Systematic Analysis of the Robustness in Complex Reaction Networks of Bacteria

Barkai and Leibler [1] demonstrated a robust property of adaptation behavior in bacterial chemotaxis, indicating that the robust adaptation was a consequence of the network’s connectivity and the chemotaxis system did not require the fine-tuning of biochemical parameters. However, they did not answer the crucial question on whether it is possible to isolate such a subsystem from the whole system composed of heterogeneous and interactive networks and to analyze it separately. If the interaction among subsystems is sensitive, one cannot analyze smaller subsystems separately. If their interaction shows a robust property, one can extract the subsystems out of the whole system, analyzing them one by one. In Escherichia coli heat shock response [2], σ32 (encoded the rpoH gene) plays a major role in controlling expression of the heat shock protein genes encoding chaperones and proteases. The level of active σ32 is regulated by complex mechanisms: chaperone-mediated regulation of σ32 activity and stability, thermoregulated-translation induction of the rpoH mRNA, and protease-mediated σ32 degradation. The numerical framework model clarifies that complexity in σ32 regulation generates a robust property of E. coli heat shock response, thereby increasing the robustness of the interconnected factors among subsystems. Complexity seems to disturb isolating a smaller subsystem out of the whole biological system. Actually, complexity generates the robustness among subsystems, thereby making it possible to extract a smaller subsystem out of the whole system and analyze it separately.