Cell-free synthetic biology: thinking outside the cell

What can we learn from the construction of in vitro replication systems?

Replication is a central function of living organisms. Several types of replication systems have been constructed in vitro from various molecules, including peptides, DNA, RNA, and proteins. In this review, I summarize the progress in the construction of replication systems over the past few decades and discuss what we can learn from their construction. I introduce various types of replication systems, supporting the feasibility of the spontaneous appearance of replication early in Earth's history. In the latter part of the review, I focus on parasitic replicators, one of the largest obstacles for sustainable replication. Compartmentalization is discussed as a possible solution.

Deconstructing Cell-Free Extract Preparation for in Vitro Activation of Transcriptional Genetic Circuitry

Recent advances in cell-free gene expression (CFE) systems have enabled their use for a host of synthetic biology applications, particularly for rapid prototyping of genetic circuits and biosensors. Despite the proliferation of cell-free protein synthesis platforms, the large number of currently existing protocols for making CFE extracts muddles the collective understanding of how the extract preparation method affects its functionality. A key aspect of extract performance relevant to many applications is the activity of the native host transcriptional machinery that can mediate protein synthesis. However, protein yields from genes transcribed in vitro by the native Escherichia coli RNA polymerase are variable for different extract preparation techniques, and specifically low in some conventional crude extracts originally optimized for expression by the bacteriophage transcriptional machinery. Here, we show that cell-free expression of genes under bacterial σ70 promoters is constrained by the rate of transcription in crude extracts, and that processing the extract with a ribosomal runoff reaction and subsequent dialysis alleviates this constraint. Surprisingly, these processing steps only enhance protein synthesis in genes under native regulation, indicating that the translation rate is unaffected. We further investigate the role of other common extract preparation process variants on extract performance and demonstrate that bacterial transcription is inhibited by including glucose in the growth culture but is unaffected by flash-freezing the cell pellet prior to lysis. Our final streamlined and detailed protocol for preparing extract by sonication generates extract that facilitates expression from a diverse set of sensing modalities including protein and RNA regulators. We anticipate that this work will clarify the methodology for generating CFE extracts that are active for biosensing using native transcriptional machinery and will encourage the further proliferation of cell-free gene expression technology for new applications.

Cell-Free Protein Synthesis and Its Perspectives for Assembling Cells from the Bottom-Up

The underlying idea of synthetic biology is that biological reactions/modules/systems can be precisely engineered and controlled toward desired products. Numerous efforts in the past decades in deciphering the complexity of biological systems in vivo have led to a variety of tools for synthetic biology, especially based on recombinant DNA. However, one generic limitation of all living systems is that the vast majority of energy input is dedicated to maintain the system as a whole, rather than the small part of interest. Cell-free synthetic biology is aiming at exactly this fundamental limitation, providing the next level of flexibility for engineering and designing biological systems in vitro. New technology has continuously inspired cell-free biology and extended its applications, including gene circuits, spatiotemporally controlled pathways, coactivated catalysts systems, and rationally designed multienzyme pathways, in particular, minimal cell construction. In the context of this special issue, discussing work being carried out in the "MaxSynBio" consortium, the advances in characterizing stochasticity and dynamics of cell-free protein synthesis within cell-sized compartments, as well as the molecular crowding effect, are discussed. The organization of spatial heterogeneity is the key prerequisite for achieving hierarchy and stepwise assembly of minimal cells from the bottom-up.

Cell-free biosynthesis of limonene using enzyme-enriched Escherichia coli lysates

Isoprenoids are an attractive class of metabolites for enzymatic synthesis from renewable substrates. However, metabolic engineering of microorganisms for monoterpenoid production is limited by the need for time-consuming, and often non-intuitive, combinatorial tuning of biosynthetic pathway variations to meet design criteria. Towards alleviating this limitation, the goal of this work was to build a modular, cell-free platform for construction and testing of monoterpenoid pathways, using the fragrance and flavoring molecule limonene as a model. In this platform, multiple Escherichia coli lysates, each enriched with a single overexpressed pathway enzyme, are mixed to construct the full biosynthetic pathway. First, we show the ability to synthesize limonene from six enriched lysates with mevalonate substrate, an adenosine triphosphate (ATP) source, and cofactors. Next, we extend the pathway to use glucose as a substrate, which relies on native metabolism in the extract to convert glucose to acetyl-CoA along with three additional enzymes to convert acetyl-CoA to mevalonate. We find that the native E. coli farnesyl diphosphate synthase (IspA) is active in the lysate and diverts flux from the pathway intermediate geranyl pyrophospahte to farnesyl pyrophsophate and the byproduct farnesol. By adjusting the relative levels of cofactors NAD+, ATP and CoA, the system can synthesize 0.66 mM (90.2 mg l-1) limonene over 24 h, a productivity of 3.8 mg l-1 h-1. Our results highlight the flexibility of crude lysates to sustain complex metabolism and, by activating a glucose-to-limonene pathway with 9 heterologous enzymes encompassing 20 biosynthetic steps, expands an approach of using enzyme-enriched lysates for constructing, characterizing and prototyping enzymatic pathways.

Acceleration of cellodextrin phosphorolysis for bioelectricity generation from cellulosic biomass by integrating a synthetic two-enzyme complex into an in vitro synthetic enzymatic biosystem

BACKGROUND

Cellulosic biomass, the earth's most abundant renewable resource, can be used as substrates for biomanufacturing biofuels or biochemicals via in vitro synthetic enzymatic biosystems in which the first step is the enzymatic phosphorolysis of cellodextrin to glucose 1-phosphate (G1P) by cellodextrin phosphorylase (CDP). However, almost all the CDPs prefer cellodextrin synthesis to phosphorolysis, resulting in the low reaction rate of cellodextrin phosphorolysis for biomanufacturing.

