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The architecture of a prototypical bacterial signaling circuit enables a single point mutation to confer novel network properties.

Ram S, Goulian M - PLoS Genet. (2013)

Bottom Line: We describe a remarkable example of this versatility in the well-studied PhoQ/PhoP bacterial signaling network, which has an architecture found in many two-component systems.We found that a single point mutation that abolishes the phosphatase activity of the sensor kinase PhoQ results in a striking change in phenotype.Our results demonstrate the remarkable versatility of the prototypical two-component signaling architecture and highlight the tradeoffs in the particular case of the PhoQ/PhoP system.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America.

ABSTRACT
Even a single mutation can cause a marked change in a protein's properties. When the mutant protein functions within a network, complex phenotypes may emerge that are not intrinsic properties of the protein itself. Network architectures that enable such dramatic changes in function from a few mutations remain relatively uncharacterized. We describe a remarkable example of this versatility in the well-studied PhoQ/PhoP bacterial signaling network, which has an architecture found in many two-component systems. We found that a single point mutation that abolishes the phosphatase activity of the sensor kinase PhoQ results in a striking change in phenotype. The mutant responds to stimulus in a bistable manner, as opposed to the wild-type, which has a graded response. Mutant cells in on and off states have different morphologies, and their state is inherited over many generations. Interestingly, external conditions that repress signaling in the wild-type drive the mutant to the on state. Mathematical modeling and experiments suggest that the bistability depends on positive autoregulation of the two key proteins in the circuit, PhoP and PhoQ. The qualitatively different characteristics of the mutant come at a substantial fitness cost. Relative to the off state, the on state has a lower fitness in stationary phase cultures in rich medium (LB). However, due to the high inheritance of the on state, a population of on cells can be epigenetically trapped in a low-fitness state. Our results demonstrate the remarkable versatility of the prototypical two-component signaling architecture and highlight the tradeoffs in the particular case of the PhoQ/PhoP system.

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A conceptual framework for priming.(A) Typical fates of OFF cells under different growth conditions. Yellow circles within cells represent PhoP-P molecules. High Mg2+ and slow growth rates lead to higher PhoP-P concentrations, which result in conversion of OFF cells to the ON state. (B) Stochastic switching and priming. Typical plot obtained by varying system parameters such as kinase rate, maximal expression rate or growth rate in a mathematical model of the phoQ (T281R) network (Text S1) is shown. Steady state PhoP-P values are depicted as a function of the system parameter being varied and the plot can be categorized into three distinct regimes as indicated. Stable OFF and ON state PhoP-P values are plotted in solid maroon. Dashed, blue line represents the unstable intermediate state in the bistable regime. Experimentally inaccessible monostable OFF steady states are shown with a dashed, cyan line. The empirically observed effect of changing magnesium concentrations and growth rate is indicated below the x-axis of the plot. Expected fates of pure OFF and pure ON populations in the three regimes are also illustrated (bottom). The bistable regime is characterized by phenotypic hysteresis and stochastic state switching, whereas priming would be seen in the monostable ON regime.
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pgen-1003706-g003: A conceptual framework for priming.(A) Typical fates of OFF cells under different growth conditions. Yellow circles within cells represent PhoP-P molecules. High Mg2+ and slow growth rates lead to higher PhoP-P concentrations, which result in conversion of OFF cells to the ON state. (B) Stochastic switching and priming. Typical plot obtained by varying system parameters such as kinase rate, maximal expression rate or growth rate in a mathematical model of the phoQ (T281R) network (Text S1) is shown. Steady state PhoP-P values are depicted as a function of the system parameter being varied and the plot can be categorized into three distinct regimes as indicated. Stable OFF and ON state PhoP-P values are plotted in solid maroon. Dashed, blue line represents the unstable intermediate state in the bistable regime. Experimentally inaccessible monostable OFF steady states are shown with a dashed, cyan line. The empirically observed effect of changing magnesium concentrations and growth rate is indicated below the x-axis of the plot. Expected fates of pure OFF and pure ON populations in the three regimes are also illustrated (bottom). The bistable regime is characterized by phenotypic hysteresis and stochastic state switching, whereas priming would be seen in the monostable ON regime.

Mentions: Taken together, the above results indicate that slow growth histories or high [Mg2+] are sufficient for en masse conversion of OFF cells to ON (Figure 3A). We call this deterministic conversion from OFF to ON state “priming”.


