Non-standard amino acid incorporation into proteins using Escherichia coli cell-free protein synthesis

Engineered mRNA-ribosome fusions for facile biosynthesis of selenoproteins

Ribosomes are often used in synthetic biology as a tool to produce desired proteins with enhanced properties or entirely new functions. However, repurposing ribosomes for producing designer proteins is challenging due to the limited number of engineering solutions available to alter the natural activity of these enzymes. In this study, we advance ribosome engineering by describing a novel strategy based on functional fusions of ribosomal RNA (rRNA) with messenger RNA (mRNA). Specifically, we create an mRNA-ribosome fusion called RiboU, where the 16S rRNA is covalently attached to selenocysteine insertion sequence (SECIS), a regulatory RNA element found in mRNAs encoding selenoproteins. When SECIS sequences are present in natural mRNAs, they instruct ribosomes to decode UGA codons as selenocysteine (Sec, U) codons instead of interpreting them as stop codons. This enables ribosomes to insert Sec into the growing polypeptide chain at the appropriate site. Our work demonstrates that the SECIS sequence maintains its functionality even when inserted into the ribosome structure. As a result, the engineered ribosomes RiboU interpret UAG codons as Sec codons, allowing easy and site-specific insertion of Sec in a protein of interest with no further modification to the natural machinery of protein synthesis. To validate this approach, we use RiboU ribosomes to produce three functional target selenoproteins in Escherichia coli by site-specifically inserting Sec into the proteins' active sites. Overall, our work demonstrates the feasibility of creating functional mRNA-rRNA fusions as a strategy for ribosome engineering, providing a novel tool for producing Sec-containing proteins in live bacterial cells.

Utilizing a cell-free protein synthesis platform for the biosynthesis of a natural product, caffeine

Natural products are a valuable source of pharmaceuticals, providing a majority of the small-molecule drugs in use today. However, their production through organic synthesis or in heterologous hosts can be difficult and time-consuming. Therefore, to allow for easier screening and production of natural products, we demonstrated the use of a cell-free protein synthesis system to partially assemble natural products in vitro using S-Adenosyl Methionine (SAM)-dependent methyltransferase enzyme reactions. The tea caffeine synthase, TCS1, was utilized to synthesize caffeine within a cell-free protein synthesis system. Cell-free systems also provide the benefit of allowing the use of substrates that would normally be toxic in a cellular environment to synthesize novel products. However, TCS1 is unable to utilize a compound like S-adenosyl ethionine as a cofactor to create ethylated caffeine analogs. The automation and reduced metabolic engineering requirements of cell-free protein synthesis systems, in combination with other synthesis methods, may enable the more efficient generation of new compounds. Graphical Abstract.

Advancing Biomimetic Functions of Synthetic Cells through Compartmentalized Cell-Free Protein Synthesis

Synthetic cells are artificial constructs that mimic the structures and functions of living cells. They are attractive for studying diverse biochemical processes and elucidating the origins of life. While creating a living synthetic cell remains a grand challenge, researchers have successfully synthesized hundreds of unique synthetic cell platforms. One promising approach to developing more sophisticated synthetic cells is to integrate cell-free protein synthesis (CFPS) mechanisms into vesicle platforms. This makes it possible to create synthetic cells with complex biomimetic functions such as genetic circuits, autonomous membrane modifications, sensing and communication, and artificial organelles. This Review explores recent advances in the use of CFPS to impart advanced biomimetic structures and functions to bottom-up synthetic cell platforms. We also discuss the potential applications of synthetic cells in biomedicine as well as the future directions of synthetic cell research.

Applications of synthetic biology in medical and pharmaceutical fields

Synthetic biology aims to design or assemble existing bioparts or bio-components for useful bioproperties. During the past decades, progresses have been made to build delicate biocircuits, standardized biological building blocks and to develop various genomic/metabolic engineering tools and approaches. Medical and pharmaceutical demands have also pushed the development of synthetic biology, including integration of heterologous pathways into designer cells to efficiently produce medical agents, enhanced yields of natural products in cell growth media to equal or higher than that of the extracts from plants or fungi, constructions of novel genetic circuits for tumor targeting, controllable releases of therapeutic agents in response to specific biomarkers to fight diseases such as diabetes and cancers. Besides, new strategies are developed to treat complex immune diseases, infectious diseases and metabolic disorders that are hard to cure via traditional approaches. In general, synthetic biology brings new capabilities to medical and pharmaceutical researches. This review summarizes the timeline of synthetic biology developments, the past and present of synthetic biology for microbial productions of pharmaceutics, engineered cells equipped with synthetic DNA circuits for diagnosis and therapies, live and auto-assemblied biomaterials for medical treatments, cell-free synthetic biology in medical and pharmaceutical fields, and DNA engineering approaches with potentials for biomedical applications.

