Cell-free synthetic biology: thinking outside the cell

Process economics evaluation of cell-free synthesis for the commercial manufacture of antibody drug conjugates

Continuous improvements of cell-free synthesis (CFS) systems have generated interest in adopting the technology for the manufacture of biologics. This paper provides an evaluation of the manufacturing cost-effectiveness of CFS for the commercial production of antibody-drug conjugates (ADCs). The evaluation was performed using an advanced techno-economic engine (TEE) built in Python. The TEE is programmed in an object-oriented environment capable of simulating a plethora of process flowsheets and predicting size and cost metrics for the process and the facility. A case study was formulated to compare the economics of whole bioprocesses based on either a CFS system or a mammalian cell system (CHO) for the manufacture of an ADC at a range of product demands. The analysis demonstrated the potential of CFS for the commercial manufacture of biologics and identified key cost drivers related to the system. The CFS system showed an approximately 80% increase in the cost of goods compared to CHO with a significant cost attributed to the in-house manufacture of the bacterial cell extract, necessary for the CFS reaction step in the process. A sensitivity and target analysis highlighted the need for further process improvements especially in the titer for the CFS process to become more competitive against well-established systems.

Development of a Cofactor Balanced, Multi Enzymatic Cascade Reaction for the Simultaneous Production of L-Alanine and L-Serine from 2-Keto-3-deoxy-gluconate

Enzymatic reaction cascades represent a powerful tool to convert biogenic resources into valuable chemicals for fuel and commodity markets. Sugars and their breakdown products constitute a significant group of possible substrates for such biocatalytic conversion strategies to value-added products. However, one major drawback of sugar cascades is the need for cofactor recycling without using additional enzymes and/or creating unwanted by-products. Here, we describe a novel, multi-enzymatic reaction cascade for the one-pot simultaneous synthesis of L-alanine and L-serine, using the sugar degradation product 2-keto-3-deoxygluconate and ammonium as precursors. To pursue this aim, we used four different, thermostable enzymes, while the necessary cofactor NADH is recycled entirely self-sufficiently. Buffer and pH optimisation in combination with an enzyme titration study yielded an optimised production of 21.3 +/− 1.0 mM L-alanine and 8.9 +/− 0.4 mM L-serine in one pot after 21 h.

A bio-inspired cell-free system for cannabinoid production from inexpensive inputs

Moving cannabinoid production away from the vagaries of plant extraction and into engineered microbes could provide a consistent, purer, cheaper and environmentally benign source of these important therapeutic molecules, but microbial production faces notable challenges. An alternative to microbes and plants is to remove the complexity of cellular systems by employing enzymatic biosynthesis. Here we design and implement a new cell-free system for cannabinoid production with the following features: (1) only low-cost inputs are needed; (2) only 12 enzymes are employed; (3) the system does not require oxygen and (4) we use a nonnatural enzyme system to reduce ATP requirements that is generally applicable to malonyl-CoA-dependent pathways such as polyketide biosynthesis. The system produces ~0.5 g l-1 cannabigerolic acid (CBGA) or cannabigerovarinic acid (CBGVA) from low-cost inputs, nearly two orders of magnitude higher than yeast-based production. Cell-free systems such as this may provide a new route to reliable cannabinoid production.

Targeted Growth Medium Dropouts Promote Aromatic Compound Synthesis in Crude E. coli Cell-Free Systems

Progress in cell-free protein synthesis (CFPS) has spurred resurgent interest in engineering complex biological metabolism outside of the cell. Unlike purified enzyme systems, crude cell-free systems can be prepared for a fraction of the cost and contain endogenous cellular pathways that can be activated for biosynthesis. Endogenous activity performs essential functions in cell-free systems including substrate biosynthesis and energy regeneration; however, use of crude cell-free systems for bioproduction has been hampered by the under-described complexity of the metabolic networks inherent to a crude lysate. Physical and chemical cultivation parameters influence the endogenous activity of the resulting lysate, but targeted efforts to engineer this activity by manipulation of these nongenetic factors has been limited. Here growth medium composition was manipulated to improve the one-pot in vitro biosynthesis of phenol from glucose via the expression of Pasteurella multocida phenol-tyrosine lyase in crude E. coli lysates. Crude cell lysate metabolic activity was focused toward the limiting precursor tyrosine by targeted growth medium dropouts guided by proteomics. The result is the activation of a 25-step enzymatic reaction cascade involving at least three endogenous E. coli metabolic pathways. Additional modification of this system, through CFPS of feedback intolerant AroG improves yield. This effort demonstrates the ability to activate a long, complex pathway in vitro and provides a framework for harnessing the metabolic potential of diverse organisms for cell-free metabolic engineering. The more than 6-fold increase in phenol yield with limited genetic manipulation demonstrates the benefits of optimizing growth medium for crude cell-free extract production and illustrates the advantages of a systems approach to cell-free metabolic engineering.

Machine learning for metabolic engineering: A review

Machine learning provides researchers a unique opportunity to make metabolic engineering more predictable. In this review, we offer an introduction to this discipline in terms that are relatable to metabolic engineers, as well as providing in-depth illustrative examples leveraging omics data and improving production. We also include practical advice for the practitioner in terms of data management, algorithm libraries, computational resources, and important non-technical issues. A variety of applications ranging from pathway construction and optimization, to genetic editing optimization, cell factory testing, and production scale-up are discussed. Moreover, the promising relationship between machine learning and mechanistic models is thoroughly reviewed. Finally, the future perspectives and most promising directions for this combination of disciplines are examined.

