Fig. 4

Incremental tuning of signal output from the aspartate chemoreceptor of E. coli (Tar). a At the C-terminal end of Tar TM2, a Trp-Tyr (red) was moved about its original position at the cytoplasmic polar/hydrophobic interface [26]. b The intracellular level of CheY-P governs the probability of clockwise flagellar rotation (P_CW). Previous results demonstrate that increased intracellular CheY-P levels lead to an enhanced probability of CW rotation. In these experiments, a sharp transition was observed, i.e., a Hill coefficient of more than 10 [65]. c Rotation of a single flagellum from roughly 200 independent E. coli cells expressing one of the aromatically tuned variants were analyzed and classified into one of five categories (from left to right): rotating exclusively CCW, rotating primarily CCW with occasional reversals, rapidly switching between both rotational directions (CW/CCW), rotating primarily CW with occasional reversals and those rotating exclusively CW. As Tar signal output increases, the number of cells in each category shifts from CCW toward CW rotational bias. In summary, the lowest overall signal output was observed from cells expressing the WY-3 variant, while the greatest was observed from cells expressing the WY + 2 or WY + 3 variants. Therefore, in the case of Tar, the absolute vertical position of the aromatic residues correlates with signal output [26]