Prolonged cell-free protein synthesis using dual energy sources: Combined use of creatine phosphate and glucose for the efficient supply of ATP and retarded accumulation of phosphate

Bioengineering lipid-based synthetic cells for therapeutic protein delivery

Synthetic cells (SCs) offer a promising approach for therapeutic protein delivery, combining principles from synthetic biology and drug delivery. Engineered to mimic natural cells, SCs provide biocompatibility and versatility, with precise control over their architecture and composition. Protein production is essential in living cells, and SCs aim to replicate this process using compartmentalized cell-free protein synthesis systems within lipid bilayers. Lipid bilayers serve as favored membranes in SC design due to their similarity to the biological cell membrane. Moreover, engineering lipidic membranes enable tissue-specific targeting and immune evasion, while stimulus-responsive SCs allow for triggered protein production and release. This Review explores lipid-based SCs as platforms for therapeutic protein delivery, discussing their design principles, functional attributes, and translational challenges and potential.

Cell-Free Protein Expression by a Reconstituted Transcription-Translation System Energized by Sugar Catabolism

Cooperation between catabolism and anabolism is crucial for maintaining homeostasis in living cells. The most fundamental systems for catabolism and anabolism are the glycolysis of sugars and the transcription-translation (TX-TL) of DNA, respectively. Despite their importance in living cells, the in vitro reconstitution of their cooperation through purified factors has not been achieved, which hinders the elucidation of the design principle in living cells. Here, we reconstituted glycolysis using sugars and integrated it with the PURE system, a commercial in vitro TX-TL kit composed of purified factors. By optimizing key parameters, such as glucokinase and initial phosphate concentrations, we determined suitable conditions for their cooperation. The optimized system showed protein synthesis at up to 33% of that of the original PURE system. We observed that ATP consumption in upstream glycolysis inhibits TX-TL and that this inhibition can be alleviated by the co-addition of glycolytic intermediates, such as glyceraldehyde 3-phosphate, with glucose. Moreover, the system developed here simultaneously synthesizes a subset of its own enzymes, that is, glycolytic enzymes, in a single test tube, which is a necessary step toward self-replication. As glycolysis and TX-TL provide building blocks for constructing cells, the integrated system can be a fundamental material for reconstituting living cells from purified factors.

Thermostable in vitro transcription-translation compatible with microfluidic droplets

BACKGROUND

In vitro expression involves the utilization of the cellular transcription and translation machinery in an acellular context to produce one or more proteins of interest and has found widespread application in synthetic biology and in pharmaceutical biomanufacturing. Most in vitro expression systems available are active at moderate temperatures, but to screen large libraries of natural or artificial genetic diversity for highly thermostable enzymes or enzyme variants, it is instrumental to enable protein synthesis at high temperatures.

OBJECTIVES

Develop an in vitro expression system operating at high temperatures compatible with enzymatic assays and with technologies that enable ultrahigh-throughput protein expression in reduced volumes, such as microfluidic water-in-oil (w/o) droplets.

RESULTS

We produced cell-free extracts from Thermus thermophilus for in vitro translation including thermostable enzymatic cascades for energy regeneration and a moderately thermostable RNA polymerase for transcription, which ultimately limited the temperature of protein synthesis. The yield was comparable or superior to other thermostable in vitro expression systems, while the preparation procedure is much simpler and can be suited to different Thermus thermophilus strains. Furthermore, these extracts have enabled in vitro expression in microfluidic droplets at high temperatures for the first time.

CONCLUSIONS

Cell-free extracts from Thermus thermophilus represent a simpler alternative to heavily optimized or pure component thermostable in vitro expression systems. Moreover, due to their compatibility with droplet microfluidics and enzyme assays at high temperatures, the reported system represents a convenient gateway for enzyme screening at higher temperatures with ultrahigh-throughput.

What remains from living cells in bacterial lysate-based cell-free systems

Because they mimic cells while offering an accessible and controllable environment, lysate-based cell-free systems (CFS) have emerged as valuable biotechnology tools for synthetic biology. Historically used to uncover fundamental mechanisms of life, CFS are nowadays used for a multitude of purposes, including protein production and prototyping of synthetic circuits. Despite the conservation of fundamental functions in CFS like transcription and translation, RNAs and certain membrane-embedded or membrane-bound proteins of the host cell are lost when preparing the lysate. As a result, CFS largely lack some essential properties of living cells, such as the ability to adapt to changing conditions, to maintain homeostasis and spatial organization. Regardless of the application, shedding light on the black-box of the bacterial lysate is necessary to fully exploit the potential of CFS. Most measurements of the activity of synthetic circuits in CFS and in vivo show significant correlations because these only require processes that are preserved in CFS, like transcription and translation. However, prototyping circuits of higher complexity that require functions that are lost in CFS (cell adaptation, homeostasis, spatial organization) will not show such a good correlation with in vivo conditions. Both for prototyping circuits of higher complexity and for building artificial cells, the cell-free community has developed devices to reconstruct cellular functions. This mini-review compares bacterial CFS to living cells, focusing on functional and cellular process differences and the latest developments in restoring lost functions through complementation of the lysate or device engineering.