Cell-free systems for accelerating glycoprotein expression and biomanufacturing

Protein glycosylation, the enzymatic modification of amino acid sidechains with sugar moieties, plays critical roles in cellular function, human health, and biotechnology. However, studying and producing defined glycoproteins remains challenging. Cell-free glycoprotein synthesis systems, in which protein synthesis and glycosylation are performed in crude cell extracts, offer new approaches to address these challenges. Here, we review versatile, state-of-the-art systems for biomanufacturing glycoproteins in prokaryotic and eukaryotic cell-free systems with natural and synthetic N-linked glycosylation pathways. We discuss existing challenges and future opportunities in the use of cell-free systems for the design, manufacture, and study of glycoprotein biomedicines.

In Vitro Selection of RNA Aptamers Binding to Nanosized DNA for Constructing Artificial Riboswitches

We here designed an in vitro selection scheme for obtaining an aptamer with which to rationally construct an artificial riboswitch as its component part. In fact, a nanosized DNA-binding aptamer obtained through this scheme allowed us to easily and successfully create eukaryotic riboswitches that upregulate internal ribosome entry site-mediated translation in response to the ligand (nanosized DNA) in wheat germ extract, a eukaryotic cell-free expression system. The induction ratio of the best riboswitch ligand-dose-dependently increased to 21 at 300 μM ligand. This switching efficiency is much higher than that of the same type of riboswitch with a widely used theophylline-binding aptamer, which was in vitro selected without considering its utility for constructing riboswitches. The selection scheme described here would facilitate obtaining various ligand/aptamer pairs suitable for constructing artificial riboswitches, which could serve as elements of synthetic gene circuits in synthetic biology.

Engineering control circuits for molecular robots using synthetic biology

The integration of molecular robots and synthetic biology allows for the creation of sophisticated behaviors at the molecular level. Similar to the synergy between bioelectronics and soft robotics, synthetic biology provides control circuitry for molecular robots. By encoding perception-action modules within synthetic circuits, molecular machines can advance beyond repeating tasks to the incorporation of complex behaviors. In particular, cell-free synthetic biology provides biomolecular circuitry independent of living cells. This research update reviews the current progress in using synthetic biology as perception-action control modules in robots from molecular robots to macroscale robots. Additionally, it highlights recent developments in molecular robotics and cell-free synthetic biology and suggests their combined use as a necessity for future molecular robot development.

A machine learning Automated Recommendation Tool for synthetic biology

Synthetic biology allows us to bioengineer cells to synthesize novel valuable molecules such as renewable biofuels or anticancer drugs. However, traditional synthetic biology approaches involve ad-hoc engineering practices, which lead to long development times. Here, we present the Automated Recommendation Tool (ART), a tool that leverages machine learning and probabilistic modeling techniques to guide synthetic biology in a systematic fashion, without the need for a full mechanistic understanding of the biological system. Using sampling-based optimization, ART provides a set of recommended strains to be built in the next engineering cycle, alongside probabilistic predictions of their production levels. We demonstrate the capabilities of ART on simulated data sets, as well as experimental data from real metabolic engineering projects producing renewable biofuels, hoppy flavored beer without hops, fatty acids, and tryptophan. Finally, we discuss the limitations of this approach, and the practical consequences of the underlying assumptions failing.

A Cell-Free Platform Based on Nisin Biosynthesis for Discovering Novel Lanthipeptides and Guiding their Overproduction In Vivo

Lanthipeptides have extensive therapeutic and industrial applications. However, because many are bactericidal, traditional in vivo platforms are limited in their capacity to discover and mass produce novel lanthipeptides as bacterial organisms are often critical components in these systems. Herein, the development of a cell-free protein synthesis (CFPS) platform that enables rapid genome mining, screening, and guided overproduction of lanthipeptides in vivo is described. For proof-of-concept studies, a type I lanthipeptide, nisin, is selected. Four novel lanthipeptides with antibacterial activity are identified among all nisin analogs in the National Center for Biotechnology Information (NCBI) database in a single day. Further, the CFPS platform is coupled with a screening assay for anti-gram-negative bacteria growth, resulting in the identification of a potent nisin mutant, M5. The titers of nisin and the nisin analog are found to be improved with CFPS platform guidance. Owing to the similarities in biosynthesis, the CFPS platform is broadly applicable to other lanthipeptides, thereby providing a universal method for lanthipeptide discovery and overproduction.

Improving the reaction mix of a Pichia pastoris cell-free system using a design of experiments approach to minimise experimental effort

A renaissance in cell-free protein synthesis (CFPS) is underway, enabled by the acceleration and adoption of synthetic biology methods. CFPS has emerged as a powerful platform technology for synthetic gene network design, biosensing and on-demand biomanufacturing. Whilst primarily of bacterial origin, cell-free extracts derived from a variety of host organisms have been explored, aiming to capitalise on cellular diversity and the advantageous properties associated with those organisms. However, cell-free extracts produced from eukaryotes are often overlooked due to their relatively low yields, despite the potential for improved protein folding and posttranslational modifications. Here we describe further development of a Pichia pastoris cell-free platform, a widely used expression host in both academia and the biopharmaceutical industry. Using a minimised Design of Experiments (DOE) approach, we were able to increase the productivity of the system by improving the composition of the complex reaction mixture. This was achieved in a minimal number of experimental runs, within the constraints of the design and without the need for liquid-handling robots. In doing so, we were able to estimate the main effects impacting productivity in the system and increased the protein synthesis of firefly luciferase and the biopharmaceutical HSA by 4.8-fold and 3.5-fold, respectively. This study highlights the P. pastoris-based cell-free system as a highly productive eukaryotic platform and displays the value of minimised DOE designs.