RESULTS

To increase the reaction rate of cellodextrin phosphorolysis, synthetic enzyme complexes containing CDP and phosphoglucomutase (PGM) were constructed to convert G1P to glucose 6-phosphate (G6P) rapidly, which is an important intermediate for biomanufacturing. Four self-assembled synthetic enzyme complexes were constructed with different spatial organizations based on the high-affinity and high-specific interaction between cohesins and dockerins from natural cellulosomes. Thus, the CDP-PGM enzyme complex with the highest enhancement of initial reaction rate was integrated into an in vitro synthetic enzymatic biosystem for generating bioelectricity from cellodextrin. The in vitro biosystem containing the best CDP-PGM enzyme complex exhibited a much higher current density (3.35-fold) and power density (2.14-fold) than its counterpart biosystem containing free CDP and PGM mixture.

CONCLUSIONS

Hereby, we first reported bioelectricity generation from cellulosic biomass via in vitro synthetic enzymatic biosystems. This work provided a strategy of how to link non-energetically favorable reaction (cellodextrin phosphorolysis) and energetically favorable reaction (G1P to G6P) together to circumvent unfavorable reaction equilibrium and shed light on improving the reaction efficiency of in vitro synthetic enzymatic biosystems through the construction of synthetic enzyme complexes.

Models for Cell-Free Synthetic Biology: Make Prototyping Easier, Better, and Faster

Cell-free TX-TL is an increasingly mature and useful platform for prototyping, testing, and engineering biological parts and systems. However, to fully accomplish the promises of synthetic biology, mathematical models are required to facilitate the design and predict the behavior of biological components in cell-free extracts. We review here the latest models accounting for transcription, translation, competition, and depletion of resources as well as genome scale models for lysate-based cell-free TX-TL systems, including their current limitations. These models will have to find ways to account for batch-to-batch variability before being quantitatively predictive in cell-free lysate-based platforms.

Expanding biological applications using cell-free metabolic engineering: An overview

Expanding the concept of cell-free biology, implemented both with purified components and crude extracts, is continuing to deepen our appreciation of biological fundamentals while enlarging the range of applications. We are no longer intimidated by the complexity of crude extracts and complicated reaction systems with hundreds of active components, and, instead, coordinately activate and inactivate metabolic processes to focus and expand the capabilities of natural biological processes. This, in turn, dramatically increases the range of benefits offered by new products, both natural and supernatural, that were previously infeasible and/or unimaginable. This overview of cell-free metabolic engineering provides a broad range of examples and insights to guide and motivate continued research that will further expand fundamental understanding and beneficial applications. However, this survey also reveals how far we are from fully unlocking the potential offered by natural and engineered biological components and systems. This is an exciting conclusion, but metabolic engineering by itself is not sufficient. Going forward, innovative metabolic engineering must be intimately combined with creative process engineering to fully realize potential contributions toward a sustainable global civilization.

Functional Surface-immobilization of Genes Using Multistep Strand Displacement Lithography

Immobilization of genes on lithographically structured surfaces allows the study of compartmentalized gene expression processes in an open microfluidic bioreactor system. In contrast to other approaches towards artificial cellular systems, such a setup allows for a continuous supply with gene expression reagents and simultaneous draining of waste products. This facilitates the implementation of cell-free gene expression processes over extended periods of time, which is important for the realization of dynamic gene regulatory feedback systems. Here we provide a detailed protocol for the fabrication of genetic biochips using a simple-to-use lithographic technique based on DNA strand displacement reactions, which exclusively uses commercially available components. We also provide a protocol on the integration of compartmentalized genes with a polydimethylsiloxane (PDMS)-based microfluidic system. Furthermore, we show that the system is compatible with total internal reflection fluorescence (TIRF) microscopy, which can be used for the direct observation of molecular interactions between DNA and molecules contained in the expression mix.

Recombinant cell-lysate-catalysed synthesis of uridine-5'-triphosphate from nucleobase and ribose, and without addition of ATP

Nucleoside triphosphates (NTPs) are important synthetic targets with diverse applications in therapeutics and diagnostics. Enzymatic routes to NTPs from simple building blocks are attractive, however the cost and complexity of assembling the requisite mixtures of multiple enzymes hinders application. Here, we describe the use of an engineered E. coli cell-free lysate as an efficient readily-prepared multi-enzyme biocatalyst for the production of uridine triphosphate (UTP) from free ribose and nucleobase. Endogenous lysate enzymes are able to support the nucleobase ribosylation and nucleotide phosphorylation steps, while uridine phosphorylation and the production of ribose phosphates (ribose 1-phosphate, ribose 5-phosphate and phosphoribosyl pyrophosphate) require recombinant enrichment of endogenous activities. Co-expression vectors encoding all required recombinant enzymes were employed for host cell transformation, such that a cell-free lysate with all necessary activities was obtained from a single bacterial culture. ATP required as phosphorylation cofactor was recycled by endogenous lysate enzymes using cheap, readily-prepared acetyl phosphate. Surprisingly, acetyl phosphate initiated spontaneous generation of ATP in the lysate, most likely from the breakdown of endogenous pools of adenosine-containing starting materials (e.g. adenosine cofactors, ribonucleic acids). The sub-stoichiometric amount of ATP produced and recycled in this way was enough to support all ATP-dependent steps without addition of any exogenous cofactor or auxiliary enzyme. Using this approach, equimolar solutions of orotic acid and ribose are transformed near quantitatively into 1.4 g L^-1 UTP within 2.5 h, using a low-cost, readily-generated biocatalytic preparation.

Quantitative and synthetic biology approaches to combat bacterial pathogens

Antibiotic resistance is one of the biggest threats to public health. The rapid emergence of resistant bacterial pathogens endangers the efficacy of current antibiotics and has led to increasing mortality and economic burden. This crisis calls for more rapid and accurate diagnosis to detect and identify pathogens, as well as to characterize their response to antibiotics. Building on this foundation, treatment options also need to be improved to use current antibiotics more effectively and develop alternative strategies that complement the use of antibiotics. We here review recent developments in diagnosis and treatment of bacterial pathogens with a focus on quantitative biology and synthetic biology approaches.

