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Figure 5.3
Transistor action of (a) a semiconductor device and (b) an enzymatic
device. The semiconductor transistor controls the flow of current between two ter-
minals using a voltage on the third terminal, while the enzyme transistor controls the
conversion of a substrate into a product using the concentration of effectors required
for the conversion.
levels of complexity where 10 levels of lithographically defined interconnects
are called for by the year 2014 [54].
A scheme similar to the enzyme networks described above could be envi-
sioned for cellular systems. Figure 5.4 shows the biochemical reactions re-
sponsible for light production in the prokaryotic
lux
system. This reaction
implements an enzyme transistor with myristyl aldehyde as the substrate, re-
duced flavin mononucleotide (FMNH
2
) and O
2
as co-effectors, and light, flavin
mononucleotide (FMN), and myristic acid as products. The entire reaction can
be thought of as several interconnected enzyme transistors at work. Addition-
ally, because the production of the enzymes that catalyze these reactions is
controlled by expression of the
lux
genes, genetic regulatory functions involv-
ing inducers, regulatory proteins, and promoters are part of the circuit.
The transistor-like devices described above are useful for realizing biosen-
sors [94] and may provide some of the conceptual framework for whole-cell
circuits with more utility. Engineered Boolean logic functions realized within
the genetic circuits of cells are more extensively treated elsewhere in this vol-
ume. We briefly review some of these efforts here.
Logic gates are devices that compute Boolean algebraic functions. The inputs
and outputs are logic levels (i.e., true/false or one/zero), and the outputs are
derived from the inputs through the application of a set of simple rules. For
example, consider the AND, OR, and XOR gates shown in Figure 5.5. The
output of an AND gate is true only if both of its inputs are true. Likewise, the
output of an OR gate is true if either of its inputs are true, whereas an XOR
gate has a true output if one, but not both, of its inputs are true. Although each
of these gates provide very modest computational power, as demonstrated by
silicon integrated circuit technology, the interconnection of these devices can
lead to exceptional functionality.
