Batchelor Eric, Goulian Mark
Department of Physics and Astronomy, Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA.
Proc Natl Acad Sci U S A. 2003 Jan 21;100(2):691-6. doi: 10.1073/pnas.0234782100. Epub 2003 Jan 9.
The EnvZ/OmpR system in Escherichia coli, which regulates the expression of the porins OmpF and OmpC, is one of the simplest and best-characterized examples of two-component signaling. Like many other histidine kinases, EnvZ is bifunctional; it phosphorylates and dephosphorylates the response regulator OmpR. We have analyzed a mathematical model of the EnvZ-mediated cycle of OmpR phosphorylation and dephosphorylation. The model predicts that when EnvZ is much less abundant than OmpR, as is the case in E. coli, the steady-state level of phosphorylated OmpR (OmpR-P) is insensitive to variations in the concentration of EnvZ. The model also predicts that the level of OmpR-P is insensitive to variations in the concentration of OmpR when the OmpR concentration is sufficiently high. To test these predictions, we have perturbed the porin regulatory circuit in E. coli by varying the expression levels of EnvZ and OmpR. We have constructed two-color fluorescent reporter strains in which ompF and ompC transcription can be easily measured in the same culture. Using these strains we have shown that, consistent with the predictions of our model, the transcription of ompC and ompF is indeed robust or insensitive to a wide range of expression levels of both EnvZ and OmpR.
大肠杆菌中的EnvZ/OmpR系统可调节孔蛋白OmpF和OmpC的表达，是双组分信号传导中最简单且特征最明确的例子之一。与许多其他组氨酸激酶一样，EnvZ具有双重功能；它使反应调节因子OmpR磷酸化和去磷酸化。我们分析了EnvZ介导的OmpR磷酸化和去磷酸化循环的数学模型。该模型预测，当EnvZ的丰度远低于OmpR时（如在大肠杆菌中那样），磷酸化OmpR（OmpR-P）的稳态水平对EnvZ浓度的变化不敏感。该模型还预测，当OmpR浓度足够高时，OmpR-P的水平对OmpR浓度的变化不敏感。为了验证这些预测，我们通过改变EnvZ和OmpR的表达水平来扰动大肠杆菌中的孔蛋白调节回路。我们构建了双色荧光报告菌株，在同一培养物中可以轻松测量ompF和ompC的转录。使用这些菌株我们已经表明，与我们模型的预测一致，ompC和ompF的转录确实稳健，即对EnvZ和OmpR的广泛表达水平不敏感。