The Long Road to a Synthetic Self-Replicating Central Dogma

The construction of a biochemical system capable of self-replication is a key objective in bottom-up synthetic biology. Throughout the past two decades, a rapid progression in the design of in vitro cell-free systems has provided valuable insight into the requirements for the development of a minimal system capable of self-replication. The main limitations of current systems can be attributed to their macromolecular composition and how the individual macromolecules use the small molecules necessary to drive RNA and protein synthesis. In this Perspective, we discuss the recent steps that have been taken to generate a minimal cell-free system capable of regenerating its own macromolecular components and maintaining the homeostatic balance between macromolecular biogenesis and consumption of primary building blocks. By following the flow of biological information through the central dogma, we compare the current versions of these systems to date and propose potential alterations aimed at designing a model system for self-replicative synthetic cells.

Bottom-Up Synthetic Biology Using Cell-Free Protein Synthesis

Technical advances in biotechnology have greatly accelerated the development of bottom-up synthetic biology. Unlike top-down approaches, bottom-up synthetic biology focuses on the construction of a minimal cell from scratch and the application of these principles to solve challenges. Cell-free protein synthesis (CFPS) systems provide minimal machinery for transcription and translation, from either a fractionated cell lysate or individual purified protein elements, thus speeding up the development of synthetic cell projects. In this review, we trace the history of the cell-free technique back to the first in vitro fermentation experiment using yeast cell lysate. Furthermore, we summarized progresses of individual cell mimicry modules, such as compartmentalization, gene expression regulation, energy regeneration and metabolism, growth and division, communication, and motility. Finally, current challenges and future perspectives on the field are outlined.

Constructing Cell-Free Expression Systems for Low-Cost Access

Cell-free systems for gene expression have gained attention as platforms for the facile study of genetic circuits and as highly effective tools for teaching. Despite recent progress, the technology remains inaccessible for many in low- and middle-income countries due to the expensive reagents required for its manufacturing, as well as specialized equipment required for distribution and storage. To address these challenges, we deconstructed processes required for cell-free mixture preparation and developed a set of alternative low-cost strategies for easy production and sharing of extracts. First, we explored the stability of cell-free reactions dried through a low-cost device based on silica beads, as an alternative to commercial automated freeze dryers. Second, we report the positive effect of lactose as an additive for increasing protein synthesis in maltodextrin-based cell-free reactions using either circular or linear DNA templates. The modifications were used to produce active amounts of two high-value reagents: the isothermal polymerase Bst and the restriction enzyme BsaI. Third, we demonstrated the endogenous regeneration of nucleoside triphosphates and synthesis of pyruvate in cell-free systems (CFSs) based on phosphoenol pyruvate (PEP) and maltodextrin (MDX). We exploited this novel finding to demonstrate the use of a cell-free mixture completely free of any exogenous nucleotide triphosphates (NTPs) to generate high yields of sfGFP expression. Together, these modifications can produce desiccated extracts that are 203-424-fold cheaper than commercial versions. These improvements will facilitate wider use of CFS for research and education purposes.

An in vitro Assay of mRNA 3' end Using the E. coli Cell-free Expression System

At the end of about 80% of the operon in Escherichia coli, translation termination decouples transcription, leading to Rho-dependent transcription termination (RDT). However, no in vitro or in vivo assay system has proven to be good enough to see the 3' end of the mRNA generated by RDT. Here, we present a cell-free assay system that could provide detailed information on the 3' end of a transcript RNA generated by RDT. Our protocol shows how to extract transcript RNA generated by transcription reactions from a cell-free extract, followed by an RNA oligomer ligation to the 3' end of a transcript RNA of interest. The 3' end of the RNA is amplified using RT-PCR. Its genetic location can be determined using a gene-specific primer extension reaction. The 3' ends of mRNA can be visualized and quantified by polyacrylamide gel electrophoresis. One significant advantage of a cell-free assay system is that factors involved in the generation of the 3' end, such as proteins and sRNA, can be directly assayed by exogenously adding factor(s) to the reaction. Graphic abstract: An illustration of the experimental methodology.

