Figure 15.

Change in conc. of membrane phoQ In order to capture the effects of varying the rate of diffusion of Mg²⁺ on the system output, the simulation was ran with increasing Mg²⁺ diffusion rates (with means of 100ms, 1ms and 10ms). The results have been reported for two key system resources, the proteins phoQ, which is the sensory protein responsible for binding to Mg²⁺, and the phoP protein, which controls the dynamics of gene expression. The figure shows how the rate of decrease in the concentration of inactive phoQ (phoQ molecule bound to a Mg²⁺) is damped with increasing delay in the diffusion of Mg²⁺ out of the membrane. Also, captured in this graph is the effect of resource starvation on the biological system. As the Mg²⁺ ion initiated signal activates the PhoPQ pathway, the sensory phoQ proteins are consumed by the system, thereby shutting down the pathway when all phoQ molecules available to the system have been used.