Microfluidic-Assisted Fabrication of Clay Microgels for Cell-Free Protein Synthesis

Cell-free protein synthesis (CFPS) is a robust platform for the simple, rapid, and cost-effective in vitro production of proteins, as well as an important tool for cell-free synthetic biology research. Here, a microfluidic clay microgel system is reported, which creates compartmentalized microenvironments for CFPS capable of high-yield and repeated protein synthesis, as well as an artificial cell-like structure. As an advantageous platform for CFPS, a modular manner to prepare clay microgels with rationally designed functions is demonstrated: (i) gene/clay microgels enhance protein expression, (ii) gene/clay/magnetic nanoparticle microgels enable a repeated protein production system, and (iii) gene/clay microgels in microfluidic droplets serve as a cell-like structure. Beyond CFPS, considering the compatibility of clay microgels with hydrophilic functional materials, our clay microgels will provide a more general platform for preparing a variety of functional materials such as encapsulating drugs and cells, enabling more biomedical applications.

Cell-free prototyping strategies for enhancing the sustainable production of polyhydroxyalkanoates bioplastics

The polyhydroxyalkanoates (PHAs) are microbially-produced biopolymers that could potentially be used as sustainable alternatives to oil-derived plastics. However, PHAs are currently more expensive to produce than oil-derived plastics. Therefore, more efficient production processes would be desirable. Cell-free metabolic engineering strategies have already been used to optimize several biosynthetic pathways and we envisioned that cell-free strategies could be used for optimizing PHAs biosynthetic pathways. To this end, we developed several Escherichia coli cell-free systems for in vitro prototyping PHAs biosynthetic operons, and also for screening relevant metabolite recycling enzymes. Furthermore, we customized our cell-free reactions through the addition of whey permeate, an industrial waste that has been previously used to optimize in vivo PHAs production. We found that the inclusion of an optimal concentration of whey permeate enhanced relative cell-free GFPmut3b production by approximately 50%. In cell-free transcription-translation prototyping reactions, gas chromatography-mass spectrometry quantification of cell-free 3-hydroxybutyrate (3HB) production revealed differences between the activities of the Native ΔPhaC_C319A (1.18 ± 0.39 µM), C104 ΔPhaC_C319A (4.62 ± 1.31 µM) and C101 ΔPhaC_C319A (2.65 ± 1.27 µM) phaCAB operons that were tested. Interestingly, the most active operon, C104 produced higher levels of PHAs (or PHAs monomers) than the Native phaCAB operon in both in vitro and in vivo assays. Coupled cell-free biotransformation/transcription-translation reactions produced greater yields of 3HB (32.87 ± 6.58 µM), and these reactions were also used to characterize a Clostridium propionicum Acetyl-CoA recycling enzyme. Together, these data demonstrate that cell-free approaches complement in vivo workflows for identifying additional strategies for optimizing PHAs production.

Sequence Specific Modeling of E. coli Cell-Free Protein Synthesis

Cell-free protein synthesis (CFPS) is a widely used research tool in systems and synthetic biology. However, if CFPS is to become a mainstream technology for applications such as point of care manufacturing, we must understand the performance limits and costs of these systems. Toward this question, we used sequence specific constraint based modeling to evaluate the performance of E. coli cell-free protein synthesis. A core E. coli metabolic network, describing glycolysis, the pentose phosphate pathway, energy metabolism, amino acid biosynthesis, and degradation was augmented with sequence specific descriptions of transcription and translation and effective models of promoter function. Model parameters were largely taken from literature; thus the constraint based approach coupled the transcription and translation of the protein product, and the regulation of gene expression, with the availability of metabolic resources using only a limited number of adjustable model parameters. We tested this approach by simulating the expression of two model proteins: chloramphenicol acetyltransferase and dual emission green fluorescent protein, for which we have data sets; we then expanded the simulations to a range of additional proteins. Protein expression simulations were consistent with measurements for a variety of cases. The constraint based simulations confirmed that oxidative phosphorylation was active in the CAT cell-free extract, as without it there was no feasible solution within the experimental constraints of the system. We then compared the metabolism of theoretically optimal and experimentally constrained CFPS reactions, and developed parameter free correlations which could be used to estimate productivity as a function of carbon number and promoter type. Lastly, global sensitivity analysis identified the key metabolic processes that controlled CFPS productivity and energy efficiency. In summary, sequence specific constraint based modeling of CFPS offered a novel means to a priori estimate the performance of a cell-free system, using only a limited number of adjustable parameters. While we modeled the production of a single protein in this study, the approach could easily be extended to multiprotein synthetic circuits, RNA circuits, or the cell-free production of small molecule products.

Cellular reagents for diagnostics and synthetic biology

We have found that the overproduction of enzymes in bacteria followed by their lyophilization leads to 'cellular reagents' that can be directly used to carry out numerous molecular biology reactions. We demonstrate the use of cellular reagents in a variety of molecular diagnostics, such as TaqMan qPCR with no diminution in sensitivity, and in synthetic biology cornerstones such as the Gibson assembly of DNA fragments, where new plasmids can be constructed solely based on adding cellular reagents. Cellular reagents have significantly reduced complexity and cost of production, storage and implementation, features that should facilitate accessibility and use in resource-poor conditions.

Auxotrophy to Xeno-DNA: an exploration of combinatorial mechanisms for a high-fidelity biosafety system for synthetic biology applications

BACKGROUND

Biosafety is a key aspect in the international Genetically Engineered Machine (iGEM) competition, which offers student teams an amazing opportunity to pursue their own research projects in the field of Synthetic Biology. iGEM projects often involve the creation of genetically engineered bacterial strains. To minimize the risks associated with bacterial release, a variety of biosafety systems were constructed, either to prevent survival of bacteria outside the lab or to hinder horizontal or vertical gene transfer.