The architecture of a prototypical bacterial signaling circuit enables a single point mutation to confer novel network properties.

Ram S, Goulian M - PLoS Genet. (2013)

A conceptual framework for priming.(A) Typical fates of OFF cells under different growth conditions. Yellow circles within cells represent PhoP-P molecules. High Mg2+ and slow growth rates lead to higher PhoP-P concentrations, which result in conversion of OFF cells to the ON state. (B) Stochastic switching and priming. Typical plot obtained by varying system parameters such as kinase rate, maximal expression rate or growth rate in a mathematical model of the phoQ (T281R) network (Text S1) is shown. Steady state PhoP-P values are depicted as a function of the system parameter being varied and the plot can be categorized into three distinct regimes as indicated. Stable OFF and ON state PhoP-P values are plotted in solid maroon. Dashed, blue line represents the unstable intermediate state in the bistable regime. Experimentally inaccessible monostable OFF steady states are shown with a dashed, cyan line. The empirically observed effect of changing magnesium concentrations and growth rate is indicated below the x-axis of the plot. Expected fates of pure OFF and pure ON populations in the three regimes are also illustrated (bottom). The bistable regime is characterized by phenotypic hysteresis and stochastic state switching, whereas priming would be seen in the monostable ON regime.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3750022&req=5

pgen-1003706-g003: A conceptual framework for priming.(A) Typical fates of OFF cells under different growth conditions. Yellow circles within cells represent PhoP-P molecules. High Mg2+ and slow growth rates lead to higher PhoP-P concentrations, which result in conversion of OFF cells to the ON state. (B) Stochastic switching and priming. Typical plot obtained by varying system parameters such as kinase rate, maximal expression rate or growth rate in a mathematical model of the phoQ (T281R) network (Text S1) is shown. Steady state PhoP-P values are depicted as a function of the system parameter being varied and the plot can be categorized into three distinct regimes as indicated. Stable OFF and ON state PhoP-P values are plotted in solid maroon. Dashed, blue line represents the unstable intermediate state in the bistable regime. Experimentally inaccessible monostable OFF steady states are shown with a dashed, cyan line. The empirically observed effect of changing magnesium concentrations and growth rate is indicated below the x-axis of the plot. Expected fates of pure OFF and pure ON populations in the three regimes are also illustrated (bottom). The bistable regime is characterized by phenotypic hysteresis and stochastic state switching, whereas priming would be seen in the monostable ON regime.
Mentions: Taken together, the above results indicate that slow growth histories or high [Mg2+] are sufficient for en masse conversion of OFF cells to ON (Figure 3A). We call this deterministic conversion from OFF to ON state “priming”.

Bottom Line: We describe a remarkable example of this versatility in the well-studied PhoQ/PhoP bacterial signaling network, which has an architecture found in many two-component systems.We found that a single point mutation that abolishes the phosphatase activity of the sensor kinase PhoQ results in a striking change in phenotype.Our results demonstrate the remarkable versatility of the prototypical two-component signaling architecture and highlight the tradeoffs in the particular case of the PhoQ/PhoP system.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America.

ABSTRACT
Even a single mutation can cause a marked change in a protein's properties. When the mutant protein functions within a network, complex phenotypes may emerge that are not intrinsic properties of the protein itself. Network architectures that enable such dramatic changes in function from a few mutations remain relatively uncharacterized. We describe a remarkable example of this versatility in the well-studied PhoQ/PhoP bacterial signaling network, which has an architecture found in many two-component systems. We found that a single point mutation that abolishes the phosphatase activity of the sensor kinase PhoQ results in a striking change in phenotype. The mutant responds to stimulus in a bistable manner, as opposed to the wild-type, which has a graded response. Mutant cells in on and off states have different morphologies, and their state is inherited over many generations. Interestingly, external conditions that repress signaling in the wild-type drive the mutant to the on state. Mathematical modeling and experiments suggest that the bistability depends on positive autoregulation of the two key proteins in the circuit, PhoP and PhoQ. The qualitatively different characteristics of the mutant come at a substantial fitness cost. Relative to the off state, the on state has a lower fitness in stationary phase cultures in rich medium (LB). However, due to the high inheritance of the on state, a population of on cells can be epigenetically trapped in a low-fitness state. Our results demonstrate the remarkable versatility of the prototypical two-component signaling architecture and highlight the tradeoffs in the particular case of the PhoQ/PhoP system.

Show MeSH
Related in: MedlinePlus