# Chemotaxis cluster-size distribution and model.

(A and B) Histograms of cluster size, measured by the number of closely spaced Eos-labeled Tar (A) and CheW (B) proteins. Smaller clusters occur much more frequently than larger clusters. Sample images of clusters are shown with arrows that indicate cluster size. To evaluate the fit in a bin-independent representation, we plotted the cumulative distribution function (CDF) (insets). The fit of our self-assembly model to our data is shown in red. (C and D) Cells with one (C) or two (D) large polar clusters (*n*≥400 proteins) have the highest density of remaining smaller clusters (*n*<400) furthest from the existing cluster(s). (E and F) Cells with two large polar clusters (F) exhibit higher Tar-receptor density at mid-cell (arrow) in comparison to cells with one polar cluster (E). *n* = 31 cells for (C and E), and 38 cells for (D and F). (G) Model of receptor self-assembly in which cluster locations are maintained within a population of growing and dividing cells. Cluster nucleation is most likely to occur where receptor density is high, which occurs far from any existing cluster. Dotted arrows denote receptor diffusion within the membrane.