MAIN BODY

Physical containment methods such as bioreactors or microencapsulation are considered the first safety level. Additionally, various systems involving auxotrophies for both natural and synthetic compounds have been utilized by iGEM teams in recent years. Combinatorial systems comprising multiple auxotrophies have been shown to reduced escape frequencies below the detection limit. Furthermore, a number of natural toxin-antitoxin systems can be deployed to kill cells under certain conditions. Additionally, parts of naturally occurring toxin-antitoxin systems can be used for the construction of 'kill switches' controlled by synthetic regulatory modules, allowing control of cell survival. Kill switches prevent cell survival but do not completely degrade nucleic acids. To avoid horizontal gene transfer, multiple mechanisms to cleave nucleic acids can be employed, resulting in 'self-destruction' of cells. Changes in light or temperature conditions are powerful regulators of gene expression and could serve as triggers for kill switches or self-destruction systems. Xenobiology-based containment uses applications of Xeno-DNA, recoded codons and non-canonical amino acids to nullify the genetic information of constructed cells for wild type organisms. A 'minimal genome' approach brings the opportunity to reduce the genome of a cell to only genes necessary for survival under lab conditions. Such cells are unlikely to survive in the natural environment and are thus considered safe hosts. If suitable for the desired application, a shift to cell-free systems based on Xeno-DNA may represent the ultimate biosafety system.

CONCLUSION

Here we describe different containment approaches in synthetic biology, ranging from auxotrophies to minimal genomes, which can be combined to significantly improve reliability. Since the iGEM competition greatly increases the number of people involved in synthetic biology, we will focus especially on biosafety systems developed and applied in the context of the iGEM competition.

Establishment of a five-enzyme cell-free cascade for the synthesis of uridine diphosphate N-acetylglucosamine

In spite of huge endeavors in cell line engineering to produce glycoproteins with desired and uniform glycoforms, it is still not possible in vivo. Alternatively, in vitro glycoengineering can be used for the modification of glycans. However, in vitro glycoengineering relies on expensive nucleotide sugars, such as uridine 5'-diphospho-N-acetylglucosamine (UDP-GlcNAc) which serves as GlcNAc donor for the synthesis of various glycans. In this work, we present a systematic study for the cell-free de novo synthesis and regeneration of UDP-GlcNAc from polyphosphate, UMP and GlcNAc by a cascade of five enzymes (N-acetylhexosamine kinase (NahK), Glc-1P uridyltransferase (GalU), uridine monophosphate kinase (URA6), polyphosphate kinase (PPK3), and inorganic diphosphatase (PmPpA). All enzymes were expressed in E. coli BL21 Gold (DE3) and purified using immobilized metal affinity chromatography (IMAC). Results from one-pot experiments demonstrate the successful production of UDP-GlcNAc with a yield approaching 100%. The highest volumetric productivity of the cascade was about 0.81 g L^-1  h^-1 of UDP-GlcNAc. A simple model based on mass action kinetics was sufficient to capture the dynamic behavior of the multienzyme pathway. Moreover, a design equation based on metabolic control analysis was established to investigate the effect of enzyme concentration on the UDP-GlcNAc flux and to demonstrate that the flux of UDP-GlcNAc can be controlled by means of the enzyme concentrations. The effect of temperature on the UDP-GlcNAc flux followed an Arrhenius equation and the optimal co-factor concentration (Mg²⁺) for high UDP-GlcNAc synthesis rates depended on the working temperature. In conclusion, the study covers the entire engineering process of a multienzyme cascade, i.e. pathway design, enzyme expression, enzyme purification, reaction kinetics and investigation of the influence of basic parameters (temperature, co-factor concentration, enzyme concentration) on the synthesis rate. Thus, the study lays the foundation for future cascade optimization, preparative scale UDP-GlcNAc synthesis and for in situ coupling of the network with UDP-GlcNAc transferases to efficiently regenerate UDP-GlcNAc. Hence, this study provides a further step towards cost-effective in vitro glycoengineering of antibodies and other glycosylated proteins.

Cell-Free Synthetic Biology for Pathway Prototyping

Engineering biological systems for the production of biofuels and bioproducts holds great potential to transform the bioeconomy, but often requires laborious, time-consuming design-build-test cycles. For decades cell-free systems have offered quick and facile approaches to study enzymes with hopes of informing cellular processes, mainly in the form of purified single-enzyme activity assays. Over the past 20 years, cell-free systems have grown to include multienzymatic systems, both purified and crude. By decoupling cellular growth objectives from enzyme pathway engineering objectives, cell-free systems provide a controllable environment to direct substrates toward a single, desired product. Cell-free approaches are being developed for prototyping and for biomanufacturing. In prototyping applications, the idea is to use cell-free systems to test and optimize biosynthetic pathways before implementation in live cells and scale-up. We present a detailed method for the generation of crude lysates for cell-free pathway prototyping, mix-and-match cell-free metabolic engineering using preenriched lysates, and cell-free protein synthesis driven cell-free metabolic engineering. The cell-free synthetic biology methods described herein are generalizable to any biosynthetic pathway of interest and provide a powerful approach to building pathways in crude lysates for the purpose of prototyping. The foundational principle of the presented approach is that we can construct discrete metabolic pathways through modular assembly of cell-free lysates containing enzyme components produced by overexpression in the lysate chassis strain or by cell-free protein synthesis (in vitro production). Overall, the modular and cell-free nature of our pathway prototyping framework is poised to facilitate multiplexed, automated study of biosynthetic pathways to inform systems-level cellular design.

Canonical translation-modulating OFF-riboswitches with a single aptamer binding to a small molecule that function in a higher eukaryotic cell-free expression system

We have found that OFF-riboswitches that ligand-dependently downregulate the canonical translation in a higher eukaryotic expression system (wheat germ extract) can be easily created by inserting a single aptamer into the 5' untranslated region (UTR) of mRNA, even if its ligand is as small as theophylline. The key is the position of the inserted aptamer: the 5' end (+0 position) is much better than other positions for inhibiting canonical translation with the aptamer-ligand complex. The data showed that ribosome loading is suppressed by a rigid structure in the 5' end, and this suppression is dependent on the structure's stability but not on its size. Although this preference of aptamer insertion point contradicts the results in a lower eukaryote, it accords with the fact that the 5'-end structural hindrance is more effective for blocking the ribosome in higher eukaryotes. Therefore, the present type of OFF-riboswitch would function in various higher eukaryotic expression systems.

