Figure 1. Fold change detection (FCD) in bacterial chemotaxis in the single input situation and its experimental test by Lazova et al.

(A) The chemoreceptor Tar binds the ligand MeAsp (L₁). Its associated kinase activity is modified by methylation. The output is CheYp which lead to bacterial tumbles. (B) Analysis of Shoval et al of the Tu et al model of chemotaxis predicted that the system shows fold change detection. For example, two steps of the same fold of L₁ should yield the same output dynamics, including the same amplitude and response time, over a wide range of background L₁ concentrations. (C) Lazova et al tested this prediction by means of E.coli strains in which the output (CheYp level) is read out optically – by means of FRET - in a flow cell in which L₁ levels can be changed over time. Over a 2-order of magnitude range of concentrations, output was invariant to multiplying L₁(t) by a constant factor. Data from Lazova et al [12].