Figure 5.

Sensitivity of chemotactic response to variations in binding protein and receptor abundances. (A) Steady-state distributions from experimental chemotaxis assays in opposing gradients created using 2 μM maltose and 10 μM MeAsp for cells obtained from different culture conditions. Cells were cultured in tryptone broth and harvested as in other experiments at mid-exponential phase (OD = 0.47), harvested at late-exponential phase (OD = 0.90) when the abundance of Tar is known to be higher, or cultured in tryptone broth supplemented with 500 μM maltose and harvested at OD = 0.47 and then placed in the microfluidic chamber with 2 μM maltose in the left channel and 10 μM MeAsp in the right channel. Curves show fits obtained using an analytical approximation of the transport-and-sensing model, with variants modifying, by the specified factor, the concentration of MalE, [BP], and the concentration of Tar, [R]. Note that for increases in [R] by 50%, we also increased n_Tar by 50% to a value of n_Tar = 6. These results demonstrate that the relative chemotactic abundance is sensitive to Tar abundance but not to binding-protein abundance. For the cells grown under the three different conditions, the corresponding CMCs are CMC = 0.11 (OD = 0.47), 0.22 (OD = 0.90), and 0.22 (500 μM maltose, OD = 0.47). (B and C) We use our fitted SPECS model to predict the peak chemotactic response, at 4 μM maltose, as a function of both binding-protein abundance (with receptor abundance held constant at 20 μM, B) and effective chemoreceptor abundance (with binding-protein abundance held constant at 1 mM, C). Here, we plot the corresponding CMC values; see Figs. S6 and S7 for some of the predicted distributions of cells. Our model suggests that the chemotactic response does not vary for sufficiently high binding-protein abundances but is very sensitive to variations in receptor abundance. The red dots indicate the responses at 4 μM maltose using our original estimate of the cells’ average binding-protein abundance and receptor abundance when grown in tryptone and harvested at OD = 0.47. To see this figure in color, go online.