Biology Reference
In-Depth Information
tandem riboswitches that could function as the Boolean logic gates AND and NAND in
response to two chemical inputs
theophylline and thiamine pyrophosphate (TPP).
The first synthetic bandpass filter in mammalian cells was reported by Weber et al.,
78
which
displayed specific modulation of target gene expression within a defined range of biotin
concentrations (input signal). This system involved BirA-mediated ligation of biotin to a
biotinylation signal-containing VP16 transactivation domain, which triggered
heterodimerization of chimeric VP16 to a streptavidin-linked tetracycline repressor (TetR).
With increasing biotin concentration up to 20 nM, there is gradual activation of tetR-
specific promoters until maximal induction is achieved at a biotin concentration range of
20 nM to 10
μ
μ
M, expression of the target gene is
shut off. Yet another mammalian bandpass filter responsive to a defined range of
tetracycline concentrations was reported by Greber and Fussenegger.
79
This consisted of a
high-detect and low-detect switch that repressed gene expression at high and low
tetracycline concentrations, respectively.
M. Above a biotin concentration of 10
Communication Modules
Synthetic gene circuits that enable communication between cells have also been
constructed, and are often referred to as communication modules. By utilizing elements of
a bacterial quorum-sensing system from the marine bacterium
Vibrio fischeri
, You and
colleagues
80
were able to program an
E. coli
population to maintain a cell density lower
than that imposed by environmental constraints; that is, a limited supply of oxygen and
nutrients as well as the accumulation of toxic metabolites. This was achieved by
engineering the expression of the LuxI/LuxR quorum-sensing system from
V. fischeri
into
E. coli
. The LuxI protein synthesizes acyl-homoserine lactone (AHL), which diffuses into
the surrounding milieu and neighboring cells. At above a certain threshold concentration,
AHL binds and activates the LuxR repressor protein, which in turn induces expression of a
'
164
suicide
gene.
'
Utilizing the same LuxI/LuxR quorum-sensing system from
V. fischeri
, Kobayashi et al.
81
were
able to program
E. coli
populations to activate synthesis of specific proteins when the cell
population exceeded a certain critical threshold density. Additionally, the same study also
engineered
E. coli
populations to form a biofilm in response to DNA-damaging chemicals by
utilizing elements of the bacterial SOS signaling response to DNA damage.
81
In yet another
study, Basu et al.
75
programmed bacterial cells to generate intricate two-dimensional patterns
through intercellular communication by splitting the LuxI/LuxR quorum-sensing system into
two distinct
sender
and
receiver
subpopulations of bacterial cells. By using quorum sensing
'
'
'
'
proteins in two distinct
E. coli
populations,
Balgadde et al.
82
were able to construct a synthetic ecosystem resembling a canonical
predator
to regulate expression of
'
killer
'
and
'
antidote
'
prey relationship. More recently, Danino et al.
83
used elements of the quorum-
sensing machineries of both
V. fischeri
and
Bacillus thuringiensis
to generate synchronized
oscillations of gene expression within bacterial populations.
Miscellaneous Synthetic Gene Circuits
Other miscellaneous synthetic gene circuits analogous to complex electronic devices have
also been constructed. This includes a diverse array of synthetic gene circuits that correspond
to various digital logic gates.
84
89
FUNCTIONAL PROTEINS AND RNA COMPONENTS OF SYNTHETIC GENE CIRCUITS
For synthetic gene circuits to be effective, they must encode for a variety of functional proteins
and RNA that exert control over diverse biological processes within cells. Proteins encoded by
synthetic gene circuits can be broadly classified into membrane components and intracellular
components.
