Comparison of the Pseudomonas aeruginosa andEscherichia coli PhoQ Sensor Domains

Abstract

The PhoP-PhoQ two-component system is present in a number of Gram-negative bacteria where it has roles in Mg 2؉ homeostasis and virulence. PhoQ is a transmembrane histidine kinase that activates PhoP-mediated regulation of a set of genes when the extracellular concentration of divalent cations is low. Divalent cations are thought to interact directly with the periplasmic PhoQ sensor domain. The PhoP-PhoQ systems of Escherichia coli and Pseudomonas aeruginosa are similar in their biological response to extracellular divalent cations; however, their sensor domains display little sequence identity. Here we have begun to explore the consequences of this sequence divergence by comparing the biophysical properties of the P. aeruginosa PhoQ sensor domain with the corresponding E. coli sensor domain. Unlike the E. coli protein, the P. aeruginosa PhoQ sensor domain undergoes changes in the circular dichroism and fluorescence spectra as well as destabilization of its dimeric form in response to divalent cations. These results suggest that distinct mechanisms of signal detection are utilized by these proteins. A hybrid protein in which the E. coli sensor domain has been substituted with the corresponding P. aeruginosa sensor domain responds normally to the presence of extracellular divalent cations in vivo in E. coli. Thus, despite apparent differences in the structural response to its stimulus, the P. aeruginosa sensor domain transduces signals to the E. coli PhoQ cytoplasmic kinase domain in a manner that mimics normal E. coli PhoQ function.The PhoP-PhoQ two-component signal transduction system is required for pathogenesis of Salmonella typhimurium, Salmonella typhi, Yersinia pestis, and Erwinia caratova (1-4). Its role in S. typhimurium virulence is due at least in part to its function as a regulator of genes responsible for lipopolysaccharide modification that increase bacterial survival in the host organism (5, 6). Similar lipopolysaccharide modification also enhances resistance to polymixin by P. aeruginosa (7-9) and occurs in isolates recovered from cystic fibrosis patients (8).Additionally, the PhoP-PhoQ system plays a role in the survival of S. typhimurium by limiting Mg 2ϩ concentrations through its regulation of magnesium transporter genes and likely plays a similar role in nonpathogenic Escherichia coli (10).PhoP-PhoQ possesses the domain structure of the archetypal two-component system (11). The transmembrane PhoQ histidine kinase consists of an extracellular sensor domain coupled to an intracellular kinase domain, and the cytoplasmic PhoP response-regulator is composed of a receiver domain and a transcriptional regulator domain. Extracellular divalent cations such as Mg 2ϩ and Ca 2ϩ have been shown to repress PhoPmediated transcriptional regulation in E. coli, S. typhimurium, and P. aeruginosa (7, 9, 10, 12). Studies in S. typhimurium and E. coli support the idea that these cations are signaling ligands that exert their effects through direct interaction with the PhoQ sensor domain (10, 12-13). Studies of the...

Temperature and Mg2+ Sensing by a Novel PhoP-PhoQ Two-component System for Regulation of Virulence in Edwardsiella tarda

“…(41). Interestingly, the P. aeruginosa PhoQ lacks this acidic patch of residues, and unlike the E. coli protein, the P. aeruginosa PhoQ sensor domain undergoes changes in its CD and fluorescence spectra in response to divalent cations (42). These data suggested that the PhoQ sensors of E. tarda, S. typhimurium, and P. aeruginosa may have very different mechanisms of signal detection.…”

Bacterial sensor kinase TodS interacts with agonistic and antagonistic signals

“…Available data on the interaction between agonists and HPKs were obtained primarily with kinases containing a periplasmic sensor domain. The affinity of HPKs for their cognate agonists was found to be in the micromolar range, as exemplified by CitA, which has a K D of 5.5 M for citrate (29), and NarX and PhoQ, which have apparent affinities of Ϸ35 M for nitrate and Ϸ300 M for Mg 2ϩ ions, respectively (30,31). Here we show that TodS binds agonists with much higher affinities than those just mentioned.…”

The CsrR/CsrS two-component system of group A Streptococcus responds to environmental Mg 2+

“…PhoQ of E. coli has been proposed to bind Mg 2ϩ on its periplasmic domain via a cluster of seven amino acids, six of which are acidic (31). Such a cluster is also found in the PhoQ periplasmic domain of S. enterica serovar Typhimurium, but not in some other PhoQ homologues that are also able to respond to Mg 2ϩ (23,32). Thus, a specific motif of acidic residues required for Mg 2ϩ binding has not been defined.…”