Preparation of a Millimeter-Sized Supergiant Liposome That Allows for Efficient, Eukaryotic Cell-Free Translation in the Interior by Spontaneous Emulsion Transfer

We sought to prepare millimeter-sized supergiant unilamellar vesicles (SGUVs) by spontaneous emulsion transfer for efficient, eukaryotic cell-free translation in the interior. Although the conventional protocols require that a considerably high concentration of sucrose be encapsulated into the SGUVs for their efficient formation, such high amounts of sucrose severely inhibited cell-free translation based on wheat germ extract (WGE). We thus optimized the preparation conditions to permit SGUV formation at a much lower concentration of sucrose that has almost no effect on WGE translation. Under the optimized conditions, we successfully prepared WGE translation system-encapsulating SGUVs that allow for protein synthesis with a high efficiency comparable to that outside a liposome. The optimization also resulted in a high rate of successful SGUV formation (>90%) and a decent stability of the formed SGUVs (>60 min). These SGUVs are expected to serve as research tools in cell-free synthetic biology and as foundations for artificial cell-based biosensors.

Development of a clostridia-based cell-free system for prototyping genetic parts and metabolic pathways

Gas fermentation by autotrophic bacteria, such as clostridia, offers a sustainable path to numerous bioproducts from a range of local, highly abundant, waste and low-cost feedstocks, such as industrial flue gases or syngas generated from biomass or municipal waste. Unfortunately, designing and engineering clostridia remains laborious and slow. The ability to prototype individual genetic part function, gene expression patterns, and biosynthetic pathway performance in vitro before implementing designs in cells could help address these bottlenecks by speeding up design. Unfortunately, a high-yielding cell-free gene expression (CFE) system from clostridia has yet to be developed. Here, we report the development and optimization of a high-yielding (236 ± 24 μg/mL) batch CFE platform from the industrially relevant anaerobe, Clostridium autoethanogenum. A key feature of the platform is that both circular and linear DNA templates can be applied directly to the CFE reaction to program protein synthesis. We demonstrate the ability to prototype gene expression, and quantitatively map aerobic cell-free metabolism in lysates from this system. We anticipate that the C. autoethanogenum CFE platform will not only expand the protein synthesis toolkit for synthetic biology, but also serve as a platform in expediting the screening and prototyping of gene regulatory elements in non-model, industrially relevant microbes.

Large scale active-learning-guided exploration for in vitro protein production optimization

Lysate-based cell-free systems have become a major platform to study gene expression but batch-to-batch variation makes protein production difficult to predict. Here we describe an active learning approach to explore a combinatorial space of ~4,000,000 cell-free buffer compositions, maximizing protein production and identifying critical parameters involved in cell-free productivity. We also provide a one-step-method to achieve high quality predictions for protein production using minimal experimental effort regardless of the lysate quality.

Integrative biology of native cell extracts: a new era for structural characterization of life processes

Advances in electron microscopy have provided unprecedented access to the structural characterization of large, flexible and heterogeneous complexes. Until recently, cryo-electron microscopy (cryo-EM) has been applied to understand molecular organization in either highly purified, isolated biomolecules or in situ. An emerging field is developing, bridging the gap between the two approaches, and focuses on studying molecular organization in native cell extracts. This field has demonstrated its potential by resolving the structure of fungal fatty acid synthase (FAS) at 4.7 Å [Fourier shell correlation (FSC) = 0.143]; FAS was not only less than 50% enriched, but also retained higher-order binders, previously unknown. Although controversial in the sense that the lysis step might introduce artifacts, cell extracts preserve aspects of cellular function. In addition, cell extracts are accessible, besides cryo-EM, to modern proteomic methods, chemical cross-linking, network biology and biophysical modeling. We expect that automation in imaging cell extracts, along with the integration of molecular/cell biology approaches, will provide remarkable achievements in the study of closer-to-life biomolecular states of pronounced biotechnological and medical importance. Such steps will, eventually, bring us a step closer to the biophysical description of cellular processes in an integrative, holistic approach.

Synthesis and Assembly of Hepatitis B Virus-Like Particles in a Pichia pastoris Cell-Free System

Virus-like particles (VLPs) are supramolecular protein assemblies with the potential for unique and exciting applications in synthetic biology and medicine. Despite the attention VLPs have gained thus far, considerable limitations still persist in their production. Poorly scalable manufacturing technologies and inconsistent product architectures continue to restrict the full potential of VLPs. Cell-free protein synthesis (CFPS) offers an alternative approach to VLP production and has already proven to be successful, albeit using extracts from a limited number of organisms. Using a recently developed Pichia pastoris-based CFPS system, we have demonstrated the production of the model Hepatitis B core antigen VLP as a proof-of-concept. The VLPs produced in the CFPS system were found to have comparable characteristics to those previously produced in vivo and in vitro. Additionally, we have developed a facile and rapid synthesis, assembly and purification methodology that could be applied as a rapid prototyping platform for vaccine development or synthetic biology applications. Overall the CFPS methodology allows far greater throughput, which will expedite the screening of optimal assembly conditions for more robust and stable VLPs. This approach could therefore support the characterization of larger sample sets to improve vaccine development efficiency.

In vitro multi-enzymatic cascades using recombinant lysates of E. coli: an emerging biocatalysis platform

In recent years, cell-free extracts (or lysates) have (re-)emerged as a third route to the traditional options of isolated or whole-cell biocatalysts. Advances in molecular biology and genetic engineering enable facile production of recombinant cell-free extracts, where endogenous enzymes are enriched with heterologous activities. These inexpensive preparations may be used to catalyse multistep enzymatic reactions without the constraints of cell toxicity and the cell membrane or the cost and complexity associated with production of isolated biocatalysts. Herein, we present an overview of the key advancements in cell-free synthetic biology that have led to the emergence of cell-free extracts as a promising biocatalysis platform.

