Biology Reference
In-Depth Information
molecules, including quorum sensing (QS) signals or essential metabolites, or using
secreted enzymes. In a natural setting, QS is used by bacteria to determine the density
of their population. 15 Each cell in the population synthesizes and secretes a small
diffusible molecule (i.e. the QS signal), the concentration of which increases with cell
density. At a sufficiently high concentration, the QS signal can activate expression of
target genes that allow the population to respond to increasing cell density. Synthetic
biology takes advantage of this property inherent to QS; activation of a gene circuit
under the regulation of QS-activated promoter only occurs at a sufficiently high bacterial
density ( Fig. 13.1a ).
Communication can also be realized by using diffusible metabolites. 13,16 Here, each cell
population cannot synthesize a required metabolite. These auxotrophic strains can be
engineered by replacing a gene required for the synthesis of the metabolite with an
antibiotic-resistant marker (e.g. 17 ). An auxotroph cannot grow in a minimal medium
lacking the corresponding essential metabolite. As such, these strains require additional
supplementation of the metabolite to survive, which may be supplied by a cocultured
population of engineered bacteria. This dependence on a partner provides a means to
enable obligate cooperation, without which a consortium is not viable.
Finally, communication can also be realized using secreted enzymes. In contrast to small
molecules, enzymes often require additional circuitry for them to be secreted and shared
(A)
Quorum sensing regulated communication
QS signal
244
Red
Red
(B)
Exchange of essential metabolites
Δ metA
Δ lysA
Requires
Requires
Secretion of enzyme components
(C)
C
C
EngB
MiniCbpA
C
FIGURE 13.1
Communication between populations in a synthetic consortium. (A) Quorum sensing (QS) systems. One population (green)
secretes a density-dependent QS signal (yellow shapes). A second population contains QS responsive promoter that drives
expression of gene that turns the cells red. When the green population is at low density (left panel) insufficient QS signal
is produced, and thus the ' red ' gene in the second population is not expressed. In contrast, when a high density of the
green population is present, sufficient concentration of QS signal is synthesized, driving expression of the red gene
(right panel). (B) Exchange of essential metabolites. Two auxotrophic populations, 22 one that cannot synthesize methionine
(purple triangles, green cells), and one that cannot synthesize lysine (blue diamonds, red cells) cannot grow (green arrow)
in monoculture in minimal medium. However, when cocultured as a consortium, both populations exchange these essential
metabolites and thus grow. (C) Exchange of enzymatic components. Two populations are engineered to synthesize and
secrete components of an enzymatic pathway. 23 The green population synthesizes and secretes EngB (blue shapes) while
the red population synthesizes and secretes MiniCbpA (orange shapes). When grown in monoculture, the hydrolysis ( )
of cellulose (C) does not occur. However, when grown in coculture, the secreted EngB and MiniCbpA form an enzymatic
complex, which allows the hydrolysis of cellulose.
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