A novel screening method for free non-standard amino acids in human plasma samples using AccQ·Tag reagents and LC-MS/MS

There are at least 500 naturally occurring amino acids, of which only 20 standard proteinogenic amino acids are used universally across all organisms in the synthesis of peptides and proteins. Non-standard amino acids can be incorporated into proteins or are intermediates and products of metabolic pathways. While the analysis of standard amino acids is well-defined, the analysis of non-standard amino acids can be challenging due to the wide range of physicochemical properties, and the lack of both reference standards and information in curated databases to aid compound identification. It has been shown that the use of an AccQ·Tag™ derivatization kit along with LC-MS/MS is an attractive option for the analysis of free standard amino acids in complex samples because it is fast, sensitive, reproducible, and selective. It has been demonstrated that the most abundant quantitative transition for MS/MS analysis of 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) derivatized amino acids corresponds to the fragmentation of the molecule at the 6-aminoquinoline carbonyl group producing a common m/z 171 fragment ion and occurs at similar mass spectrometry collision energy and cone voltages. In this study, the unique properties of AQC derivatized amino acids producing high intensity common fragment ions, along with chromatographic separation of amino acids under generic chromatography conditions, were used to develop a novel screening method for the detection of trace levels of non-standard amino acids in complex matrices. Structural elucidation was carried out by comparing the MS/MS fragment ion mass spectra generated with in silico predicted fragmentation spectra to enable a putative identification, which was confirmed using an appropriate analytical standard. This workflow was applied to screen human plasma samples for bioactive thiol-group modified cysteine amino acids and S-allylmercaptocysteine (SAMC), S-allylcysteine sulfoxide (SACS or alliin) and S-propenylcysteine (S1PC) are reported for the first time to be present in human plasma samples after the administration of garlic supplements.

Customized synthesis of phosphoprotein bearing phosphoserine or its nonhydrolyzable analog

Studies on the mechanism of protein phosphorylation and therapeutic interventions of its related molecular processes are limited by the difficulty in the production of purpose-built phosphoproteins harboring site-specific phosphorylated amino acids or their nonhydrolyzable analogs. Here we address this limitation by customizing the cell-free protein synthesis (CFPS) machinery via chassis strain selection and orthogonal translation system (OTS) reconfiguration screening. The suited chassis strains and reconfigured OTS combinations with high orthogonality were consequently picked out for individualized phosphoprotein synthesis. Specifically, we synthesized the sfGFP protein and MEK1 protein with site-specific phosphoserine (O-pSer) or its nonhydrolyzable analog, 2-amino-4-phosphonobutyric acid (C-pSer). This study successfully realized building cell-free systems for site-specific incorporation of phosphonate mimics into the target protein. Our work lays the foundation for developing a highly expansible CFPS platform and the streamlined production of user-defined phosphoproteins, which can facilitate research on the physiological mechanism and potential interference tools toward protein phosphorylation.

Recombinant and genetic code expanded collagen-like protein as a tailorable biomaterial

Collagen occurs in nature with a dedicated triple helix structure and is the most preferred biomaterial in commercialized medical products. However, concerns on purity, disease transmission, and the reproducibility of animal derived collagen restrict its applications and warrants alternate recombinant sources. The expression of recombinant collagen in different prokaryotic and eukaryotic hosts has been reported with varying degrees of success, however, it is vital to elucidate the structural and biological characteristics of natural collagen. The recombinant production of biologically functional collagen is restricted by its high molecular weight and post-translational modification (PTM), especially the hydroxylation of proline to hydroxyproline. Hydroxyproline plays a key role in the structural stability and higher order self-assembly to form fibrillar matrices. Advancements in synthetic biology and recombinant technology are being explored for improving the yield and biomimicry of recombinant collagen. It emerges as reliable, sustainable source of collagen, promises tailorable properties and thereby custom-made protein biomaterials. Remarkably, the evolutionary existence of collagen-like proteins (CLPs) has been identified in single-cell organisms. Interestingly, CLPs exhibit remarkable ability to form stable triple helical structures similar to animal collagen and have gained increasing attention. Strategies to expand the genetic code of CLPs through the incorporation of unnatural amino acids promise the synthesis of highly tunable next-generation triple helical proteins required for the fabrication of smart biomaterials. The review outlines the importance of collagen, sources and diversification, and animal and recombinant collagen-based biomaterials and highlights the limitations of the existing collagen sources. The emphasis on genetic code expanded tailorable CLPs as the most sought alternate for the production of functional collagen and its advantages as translatable biomaterials has been highlighted.

