Protein exchange at the cluster core. (a-b) Recovery of YFP-CheAΔ258 (a) and CheW-YFP (b) in strain LL4 (CheR+ CheB+) where receptors are in the low modification state (filled circles) and in strain LL5 (ΔcheR ΔcheB) where receptors are in the intermediate modification state (white squares). (c) Recovery of unmodified TarEEEE-YFP (filled circles) and fully modified TarQQQQ-YFP (white squares) receptors in strain LL5. Curves represent means of 14 to 27 experiments, with error bars indicating standard errors. To reduce variability associated with the varying depth of bleaching, the value of the first post-bleach point was subtracted prior to normalization to the relative intensity before photobleaching (see Methods). Grey shading indicates the initial rapid recovery of the fusion protein that is not incorporated into the cluster and freely diffuses in the cytoplasm or in the plasma membrane (see text).