414 9.3  Synthetic Biology, Biomimicry, and Bionanotechnology

along a DNA molecule whose associated gene is being expressed. Transcriptors use combin­

ations of enzymes called “integrases” that control RNA movement as it is fabricated from the

DNA template.

Transistors amplify a relatively small current signal at the base input (or gate in the case

of field effect transistors [FETs]) into a much larger current between the emitter/​collector

(or equivalent voltage between the source/​drain in FETs). Similarly, transcriptors respond

to small changes in the integrase activity, and these can result in a very large change in the

flux of RNA polymerase resulting in significant increases in the level of expression of specific

genes. Multiple transcriptors can be cloned on different plasmids and controlled using

different chemical induction (see Chapter 7) and then combined in the same way as multiple

electronic transistors to generate all the standard logic gates, but inside a living cell. Thus,

this presents the opportunity for genuine biocomputation, that is, a biological computer

inside a functional cell. An appealing potential application is toward in situ diagnostics and

associated therapeutics of human diseases, that is, cells with a capacity to detect the presence

of diseases and regulate cellular level interventions as bespoke treatment with no external

intervention required (see the section on personalizing healthcare).

Worked Case Example 9.1: Biological Circuits

A biological circuit was found to consist of a simple feedback system where protein X

binds to another molecule of X to generate a dimer, concentration X2, at forward rate k

and backward rate for the reverse process k−2. This dimer binds to its own promoter at for­

ward rate k2 and backward rate for the reverse process k−2 and activates the promotor at a

rate k3, whereas the monomer subunits have no effect on gene expression. Protein X is also

degraded by an enzyme to form an enzyme product E·X at a forward rate k4 with back­

ward rate for the reverse process k−4. In addition, the promotor is “leaky” meaning that it

spontaneously results in expression of X at a rate k5, which depends on the concentration

of the promotor but not of X.

E·X decays irreversibly into a degraded product at a rate k6, and X also spontaneously

irreversibly degrades without the requirement of E at a rate k7, which depends on C.

a What do we mean by a reaction-​limited regime? Assuming a reaction-​limited regime

for this biological circuit, write down a rate equation for the concentration C after a

time t, assuming a promotor concentration P and enzyme concentration E.

FIGURE 9.4  Synthetic biology combined with fabricated solid-​state micro and nanoscale

structures. Cutaway schematic of a hybrid bionanotechnology/​non-​bionanotechnology device.

Here, a 20 μm diameter silicon dioxide rotor is pushed around by the swimming action of the

bacterium Mycoplasma mobile, whose surface has been tagged with biotin while that of the

rotor has been labeled with streptavidin. The bacterial motion is energized by glucose, and this

cell motion is coupled to rotation of the rotor via chemical bonds formed between the biotin

and streptavidin.

KEY BIOLOGICAL

APPLICATIONS:

SYNTHETIC BIOLOGY

AND

BIONANOTECHNOLOGY

TOOLS

Biosensing; Smart cell–​based

diagnostics; Biomimetization.