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may still be more efficient to synthesize artificial DNA-binding proteins for
use as signals rather than finding natural sources. In the future, combinatorial
chemistry techniques, along with a method such as phage display, will yield
large libraries of novel DNA-binding proteins and corresponding operators.
One potential source of a very large set of noninteracting signals is engineered
zinc finger DNA binding proteins [3].
Intercellular Gates
Although the biochemical inversion mechanism suffices for building intra-
cellular circuits, external interaction with the cells requires additional logic
gates. Small molecules known as inducers freely diffuse through cellular mem-
branes and interact with DNA-binding proteins. This section describes how the
inducer-protein interactions implement two different intercellular gates. Chap-
ter 7 reports on experimental results where the IMPLIES gate enables human-to-
cell communications, and the AND gate facilitates cell-to-cell communications.
The IMPLIES Gate
The IMPLIES gate allows cells to receive control messages sent by humans or to
detect certain environmental conditions. Figure 4.7 illustrates the biochemical
reactions, the logic symbol, and the logic truth table for an intercellular gate that
implements the IMPLIES logic function. In the absence of the input mRNA
and its corresponding repressor, RNAp, binds to the promoter and transcribes
the output gene, yielding a high output. As with the inverter, if only the input
repressor is present, it binds to the promoter and prevents transcription, yielding
a low output. Finally, if both the repressor and the inducer are present, the
Repressor
Inducer
Output
1
1
0
1
0
0
1
1
0
1
0
1
Repressor
Inducer
Output
Figure 4.7
A genetic gate for the IMPLIES logic function using repressors and in-
ducers. Shown here are the two states when the repressor protein is present, the logic
symbol for the gate, and the logic truth table.