Conversion of d-glucose to l-lactate via pyruvate by an optimized cell-free enzymatic biosystem containing minimized reactions

Cell-free synthetic enzymatic biosystem is emerging to expand the traditional biotechnological mode by utilizing a number of purified/partially purified enzymes and coenzymes in a single reaction vessel for the production of desired products from low-cost substrates. Here, a cell-free synthetic biosystem containing minimized number of reactions was designed for the conversion of d-glucose to l-lactate via pyruvate. This NADH-balanced biosystem was comprised of only 5 thermophilic enzymes without ATP supplementation. After optimization of enzyme loading amounts, buffer concentration and cofactor concentration, d-glucose was converted to l-lactate with a product yield of ∼90%. Our study has provided an emerging platform with potentials in producing pyruvate-derived chemicals, and may promote the development of cell-free synthetic enzymatic biosystems for biomanufacturing.

Rapid production and characterization of antimicrobial colicins using Escherichia coli-based cell-free protein synthesis

Colicins are antimicrobial proteins produced by Escherichia coli, which, upon secretion from the host, kill non-host E. coli strains by forming pores in the inner membrane and degrading internal cellular components such as DNA and RNA. Due to their unique cell-killing activities, colicins are considered viable alternatives to conventional antibiotics. Recombinant production of colicins requires co-production of immunity proteins to protect host cells; otherwise, the colicins are lethal to the host. In this study, we used cell-free protein synthesis (CFPS) to produce active colicins without the need for protein purification and co-production of immunity proteins. Cell-free synthesized colicins were active in killing model E. coli cells with different modes of cytotoxicity. Pore-forming colicins E1 and nuclease colicin E2 killed actively growing cells in a nutrient-rich medium, but the cytotoxicity of colicin Ia was low compared to E1 and E2. Moreover, colicin E1 effectively killed cells in a nutrient-free solution, while the activity of E2 was decreased compared to nutrient-rich conditions. Both colicins E1 and E2 decreased the level of persister cells (metabolically dormant cell populations that are insensitive to antibiotics) by up to six orders of magnitude compared to that of the rifampin pretreated persister cells. This study finds that colicins can eradicate non-growing cells including persisters, and that CFPS is a promising platform for rapid production and characterization of toxic proteins.

Simultaneous monitoring of transcription and translation in mammalian cell-free expression in bulk and in cell-sized droplets

Transcription and translation are two critical processes during eukaryotic gene expression that regulate cellular activities. The development of mammalian cell-free expression (CFE) systems provides a platform for studying these two critical processes in vitro for bottom-up synthetic biology applications such as construction of an artificial cell. Moreover, real-time monitoring of the dynamics of synthesized mRNA and protein is key to characterize and optimize gene circuits before implementing in living cells or in artificial cells. However, there are few tools for measurement of mRNA and protein dynamics in mammalian CFE systems. Here, we developed a locked nucleic acid (LNA) probe for monitoring transcription in a HeLa-based CFE system in real-time. By using this LNA probe in conjunction with a fluorescent reporter protein, we were able to simultaneously monitor mRNA and protein dynamics in bulk reactions and cell-sized single-emulsion droplets. We found rapid production of mRNA transcripts that decreased over time as protein production ensued in bulk reactions. Our results also showed that transcription in cell-sized droplets has different dynamics compared to the transcription in bulk reactions. The use of this LNA probe in conjunction with fluorescent proteins in HeLa-based mammalian CFE system provides a versatile in vitro platform for studying mRNA dynamics for bottom-up synthetic biology applications.

Cell-Free Optogenetic Gene Expression System

Optogenetic tools provide a new and efficient way to dynamically program gene expression with unmatched spatiotemporal precision. To date, their vast potential remains untapped in the field of cell-free synthetic biology, largely due to the lack of simple and efficient light-switchable systems. Here, to bridge the gap between cell-free systems and optogenetics, we studied our previously engineered one component-based blue light-inducible Escherichia coli promoter in a cell-free environment through experimental characterization and mathematical modeling. We achieved >10-fold dynamic expression and demonstrated rapid and reversible activation of the target gene to generate oscillatory response. The deterministic model developed was able to recapitulate the system behavior and helped to provide quantitative insights to optimize dynamic response. This in vitro optogenetic approach could be a powerful new high-throughput screening technology for rapid prototyping of complex biological networks in both space and time without the need for chemical induction.

Cell-Free Approaches in Synthetic Biology Utilizing Microfluidics

Synthetic biology is a rapidly growing multidisciplinary branch of science which aims to mimic complex biological systems by creating similar forms. Constructing an artificial system requires optimization at the gene and protein levels to allow the formation of entire biological pathways. Advances in cell-free synthetic biology have helped in discovering new genes, proteins, and pathways bypassing the complexity of the complex pathway interactions in living cells. Furthermore, this method is cost- and time-effective with access to the cellular protein factory without the membrane boundaries. The freedom of design, full automation, and mimicking of in vivo systems reveal advantages of synthetic biology that can improve the molecular understanding of processes, relevant for life science applications. In parallel, in vitro approaches have enhanced our understanding of the living system. This review highlights the recent evolution of cell-free gene design, proteins, and cells integrated with microfluidic platforms as a promising technology, which has allowed for the transformation of the concept of bioprocesses. Although several challenges remain, the manipulation of biological synthetic machinery in microfluidic devices as suitable 'homes' for in vitro protein synthesis has been proposed as a pioneering approach for the development of new platforms, relevant in biomedical and diagnostic contexts towards even the sensing and monitoring of environmental issues.

