The circuit-like connectivity of biological parts and their ability to collectively process logical operations was first appreciated nearly 50 years ago1
. This inspired attempts to describe biological regulation schemes with mathematical models2–5
and to apply circuit analogies from established frameworks in electrical engineering6, 7
. Meanwhile, breakthroughs in genomic research and genetic engineering (e.g., recombinant DNA technology) were supplying the inventory and methods necessary to physically construct and assemble biomolecular parts. As a result, synthetic biology was born with the broad goal of engineering or “wiring” biological circuitry—be it genetic, protein, viral, pathway, or genomic—for manifesting logical forms of cellular control. Synthetic biology, equipped with the engineering-driven approaches of modularization, rationalization, and modeling, has progressed rapidly and generated an ever-increasing suite of genetic devices and biological modules.
The successful design and construction of the first synthetic gene networks—the genetic toggle switch8
and the repressilator9
)—demonstrated that engineering-based methodology could indeed be applied to build sophisticated, computing-like behaviour into biological systems. In these two cases, basic transcriptional regulatory elements were designed and assembled to realize the biological equivalents of electronic memory storage and timekeeping (Box 1
). Within the framework provided by these two synthetic systems, biological circuits can be built from smaller well-defined parts according to model blueprints, they can be studied and tested in isolation, and their behaviour can be evaluated against model predictions of the system dynamics. This methodology has subsequently been applied in the synthetic construction of additional genetic switches8, 10–18
, memory elements8, 14, 15, 19
, and oscillators9, 10, 20–23
, as well as of other electronics-inspired genetic devices, including pulse generators24
, digital logic gates25–30
, and communication modules23, 31, 34, 35
Box 1. Early synthetic biology designs: switches and oscillators
Switches and oscillators that occur in electronic systems are also seen in biology and have been engineered into synthetic biological systems.
In electronics, one of the most basic elements for storing memory is the reset-set (RS) latch based on logical NOR gates. This device is bistable, in that it possesses two stable states that can be toggled with the delivery of specified inputs. Upon removal of the input, the circuit retains memory of its current state indefinitely. These forms of memory and state switching have significant implications in biology, such as in the differentiation of cells from an initially undifferentiated state. One means by which cellular systems can achieve bistability is through genetic mutual repression. The natural PR
genetic switch from bacteriophage lambda, which uses this network architecture to govern the lysis/lysogeny decision, consists of two promoters that are each repressed by the gene product of the other (that is, by the Cro and CI repressor proteins). The genetic toggle switch8
, constructed by our research group, is a synthetically engineered version of this co-repressed gene regulation scheme. In one version of the genetic toggle, the PL
promoter from lambda phage was used to drive transcription of lacI
, whose product represses a second promoter Ptrc-2 (a lac
promoter variant). Ptrc-2, on the other hand, drives expression of a temperature-sensitive λ CI repressor protein (cI-ts), which inhibits the PL
promoter. The activity of the circuit is monitored through the expression of GFP. The system can be toggled in one direction with the exogenous addition of the chemical inducer isopropyl-β-D-thiogalactopyranoside (IPTG) or in the other with a transient increase in temperature. Importantly, upon removal of these exogenous signals, the system retains its current state, creating a cellular form of memory.
Timing mechanisms, much like memory, are also fundamental to many electronic and biological systems. Electronic timekeeping can be achieved with basic oscillator circuits, such as the LC circuit (inductor L and capacitor C), which act as resonators for producing periodic electronic signals. Biological timekeeping, which is widespread among living organisms120
, is achieved with circadian clocks and similar oscillator circuits, such as the one responsible for synchronizing the critical processes of photosynthesis and nitrogen fixation in cyanobacteria. The circadian clock of cyanobacteria is based on, among other regulatory mechanisms, intertwined positive and negative feedback loops on the clock genes kaiA
, and kaiC
. Elowitz and Leibler constructed a synthetic genetic oscillator based, not on clock genes, but on standard transcriptional repressors (the repressilator)9
. Here, a cyclic negative feedback loop composed of three promoter–gene pairs, in which the ‘first’ promoter in the cascade drives expression of the ‘second’ promoter’s repressor and so on, was used to drive oscillatory output in gene expression.
Now, ten years after the demonstration of synthetic biology’s inaugural devices8, 9
, engineered biomolecular networks are beginning to move into the application stage and yield solutions to many complex societal problems. While much work remains on elucidating biological design principles36
, this foray into practical applications signals an exciting coming-of-age time for the field.
Here, we review the practical application of synthetic biology to biosensing, therapeutics, and the production of biofuels, pharmaceuticals and novel biomaterials. Many of the examples herein do not fit exclusively or neatly into only one of these three application categories; however, it is precisely this multivalent applicability that makes synthetic biology platforms so powerful and promising.