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lacI
Operator
lacZ
lacY
lacA
Promoter
Repressor
Figure 1.4 Repression of the lac operon by the lacI gene product.
ENCODING NETWORKS
So far, we have used computation only as a metaphor for intracellular pro-
cesses. Since Adleman's original experiment, several other attempts to imple-
ment computations using DNA have been reported [5, 13, 20]. However, these
experiments, although different in many ways, are characterized by the fact
that they are all implemented in vitro : information is encoded as strands of
DNA, and these are then manipulated in solution to perform a computation. It
may be argued that this approach is suboptimal in that it fails to utilize the true
potential of the DNA molecule in its natural environment (i.e., in the cell). The
advantages of working in vivo as opposed to in vitro are numerous; rather than
using DNA as a passive information carrier, we may take advantage of the fact
that it can also be meaningful in a biological context. By reprogramming part
of the cellular machinery to our advantage, the DNA “program” can affect its
own execution.
In 1999, Weiss et al. [21] described a technique for mapping digital logic
circuits onto genetic regulatory networks such that the resulting chemical ac-
tivity within the cell corresponds to the computations specified by the digital
Cap binding site
Promoter
lacI
lacZ
lacY
lacA
RNA polymerase
CAP-cAMP
complex
Repressor
Inducer
Figure 1.5 Positive control of the lac operon.
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