Coherent Type I Feed-Forward Loop

A playground for understanding C1-FFL circuits

A coherent type-1 feed-forward loop (C1-FFL) is a common regulatory circuit where an input signal activates both a target gene and an intermediate regulator, which also activates the target gene. In the interactive below, the input signal is Sx, the intermediate regulator is Y*, and the target gene is Z.

The C1-FFL is the most commonly found type of feed-forward loop in biological circuits (there are 8 possible combinations for 3 nodes). This two paths to activation — a direct path and a delayed path through the intermediate — allow it to function as a biological AND gate, which can generate interesting behaviors like sign-sensitive delay and persistence detection.

Try it out below and notice it yourself! Drag signal Sx near protein X to activate it; click Run to begin the simulation. Remove/add the signal how many ever times to see how the circuit behaves. There's more info on sign-sensitive delay and persistence detection below.

Sign-sensitive delay here refers to the fact that target gene (Z) accumulation (positive sign) has a delay, but its depletion (negative sign) does not; i.e. it begins depleting immediately after either of the two regulators is removed from the circuit. The delays are shown by yellow boxes in the Z concentration chart. Persistence detection refers to the fact that target gene only begins accumulating after Y* crosses a threshold, making it filter out noise in the circuit, or, in other words, it only responds to a sustained, persistent signal.

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