Experimental validation that human microbiome phages use alternative genetic coding

Previous bioinformatic analyses of metagenomic data have indicated that bacteriophages can use genetic codes different from those of their host bacteria. In particular, reassignment of stop codon TAG to glutamine (a variation known as 'genetic code 15') has been predicted. Here, we use LC-MS/MS-based metaproteomics of human fecal samples to provide experimental evidence of the use of genetic code 15 in two crAss-like phages. Furthermore, the proteomic data from several phage structural proteins supports the reassignment of the TAG stop codon to glutamine late in the phage infection cycle. Thus, our work experimentally validates the expression of genetic code 15 in human microbiome phages.

Computational design and engineering of an Escherichia coli strain producing the nonstandard amino acid para-aminophenylalanine

Introducing heterologous pathways into host cells constitutes a promising strategy for synthesizing nonstandard amino acids (nsAAs) to enable the production of proteins with expanded chemistries. However, this strategy has proven challenging, as the expression of heterologous pathways can disrupt cellular homeostasis of the host cell. Here, we sought to optimize the heterologous production of the nsAA para-aminophenylalanine (pAF) in Escherichia coli. First, we incorporated a heterologous pAF biosynthesis pathway into a genome-scale model of E. coli metabolism and computationally identified metabolic interventions in the host’s native metabolism to improve pAF production. Next, we explored different approaches of imposing these flux interventions experimentally and found that the upregulation of flux in the chorismate biosynthesis pathway through the elimination of feedback inhibition mechanisms could significantly raise pAF titers (∼20-fold) while maintaining a reasonable pAF production-growth rate trade-off. Overall, this study provides a promising strategy for the biosynthesis of nsAAs in engineered cells.

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Sought to optimize para-aminophenylalanine (pAF) production and growth in E. coli

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Identified interventions in the host native metabolism using genome-scale models

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Constructed multiple mutant strains involving gene knockouts and/or overexpressions

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Flux modification in chorismate biosynthesis pathway significantly raised pAF titer

Efficient multi-gene expression in cell-free droplet microreactors

Cell-free transcription and translation systems promise to accelerate and simplify the engineering of proteins, biological circuits and metabolic pathways. Their encapsulation on microfluidic platforms can generate millions of cell-free reactions in picoliter volume droplets. However, current methods struggle to create DNA diversity between droplets while also reaching sufficient protein expression levels. In particular, efficient multi-gene expression has remained elusive. We here demonstrate that co-encapsulation of DNA-coated beads with a defined cell-free system allows high protein expression while also supporting genetic diversity between individual droplets. We optimize DNA loading on commercially available microbeads through direct binding as well as through the sequential coupling of up to three genes via a solid-phase Golden Gate assembly or BxB1 integrase-based recombineering. Encapsulation with an off-the-shelf microfluidics device allows for single or multiple protein expression from a single DNA-coated bead per 14 pL droplet. We envision that this approach will help to scale up and parallelize the rapid prototyping of more complex biological systems.

Simple Extract Preparation Methods for E. coli-Based Cell-Free Expression

Cell-free protein synthesis (CFPS) is a powerful platform for synthetic biology, allowing for the controlled expression of proteins without reliance on living cells. However, the process of producing the cell extract, a key component of cell-free reactions, can be a bottleneck for new users to adopt CFPS as it requires technical knowledge and significant researcher oversight. Here, we provide a detailed method for implementing a simplified cell extract preparation workflow using CFAI media. We also provide a detailed protocol for the alternative, 2x YPTG media-based preparation process, as it represents a useful benchmark within the cell-free community.