Cell-free protein synthesis: The transition from batch reactions to minimal cells and microfluidic devices

Thanks to the synthetic biology, the laborious and restrictive procedure for producing a target protein in living microorganisms by biotechnological approaches can now experience a robust, pliant yet efficient alternative. The new system combined with lab-on-chip microfluidic devices and nanotechnology offers a tremendous potential envisioning novel cell-free formats such as DNA brushes, hydrogels, vesicular particles, droplets, as well as solid surfaces. Acting as robust microreactors/microcompartments/minimal cells, the new platforms can be tuned to perform various tasks in a parallel and integrated manner encompassing gene expression, protein synthesis, purification, detection, and finally enabling cell-cell signaling to bring a collective cell behavior, such as directing differentiation process, characteristics of higher order entities, and beyond. In this review, we issue an update on recent cell-free protein synthesis (CFPS) formats. Furthermore, the latest advances and applications of CFPS for synthetic biology and biotechnology are highlighted. In the end, contemporary challenges and future opportunities of CFPS systems are discussed.

Advances in biological conversion technologies: new opportunities for reaction engineering

Reaction engineering needs to embrace biological conversion technologies, on the road to identify more sustainable routes for chemical manufacture.

Cell-free protein synthesis in hydrogel materials

Fabrication of macro-scale polysaccharide, proteinaceous, micellular and covalently crosslinked hydrogels for housing cell-free protein synthesis reactions.

One-pot efficient biosynthesis of (3R)-acetoin from pyruvate by a two-enzyme cascade

Opening the possibility of sustainable industrial (3R)-acetoin biomanufacturing in vitro.

In Vitro Naringenin Biosynthesis from p-Coumaric Acid Using Recombinant Enzymes

Naringenin is an important precursor for the production of a wide spectrum of flavonoids, and its production is of great interest in metabolic engineering. However, in cellular systems, identification of rate-limiting factors is often difficult because of complex regulatory networks. Cell-free catalytic systems emerge as a promising method to address this issue. Here, we explored the cell-free biosystem for naringenin production by combining different sources of 4-coumaroyl-CoA ligase (4CL), chalcone synthase (CHS), and chalcone isomerase (CHI). After systematic analysis of enzyme levels, substrate concentrations, and cofactors, 4CL and CHS were found to be crucial to the reaction. The best loading ratio of 4CL/CHS/CHI was 10:10:1, and malonyl-CoA was the limiting factor, as identified previously in fermentation. For the first time, we successfully constructed the system for naringenin production in vitro. Our study will deepen our understanding of the key factors in naringenin production and guide further engineering.

Toward a genome scale sequence specific dynamic model of cell-free protein synthesis in Escherichia coli

In this study, we developed a dynamic mathematical model of E. coli cell-free protein synthesis (CFPS). Model parameters were estimated from a dataset consisting of glucose, organic acids, energy species, amino acids, and protein product, chloramphenicol acetyltransferase (CAT) measurements. The model was successfully trained to simulate these measurements, especially those of the central carbon metabolism. We then used the trained model to evaluate the performance, e.g., the yield and rates of protein production. CAT was produced with an energy efficiency of 12%, suggesting that the process could be further optimized. Reaction group knockouts showed that protein productivity was most sensitive to the oxidative phosphorylation and glycolysis/gluconeogenesis pathways. Amino acid biosynthesis was also important for productivity, while overflow metabolism and TCA cycle affected the overall system state. In addition, translation was more important to productivity than transcription. Finally, CAT production was robust to allosteric control, as were most of the predicted metabolite concentrations; the exceptions to this were the concentrations of succinate and malate, and to a lesser extent pyruvate and acetate, which varied from the measured values when allosteric control was removed. This study is the first to use kinetic modeling to predict dynamic protein production in a cell-free E. coli system, and could provide a foundation for genome scale, dynamic modeling of cell-free E. coli protein synthesis.

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Protein production is biphasic, powered initially by glucose and later by pyruvate.

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Protein is produced with an energy efficiency of only 12%.

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Protein productivity is most sensitive to oxidative phosphorylation and glycolysis.

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Protein production is robust to allosteric control.

Development of a Split Esterase for Protein-Protein Interaction-Dependent Small-Molecule Activation

Split reporters based on fluorescent proteins and luciferases have emerged as valuable tools for measuring interactions in biological systems. Relatedly, biosensors that transduce measured input signals into outputs that influence the host system are key components of engineered gene circuits for synthetic biology applications. While small-molecule-based imaging agents are widely used in biological studies, and small-molecule-based drugs and chemical probes can target a range of biological processes, a general method for generating a target small molecule in a biological system based on a measured input signal is lacking. Here, we develop a proximity-dependent split esterase that selectively unmasks ester-protected small molecules in an interaction-dependent manner. Exploiting the versatility of an ester-protected small-molecule output, we demonstrate fluorescent, chemiluminescent, and pharmacological probe generation, each created by masking key alcohol functional groups on a target small molecule. We show that the split esterase system can be used in combination with ester-masked fluorescent or luminescent probes to measure protein-protein interactions and protein-protein interaction inhibitor engagement. We demonstrate that the esterase-based reporter system is compatible with other commonly used split reporter imaging systems for the simultaneous detection of multiple protein-protein interactions. Finally, we develop a system for selective small-molecule-dependent cell killing by unmasking a cytotoxic molecule using an inducible split esterase. Presaging utility in future synthetic biology-based therapeutic applications, we also show that the system can be used for intercellular cell killing via a bystander effect, where one activated cell unmasks a cytotoxic molecule and kills cells physically adjacent to the activated cells. Collectively, this work illustrates that the split esterase system is a valuable new addition to the split protein toolbox, with particularly exciting potential in synthetic biology applications.

