Text S1 Differential affinity and catalytic activity of CheZ in E. coli chemotaxis

The experiments of Vaknin and Berg [1] allow us to verify whether the receptor cluster and the intracellular signaling pathway are indeed two independent modules connected in series. Vaknin and Berg performed experiments on four bacterial strains: wild type CheRCheBCheZ cells, CheR−CheB−CheZ cells lacking CheR and CheB, CheR+CheB+CheZ∗ cells with mutant CheZ proteins that cannot bind the receptor cluster, and CheR−CheB−CheZ∗ cells lacking CheR and CheB and with CheZ∗ (mutated CheZ). As discussed in the main text, we assume that βk0[L] is the same for wild type and CheZ mutant cells, but not for wild type and cheRcheB cells; in contrast, [YpZ](βk0) is the same for wild type and cheRcheB cells, but not for wild type and CheZ mutant cells. If the receptor cluster and the chemotaxis network are two independent modules connected in series, then it should be possible to describe the response curve [YpZ]([L]) of each of the four strains using two of the following response curves: [YpZ] wt(βk0) or [YpZ] ∗(βk0), and βk +B+ 0 ([L]) or βk R−B− 0 ([L]). In other words, all four response curves measured in [1] should be of the compos-