Modularize and Unite: Toward Creating a Functional Artificial Cell

An artificial cell is a simplified model of a living system, bringing breakthroughs into both basic life science and applied research. The bottom-up strategy instructs the construction of an artificial cell from nonliving materials, which could be complicated and interdisciplinary considering the inherent complexity of living cells. Although significant progress has been achieved in the past 2 decades, the area is still facing some problems, such as poor compatibility with complex bio-systems, instability, and low standardization of the construction method. In this review, we propose creating artificial cells through the integration of different functional modules. Furthermore, we divide the function requirements of an artificial cell into four essential parts (metabolism, energy supplement, proliferation, and communication) and discuss the present researches. Then we propose that the compartment and the reestablishment of the communication system would be essential for the reasonable integration of functional modules. Although enormous challenges remain, the modular construction would facilitate the simplification and standardization of an artificial cell toward a natural living system. This function-based strategy would also broaden the application of artificial cells and represent the steps of imitating and surpassing nature.

Production of Recombinant Horseradish Peroxidase in an Engineered Cell-free Protein Synthesis System

One of the main advantages of a cell-free synthesis system is that the synthetic machinery of cells can be modularized and re-assembled for desired purposes. In this study, we attempted to combine the translational activity of Escherichia coli extract with a heme synthesis pathway for the functional production of horseradish peroxidase (HRP). We first optimized the reaction conditions and the sequence of template DNA to enhance protein expression and folding. The reaction mixture was then supplemented with 5-aminolevulinic acid synthase to facilitate co-synthesis of the heme prosthetic group from glucose. Combining the different synthetic modules required for protein synthesis and cofactor generation led to successful production of functional HRP in a cell-free synthesis system.

sRNA-mediated regulation of gal mRNA in E. coli: Involvement of transcript cleavage by RNase E together with Rho-dependent transcription termination

In bacteria, small non-coding RNAs (sRNAs) bind to target mRNAs and regulate their translation and/or stability. In the polycistronic galETKM operon of Escherichia coli, binding of the Spot 42 sRNA to the operon transcript leads to the generation of galET mRNA. The mechanism of this regulation has remained unclear. We show that sRNA-mRNA base pairing at the beginning of the galK gene leads to both transcription termination and transcript cleavage within galK, and generates galET mRNAs with two different 3'-OH ends. Transcription termination requires Rho, and transcript cleavage requires the endonuclease RNase E. The sRNA-mRNA base-paired segments required for generating the two galET species are different, indicating different sequence requirements for the two events. The use of two targets in an mRNA, each of which causes a different outcome, appears to be a novel mode of action for a sRNA. Considering the prevalence of potential sRNA targets at cistron junctions, the generation of new mRNA species by the mechanisms reported here might be a widespread mode of bacterial gene regulation.

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.

In Vitro Use of Cellular Synthetic Machinery for Biosensing Applications

The application of biosensors is expanding in diverse fields due to their high selectivity and sensitivity. Biosensors employ biological components for the recognition of target analytes. In addition, the amplifying nature of biosynthetic processes can potentially be harnessed to for biological transduction of detection signals. Recent advances in the development of highly productive and cost-effective cell-free synthesis systems make it possible to use these systems as the biological transducers to generate biosensing signals. This review surveys recent developments in cell-free biosensors, focusing on the newly devised mechanisms for the biological recognition of analytes to initiate the amplification processes of transcription and translation.

High-Throughput Optimization Cycle of a Cell-Free Ribosome Assembly and Protein Synthesis System

Building variant ribosomes offers opportunities to reveal fundamental principles underlying ribosome biogenesis and to make ribosomes with altered properties. However, cell viability limits mutations that can be made to the ribosome. To address this limitation, the in vitro integrated synthesis, assembly and translation (iSAT) method for ribosome construction from the bottom up was recently developed. Unfortunately, iSAT is complex, costly, and laborious to researchers, partially due to the high cost of reaction buffer containing over 20 components. In this study, we develop iSAT in Escherichia coli BL21Rosetta2 cell lysates, a commonly used bacterial strain, with a cost-effective poly sugar and nucleotide monophosphate-based metabolic scheme. We achieved a 10-fold increase in protein yield over our base case with an evolutionary design of experiments approach, screening 490 reaction conditions to optimize the reaction buffer. The computationally guided, cell-free, high-throughput technology presented here augments the way we approach multicomponent synthetic biology projects and efforts to repurpose ribosomes.