Cell-free synthetic biology for in vitro biosynthesis of pharmaceutical natural products

Natural products with significant biological activities continuously act as rich sources for drug discovery and development. To harness the potential of these valuable compounds, robust methods need to be developed for their rapid and sustainable production. Cell-free biosynthesis of pharmaceutical natural products by in vitro reconstruction of the entire biosynthetic pathways represents one such solution. In this review, we focus on in vitro biosynthesis of two important classes of natural products, polyketides (PKs) and nonribosomal peptides (NRPs). First, we summarize purified enzyme-based systems for the biosynthesis of PKs, NRPs, and PK/NRP hybrids. Then, we introduce the cell-free protein synthesis (CFPS)-based technology for natural product production. With that, we discuss challenges and opportunities of cell-free synthetic biology for in vitro biosynthesis of natural products.

Synthetic Biology with an All E. coli TXTL System: Quantitative Characterization of Regulatory Elements and Gene Circuits

Over the past decade, a new generation of cell-free transcription-translation (TXTL) systems has been devised for emerging multidisciplinary applications. The DNA-dependent in vitro protein synthesis technology has been developed to tackle applications in synthetic biology, biological and chemical engineering, as well as quantitative disciplines such as biophysics. In addition to being convenient at the biosafety level, the new TXTL platforms are user-friendly; more affordable; more versatile at the level of transcription, with a TX repertoire covering hundreds of parts; and more powerful, with protein production reaching a few mg/mL in batch and continuous modes. As a consequence, TXTL is rising up as a popular research tool and is used by a growing research community. While TXTL is proving reliable for an increasing number of applications, it is important to gain appropriate TXTL skills, especially for quantitative applications. TXTL has become particularly useful to rapidly prototype genetic devices , from single regulatory elements to elementary circuit motifs . In this chapter, we describe the basic procedures to develop appropriate TXTL practices for the characterization of such genetic parts. We use an all E. coli TXTL system developed in our lab, now commercialized by Arbor Biosciences under the name myTXTL.

Generation of thermostable Moloney murine leukemia virus reverse transcriptase variants using site saturation mutagenesis library and cell-free protein expression system

We attempted to increase the thermostability of Moloney murine leukemia virus (MMLV) reverse transcriptase (RT). The eight-site saturation mutagenesis libraries corresponding to Ala70−Arg469 in the whole MMLV RT (Thr24−Leu671), in each of which 1 out of 50 amino acid residues was replaced with other amino acid residue, were constructed. Seven-hundred and sixty eight MMLV RT clones were expressed using a cell-free protein expression system, and their thermostabilities were assessed by the temperature of thermal treatment at which they retained cDNA synthesis activity. One clone D200C was selected as the most thermostable variant. The highest temperature of thermal treatment at which D200C exhibited cDNA synthesis activity was 57ºC, which was higher than for WT (53ºC). Our results suggest that a combination of site saturation mutagenesis library and cell-free protein expression system might be useful for generation of thermostable MMLV RT in a short period of time for expression and selection.

Deacidification of grass silage press juice by continuous production of acetoin from its lactate via an immobilized enzymatic reaction cascade

An immobilized enzymatic reaction cascade was designed and optimized for the deacidification of grass silage press juice (SPJ), thus facilitating the production of bio-based chemicals. The cascade involves a three-step process using four enzymes immobilized in a Ca-alginate gel and uses lactic acid to form acetoin, a value-added product. The reaction is performed with a continuous, pH-dependent substrate feed under oxygenation. With titrated lactic acid yields of up to 91% and reaction times of ca. 6h was achieved. Using SPJ as titrant yields of 49% were obtained within 6h. In this deacidification process, with acetoin one value-added bio-based chemical is produced while simultaneously the remaining press juice can be used in applications that require a higher pH. Such, this system can be applied in a multi-product biorefinery concept to take full advantage of nutrient-rich SPJ, which is a widely available and easily storable renewable resource.

Cell-free protein synthesis from non-growing, stressed Escherichia coli

Cell-free protein synthesis is a versatile protein production system. Performance of the protein synthesis depends on highly active cytoplasmic extracts. Extracts from E. coli are believed to work best; they are routinely obtained from exponential growing cells, aiming to capture the most active translation system. Here, we report an active cell-free protein synthesis system derived from cells harvested at non-growth, stressed conditions. We found a downshift of ribosomes and proteins. However, a characterization revealed that the stoichiometry of ribosomes and key translation factors was conserved, pointing to a fully intact translation system. This was emphasized by synthesis rates, which were comparable to those of systems obtained from fast-growing cells. Our approach is less laborious than traditional extract preparation methods and multiplies the yield of extract per cultivation. This simplified growth protocol has the potential to attract new entrants to cell-free protein synthesis and to broaden the pool of applications. In this respect, a translation system originating from heat stressed, non-growing E. coli enabled an extension of endogenous transcription units. This was demonstrated by the sigma factor depending activation of parallel transcription. Our cell-free expression platform adds to the existing versatility of cell-free translation systems and presents a tool for cell-free biology.

Sequence-based prediction of permissive stretches for internal protein tagging and knockdown

BACKGROUND

Internal tagging of proteins by inserting small functional peptides into surface accessible permissive sites has proven to be an indispensable tool for basic and applied science. Permissive sites are typically identified by transposon mutagenesis on a case-by-case basis, limiting scalability and their exploitation as a system-wide protein engineering tool.

METHODS

We developed an apporach for predicting permissive stretches (PSs) in proteins based on the identification of length-variable regions (regions containing indels) in homologous proteins.

RESULTS

We verify that a protein's primary structure information alone is sufficient to identify PSs. Identified PSs are predicted to be predominantly surface accessible; hence, the position of inserted peptides is likely suitable for diverse applications. We demonstrate the viability of this approach by inserting a Tobacco etch virus protease recognition site (TEV-tag) into several PSs in a wide range of proteins, from small monomeric enzymes (adenylate kinase) to large multi-subunit molecular machines (ATP synthase) and verify their functionality after insertion. We apply this method to engineer conditional protein knockdowns directly in the Escherichia coli chromosome and generate a cell-free platform with enhanced nucleotide stability.