Enzyme alchemy: cell-free synthetic biochemistry for natural products

Cell-free synthetic biochemistry aims to engineer chemical biology by exploiting biosynthetic dexterity outside of the constraints of a living cell. One particular use is for making natural products, where cell-free systems have initially demonstrated feasibility in the biosynthesis of a range of complex natural products classes. This has shown key advantages over total synthesis, such as increased yield, enhanced regioselectivity, use of reduced temperatures and less reaction steps. Uniquely, cell-free synthetic biochemistry represents a new area that seeks to advance upon these efforts and is particularly useful for defining novel synthetic pathways to replace natural routes and optimising the production of complex natural product targets from low-cost precursors. Key challenges and opportunities will include finding solutions to scaled-up cell-free biosynthesis, as well as the targeting of high value and toxic natural products that remain challenging to make either through whole-cell biotransformation platforms or total synthesis routes. Although underexplored, cell-free synthetic biochemistry could also be used to develop 'non-natural' natural products or so-called xenobiotics for novel antibiotics and drugs, which can be difficult to engineer directly within a living cell.

A Highly Productive, One-Pot Cell-Free Protein Synthesis Platform Based on Genomically Recoded Escherichia coli

The site-specific incorporation of non-canonical amino acids (ncAAs) into proteins via amber suppression provides access to novel protein properties, structures, and functions. Historically, poor protein expression yields resulting from release factor 1 (RF1) competition has limited this technology. To address this limitation, we develop a high-yield, one-pot cell-free platform for synthesizing proteins bearing ncAAs based on genomically recoded Escherichia coli lacking RF1. A key feature of this platform is the independence on the addition of purified T7 DNA-directed RNA polymerase (T7RNAP) to catalyze transcription. Extracts derived from our final strain demonstrate high productivity, synthesizing 2.67 ± 0.06 g/L superfolder GFP in batch mode without supplementation of purified T7RNAP. Using an optimized one-pot platform, we demonstrate multi-site incorporation of the ncAA p-acetyl-L-phenylalanine into an elastin-like polypeptide with high accuracy of incorporation and yield. Our work has implications for chemical and synthetic biology.

Coupled chemistry kinetics demonstrate the utility of functionalized Sup35 amyloid nanofibrils in biocatalytic cascades

Concerns over the environment are a central driver for designing cell-free enzymatic cascade reactions that synthesize non-petrol-based commodity compounds. An often-suggested strategy that would demonstrate the economic competitiveness of this technology is recycling of valuable enzymes through their immobilization. For this purpose, amyloid nanofibrils are an ideal scaffold to realize chemistry-free covalent enzyme immobilization on a material that offers a large surface area. However, in most instances, only single enzyme-functionalized amyloid fibrils have so far been studied. To embark on the next stage, here we displayed xylanase A, β-xylosidase, and an aldose sugar dehydrogenase on Sup35(1-61) nanofibrils to convert beechwood xylan to xylonolactone. We characterized this enzymatic cascade by measuring the time-dependent accumulation of xylose, xylooligomers, and xylonolactone. Furthermore, we studied the effects of relative enzyme concentrations, pH, temperature, and agitation on product formation. Our investigations revealed that a modular cascade with a mixture of xylanase and β-xylosidase, followed by product removal and separate oxidation of xylose with the aldose sugar dehydrogenase, is more productive than an enzyme mix containing all of these enzymes together. Moreover, we found that the nanofibril-coupled enzymes do not lose activity compared with their native state. These findings provide proof of concept of the feasibility of functionalized Sup35(1-61) fibrils as a molecular scaffold for biocatalytic cascades consisting of reusable enzymes that can be used in biotechnology.

Exploring the Potential of Cell-Free Protein Synthesis for Extending the Abilities of Biological Systems

Cell-free protein synthesis (CFPS) system is a simple, rapid, and sensitive tool that is devoid of membrane-bound barriers, yet contains all the mandatory substrates, biomolecules, and machineries required for the synthesis of the desired proteins. It has the potential to overcome loopholes in the current in vivo production systems and is a promising tool in both basic and applied scientific research. It facilitates a simplified organization of desired experiments with a variety of reaction conditions, making CFPS a powerful tool in biological research. It has been used for the expansion of genetic code, assembly of viruses, and in metabolic engineering for production of toxic and complex proteins. Subsequently, CFPS systems have emerged as potent technology for high-throughput production of membrane proteins, enzymes, and therapeutics. The present review highlights the recent advances and uses of CFPS systems in biomedical, therapeutic, and biotechnological applications. Additionally, we highlight possible solutions to the potential biosafety issues that may be encountered while using CFPS technology.

PARAGEN 1.0: A Standardized Synthetic Gene Library for Fast Cell-Free Bacteriocin Synthesis

The continuous emergence of microbial resistance to our antibiotic arsenal is widely becoming recognized as an imminent threat to global human health. Bacteriocins are antimicrobial peptides currently under consideration as real alternatives or complements to common antibiotics. These peptides have been much studied, novel bacteriocins are regularly reported and several genomic databases on these peptides are currently updated. Despite this, to our knowledge, a physical collection of bacteriocins that would allow testing and comparing them for different applications does not exist. Rapid advances in synthetic biology in combination with cell-free protein synthesis technologies offer great potential for fast protein production. Based on the amino acid sequences of the mature peptide available in different databases, we have built a bacteriocin gene library, called PARAGEN 1.0, containing all the genetic elements required for in vitro cell-free peptide synthesis. Using PARAGEN 1.0 and a commercial kit for cell-free protein synthesis we have produced 164 different bacteriocins. Of the bacteriocins synthesized, 54% have shown antimicrobial activity against at least one of the indicator strains tested, including Gram-positive and Gram-negative bacteria representing commonly used lab strains, industrially relevant microorganisms, and known pathogens. This bacteriocin collection represents a streamlined pipeline for selection, production, and screening of bacteriocins as well as a reservoir of ready-to-use antimicrobials against virtually any class of relevant bacteria.