Unexpected instabilities explain batch-to-batch variability in cell-free protein expression systems

Cell-free methods of protein synthesis offer rapid access to expressed proteins. Though the amounts produced are generally only at a small scale, these are sufficient to perform protein-protein interaction assays and tests of enzymatic activity. As such they are valuable tools for the biochemistry and bioengineering community. However the most complex, eukaryotic cell-free systems are difficult to manufacture in house and can be prohibitively expensive to obtain from commercial sources. The Leishmania tarentolae system offers a relatively cheap alternative which is capable of producing difficult to express proteins, but which is simpler to produce in large scale. However, this system suffers from batch-to-batch variability, which has been accepted as a consequence of the complexity of the extracts. Here we show an unexpected origin for the variability observed and demonstrate that small variations in a single parameter can dramatically affect expression, such that minor pipetting errors can have major effects on yields. L. tarentolae cell-free lysate activity is shown to be more stable to changes in Mg²⁺ concentration at a lower ratio of feed solution to lysate in the reaction than typically used, and a higher Mg²⁺ optimum. These changes essentially eliminate batch-to-batch variability of L. tarentolae lysate activity and permit their full potential to be realized.

Experimentally Validated Model Enables Debottlenecking of in Vitro Protein Synthesis and Identifies a Control Shift under in Vivo Conditions

Cell-free (in vitro) protein synthesis (CFPS) systems provide a versatile tool that can be used to investigate different aspects of the transcription-translation machinery by reducing cells to the basic functions of protein formation. Recent improvements in reaction stability and lysate preparation offer the potential to expand the scope of in vitro biosynthesis from a research tool to a multifunctional and versatile platform for protein production and synthetic biology. To date, even the best-performing CFPS systems are drastically slower than in vivo references. Major limitations are imposed by ribosomal activities that progress in an order of magnitude slower on the mRNA template. Owing to the complex nature of the ribosomal machinery, conventional "trial and error" experiments only provide little insight into how the desired performance could be improved. By applying a DNA-sequence-oriented mechanistic model, we analyzed the major differences between cell-free in vitro and in vivo protein synthesis. We successfully identified major limiting elements of in vitro translation, namely the supply of ternary complexes consisting of EFTu and tRNA. Additionally, we showed that diluted in vitro systems suffer from reduced ribosome numbers. On the basis of our model, we propose a new experimental design predicting 90% increased translation rates, which were well achieved in experiments. Furthermore, we identified a shifting control in the translation rate, which is characterized by availability of the ternary complex under in vitro conditions and the initiation of translation in a living cell. Accordingly, the model can successfully be applied to sensitivity analyses and experimental design.

Optimizing cell-free protein expression in CHO: Assessing small molecule mass transfer effects in various reactor configurations

Cell-free protein synthesis (CFPS) is an ideal platform for rapid and convenient protein production. However, bioreactor design remains a critical consideration in optimizing protein expression. Using turbo green fluorescent protein (tGFP) as a model, we tracked small molecule components in a Chinese Hamster Ovary (CHO) CFPS system to optimize protein production. Here, three bioreactors in continuous-exchange cell-free (CECF) format were characterized. A GFP optical sensor was built to monitor the product in real-time. Mass transfer of important substrate and by-product components such as nucleoside triphosphates (NTPs), creatine, and inorganic phosphate (Pi) across a 10-kDa MWCO cellulose membrane was calculated. The highest efficiency measured by tGFP yields were found in a microdialysis device configuration; while a negative effect on yield was observed due to limited mass transfer of NTPs in a dialysis cup configuration. In 24-well plate high-throughput CECF format, addition of up to 40 mM creatine phosphate in the system increased yields by up to ∼60% relative to controls. Direct ATP addition, as opposed to creatine phosphate addition, negatively affected the expression. Pi addition of up to 30 mM to the expression significantly reduced yields by over ∼40% relative to controls. Overall, data presented in this report serves as a valuable reference to optimize the CHO CFPS system for next-generation bioprocessing. Biotechnol. Bioeng. 2017;114: 1478-1486. © 2017 Wiley Periodicals, Inc.

Site-Specific Cleavage of Ribosomal RNA in Escherichia coli-Based Cell-Free Protein Synthesis Systems

