ia
Available online 15th September 2006
gram bacteria [1]. Sensors have been developed that
respond to small molecules, light and temperature
vided. This is an empirical measurement that describes
how the output changes as a function of the input [4,13][2,3



]. Genetic circuits are available that function as
inverters, logic gates, pulse generators, band pass filters
and oscillators [4,5,6


,7]. Sender and receiver components
enable cells to communicate [8



]. Based on these genetic
parts, strains of bacteria have been developed that can
communicate to form two-dimensional patterns [8



],
(Box 1). The focus of this review is on bacteria, although
there has been much recent work in eukaryotes [14].
Sensors and inputs
Cell-based sensors can be used to identify a microenvir-
onment, to direct communication between cells or to
Current Opinion in Biotechnology 2006, 17:548–557 www.sciencedirect.com0958-1669/$ – see front matter
# 2006 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.copbio.2006.09.001
Introduction
The genome contains commands dictating how cells eat,
reproduce, communicate, move and interact with their
environment. Cells can be programmed by introducing
synthetic DNA containing new commands that instruct
the cell to perform a set of artificial tasks in series or in
parallel. These programs consist of multiple genes and
regulatory elements that function as a system composed
of sensors, circuits and converters to control biological
responses.
A rudimentary language is emerging to genetically pro-Genetic parts to program bacter
Christopher A Voigt
Genetic engineering is entering a new era, where
microorganisms can be programmed using synthetic
constructs of DNA encoding logic and operational commands.
A toolbox of modular genetic parts is being developed,
comprised of cell-based environmental sensors and genetic
circuits. Systems have already been designed to be
interconnected with each other and interfaced with the control
of cellular processes. Engineering theory will provide a
predictive framework to design operational multicomponent
systems. On the basis of these developments, increasingly
complex cellular machines are being constructed to build
specialty chemicals, weave biomaterials, and to deliver
therapeutics.
Addresses
Biophysics and Chemistry & Chemical Biology, Department of
Pharmaceutical Chemistry, University of California San Francisco,
QB3 Box 2540, 1700 4th Street, San Francisco, CA 94158, USA
Corresponding author: Voigt, Christopher A (cavoigt@picasso.ucsf.edu)
Current Opinion in Biotechnology 2006, 17:548–557
This review comes from a themed issue on
Tissue and cell engineering
Edited by James L Sherleycontrol their population density [9



], synthesize antima-
larial and cancer-fighting drugs [10,11], and attack malig-
nant cells in response to environmental cues present in a
tumor [12



](Figure 1).
The analogy with electronic parts is useful in constructing
genetic circuits that perform signal processing tasks. How-
ever, the analogy is less applicable for the design of systems
composed of many parts. Genetic parts have problems with
interference — where one part inadvertently affects
another part — because their functions are carried out
by molecular interactions and reactions that occur in the
same confined space of the cell. This imposes the restric-
tion that a particular genetic circuit can only be used once
in a design. Thus, the language to program cells is going to
require redundancy, or breadth, in the available parts.
A second problem is that cells are alive. They eat, grow,
avoid stress and evolve. Bacteria undergo remarkable
changes in cell state as a function of their growth stage.
The cell volume, metabolism, membrane composition,
and global regulators change in response to the growth
media and cell state. All of these factors can impact the
function of synthetic sensors and circuits. Some are more
fragile than others and recent designs have attempted to
build genetic parts whose function is as detached from the
cell state as possible. Also, evolution can effectively
‘break’ a synthetic part by introducing mutations over
many generations.
This review has been written to introduce readers to the
most robust genetic parts that have been reused in differ-
ent designs. They have been loosely divided into three
categories (Table 1 and Supplementary material). Sensors
encompass all means by which information is received by
the cell. Genetic circuits represent how information is
processed and decisions made. Actuators describe how
the circuits and sensors can be used to control processes
in the cell. The sequences and performance characteristics
for many of these parts are available at the Massachusetts
Institute of Technology (MIT) Registry of Standard Bio-
logical Parts (http://parts.mit.edu). When given, the part
number refers to the Registry numbering system. When
available, the transfer function of a genetic part is pro-

Genetic parts to program bacteria Voigt 549
Figure 1
Using genetic parts to program bacteria. Programmed bacteria can (a) autonomously form spatial patterns [8



], (b) record images of light [3



],
(c) form a biofilm in response to UV light [35



], and (d) commit suicide (left-hand panel) or kill tumor cells (right-hand panel) after reaching
a critical population density [9



,12



]. Each design involves the linkage of cellular sensors to the control of biological processes, mediated
by genetic circuits. In (a), spatial patterns are formed by using a quorum sensing system to program the communication between bacteria.
The enzyme LuxI produces a small molecule (green dots) that diffuses through the cell membrane. Once the molecule accumulates to a
sufficient concentration in the media, it binds to a regulatory protein (LuxR). This regulatory protein is then connected to a pulse generator,
which controls the expression of green fluorescent protein. Thus, cells only turn green at an intermediate concentration of the signal. This forms
rings of gene expression (green, red) around the source of the signal (blue dot). In (b), bacterial photography was achieved using a light-sensing
sensor from a cyanobacterial two-component system. The protein domain that responds to light (light blue) was fused to a signal transduction
domain from E. coli (dark blue). In addition, the metabolic enzymes (green) that produce the required chromophore (pentagons) were included.
The output of the light sensor was connected to the expression of an enzyme that turns the media black. In (c), the toggle switch (yellow, magenta)
was used to control the expression of a protein that causes the bacteria to form a biofilm. One of the repressors in the toggle switch is sensitive
to UV. Thus, in the presence of UV light, the bacteria will form a biofilm. In (d), two similar quorum sensing systems are used to control
different responses as outputs. On the left, a gene is controlled that causes the cell to commit suicide (ccdB). Once the cell density reaches a
critical threshold, the cells begin to die. On the right, a gene is controlled that causes E. coli to invade malignant cells (invasin). This gene is only
turned on when there is a high concentration of bacteria. (Note that the same parts are reused in different designs and appear in Table 1 or
Supplementary material.)
www.sciencedirect.com Current Opinion in Biotechnology 2006, 17:548–557