A Multiplexed Cell‐Free Assay to Screen for Antimicrobial Peptides in Double Emulsion Droplets

The global surge in bacterial resistance against traditional antibiotics triggered intensive research for novel compounds, with antimicrobial peptides (AMPs) identified as a promising candidate. Automated methods to systematically generate and screen AMPs according to their membrane preference, however, are still lacking. We introduce a novel microfluidic system for the simultaneous cell‐free production and screening of AMPs for their membrane specificity. On our device, AMPs are cell‐free produced within water‐in‐oil‐in‐water double emulsion droplets, generated at high frequency. Within each droplet, the peptides can interact with different classes of co‐encapsulated liposomes, generating a membrane‐specific fluorescent signal. The double emulsions can be incubated and observed in a hydrodynamic trapping array or analyzed via flow cytometry. Our approach provides a valuable tool for the discovery and development of membrane‐active antimicrobials.

Recent Technologies for Genetic Code Expansion and their Implications on Synthetic Biology Applications

Genetic code expansion (GCE) enables the site-specific incorporation of non-canonical amino acids as novel building blocks for the investigation and manipulation of proteins. The advancement of genetic code expansion has been benefited from the development of synthetic biology, while genetic code expansion also helps to create more synthetic biology tools. In this review, we summarize recent advances in genetic code expansion brought by synthetic biology progresses, including engineering of the translation machinery, genome-wide codon reassignment, and the biosynthesis of non-canonical amino acids. We highlight the emerging application of this technology in construction of new synthetic biology parts, circuits, chassis, and products.

Engineering Translation Components for Genetic Code Expansion

The expansion of the genetic code consisting of four bases and 20 amino acids into diverse building blocks has been an exciting topic in synthetic biology. Many biochemical components are involved in gene expression; therefore, adding a new component to the genetic code requires engineering many other components that interact with it. Genetic code expansion has advanced significantly for the last two decades with the engineering of several components involved in protein synthesis. These components include tRNA/aminoacyl-tRNA synthetase, new codons, ribosomes, and elongation factor Tu. In addition, biosynthesis and enhanced uptake of non-canonical amino acids have been attempted and have made meaningful progress. This review discusses the efforts to engineer these translation components, to improve the genetic code expansion technology.

A linear DNA template-based framework for site-specific unnatural amino acid incorporation

Site-specific incorporation of unnatural amino acids (UNAAs) into proteins using an orthogonal translation system (OTS) has expanded the scope of protein-coding chemistry. The key factor affecting UNAA embedding efficiency is the orthogonality of the OTS. Compared to traditional cell systems, cell-free systems are more convenient to control the reaction process and improve the utilization rate of UNAA. In this study, a linear DNA template-based cell-free unnatural protein synthesis system for rapid high-throughput screening and evolution was proposed. A total of 14 cell extracts were selected for screening out cell extract with high expression level. The result showed that EcAR7 ΔA ΔSer cell extract was optimal for the cell-free system. In addition, the screening results of four UNAAs, p-propargyloxy-l-phenylalanine (pPaF), p-azyl-phenylalanine (pAzF), p-acetyl-l-phenylalanine (pAcF), and p-benzoyl-l-phenylalanine (pBpF), showed that o-aaRS and o-tRNA of pPaF had good orthogonality. A new pair of corresponding o-aaRS and o-tRNA for pBpF was screened out. These results proved that this method could speed up the screening of optimal OTS components for UNAAs with versatile functions.

Combining flavin photocatalysis with parallel synthesis: a general platform to optimize peptides with non-proteinogenic amino acids

Most peptide drugs contain non-proteinogenic amino acids (NPAAs), born out through extensive structure-activity relationship (SAR) studies using solid-phase peptide synthesis (SPPS). Synthetically laborious and expensive to manufacture, NPAAs also can have poor coupling efficiencies allowing only a small fraction to be sampled by conventional SPPS. To gain general access to NPAA-containing peptides, we developed a first-generation platform that merges contemporary flavin photocatalysis with parallel synthesis to simultaneously make, purify, quantify, and even test up to 96 single-NPAA peptide variants via the unique combination of boronic acids and a dehydroalanine residue in a peptide. We showcase the power of our newly minted platform to introduce NPAAs of diverse chemotypes-aliphatic, aromatic, heteroaromatic-directly into peptides, including 15 entirely new residues, and to evolve a simple proteinogenic peptide into an unnatural inhibitor of thrombin by non-classical peptide SAR.