Cell-free protein synthesis, which mimics the biological protein production system, allows rapid expression of proteins without the need to maintain a viable cell. Nevertheless, cell-free protein expression relies on active in vivo translation machinery including ribosomes and translation factors. Here, we examined the integrity of the protein synthesis machinery, namely the functionality of ribosomes, during (i) the cell-free extract preparation and (ii) the performance of in vitro protein synthesis by analyzing crucial components involved in translation. Monitoring the 16S rRNA, 23S rRNA, elongation factors and ribosomal protein S1, we show that processing of a cell-free extract results in no substantial alteration of the translation machinery. Moreover, we reveal that the 16S rRNA is specifically cleaved at helix 44 during in vitro translation reactions, resulting in the removal of the anti-Shine-Dalgarno sequence. These defective ribosomes accumulate in the cell-free system. We demonstrate that the specific cleavage of the 16S rRNA is triggered by the decreased concentrations of Mg2+. In addition, we provide evidence that helix 44 of the 30S ribosomal subunit serves as a point-of-entry for ribosome degradation in Escherichia coli. Our results suggest that Mg2+ homeostasis is fundamental to preserving functional ribosomes in cell-free protein synthesis systems, which is of major importance for cell-free protein synthesis at preparative scale, in order to create highly efficient technical in vitro systems.

Energizing eukaryotic cell-free protein synthesis with glucose metabolism

Eukaryotic cell-free protein synthesis (CFPS) is limited by the dependence on costly high-energy phosphate compounds and exogenous enzymes to power protein synthesis (e.g., creatine phosphate and creatine kinase, CrP/CrK). Here, we report the ability to use glucose as a secondary energy substrate to regenerate ATP in a Saccharomyces cerevisiae crude extract CFPS platform. We observed synthesis of 3.64±0.35 μg mL(-1) active luciferase in batch reactions with 16 mM glucose and 25 mM phosphate, resulting in a 16% increase in relative protein yield (μg protein/$ reagents) compared to the CrP/CrK system. Our demonstration provides the foundation for development of cost-effective eukaryotic CFPS platforms.

A cost-effective polyphosphate-based metabolism fuels an all E. coli cell-free expression system

A new cost-effective metabolism providing an ATP-regeneration system for cell-free protein synthesis is presented. Hexametaphosphate, a polyphosphate molecule, is used as phosphate donor together with maltodextrin, a polysaccharide used as carbon source to stimulate glycolysis. Remarkably, addition of enzymes is not required for this metabolism, which is carried out by endogenous catalysts present in the Escherichia coli crude extract. This new ATP regeneration system allows efficient recycling of inorganic phosphate, a strong inhibitor of protein synthesis. We show that up to 1.34-1.65mg/mL of active reporter protein is synthesized in batch-mode reaction after 5h of incubation. Unlike typical hybrid in vitro protein synthesis systems based on bacteriophage transcription, expression is carried out through E. coli promoters using only the endogenous transcription-translation molecular machineries provided by the extract. We demonstrate that traditional expensive energy regeneration systems, such as creatine phosphate, phosphoenolpyruvate or phosphoglycerate, can be replaced by a cost-effective metabolic scheme suitable for cell-free protein synthesis applications. Our work also shows that cell-free systems are useful platforms for metabolic engineering.

Soluble full-length expression and characterization of snRNP protein U1-68/70K

The autoantigen U1-68/70K is the dominant diagnostic marker in Mixed Connective Tissue Disease (MCTD) that until recently could not be expressed in its full-length form (Northemann et al., 1995, [16]). Using cell-free expression screening, we successfully produced the snRNP protein U1-68/70K in a soluble full-length form in Escherichia coli cells. The protein length and identity was determined by Western Blot and MS/MS analysis. Additionally, its reactivity in the autoimmune diagnostic was confirmed. Establishment of a cell-free expression system for this protein was important for further elucidation of protein expression properties such as the cDNA construct, expression temperature and folding properties; these parameters can now be determined in a fast and resource-conserving manner.

Reconstitution of intracellular environments in vitro and in artificial cells

Toward reconstitution of living cells by artificial cells technology, it is critical process to understand the differences between mixtures of biomolecules and living cells. For the aim, we have developed procedures for preparation of an additive-free cell extract (AFCE) and for concentrating biomacromolecules in artificial cells. In this review, we introduce our recent progress to reconstitute intracellular environments in vitro and in artificial cells.

Synthesis and site-directed fluorescence labeling of azido proteins using eukaryotic cell-free orthogonal translation systems

Eukaryotic cell-free systems based on wheat germ and Spodoptera frugiperda insect cells were equipped with an orthogonal amber suppressor tRNA-synthetase pair to synthesize proteins with a site-specifically incorporated p-azido-l-phenylalanine residue in order to provide their chemoselective fluorescence labeling with azide-reactive dyes by Staudinger ligation. The specificity of incorporation and bioorthogonality of labeling within complex reaction mixtures was shown by means of translation and fluorescence detection of two model proteins: β-glucuronidase and erythropoietin. The latter contained the azido amino acid in proximity to a signal peptide for membrane translocation into endogenous microsomal vesicles of the insect cell-based system. The results indicate a stoichiometric incorporation of the azido amino acid at the desired position within the proteins. Moreover, the compatibility of cotranslational protein translocation, including glycosylation and amber suppression-based incorporation of p-azido-l-phenylalanine within a cell-free system, is demonstrated. The presented approach should be particularly useful for providing eukaryotic and membrane-associated proteins for investigation by fluorescence-based techniques.