CONCLUSIONS

Functional internally tagged proteins can be rationally designed and directly chromosomally implemented. Critical for the successful design of protein knockdowns was the incorporation of surface accessibility and secondary structure predictions, as well as the design of an improved TEV-tag that enables efficient hydrolysis when inserted into the middle of a protein. This versatile and portable approach can likely be adapted for other applications, and broadly adopted. We provide guidelines for the design of internally tagged proteins in order to empower scientists with little or no protein engineering expertise to internally tag their target proteins.

Recent advances in production of 5-aminolevulinic acid using biological strategies

5-Aminolevulinic acid (5-ALA) is the precursor for the biosynthesis of tetrapyrrole compounds and has broad applications in the medical and agricultural fields. Because of the disadvantages of chemical synthesis methods, microbial production of 5-ALA has drawn intensive attention and has been regarded as an alternative in the last years, especially with the rapid development of metabolic engineering and synthetic biology. In this mini-review, recent advances on the application and microbial production of 5-ALA using novel biological approaches (such as whole-cell enzymatic-transformation, metabolic pathway engineering and cell-free process) are described and discussed in detail. In addition, the challenges and prospects of synthetic biology are discussed.

Protein engineering of oxidoreductases utilizing nicotinamide-based coenzymes, with applications in synthetic biology

Two natural nicotinamide-based coenzymes (NAD and NADP) are indispensably required by the vast majority of oxidoreductases for catabolism and anabolism, respectively. Most NAD(P)-dependent oxidoreductases prefer one coenzyme as an electron acceptor or donor to the other depending on their different metabolic roles. This coenzyme preference associated with coenzyme imbalance presents some challenges for the construction of high-efficiency in vivo and in vitro synthetic biology pathways. Changing the coenzyme preference of NAD(P)-dependent oxidoreductases is an important area of protein engineering, which is closely related to product-oriented synthetic biology projects. This review focuses on the methodology of nicotinamide-based coenzyme engineering, with its application in improving product yields and decreasing production costs. Biomimetic nicotinamide-containing coenzymes have been proposed to replace natural coenzymes because they are more stable and less costly than natural coenzymes. Recent advances in the switching of coenzyme preference from natural to biomimetic coenzymes are also covered in this review. Engineering coenzyme preferences from natural to biomimetic coenzymes has become an important direction for coenzyme engineering, especially for in vitro synthetic pathways and in vivo bioorthogonal redox pathways.

Artificial OFF-Riboswitches That Downregulate Internal Ribosome Entry without Hybridization Switches in a Eukaryotic Cell-Free Translation System

We constructed novel artificial riboswitches that function in a eukaryotic translation system (wheat germ extract), by rationally implanting an in vitro-selected aptamer into the intergenic internal ribosome entry site (IRES) of Plautia stali intestine virus. These eukaryotic OFF-riboswitches (OFF-eRSs) ligand-dose-dependently downregulate IRES-mediated translation without hybridization switches, which typical riboswitches utilize for gene regulation. The hybridization-switch-free mechanism not only allows for easy design but also requires less energy for regulation, resulting in a higher switching efficiency than hybridization-switch-based OFF-eRSs provide. In addition, even a small ligand such as theophylline can induce satisfactory repression, in contrast to other types of OFF-eRSs that modulate the 5' cap-dependent canonical translation. Because our proposed hybridization-switch-free OFF-eRSs are based on a versatile IRES that functions well in many types of eukaryotic translation systems, they would be widely usable elements for synthetic gene circuits in both cell-free and cell-based synthetic biology.

In Vitro Metabolic Engineering of Amorpha-4,11-diene Biosynthesis at Enhanced Rate and Specific Yield of Production

In vitro metabolic engineering is an alternative approach to cell-based biosynthesis. It offers unprecedented flexibility for the study of biochemical pathways, thus providing useful information for the rational design and assembly of reaction modules. Herein, we took the advantage of in vitro metabolic engineering to initially gain insight into the regulatory network of a reconstituted amorpha-4,11-diene (AD) synthetic pathway. Guided by lin-log approximation, we rapidly identified the hitherto unrecognized inhibition of adenosine triphosphate (ATP) on both farnesyl pyrophosphate synthase (FPPS) and amorpha-4,11-diene synthase (ADS). Furthermore, the byproduct, pyrophosphate (PPi), potently inhibits ADS, but not FPPS. To lower the inhibition, an ATP recycling system and pyrophosphatase were used and resulted in a significant (∼3-fold) enhancement in the rate of AD production (∼5.7 μmol L^-1 min^-1). A coimmobilized multienzyme reaction system was then developed to recycle the enzymes. When inhibitory metabolites concentrations were reduced, the specific enzymatic yield of AD was further enhanced (>6-fold). This study demonstrated that mitigating the accumulation of inhibitory metabolites can result in higher yields of AD production by in vitro multienzymatic reactions.

Synthesis of Infectious Bacteriophages in an E. coli-based Cell-free Expression System

A new generation of cell-free transcription-translation (TXTL) systems, engineered to have a greater versatility and modularity, provide novel capabilities to perform basic and applied sciences in test tube reactions. Over the past decade, cell-free TXTL has become a powerful technique for a broad range of novel multidisciplinary research areas related to quantitative and synthetic biology. The new TXTL platforms are particularly useful to construct and interrogate biochemical systems through the execution of synthetic or natural gene circuits. In vitro TXTL has proven convenient to rapidly prototype regulatory elements and biological networks as well as to recapitulate molecular self-assembly mechanisms found in living systems. In this article, we describe how infectious bacteriophages, such as MS2 (RNA), ΦΧ174 (ssDNA), and T7 (dsDNA), are entirely synthesized from their genome in one-pot reactions using an all Escherichia coli, cell-free TXTL system. Synthesis of the three coliphages is quantified using the plaque assay. We show how the yield of synthesized phage depends on the biochemical settings of the reactions. Molecular crowding, emulated through a controlled concentration of PEG 8000, affects the amount of synthesized phages by orders of magnitudes. We also describe how to amplify the phages and how to purify their genomes. The set of protocols and results presented in this work should be of interest to multidisciplinary researchers involved in cell-free synthetic biology and bioengineering.

