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Biological Signal Processing with a Genetic Toggle Switch
Figure 1
Transfer of signal processing concepts from digital electronics to molecular biology.
(A) Schematic of a general sequential logic circuit, consisting of a combinatorial-front end and a memory unit. The front-end combines external inputs as well as the current memory state to determine the next state of the memory unit. Here, the state of the memory unit directly serves as the output of the circuit. (B) Wiring diagram of the JK-latch known from digital electronics, where the memory unit is typically implemented by a Set-Reset latch (composed of two cross-coupled NOR gates; dark grey box) and the combinatorial front-end consists of two AND gates, each of which feeds to one input of the SR-latch and integrates both the primary inputs 
and 
of the circuit, as well as the current state 
and its complement 
of the SR-latch (light grey box). (C) The operational truth table defines a mapping between the input signals 
and 
and the operation to be performed on the internal state. Each operation corresponds to a ‘state transition’ from the current state 
of the circuit to the follow-up state 
. (D) Possible realization of a genetic JK-latch, in which the AND gates are implemented by heterodimerization between the input proteins 
and 
and the proteins of the memory element 
and 
. The memory element (SR-latch) is translated into a genetic toggle switch, which can be set to ON (
high, 
low) and reset to OFF (
low, 
high) by additional repressor sites, binding the heterodimers 
and 
, respectively.
doi: https://doi.org/10.1371/journal.pone.0068345.g001