Substrate replenishment and byproduct removal improve yeast cell-free protein synthesis

Cell-free protein synthesis (CFPS) platforms are now considered a powerful tool for synthesizing a variety of proteins at scales from pL to 100 L with accelerated process development pipelines. We previously reported the advancement of a novel yeast-based CFPS platform. Here, we studied factors that cause termination of yeast CFPS batch reactions. Specifically, we characterized the substrate and byproduct concentrations in batch, fed-batch, and semi-continuous reaction formats through high-performance liquid chromatography (HPLC) and chemical assays. We discovered that creatine phosphate, the secondary energy substrate, and nucleoside triphosphates were rapidly degraded during batch CFPS, causing a significant drop in the reaction's energy charge (E.C.) and eventual termination of protein synthesis. As a consequence of consuming creatine phosphate, inorganic phosphate accumulated as a toxic byproduct. Additionally, we measured amino acid concentrations and found that aspartic acid was rapidly consumed. By adopting a semi-continuous reaction format, where passive diffusion enables substrate replenishment and byproduct removal, we achieved over a 70% increase in active superfolder green fluorescent protein (sfGFP) as compared with the batch system. This study identifies targets for the future improvement of the batch yeast CFPS reaction. Moreover, it outlines a detailed, generalized method to characterize and improve other CFPS platforms.

Synthesis of 2.3 mg/ml of protein with an all Escherichia coli cell-free transcription-translation system

Cell-free protein synthesis is becoming a useful technique for synthetic biology. As more applications are developed, the demand for novel and more powerful in vitro expression systems is increasing. In this work, an all Escherichia coli cell-free system, that uses the endogenous transcription and translation molecular machineries, is optimized to synthesize up to 2.3 mg/ml of a reporter protein in batch mode reactions. A new metabolism based on maltose allows recycling of inorganic phosphate through its incorporation into newly available glucose molecules, which are processed through the glycolytic pathway to produce more ATP. As a result, the ATP regeneration is more efficient and cell-free protein synthesis lasts up to 10 h. Using a commercial E. coli strain, we show for the first time that more than 2 mg/ml of protein can be synthesized in run-off cell-free transcription-translation reactions by optimizing the energy regeneration and waste products recycling. This work suggests that endogenous enzymes present in the cytoplasmic extract can be used to implement new metabolic pathways for increasing protein yields. This system is the new basis of a cell-free gene expression platform used to construct and to characterize complex biochemical processes in vitro such as gene circuits.

Cell-free unnatural amino acid incorporation with alternative energy systems and linear expression templates

Site-specific incorporation of unnatural amino acids (uAAs) during protein synthesis expands the proteomic code through the addition of unique residue chemistry. This field provides a unique tool to improve pharmacokinetics, cancer treatments, vaccine development, proteomics and protein engineering. The limited ability to predict the characteristics of proteins with uAA-incorporation creates a need for a low-cost system with the potential for rapid screening. Escherichia coli-based cell-free protein synthesis is a compelling platform for uAA incorporation due to the open and accessible nature of the reaction environment. However, typical cell-free systems can be expensive due to the high cost of energizing reagents. By employing alternative energy sources, we reduce the cost of uAA-incorporation in CFPS by 55%. While alternative energy systems reduce cost, the time investment to develop gene libraries can remain cumbersome. Cell-free systems allow the direct use of PCR products known as linear expression templates, thus alleviating tedious plasmid library preparations steps. We report the specific costs of CFPS with uAA incorporation, demonstrate that LETs are suitable expression templates with uAA-incorporation, and consider the substantial reduction in labor intensity using LET-based expression for CFPS uAA incorporation.

Cell-free protein synthesis: the state of the art

Cell-free protein synthesis harnesses the synthetic power of biology, programming the ribosomal translational machinery of the cell to create macromolecular products. Like PCR, which uses cellular replication machinery to create a DNA amplifier, cell-free protein synthesis is emerging as a transformative technology with broad applications in protein engineering, biopharmaceutical development, and post-genomic research. By breaking free from the constraints of cell-based systems, it takes the next step towards synthetic biology. Recent advances in reconstituted cell-free protein synthesis (Protein synthesis Using Recombinant Elements expression systems) are creating new opportunities to tailor the reactions for specialized applications including in vitro protein evolution, printing protein microarrays, isotopic labeling, and incorporating nonnatural amino acids.