Development of SimCells as a novel chassis for functional biosensors

This work serves as a proof-of-concept for bacterially derived SimCells (Simple Cells), which contain the cell machinery from bacteria and designed DNA (or potentially a simplified genome) to instruct the cell to carry out novel, specific tasks. SimCells represent a reprogrammable chassis without a native chromosome, which can host designed DNA to perform defined functions. In this paper, the use of Escherichia coli MC1000 ∆minD minicells as a non-reproducing chassis for SimCells was explored, as demonstrated by their ability to act as sensitive biosensors for small molecules. Highly purified minicells derived from E. coli strains containing gene circuits for biosensing were able to transduce the input signals from several small molecules (glucarate, acrylate and arabinose) into the production of green fluorescent protein (GFP). A mathematical model was developed to fit the experimental data for induction of gene expression in SimCells. The intracellular ATP level was shown to be important for SimCell function. A purification and storage protocol was developed to prepare SimCells which could retain their functions for an extended period of time. This study demonstrates that SimCells are able to perform as ‘smart bioparticles’ controlled by designed gene circuits.

An In Vitro Enzyme System for the Production of myo-Inositol from Starch

We developed an in vitro enzyme system to produce myo-inositol from starch. Four enzymes were used, maltodextrin phosphorylase (MalP), phosphoglucomutase (PGM), myo-inositol-3-phosphate synthase (MIPS), and inositol monophosphatase (IMPase). The enzymes were thermostable: MalP and PGM from the hyperthermophilic archaeon Thermococcus kodakarensis, MIPS from the hyperthermophilic archaeon Archaeoglobus fulgidus, and IMPase from the hyperthermophilic bacterium Thermotoga maritima The enzymes were individually produced in Escherichia coli and partially purified by subjecting cell extracts to heat treatment and removing denatured proteins. The four enzyme samples were incubated at 90°C with amylose, phosphate, and NAD⁺, resulting in the production of myo-inositol with a yield of over 90% at 2 h. The effects of varying the concentrations of reaction components were examined. When the system volume was increased and NAD⁺ was added every 2 h, we observed the production of 2.9 g myo-inositol from 2.9 g amylose after 7 h, achieving gram-scale production with a molar conversion of approximately 96%. We further integrated the pullulanase from T. maritima into the system and observed myo-inositol production from soluble starch and raw potato with yields of 73% and 57 to 61%, respectively.IMPORTANCEmyo-Inositol is an important nutrient for human health and provides a wide variety of benefits as a dietary supplement. This study demonstrates an alternative method to produce myo-inositol from starch with an in vitro enzyme system using thermostable maltodextrin phosphorylase (MalP), phosphoglucomutase (PGM), myo-inositol-3-phosphate synthase, and myo-inositol monophosphatase. By utilizing MalP and PGM to generate glucose 6-phosphate, we can avoid the addition of phosphate donors such as ATP, the use of which would not be practical for scaled-up production of myo-inositol. myo-Inositol was produced from amylose on the gram scale with yields exceeding 90%. Conversion rates were also high, producing over 2 g of myo-inositol within 4 h in a 200-ml reaction mixture. By adding a thermostable pullulanase, we produced myo-inositol from raw potato with yields of 57 to 61% (wt/wt). The system developed here should provide an attractive alternative to conventional methods that rely on extraction or microbial production of myo-inositol.

Cell-Free Synthetic Biology Chassis for Nanocatalytic Photon-to-Hydrogen Conversion

We report on an entirely man-made nano-bio architecture fabricated through noncovalent assembly of a cell-free expressed transmembrane proton pump and TiO₂ semiconductor nanoparticles as an efficient nanophotocatalyst for H₂ evolution. The system produces hydrogen at a turnover of about 240 μmol of H₂ (μmol protein)^-1 h^-1 and 17.74 mmol of H₂ (μmol protein)^-1 h^-1 under monochromatic green and white light, respectively, at ambient conditions, in water at neutral pH and room temperature, with methanol as a sacrificial electron donor. Robustness and flexibility of this approach allow for systemic manipulation at the nanoparticle-bio interface toward directed evolution of energy transformation materials and artificial systems.

Synthetic CO2-fixation enzyme cascades immobilized on self-assembled nanostructures that enhance CO2/O2 selectivity of RubisCO

BACKGROUND

With increasing concerns over global warming and depletion of fossil-fuel reserves, it is attractive to develop innovative strategies to assimilate CO₂, a greenhouse gas, into usable organic carbon. Cell-free systems can be designed to operate as catalytic platforms with enzymes that offer exceptional selectivity and efficiency, without the need to support ancillary reactions of metabolic pathways operating in intact cells. Such systems are yet to be exploited for applications involving CO₂ utilization and subsequent conversion to valuable products, including biofuels. The Calvin-Benson-Bassham (CBB) cycle and the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) play a pivotal role in global CO₂ fixation.

RESULTS

We hereby demonstrate the co-assembly of two RubisCO-associated multienzyme cascades with self-assembled synthetic amphiphilic peptide nanostructures. The immobilized enzyme cascades sequentially convert either ribose-5-phosphate (R-5-P) or glucose, a simpler substrate, to ribulose 1,5-bisphosphate (RuBP), the acceptor for incoming CO₂ in the carboxylation reaction catalyzed by RubisCO. Protection from proteolytic degradation was observed in nanostructures associated with the small dimeric form of RubisCO and ancillary enzymes. Furthermore, nanostructures associated with a larger variant of RubisCO resulted in a significant enhancement of the enzyme's selectivity towards CO₂, without adversely affecting the catalytic activity.

CONCLUSIONS

The ability to assemble a cascade of enzymes for CO₂ capture using self-assembling nanostructure scaffolds with functional enhancements show promise for potentially engineering entire pathways (with RubisCO or other CO₂-fixing enzymes) to redirect carbon from industrial effluents into useful bioproducts.