Additive CHARMM36 Force Field for Nonstandard Amino Acids

Nonstandard amino acids are both abundant in nature, where they play a key role in various cellular processes, and can be synthesized in laboratories, for example, for the manufacture of a range of pharmaceutical agents. In this work, we have extended the additive all-atom CHARMM36 and CHARMM General force field (CGenFF) to a large set of 333 nonstandard amino acids. These include both amino acids with nonstandard side chains, such as post-translationally modified and artificial amino acids, as well as amino acids with modified backbone groups, such as chromophores composed of several amino acids. Model compounds representative of the nonstandard amino acids were parametrized for protonation states that are likely at the physiological pH of 7 and, for some more common residues, in both d- and l-stereoisomers. Considering all protonation, tautomeric, and stereoisomeric forms, a total of 406 nonstandard amino acids were parametrized. Emphasis was placed on the quality of both intra- and intermolecular parameters. Partial charges were derived using quantum mechanical (QM) data on model compound dipole moments, electrostatic potentials, and interactions with water. Optimization of all intramolecular parameters, including torsion angle parameters, was performed against information from QM adiabatic potential energy surface (PES) scans. Special emphasis was put on the quality of terms corresponding to PES around rotatable dihedral angles. Validation of the force field was based on molecular dynamics simulations of 20 protein complexes containing different nonstandard amino acids. Overall, the presented parameters will allow for computational studies of a wide range of proteins containing nonstandard amino acids, including natural and artificial residues.

Easy Synthesis of Complex Biomolecular Assemblies: Wheat Germ Cell-Free Protein Expression in Structural Biology

Cell-free protein synthesis (CFPS) systems are gaining more importance as universal tools for basic research, applied sciences, and product development with new technologies emerging for their application. Huge progress was made in the field of synthetic biology using CFPS to develop new proteins for technical applications and therapy. Out of the available CFPS systems, wheat germ cell-free protein synthesis (WG-CFPS) merges the highest yields with the use of a eukaryotic ribosome, making it an excellent approach for the synthesis of complex eukaryotic proteins including, for example, protein complexes and membrane proteins. Separating the translation reaction from other cellular processes, CFPS offers a flexible means to adapt translation reactions to protein needs. There is a large demand for such potent, easy-to-use, rapid protein expression systems, which are optimally serving protein requirements to drive biochemical and structural biology research. We summarize here a general workflow for a wheat germ system providing examples from the literature, as well as applications used for our own studies in structural biology. With this review, we want to highlight the tremendous potential of the rapidly evolving and highly versatile CFPS systems, making them more widely used as common tools to recombinantly prepare particularly challenging recombinant eukaryotic proteins.

Methodologies for preparation of prokaryotic extracts for cell-free expression systems

Cell-free systems that mimic essential cell functions, such as gene expression, have dramatically expanded in recent years, both in terms of applications and widespread adoption. Here we provide a review of cell-extract methods, with a specific focus on prokaryotic systems. Firstly, we describe the diversity of Escherichia coli genetic strains available and their corresponding utility. We then trace the history of cell-extract methodology over the past 20 years, showing key improvements that lower the entry level for new researchers. Next, we survey the rise of new prokaryotic cell-free systems, with associated methods, and the opportunities provided. Finally, we use this historical perspective to comment on the role of methodology improvements and highlight where further improvements may be possible.

