a periplasmic domain. 5) In fact, the purified periplasmic domains (amino acid residue 45-190) of the PhoQ proteins from S. enterica and E. coli specifically bind Mg 2þ in vitro. 6,7) Many mutants of the periplasmic domain of PhoQ have previously been isolated and characterized for a functional analysis of PhoQ.…”

“…The S. enterica strain expressing a PhoQ mutant harboring the T48I substitution in the periplasmic domain is less sensitive to repression by Ca 2þ , but responds to Mg 2þ in the same way as wild-type S. enterica, consistent with the assumption that the PhoQ protein has distinct binding sites for Mg 2þ and Ca 2þ . 5,6) S. enterica membranes containing the T48I PhoQ mutant protein promoted more PhoP phosphorylation than those harboring wildtype PhoQ. 8) On the other hand, the equivalent mutation in the E. coli PhoQ protein resulted in a strain that responded to both Ca 2þ and Mg 2þ .…”

Phylogenetic and Functional Analysis of Aspergillus fumigatus MGTC, a Fungal Protein Homologous to a Bacterial Virulence Factor

“…The function of MgtCp is unknown, but it seems to facilitate or regulate Mg 2ϩ transport since it is required for growth at low Mg 2ϩ concentrations (11). MgtC-like proteins are found in a limited number of bacterial genomes, and phylogenetic analysis suggests that MgtC has been acquired by horizontal gene transfer (HGT) repeatedly throughout bacterial evolution.…”