Adaptation Dynamics in Densely Clustered Chemoreceptors
Figure 2
Processive receptor methylation compromises adaptation and decreases signaling noise.
Compared are three simulated models of the chemotaxis adaptation system: M1 with assistance neighborhoods and efficient brachiation (black traces), M2 with no assistance neighborhoods or brachiation (light gray), and M3 with assistance neighborhoods but inefficient brachiation (dark gray). Methylation is more processive in M2 and M3 than in M1. As processivity increases, enzymes become more localized to receptors that are already highly methylated (CheR) or demethylated (CheB), limiting their effectiveness. (A) The kinetics of M1 were calibrated by comparison to population-level measurements (gray) [43]. The model was exposed to simulated time-varying exponential ramps of methyl-aspartate and the resulting steady-state activity a0 recorded (black). (B) Response to small (5 µM) and large (1 mM) MeAsp step stimulus at applied at t = 200 s as measured by receptor activity a(t). While all models adapt to the small stimulus (top), they fail to adapt precisely to the large stimulus (bottom). For the large stimulus, higher processivity leads to less precise adaptation with M1 performing best and M2 worst. Activities have been scaled and recentered with steady-state values at 0. (C) Increasing processivity also decreases the magnitude of fluctuations in a(t) in the adapted state around the mean value a0. Plotted is the variance σaa of a(t) and the noise relative to the mean output σa/a0 (inset) for different expression levels of the enzyme CheR. Fluctuations are largest in M1 and smallest in model M2.