Cell-free synthesis of functional and endotoxin-free antibody Fab fragments by translocation into microsomes

A eukaryotic cell-free system based on Spodoptera frugiperda cells was developed for the convenient synthesis of Fab antibody fragments and other disulfide bridge containing proteins. The system uses (i) a cell lysate that is mildly prepared under slightly reduced conditions, thus maintaining the activity of vesicles derived from the endoplasmic reticulum, (ii) signal peptide dependent translocation into these vesicles, and (iii) a redox potential based on reduced and oxidized glutathione. Monomeric heavy and light immunoglobulin chains are almost completely converted to highly active dimeric Fab joined by intermolecular disulfide bridges without supplementation of chaperones or protein disulfide isomerase. The applicability of the system is demonstrated by the synthesis of anti-lysozyme and anti-CD4 Fab antibody fragments yielding approximately 10 µg Fab per milliliter reaction mixture. The lack of endotoxins in this system is a prerequisite that synthesized Fab can be applied directly using whole synthesis reactions in cell-based assays that are sensitive to this substance class. Moreover, the system is compatible with PCR-generated linear templates enabling automated generation of antibody fragments in a high-throughput manner, and facilitating its application for screening and validation purposes.

Flexible programming of cell-free protein synthesis using magnetic bead-immobilized plasmids

The use of magnetic bead-immobilized DNA as movable template for cell-free protein synthesis has been investigated. Magnetic microbeads containing chemically conjugated plasmids were used to direct cell-free protein synthesis, so that protein generation could be readily programmed, reset and reprogrammed. Protein synthesis by using this approach could be ON/OFF-controlled through repeated addition and removal of the microbead-conjugated DNA and employed in sequential expression of different genes in a same reaction mixture. Since the incubation periods of individual template plasmids are freely controllable, relative expression levels of multiple proteins can be tuned to desired levels. We expect that the presented results will find wide application to the flexible design and execution of synthetic pathways in cell-free chassis.

Fermentative glycolysis with purified Escherichia coli enzymes for in vitro ATP production and evaluating an engineered enzyme

Each of the twelve enzymes for glycolytic fermentation, eleven from Escherichia coli and one from Saccharomyces cerevisiae, have been over-expressed in E. coli and purified with His-tags. Simple assays have been developed for each enzyme and they have been assembled for fermentation of glucose to ethanol. Phosphorus-31 NMR revealed that this in vitro reaction accumulates fructose 1,6-bisphosphate while recycling the cofactors NAD(+) and ATP. This reaction represents a defined ATP-regeneration system that can be tailored to suit in vitro biochemical reactions such as cell-free protein synthesis. The enzyme from S. cerevisiae, pyruvate decarboxylase 1 (Pdc1; EC 4.1.1.1), was identified as one of the major 'flux controlling' enzymes for the reaction and was replaced with an evolved version of Pdc1 that has over 20-fold greater activity under glycolysis reaction conditions. This substitution was only beneficial when the ratio of glycolytic enzymes was adjusted to suit greater Pdc1 activity.

Development of an artificial cell, from self-organization to computation and self-reproduction

This article describes the state and the development of an artificial cell project. We discuss the experimental constraints to synthesize the most elementary cell-sized compartment that can self-reproduce using synthetic genetic information. The original idea was to program a phospholipid vesicle with DNA. Based on this idea, it was shown that in vitro gene expression could be carried out inside cell-sized synthetic vesicles. It was also shown that a couple of genes could be expressed for a few days inside the vesicles once the exchanges of nutrients with the outside environment were adequately introduced. The development of a cell-free transcription/translation toolbox allows the expression of a large number of genes with multiple transcription factors. As a result, the development of a synthetic DNA program is becoming one of the main hurdles. We discuss the various possibilities to enrich and to replicate this program. Defining a program for self-reproduction remains a difficult question as nongenetic processes, such as molecular self-organization, play an essential and complementary role. The synthesis of a stable compartment with an active interface, one of the critical bottlenecks in the synthesis of artificial cell, depends on the properties of phospholipid membranes. The problem of a self-replicating artificial cell is a long-lasting goal that might imply evolution experiments.

Rational improvement of cell-free protein synthesis

Experimental design principles were applied on cell-free protein synthesis to optimize performance with regard to the expression yield and the incorporation efficiency of amino acid precursors. A versatile screening platform based on batch-mode cell-free expression and central composite design was used. The performance of different extracts (S12 and S30), the concentration dependence of key components and the effect of different additives were investigated. We find that the initial expression yield can be enhanced twofold to threefold in this manner. The improved conditions comprise a modified S12 extract, optimized concentrations of creatine phosphate and key amino acids, as well as introduction of ketoacid additives. Our results show that current cell-free expression technology is far from optimal and that higher yields and increased utilization of the provided precursors are attainable with further optimization.

