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The role of coupled positive feedback in the expression of the SPI1 type three secretion system in Salmonella.

Saini S, Ellermeier JR, Slauch JM, Rao CV - PLoS Pathog. (2010)

Bottom Line: While the core architecture of the SPI1 gene circuit has been determined, the relative roles of these interacting regulators and associated feedback loops are still unknown.This enabled us to directly test our predictions regarding the function of the circuit by varying the strength and dynamics of the activating signal.Collectively, our experimental and computational results enable us to deconstruct this complex circuit and determine the role of its individual components in regulating SPI1 gene expression dynamics.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA.

ABSTRACT
Salmonella enterica serovar Typhimurium is a common food-borne pathogen that induces inflammatory diarrhea and invades intestinal epithelial cells using a type three secretion system (T3SS) encoded within Salmonella pathogenicity island 1 (SPI1). The genes encoding the SPI1 T3SS are tightly regulated by a network of interacting transcriptional regulators involving three coupled positive feedback loops. While the core architecture of the SPI1 gene circuit has been determined, the relative roles of these interacting regulators and associated feedback loops are still unknown. To determine the function of this circuit, we measured gene expression dynamics at both population and single-cell resolution in a number of SPI1 regulatory mutants. Using these data, we constructed a mathematical model of the SPI1 gene circuit. Analysis of the model predicted that the circuit serves two functions. The first is to place a threshold on SPI1 activation, ensuring that the genes encoding the T3SS are expressed only in response to the appropriate combination of environmental and cellular cues. The second is to amplify SPI1 gene expression. To experimentally test these predictions, we rewired the SPI1 genetic circuit by changing its regulatory architecture. This enabled us to directly test our predictions regarding the function of the circuit by varying the strength and dynamics of the activating signal. Collectively, our experimental and computational results enable us to deconstruct this complex circuit and determine the role of its individual components in regulating SPI1 gene expression dynamics.

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Parametric analysis of model predicts that SPI1 gene circuit functions as an amplifier and encodes an activation threshold.(A) Effect of positive feedback on HilD expression. Plot shows steady-state concentration of HilD as a function of the parameters  and . The parameter  specifies the degree by which the SPI1 regulators - HilC, HilD, and RtsA - can activate HilD expression, effectively the strength of positive feedback on HilD expression. The parameter  specifies the strength of the signal activating HilD expression. (B) Effect of HilC and RtsA on HilD expression. Plot shows the steady-state concentration of HilD as a function of the parameters , , and . The parameters  and  specify the degree by which the SPI1 regulators - HilC, HilD, and RtsA - can activate HilC and RtsA expression, respectively. In other words, these parameters set the strength of feedback on HilC and RtsA expression. In these simulations, the parameters  and  were both varied in tandem: the numerical values for the two are the same. (C) Effect of HilE on HilD expression. Plot shows the steady-state concentration of HilD as a function of the parameters  and . The parameter  specifies the rate of HilE expression. Results for HilA are shown in Figures S6A–C. The black lines in the plots are used to denote the results obtained using the nominal parameters (aside from ). A detailed description of the model is provided in the Materials and Methods.
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ppat-1001025-g006: Parametric analysis of model predicts that SPI1 gene circuit functions as an amplifier and encodes an activation threshold.(A) Effect of positive feedback on HilD expression. Plot shows steady-state concentration of HilD as a function of the parameters and . The parameter specifies the degree by which the SPI1 regulators - HilC, HilD, and RtsA - can activate HilD expression, effectively the strength of positive feedback on HilD expression. The parameter specifies the strength of the signal activating HilD expression. (B) Effect of HilC and RtsA on HilD expression. Plot shows the steady-state concentration of HilD as a function of the parameters , , and . The parameters and specify the degree by which the SPI1 regulators - HilC, HilD, and RtsA - can activate HilC and RtsA expression, respectively. In other words, these parameters set the strength of feedback on HilC and RtsA expression. In these simulations, the parameters and were both varied in tandem: the numerical values for the two are the same. (C) Effect of HilE on HilD expression. Plot shows the steady-state concentration of HilD as a function of the parameters and . The parameter specifies the rate of HilE expression. Results for HilA are shown in Figures S6A–C. The black lines in the plots are used to denote the results obtained using the nominal parameters (aside from ). A detailed description of the model is provided in the Materials and Methods.

Mentions: We first considered the role of positive feedback on HilD expression, given the central role of this SPI1 regulator. To perform this analysis, we varied the degree by which the SPI1 regulators - HilC, HilD, and RtsA - could activate HilD expression by simulating the model at different values for the parameter . When interpreting these results, we found it informative to also vary the strength of the activating signal in our simulations, given by the parameter in the model. As shown in Figure 6A, HilD expression increases as the value of the parameter increases, equivalent to increasing the strength of the feedback on HilD expression. When this feedback is sufficiently strong, the response to the activating signal becomes discontinuous and switch-like. These results suggest that, in addition to amplifying the response, positive feedback may serve, along with HilE as described below, to endow the SPI1 circuit with an activation threshold. This threshold would ensure that SPI1 gene expression occurs only when a sufficiently strong activating signal is present. Moreover, the threshold decreases as the strength of the feedback increases, indicating that there is a tradeoff between the degree of amplification and the size of the threshold.