Quantitative modeling of transcription and translation of an all-E. coli cell-free system

Cell-free transcription-translation (TXTL) is expanding as a polyvalent experimental platform to engineer biological systems outside living organisms. As the number of TXTL applications and users is rapidly growing, some aspects of this technology could be better characterized to provide a broader description of its basic working mechanisms. In particular, developing simple quantitative biophysical models that grasp the different regimes of in vitro gene expression, using relevant kinetic constants and concentrations of molecular components, remains insufficiently examined. In this work, we present an ODE (Ordinary Differential Equation)-based model of the expression of a reporter gene in an all E. coli TXTL that we apply to a set of regulatory elements spanning several orders of magnitude in strengths, far beyond the T7 standard system used in most of the TXTL platforms. Several key biochemical constants are experimentally determined through fluorescence assays. The robustness of the model is tested against the experimental parameters, and limitations of TXTL resources are described. We establish quantitative references between the performance of E. coli and synthetic promoters and ribosome binding sites. The model and the data should be useful for the TXTL community interested either in gene network engineering or in biomanufacturing beyond the conventional platforms relying on phage transcription.

Recent advances in plasmid-based tools for establishing novel microbial chassis

A key challenge for domesticating alternative cultivable microorganisms with biotechnological potential lies in the development of innovative technologies. Within this framework, a myriad of genetic tools has flourished, allowing the design and manipulation of complex synthetic circuits and genomes to become the general rule in many laboratories rather than the exception. More recently, with the development of novel technologies such as DNA automated synthesis/sequencing and powerful computational tools, molecular biology has entered the synthetic biology era. In the beginning, most of these technologies were established in traditional microbial models (known as chassis in the synthetic biology framework) such as Escherichia coli and Saccharomyces cerevisiae, enabling fast advances in the field and the validation of fundamental proofs of concept. However, it soon became clear that these organisms, although extremely useful for prototyping many genetic tools, were not ideal for a wide range of biotechnological tasks due to intrinsic limitations in their molecular/physiological properties. Over the last decade, researchers have been facing the great challenge of shifting from these model systems to non-conventional chassis with endogenous capacities for dealing with specific tasks. The key to address these issues includes the generation of narrow and broad host plasmid-based molecular tools and the development of novel methods for engineering genomes through homologous recombination systems, CRISPR/Cas9 and other alternative methods. Here, we address the most recent advances in plasmid-based tools for the construction of novel cell factories, including a guide for helping with "build-your-own" microbial host.

Optimization of a reduced enzymatic reaction cascade for the production of L-alanine

Cell-free enzymatic reaction cascades combine the advantages of well-established in vitro biocatalysis with the power of multi-step in vivo pathways. The absence of a regulatory cell environment enables direct process control including methods for facile bottleneck identification and process optimization. Within this work, we developed a reduced, enzymatic reaction cascade for the direct production of L-alanine from D-glucose and ammonium sulfate. An efficient, activity based enzyme selection is demonstrated for the two branches of the cascade. The resulting redox neutral cascade is composed of a glucose dehydrogenase, two dihydroxyacid dehydratases, a keto-deoxy-aldolase, an aldehyde dehydrogenase and an L-alanine dehydrogenase. This artificial combination of purified biocatalysts eliminates the need for phosphorylation and only requires NAD as cofactor. We provide insight into in detail optimization of the process parameters applying a fluorescamine based L-alanine quantification assay. An optimized enzyme ratio and the necessary enzyme load were identified and together with the optimal concentrations of cofactor (NAD), ammonium and buffer yields of >95% for the main branch and of 8% for the side branch were achieved.

Multiplex transcriptional characterizations across diverse bacterial species using cell-free systems

Cell-free expression systems enable rapid prototyping of genetic programs in vitro. However, current throughput of cell-free measurements is limited by the use of channel-limited fluorescent readouts. Here, we describe DNA Regulatory element Analysis by cell-Free Transcription and Sequencing (DRAFTS), a rapid and robust in vitro approach for multiplexed measurement of transcriptional activities from thousands of regulatory sequences in a single reaction. We employ this method in active cell lysates developed from ten diverse bacterial species. Interspecies analysis of transcriptional profiles from > 1,000 diverse regulatory sequences reveals functional differences in promoter activity that can be quantitatively modeled, providing a rich resource for tuning gene expression in diverse bacterial species. Finally, we examine the transcriptional capacities of dual-species hybrid lysates that can simultaneously harness gene expression properties of multiple organisms. We expect that this cell-free multiplex transcriptional measurement approach will improve genetic part prototyping in new bacterial chassis for synthetic biology.

