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In the Single Input Module (SIM) one master regulator controls a whole group of genes. They often control genes that encode enzymes for biochemical pahtways and are capable of executing a temporal program of gene expression to produce proteins just in time when they are needed. The SIM is a network motif and occurs more often than expected at random.

The genes $Z_i$ recieve a single input from the master transcription factor $Y$ that is usually autoregulated. The order of activation of the $Z_i$s can be tuned by the respective activation threshold $K_{yi}$. In an enzyme pathway that produces a desired product it is convinient to first start producing the first enzyme of the pathway, then the second, thrid ans so on. Thus the according activation thresholds of the enzyme coding genes are tuned to satisfy $K_{yi-1} < K_{yi} < K_{yi+1}$. The activation threshold increases sucessively if one follows the pathway downstream.

Solving the differential equations of this motif and plotting the time course gives the following dynamics

The last gene activated is the first that shuts down production. We call such a circuit a LIFO (last in first out) SIM circuit.

Further Reading

• Uri Alon - An Introduction to Systems Biology: Design Principles of Biological Circuits (Link)