Figure 7. Rewiring SPI1 gene circuits demonstrates that HilE imposes threshold on activation.

(A) Comparison of time-course dynamics for P_hilA (pSS077) promoter activities in wild type (black), CR355 (ΔP_hilD::P_hilC ΔhilC, red) and CR356 (ΔP_hilD::P_hilC ΔhilC ΔhilE, blue) as determined using luciferase transcriptional reporters. In strain CR355, the P_hilD promoter was replaced with the P_hilC promoter in an otherwise ΔhilC background. In this strain, hilD is transcriptionally regulated in a manner identical to hilC. Strain CR356 is the same as CR355 except that it lacks HilE. (B) Dynamics of P_hilA promoter activity in CR356 as determined using green fluorescent protein (GFP) transcriptional fusions and flow cytometry. Note that the activation of the P_hilA promoter in CR356 is no longer switch-like but rather rheostatic in nature. Similar dynamics are seen with the P_hilC promoter (Figure S2C). Experiments were performed as described in Figure 1 .