Figure 4.

A simulation of the relationship between CheY-P propagation and CheZ intracellular localization. In this simulation, a rectangle with 2-μm long and 1-μm wide was assumed the cell; 25,000 CheY-P molecules were generated at the left side edge of the rectangle (shown as receptors in the illustrations in the top panels) and diffused in two dimensions with a diffusion coefficient of 5 μm²/s. The motors were positioned 0.7 (motor 1) and 1.2 μm (motor 2) from the receptor patch, and the number of CheY-P molecules around the position of each motor was counted. Graphs shown in the middle panels indicate the time courses for the number of CheY-P molecules that was counted near motor 1 (red lines) and motor 2 (blue lines). The horizontal broken lines in the graphs indicate the arbitrary threshold of CheY-P molecules set to trigger the rotational direction switching of the motors (260 counts was assumed as the threshold). The bottom panels show the time course of the rotational direction of motors inferred from the corresponding changes in CheY-P counts shown in the middle panels. (a) Simulations in the absence of CheZ (averaged trace of 3 independent calculations). (b) Simulations in which CheZ was assumed to be uniformly distributed in the rectangle (averaged trace of 3 independent calculations). The probability of dephosphorylation was set to 0.0004 in the cytoplasm (2 μm in length and 1 μm in width). (c) Simulations in which CheZ was assumed to be localized within 20 nm of one rectangle edge (averaged trace of 5 independent calculations). In this 20-nm area, the probability of dephosphorylation was set to 0.04. The probability of dephosphorylation in the case of (c) was set 100 times higher than for (b), because we assumed that the same number of CheZ molecules was distributed uniformly in the 2-μm long of the rectangle in (b) and within 20 nm of the rectangle edge in (c).