Multi-enzymatic recycling of ATP and NADPH for the synthesis of 5-aminolevulinic acid using a semipermeable reaction system

5-Aminolevulinic acid (ALA) is an important cellular metabolic intermediate that has broad agricultural and medical applications. Previously, attempts have been made to synthesize ALA by multiple enzymes in cell free systems. Here we report the development of a semi-permeable system for ALA production using stable enzymes. Glucose, sodium polyphosphate, ATP, tRNA, glutamate and NADPH were used as substrates for ALA synthesis by a total of nine enzymes: adenylate kinase, polyphosphate kinase, glucose-6-phosphate dehydrogenase, phosphogluconolactonase, 6-phosphogluconate dehydrogenase, glutamyl-tRNA synthetase and glutamate-1-semialdehyde aminotransferase from E. coli, hexokinase from yeast, as well as glutamyl-tRNA reductase and its stimulator protein glutamyl-tRNA reductase binding protein (GBP) from Arabidopsis in a semi-permeable system. After reaction for 48 h, the glutamate conversion reached about 95%. This semi-permeable system facilitated the reuse of enzymes, and was helpful for the separation and purification of the product. The ALA production could be further improved by process optimization and enzyme engineering.Abbreviations: PPK: polyphosphate kinase; ADK: adenylate kinase; ALA: 5-Aminolevulinic acid; HK: hexokinase; ZWF: glucose-6-phosphatedehydrogenase; PGL: phosphogluconolactonase; GND: 6-phosphogluconate dehydrogenase; GTS: glutamyl-tRNA synthetase; GTR: glutamyl-tRNA reductase; GBP: GTR binding protein; GSAAT: glutamate-1-semialdehyde aminotransferase.

Opportunities at the Intersection of Synthetic Biology, Machine Learning, and Automation

Our inability to predict the behavior of biological systems severely hampers progress in bioengineering and biomedical applications. We cannot predict the effect of genotype changes on phenotype, nor extrapolate the large-scale behavior from small-scale experiments. Machine learning techniques recently reached a new level of maturity, and are capable of providing the needed predictive power without a detailed mechanistic understanding. However, they require large amounts of data to be trained. The amount and quality of data required can only be produced through a combination of synthetic biology and automation, so as to generate a large diversity of biological systems with high reproducibility. A sustained investment in the intersection of synthetic biology, machine learning, and automation will drive forward predictive biology, and produce improved machine learning algorithms.

Facile synthesis of (-)-vibo-quercitol from maltodextrin via an in vitro synthetic enzymatic biosystem

(-)-vibo-Quercitol (VQ: 1L-1,2,4/3,5-cyclohexanepentol), a form of deoxyinositol, is an alternative chiral building block in the synthesis of bioactive compounds to control diabetes. In this study, an adenosine triphosphate-free in vitro synthetic enzymatic biosystem composed of five enzymes (including one enzyme for NADH regeneration) was constructed to produce VQ from maltodextrin in one-pot. After optimization of reaction conditions, 7.6 g/L VQ was produced from 10 g/L maltodextrin with a product yield (mol/mol) of 77%, and 25.3 g/L VQ with a purity of 87% was produced from 50 g/L maltodextrin through simple scaling up of this nonfermentative enzymatic biosystem. Therefore, this study provides an economical and environmentally friendly method for the envisioned quercitol biosynthesis.

Lessons from Two Design-Build-Test-Learn Cycles of Dodecanol Production in Escherichia coli Aided by Machine Learning

The Design-Build-Test-Learn (DBTL) cycle, facilitated by exponentially improving capabilities in synthetic biology, is an increasingly adopted metabolic engineering framework that represents a more systematic and efficient approach to strain development than historical efforts in biofuels and biobased products. Here, we report on implementation of two DBTL cycles to optimize 1-dodecanol production from glucose using 60 engineered Escherichia coli MG1655 strains. The first DBTL cycle employed a simple strategy to learn efficiently from a relatively small number of strains (36), wherein only the choice of ribosome-binding sites and an acyl-ACP/acyl-CoA reductase were modulated in a single pathway operon including genes encoding a thioesterase (UcFatB1), an acyl-ACP/acyl-CoA reductase (Maqu_2507, Maqu_2220, or Acr1), and an acyl-CoA synthetase (FadD). Measured variables included concentrations of dodecanol and all proteins in the engineered pathway. We used the data produced in the first DBTL cycle to train several machine-learning algorithms and to suggest protein profiles for the second DBTL cycle that would increase production. These strategies resulted in a 21% increase in dodecanol titer in Cycle 2 (up to 0.83 g/L, which is more than 6-fold greater than previously reported batch values for minimal medium). Beyond specific lessons learned about optimizing dodecanol titer in E. coli, this study had findings of broader relevance across synthetic biology applications, such as the importance of sequencing checks on plasmids in production strains as well as in cloning strains, and the critical need for more accurate protein expression predictive tools.

High-throughput mapping of CoA metabolites by SAMDI-MS to optimize the cell-free biosynthesis of HMG-CoA

Metabolic engineering uses enzymes to produce small molecules with industrial, pharmaceutical, and energy applications. However, efforts to optimize enzymatic pathways for commercial production are limited by the throughput of assays for quantifying metabolic intermediates and end products. We developed a multiplexed method for profiling CoA-dependent pathways that uses a cysteine-terminated peptide to covalently capture CoA-bound metabolites. Captured metabolites are then rapidly separated from the complex mixture by immobilization onto arrays of self-assembled monolayers and directly quantified by SAMDI mass spectrometry. We demonstrate the throughput of the assay by characterizing the cell-free synthesis of HMG-CoA, a key intermediate in the biosynthesis of isoprenoids, collecting over 10,000 individual spectra to map more than 800 unique reaction conditions. We anticipate that our rapid and robust analytical method will accelerate efforts to engineer metabolic pathways.

