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Some recent work has centered on constructing gene transcription modules
to create biochemical devices that can be combined to create logic circuits 51,
94, 114, 115]. The transcriptional unit consists of a promoter that is inducible
or controllable and a gene. The gene product can be a regulator or an enzyme.
The inputs are molecular signals that control gene expression, transcription, or
translation, whereas the output of the logic gate can be the enzyme itself or the
activity of the enzyme.
As a simple example, consider the implementation of a transcriptional OR
gate. There are at least two strategies for implementing the OR gate: (1) use two
promoters that have identical gene transcriptional effects but respond to two
different inducers; or (2) use a single promoter that responds in a similar manner
to two different inducers. The latter strategy has been implemented using a tod-
lux fusion in Pseudomonas putida TVA8 [94]. This operon is similarly induced
by both trichloroethylene (TCE) and toluene [91]. The result is the production of
bioluminescence if either inducer is present. As Figure 5.6 shows, this construct
implements the logical OR function.
The AND, OR, and XOR functions described above are functionally com-
plete as they can be combined to implement any combinatorial logic function.
However, these components cannot be used to implement sequential circuits
that require memory of past logic states and clock signals for synchronization.
Recently, Gardner et al. [42] demonstrated a two-state genetic latch in E. coli
Figure 5.6 A simple OR gate constructed using a tod-lux fusion. (a) The todR reg-
ulatory protein reacts similarly to either trichloroethane (TCE) or toluene, thereby
upregulating the expression of the lux CDABE genes and producing bioluminescence.
(b) A truth table for this simple genetic logic gate where the presence (1) or absence
(0) of bioluminescence represents the logic state of the output. (c) Logic symbol
representation of this genetic circuit.
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