Predicting Temporal Fluctuations in an Intracellular Signalling Pathway

We used a newly developed stochastic-based program to predict the fluctuations in numbers of molecules in a chemotactic signalling pathway of coliform bacteria. Specifically, we examined temporal changes in molecules of CheYp, a cytoplasmic protein known to influence the direction of rotation of the flagellar motor. Signalling molecules in the vicinity of a flagellar motor were represented as individual software objects interacting according to probabilities derived from experimentally-observed concentrations and rate constants. The simulated CheYp molecules were found to undergo random fluctuations in number about an average corresponding to the deterministically calculated concentration. Both the relative amplitude of the fluctuations, as a proportion of the total number of molecules, and their average duration, increased as the simulated volume was reduced. In a simulation corresponding to 10% of the volume of a bacterium, the average duration of fluctuations was found to be 80.7 ms, which is much shorter than the observed alternations between clockwise and counter-clockwise rotations of tethered bacteria (typically 2.6 s). Our results are therefore not in agreement with a simple threshold-crossing model for motor switching. However, it is possible to filter the CheYp fluctuations to produce temporal distributions closer to the observed swimming behaviour and we discuss the possible implications for the control of motor rotation.