Biocatalytic synthesis of polylactate and its copolymers by engineered microorganisms

Poly(lactate), also called poly(lactic acid) or poly(lactide) [PLA], has been one of the most attractive bio-based polymers since it possesses desirable material properties for its use in general performance plastics in addition to biodegradability and biocompatibility. PLA has been produced by biological and chemical hybrid process comprising microbial fermentation for lactate (LA) production followed by purification and chemical polymerization process of LA. Recently, the direct one-step fermentative processes for production of PLA and several LA-containing polyesters have been developed by employing metabolically engineered microorganisms. Since natural microorganisms cannot produce the LA-containing polymers, several engineering strategies have been employed together based on the polyhydroxyalkanoate (PHA) biosynthesis system. In this chapter, we summarize strategies and procedures on developing the engineered microorganisms producing PLA and its copolymers, cultivating the cells, and extracting the polymers from the cells. Focuses were given on construction of enzymatic polymerization process of LA: design of metabolic pathway for PLA by mimicking PHA biosynthetic pathway, examination of possible enzymes, and engineering of the enzymes for better performances. This synthetic pathway has been established in a microorganism producing LA that enabled one-step fermentative production of LA-containing polyesters from carbohydrates derived from renewable biomass. Polymer production has been further enhanced by implementing strain engineering to concentrate the metabolic fluxes toward PLA formation. In addition, various monomers such as glycolate, 2-hydroxybutyrate, and phenyllactate have been copolymerized with LA by the microbial system. These fermentative production systems developed by using the engineered microorganisms can be versatile and sustainable platforms for the production of LA-containing polyesters and other non-natural polymers.

Characterization of the all-E. coli transcription-translation system myTXTL by mass spectrometry

RATIONALE

Cell-free transcription-translation (TXTL) is becoming a popular technology to prototype and engineer biological systems outside living organisms. TXTL relies commonly on a cytoplasmic extract that provides the molecular components necessary to recapitulate gene expression in vitro, where most of the available systems are derived from E. coli. The proteinic and enzymatic composition of lysates, however, is typically unknown. In this work, we analyzed by mass spectrometry the molecular constituents of the all-E. coli TXTL platform myTXTL prepared from the E. coli strain BL21 Rosetta2.

METHODS

Standard TXTL reactions were assembled and executed for 10-12 hours at 29°C. In addition to a no-DNA control, four DNA programs were executed in separate reactions to synthesize the reporter protein deGFP as well as the phages MS2, phix174 and T7. The reactions were treated according to standard procedures (trypsin treatment, cleaning) before performing liquid chromatography/mass spectrometry (LC/MS). Data analysis was performed using Sequest and protein identification using Scaffold.

RESULTS

A total of 500-800 proteins were identified by LC/MS in the blank reactions. We organized the most abundant protein sets into several categories pertaining, in particular, to transcription, translation and ATP regeneration. The synthesis of deGFP was easily measured. The major structural proteins that compose the three phages MS2, phix174 and T7 were also identified.

CONCLUSIONS

Mass spectrometry is a practical tool to characterize biochemical solutions as complex as a cell-free TXTL reaction and to determine the presence of synthesized proteins. The data presented demonstrate that the composition of TXTL based on lysates can be used to validate some underlying molecular mechanisms implicated in cell-free protein synthesis. The composition of the lysate shows significant differences with respect to similar studies on other E. coli strains.

Microfluidics for Artificial Life: Techniques for Bottom-Up Synthetic Biology

Synthetic biology is a rapidly growing multidisciplinary branch of science that exploits the advancement of molecular and cellular biology. Conventional modification of pre-existing cells is referred to as the top-down approach. Bottom-up synthetic biology is an emerging complementary branch that seeks to construct artificial cells from natural or synthetic components. One of the aims in bottom-up synthetic biology is to construct or mimic the complex pathways present in living cells. The recent, and rapidly growing, application of microfluidics in the field is driven by the central tenet of the bottom-up approach-the pursuit of controllably generating artificial cells with precisely defined parameters, in terms of molecular and geometrical composition. In this review we survey conventional methods of artificial cell synthesis and their limitations. We proceed to show how microfluidic approaches have been pivotal in overcoming these limitations and ushering in a new generation of complexity that may be imbued in artificial cells and the milieu of applications that result.

Direct Aromatic Nitration System for Synthesis of Nitrotryptophans in Escherichia coli

Nitrotryptophan and its analogues are useful building blocks for synthesizing bioactive and biotechnologically relevant chemicals, materials, and proteins. However, synthetic routes to enantiopure nitro-containing tryptophan derivatives are either complex and polluting or even unestablished yet. Herein, we describe microbial production of 4-NO₂-l-tryptophan (Nitrotrp) and its analogues by designing and expressing the biosynthetic pathway in Escherichia coli. The biosynthetic pathway comprised one engineered self-sufficient P450 TB14 of Streptomyces origin for direct nitration of the C-4 of l-Trp indole and one nitric oxide synthase from Bacillus subtilis (BsNOS) for the production of nitric oxide (NO) from l-Arg to support the direct aromatic nitration. As both TB14 and BsNOS require reducing agent NADPH for their reactions, we also included one glucose dehydrogenase (GDH) from B. subtilis for in situ NADPH regeneration. The initially designed pathway led to 16.2 ± 2.3 mg/L of Nitrotrp by the engineered E. coli fermented in the M9 minimal medium for 3 days. A combination of the design and screening of three additional pathways, fermentation optimization and the knockout of competitive metabolic pathways together improved the Nitrotrp titer to around 192 mg/L within 20 h. Finally, the whole-cell biotransformation system produced eight Nitrotrp analogues with their titers varying from 2.5 to 61.5 mg/L. This work provides the first microbial direct aromatic nitration processes and sets the stage for the development of biocatalytic routes to other useful nitroaromatics in the future.