Signal-dependent Binding of the Response Regulators PhoP and PmrA to Their Target Promoters in Vivo

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111

122

The sigma factor RpoS regulates the expression of many stress response genes and is required for virulence in several bacterial species. We now report that RpoS accumulates when Salmonella enterica serovar Typhimurium is growing logarithmically in media with low Mg 2؉ concentrations. This process requires the twocomponent regulatory system PhoP͞PhoQ, which is specifically activated in low Mg 2؉ . We show that PhoP controls RpoS protein turnover by serving as a transcriptional activator of the iraP (yaiB) gene, which encodes a product that enhances RpoS stability by interacting with RssB, the protein that normally delivers RpoS to the ClpXP protease for degradation. Mutation of the phoP gene rendered Salmonella as sensitive to hydrogen peroxide as an rpoS mutant after growth in low Mg 2؉ . In Escherichia coli, low Mg 2؉ leads to only modest RpoS stabilization, and iraP is not regulated by PhoP͞PhoQ. These findings add the sigma factor RpoS to the regulatory proteins and two-component systems that are elevated in a PhoP͞PhoQ-dependent fashion when Salmonella face low Mg 2؉ environments. Our data also exemplify the critical differences in regulatory circuits that exist between the closely related enteric bacteria Salmonella and E. coli.IraP ͉ magnesium ͉ Escherichia coli

Section: Resultssupporting

confidence: 58%

“…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).…”

mentioning

confidence: 99%

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

Latifi²,

Bougdour³

et al. 2006

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111

122

“…Furthermore, the steady-state behavior of the PhoQ/PhoP circuit with such a mutant should be strongly dependent on autoregulation, even for levels of stimulus where autoregulation has no effect on the behavior of the circuit with wild-type PhoQ. To test this, we used a PhoQ(T281R) mutant, which was predicted to have defective phosphatase activity (29). When we compared the activities of cytoplasmic fragments of wild-type PhoQ and PhoQ(T281R) in vitro, we found that PhoQ(T281R) showed slower PhoP phosphorylation and no observable PhoP-P dephosphorylation activity (Fig.…”

Section: Resultsmentioning

confidence: 99%

High stimulus unmasks positive feedback in an autoregulated bacterial signaling circuit

Miyashiro¹,

Goulian

2008

139

We examined the effect of positive autoregulation on the steadystate behavior of the PhoQ/PhoP two-component signaling system in Escherichia coli. We found that autoregulation has no effect on the steady-state output for a large range of input stimulus, which was modulated by varying the concentration of magnesium in the growth medium. We provide an explanation for this finding with a simple model of the PhoQ/PhoP circuit. The model predicts that even when autoregulation is manifest across a range of stimulus levels, the effects of positive feedback on the steady-state output emerge only in the limit that the system is strongly stimulated. Consistent with this prediction, amplification associated with autoregulation was observed in growth-limiting levels of magnesium, a condition that strongly activates PhoQ/PhoP. In a further test of the model, we found that strains harboring a phosphatasedefective PhoQ showed strong positive feedback and considerable cell-to-cell variability under growth conditions where the wildtype circuit did not show this behavior. Our results demonstrate a simple and general mechanism for regulating the positive feedback associated with autoregulation within a bacterial signaling circuit to boost response range and maintain a relatively uniform and graded output.autogenous control ͉ PhoP-PhoQ ͉ two-component signal transduction

“…The PmrD protein is produced when the sensor protein PhoQ responds to low extracytoplasmic Mg 2ϩ by promoting phosphorylation of the DNA-binding protein PhoP. The phosphorylated PhoP protein then binds to a large number of target promoters, including that of the pmrD gene, and activates or represses gene transcription (9,10). Three of the PmrA-activated loci mediate resistance to the antibiotic polymyxin B.…”

mentioning

confidence: 99%

A connector of two-component regulatory systems promotes signal amplification and persistence of expression

Kato

Mitrophanov²,

Groisman³

2007

Self Cite

Organisms rely on a variety of regulatory architectures to control gene transcription. Whereas the functional characteristics of particular architectures are well understood, the properties of newly discovered regulatory designs cannot be easily predicted. One emerging design depends on small proteins that connect twocomponent regulatory systems, which constitute the dominant form of bacterial signal transduction. These connectors enable one system to respond to the signal perceived by a different system. To understand the functional properties of such connector-mediated architectures, we investigated the pathway controlled by the PhoP-dependent connector protein PmrD of Salmonella enterica and contrasted it to the circuit in which genes are regulated directly by the transcription factor PhoP. The PmrD-mediated pathway displayed both signal amplification and persistence of expression when compared with the direct pathway. Mathematical modeling of the two pathways allowed us to identify critical factors responsible for signal amplification. mathematical modeling ͉ Salmonella ͉ signal transduction ͉ transcriptional regulatory circuit ͉ two-component system