Study of messenger RNA inactivation and protein degradation in an Escherichia coli cell-free expression system

Background

A large amount of recombinant proteins can be synthesized in a few hours with Escherichia coli cell-free expression systems based on bacteriophage transcription. These cytoplasmic extracts are used in many applications that require large-scale protein production such as proteomics and high throughput techniques. In recent years, cell-free systems have also been used to engineer complex informational processes. These works, however, have been limited by the current available cell-free systems, which are not well adapted to these types of studies. In particular, no method has been proposed to increase the mRNA inactivation rate and the protein degradation rate in cell-free reactions. The construction of in vitro informational processes with interesting dynamics requires a balance between mRNA and protein synthesis (the source), and mRNA inactivation and protein degradation (the sink).

Results

Two quantitative studies are presented to characterize and to increase the global mRNA inactivation rate, and to accelerate the degradation of the synthesized proteins in an E. coli cell-free expression system driven by the endogenous RNA polymerase and sigma factor 70. The E. coli mRNA interferase MazF was used to increase and to adjust the mRNA inactivation rate of the Firefly luciferase (Luc) and of the enhanced green fluorescent protein (eGFP). Peptide tags specific to the endogenous E. coli AAA + proteases were used to induce and to adjust the protein degradation rate of eGFP. Messenger RNA inactivation rate, protein degradation rate, maturation time of Luc and eGFP were measured.

Conclusions

The global mRNA turnover and the protein degradation rate can be accelerated and tuned in a biologically relevant range in a cell-free reaction with quantitative procedures easy to implement. These features broaden the capabilities of cell-free systems with a better control of gene expression. This cell-free extract could find some applications in new research areas such as in vitro synthetic biology and systems biology where engineering informational processes requires a quantitative control of mRNA inactivation and protein degradation.

Efficient cell-free expression with the endogenous E. Coli RNA polymerase and sigma factor 70

Background

Escherichia coli cell-free expression systems use bacteriophage RNA polymerases, such as T7, to synthesize large amounts of recombinant proteins. These systems are used for many applications in biotechnology, such as proteomics. Recently, informational processes have been reconstituted in vitro with cell-free systems. These synthetic approaches, however, have been seriously limited by a lack of transcription modularity. The current available cell-free systems have been optimized to work with bacteriophage RNA polymerases, which put significant restrictions to engineer processes related to biological information. The development of efficient cell-free systems with broader transcription capabilities is required to study complex informational processes in vitro.

Results

In this work, an efficient cell-free expression system that uses the endogenous E. coli RNA polymerase only and sigma factor 70 for transcription was prepared. Approximately 0.75 mg/ml of Firefly luciferase and enhanced green fluorescent protein were produced in batch mode. A plasmid was optimized with different regulatory parts to increase the expression. In addition, a new eGFP was engineered that is more translatable in cell-free systems than the original eGFP. The protein production was characterized with three different adenosine triphosphate (ATP) regeneration systems: creatine phosphate (CP), phosphoenolpyruvate (PEP), and 3-phosphoglyceric acid (3-PGA). The maximum protein production was obtained with 3-PGA. Preparation of the crude extract was streamlined to a simple routine procedure that takes 12 hours including cell culture.

Conclusions

Although it uses the endogenous E. coli transcription machinery, this cell-free system can produce active proteins in quantities comparable to bacteriophage systems. The E. coli transcription provides much more possibilities to engineer informational processes in vitro. Many E. coli promoters/operators specific to sigma factor 70 are available that form a broad library of regulatory parts. In this work, cell-free expression is developed as a toolbox to design and to study synthetic gene circuits in vitro.

Ribosomal synthesis and in situ isolation of peptide molecules in a cell-free translation system

Although the cell-free translation system is now widely accepted as an efficient platform for production, engineering and screening of recombinant proteins, it has not been successfully used for the synthesis of peptide molecules mainly due to low expression yields and rapid proteolysis of the expressed peptides. In this study, we propose a novel strategy for rapid expression and recovery of peptide molecules which involves the rational design of template DNA and heterogenous cell-free translation reaction in the presence of affinity beads. Various peptide molecules which were not expressed in a detectable level were successfully expressed and recovered in situ in a substantial yield. We expect that the presented approach will be widely used as a versatile platform for the generation of a variety of peptide molecules.