5 research outputs found
GPCR-Based Chemical Biosensors for Medium-Chain Fatty Acids
A key limitation to engineering microbes
for chemical production
is a reliance on low-throughput chromatography-based screens for chemical
detection. While colorimetric chemicals are amenable to high-throughput
screens, many value-added chemicals are not colorimetric and require
sensors for high-throughput screening. Here, we use G-protein coupled
receptors (GPCRs) known to bind medium-chain fatty acids in mammalian
cells to rapidly construct chemical sensors in yeast. Medium-chain
fatty acids are immediate precursors to the advanced biofuel fatty
acid methyl esters, which can serve as a ādrop-inā replacement
for D2 diesel. One of the sensors detects even-chain C8āC12
fatty acids with a 13- to 17-fold increase in signal after activation,
with linear ranges up to 250 Ī¼M. Introduction of a synthetic
response unit alters both dynamic and linear range, improving the
sensor response to decanoic acid to a 30-fold increase in signal after
activation, with a linear range up to 500 Ī¼M. To our knowledge,
this is the first report of a whole-cell medium-chain fatty acid biosensor,
which we envision could be applied to the evolutionary engineering
of fatty acid-producing microbes. Given the affinity of GPCRs for
a wide range of chemicals, it should be possible to rapidly assemble
new biosensors by simply swapping the GPCR sensing unit. These sensors
should be amenable to a variety of applications that require different
dynamic and linear ranges, by introducing different response units
Medium-Throughput Screen of Microbially Produced Serotonin via a GāProtein-Coupled Receptor-Based Sensor
Chemical
biosensors, for which chemical detection triggers a fluorescent
signal, have the potential to accelerate the screening of noncolorimetric
chemicals produced by microbes, enabling the high-throughput engineering
of enzymes and metabolic pathways. Here, we engineer a G-protein-coupled
receptor (GPCR)-based sensor to detect serotonin produced by a producer
microbe in the producer microbeās supernatant. Detecting a
chemical in the producer microbeās supernatant is nontrivial
because of the number of other metabolites and proteins present that
could interfere with sensor performance. We validate the two-cell
screening system for medium-throughput applications, opening the door
to the rapid engineering of microbes for the increased production
of serotonin. We focus on serotonin detection as serotonin levels
limit the microbial production of hydroxystrictosidine, a modified
alkaloid that could accelerate the semisynthesis of camptothecin-derived
anticancer pharmaceuticals. This work shows the ease of generating
GPCR-based chemical sensors and their ability to detect specific chemicals
in complex aqueous solutions, such as microbial spent medium. In addition,
this work sets the stage for the rapid engineering of serotonin-producing
microbes
A Heritable Recombination System for Synthetic Darwinian Evolution in Yeast
Genetic recombination is central to the generation of
molecular
diversity and enhancement of evolutionary fitness in living systems.
Methods such as DNA shuffling that recapitulate this diversity mechanism <i>in vitro</i> are powerful tools for engineering biomolecules
with useful new functions by directed evolution. Synthetic biology
now brings demand for analogous technologies that enable the controlled
recombination of beneficial mutations in living cells. Thus, here
we create a Heritable Recombination system centered around a library
cassette plasmid that enables inducible mutagenesis <i>via</i> homologous recombination and subsequent combination of beneficial
mutations through sexual reproduction in <i>Saccharomyces cerevisiae</i>. Using repair of nonsense codons in auxotrophic markers as a model,
Heritable Recombination was optimized to give mutagenesis efficiencies
of up to 6% and to allow successive repair of different markers through
two cycles of sexual reproduction and recombination. Finally, Heritable
Recombination was employed to change the substrate specificity of
a biosynthetic enzyme, with beneficial mutations in three different
active site loops crossed over three continuous rounds of mutation
and selection to cover a total sequence diversity of 10<sup>13</sup>. Heritable Recombination, while at an early stage of development,
breaks the transformation barrier to library size and can be immediately
applied to combinatorial crossing of beneficial mutations for cell
engineering, adding important features to the growing arsenal of next
generation molecular biology tools for synthetic biology
Matching Protein Interfaces for Improved Medium-Chain Fatty Acid Production
Medium-chain fatty
acids (MCFAs) are key intermediates in the synthesis
of medium-chain chemicals including Ī±-olefins and dicarboxylic
acids. In bacteria, microbial production of MCFAs is limited by the
activity and product profile of fatty acyl-ACP thioesterases. Here,
we engineer a heterologous bacterial medium-chain fatty acyl-ACP thioesterase
for improved MCFA production in <i>Escherichia coli</i>.
Electrostatically matching the interface between the heterologous
medium-chain <i>Acinetobacter baylyi</i> fatty acyl-ACP
thioesterase (AbTE) and the endogenous <i>E.Ā coli</i> fatty acid ACP (<i>E.Ā coli</i> AcpP) by replacing
small nonpolar amino acids on the AbTE surface for positively charged
ones increased secreted MCFA titers more than 3-fold. Nuclear magnetic
resonance titration of <i>E.Ā coli</i> <sup>15</sup>N-octanoyl-AcpP with a single AbTE point mutant and the best double
mutant showed a progressive and significant increase in the number
of interactions when compared to AbTE wildtype. The best AbTE mutant
produced 131 mg/L of MCFAs, with MCFAs being 80% of all secreted fatty
acid chain lengths after 72 h. To enable the future screening of larger
numbers of AbTE variants to further improve MCFA titers, we show that
a previously developed G-protein coupled receptor (GPCR)-based MCFA
sensor differentially detects MCFAs secreted by <i>E.Ā coli</i> expressing different AbTE variants. This work demonstrates that
engineering the interface of heterologous enzymes to better couple
with endogenous host proteins is a useful strategy to increase the
titers of microbially produced chemicals. Further, this work shows
that GPCR-based sensors are producer microbe agnostic and can detect
chemicals directly in the producer microbe supernatant, setting the
stage for the sensor-guided engineering of MCFA producing microbes
Microbial Synthesis of Pinene
The volumetric heating values of
todayās biofuels are too
low to power energy-intensive aircraft, rockets, and missiles. Recently,
pinene dimers were shown to have a volumetric heating value similar
to that of the tactical fuel JP-10. To provide a sustainable source
of pinene, we engineered <i>Escherichia coli</i> for pinene
production. We combinatorially expressed three pinene synthases (PS)
and three geranyl diphosphate synthases (GPPS), with the best combination
achieving ā¼28 mg/L of pinene. We speculated that pinene toxicity
was limiting production; however, toxicity should not be limiting
at current titers. Because GPPS is inhibited by geranyl diphosphate
(GPP) and to increase flux through the pathway, we combinatorially
constructed GPPS-PS protein fusions. The <i>Abies grandis</i> GPPS-PS fusion produced 32 mg/L of pinene, a 6-fold improvement
over the highest titer previously reported in engineered <i>E.
coli</i>. Finally, we investigated the pinene isomer ratio of
our pinene-producing microbe and discovered that the isomer profile
is determined not only by the identity of the PS used but also by
the identity of the GPPS with which the PS is paired. We demonstrated
that the GPP concentration available to PS for cyclization alters
the pinene isomer ratio