The role of coupled positive feedback in the expression of the SPI1 type three secretion system in Salmonella.

Saini S, Ellermeier JR, Slauch JM, Rao CV - PLoS Pathog. (2010)

Parametric analysis of model predicts that SPI1 gene circuit functions as an amplifier and encodes an activation threshold.(A) Effect of positive feedback on HilD expression. Plot shows steady-state concentration of HilD as a function of the parameters  and . The parameter  specifies the degree by which the SPI1 regulators - HilC, HilD, and RtsA - can activate HilD expression, effectively the strength of positive feedback on HilD expression. The parameter  specifies the strength of the signal activating HilD expression. (B) Effect of HilC and RtsA on HilD expression. Plot shows the steady-state concentration of HilD as a function of the parameters , , and . The parameters  and  specify the degree by which the SPI1 regulators - HilC, HilD, and RtsA - can activate HilC and RtsA expression, respectively. In other words, these parameters set the strength of feedback on HilC and RtsA expression. In these simulations, the parameters  and  were both varied in tandem: the numerical values for the two are the same. (C) Effect of HilE on HilD expression. Plot shows the steady-state concentration of HilD as a function of the parameters  and . The parameter  specifies the rate of HilE expression. Results for HilA are shown in Figures S6A–C. The black lines in the plots are used to denote the results obtained using the nominal parameters (aside from ). A detailed description of the model is provided in the Materials and Methods.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2912647&req=5

ppat-1001025-g006: Parametric analysis of model predicts that SPI1 gene circuit functions as an amplifier and encodes an activation threshold.(A) Effect of positive feedback on HilD expression. Plot shows steady-state concentration of HilD as a function of the parameters and . The parameter specifies the degree by which the SPI1 regulators - HilC, HilD, and RtsA - can activate HilD expression, effectively the strength of positive feedback on HilD expression. The parameter specifies the strength of the signal activating HilD expression. (B) Effect of HilC and RtsA on HilD expression. Plot shows the steady-state concentration of HilD as a function of the parameters , , and . The parameters and specify the degree by which the SPI1 regulators - HilC, HilD, and RtsA - can activate HilC and RtsA expression, respectively. In other words, these parameters set the strength of feedback on HilC and RtsA expression. In these simulations, the parameters and were both varied in tandem: the numerical values for the two are the same. (C) Effect of HilE on HilD expression. Plot shows the steady-state concentration of HilD as a function of the parameters and . The parameter specifies the rate of HilE expression. Results for HilA are shown in Figures S6A–C. The black lines in the plots are used to denote the results obtained using the nominal parameters (aside from ). A detailed description of the model is provided in the Materials and Methods.
Mentions: We first considered the role of positive feedback on HilD expression, given the central role of this SPI1 regulator. To perform this analysis, we varied the degree by which the SPI1 regulators - HilC, HilD, and RtsA - could activate HilD expression by simulating the model at different values for the parameter . When interpreting these results, we found it informative to also vary the strength of the activating signal in our simulations, given by the parameter in the model. As shown in Figure 6A, HilD expression increases as the value of the parameter increases, equivalent to increasing the strength of the feedback on HilD expression. When this feedback is sufficiently strong, the response to the activating signal becomes discontinuous and switch-like. These results suggest that, in addition to amplifying the response, positive feedback may serve, along with HilE as described below, to endow the SPI1 circuit with an activation threshold. This threshold would ensure that SPI1 gene expression occurs only when a sufficiently strong activating signal is present. Moreover, the threshold decreases as the strength of the feedback increases, indicating that there is a tradeoff between the degree of amplification and the size of the threshold.

Bottom Line: While the core architecture of the SPI1 gene circuit has been determined, the relative roles of these interacting regulators and associated feedback loops are still unknown.This enabled us to directly test our predictions regarding the function of the circuit by varying the strength and dynamics of the activating signal.Collectively, our experimental and computational results enable us to deconstruct this complex circuit and determine the role of its individual components in regulating SPI1 gene expression dynamics.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA.

ABSTRACT
Salmonella enterica serovar Typhimurium is a common food-borne pathogen that induces inflammatory diarrhea and invades intestinal epithelial cells using a type three secretion system (T3SS) encoded within Salmonella pathogenicity island 1 (SPI1). The genes encoding the SPI1 T3SS are tightly regulated by a network of interacting transcriptional regulators involving three coupled positive feedback loops. While the core architecture of the SPI1 gene circuit has been determined, the relative roles of these interacting regulators and associated feedback loops are still unknown. To determine the function of this circuit, we measured gene expression dynamics at both population and single-cell resolution in a number of SPI1 regulatory mutants. Using these data, we constructed a mathematical model of the SPI1 gene circuit. Analysis of the model predicted that the circuit serves two functions. The first is to place a threshold on SPI1 activation, ensuring that the genes encoding the T3SS are expressed only in response to the appropriate combination of environmental and cellular cues. The second is to amplify SPI1 gene expression. To experimentally test these predictions, we rewired the SPI1 genetic circuit by changing its regulatory architecture. This enabled us to directly test our predictions regarding the function of the circuit by varying the strength and dynamics of the activating signal. Collectively, our experimental and computational results enable us to deconstruct this complex circuit and determine the role of its individual components in regulating SPI1 gene expression dynamics.

Show MeSH
Related in: MedlinePlus