Cell-free systems for accelerating glycoprotein expression and biomanufacturing

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

Cell-Free Approach for Non-canonical Amino Acids Incorporation Into Polypeptides

Synthetic biology holds promise to revolutionize the life sciences and biomedicine via expansion of macromolecular diversity outside the natural chemical space. Use of non-canonical amino acids (ncAAs) via codon reassignment has found diverse applications in protein structure and interaction analysis, introduction of post-translational modifications, production of constrained peptides, antibody-drug conjugates, and novel enzymes. However, simultaneously encoding multiple ncAAs in vivo requires complex engineering and is sometimes restricted by the cell's poor uptake of ncAAs. In contrast the open nature of cell-free protein synthesis systems offers much greater freedom for manipulation and repurposing of the biosynthetic machinery by controlling the level and identity of translational components and reagents, and allows simultaneous incorporation of multiple ncAAs with non-canonical side chains and even backbones (N-methyl, D-, β-amino acids, α-hydroxy acids etc.). This review focuses on the two most used Escherichia coli-based cell-free protein synthesis systems; cell extract- and PURE-based systems. The former is a biological mixture with >500 proteins, while the latter consists of 38 individually purified biomolecules. We delineate compositions of these two systems and discuss their respective advantages and applications. Also, we dissect the translational components required for ncAA incorporation and compile lists of ncAAs that can be incorporated into polypeptides via different acylation approaches. We highlight the recent progress in using unnatural nucleobase pairs to increase the repertoire of orthogonal codons, as well as using tRNA-specific ribozymes for in situ acylation. We summarize advances in engineering of translational machinery such as tRNAs, aminoacyl-tRNA synthetases, elongation factors, and ribosomes to achieve efficient incorporation of structurally challenging ncAAs. We note that, many engineered components of biosynthetic machinery are developed for the use in vivo but are equally applicable to the in vitro systems. These are included in the review to provide a comprehensive overview for ncAA incorporation and offer new insights for the future development in cell-free systems. Finally, we highlight the exciting progress in the genomic engineering, resulting in E. coli strains free of amber and some redundant sense codons. These strains can be used for preparation of cell extracts offering multiple reassignment options.

Holistic engineering of cell-free systems through proteome-reprogramming synthetic circuits

Synthetic biology has focused on engineering genetic modules that operate orthogonally from the host cells. A synthetic biological module, however, can be designed to reprogram the host proteome, which in turn enhances the function of the synthetic module. Here, we apply this holistic synthetic biology concept to the engineering of cell-free systems by exploiting the crosstalk between metabolic networks in cells, leading to a protein environment more favorable for protein synthesis. Specifically, we show that local modules expressing translation machinery can reprogram the bacterial proteome, changing the expression levels of more than 700 proteins. The resultant feedback generates a cell-free system that can synthesize fluorescent reporters, protein nanocages, and the gene-editing nuclease Cas9, with up to 5-fold higher expression level than classical cell-free systems. Our work demonstrates a holistic approach that integrates synthetic and systems biology concepts to achieve outcomes not possible by only local, orthogonal circuits.

Strategies for in vitro engineering of the translation machinery

Engineering the process of molecular translation, or protein biosynthesis, has emerged as a major opportunity in synthetic and chemical biology to generate novel biological insights and enable new applications (e.g. designer protein therapeutics). Here, we review methods for engineering the process of translation in vitro. We discuss the advantages and drawbacks of the two major strategies-purified and extract-based systems-and how they may be used to manipulate and study translation. Techniques to engineer each component of the translation machinery are covered in turn, including transfer RNAs, translation factors, and the ribosome. Finally, future directions and enabling technological advances for the field are discussed.

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

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

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

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

"Cell-Free Synthetic Biology": Synthetic Biology Meets Cell-Free Protein Synthesis

Since Nirenberg and Matthaei used cell-free protein synthesis (CFPS) to elucidate the genetic code in the early 1960s [1], the technology has been developed over the course of decades and applied to studying both fundamental and applied biology [2]. Cell-free synthetic biology integrating CFPS with synthetic biology has received attention as a powerful and rapid approach to characterize and engineer natural biological systems. The open nature of cell-free (or in vitro) biological platforms compared to in vivo systems brings an unprecedented level of control and freedom in design [3]. This versatile engineering toolkit has been used for debugging biological networks, constructing artificial cells, screening protein libraries, prototyping genetic circuits, developing biosensors, producing metabolites, and synthesizing complex proteins including antibodies, toxic proteins, membrane proteins, and novel proteins containing nonstandard (unnatural) amino acids. The Methods and Protocols "Cell-Free Synthetic Biology" Special Issue consists of a series of reviews, protocols, benchmarks, and research articles describing the current development and applications of cell-free synthetic biology in diverse areas. [...].

A Crude Extract Preparation and Optimization from a Genomically Engineered Escherichia coli for the Cell-Free Protein Synthesis System: Practical Laboratory Guideline

With the advancement of synthetic biology, the cell-free protein synthesis (CFPS) system has been receiving the spotlight as a versatile toolkit for engineering natural and unnatural biological systems. The CFPS system reassembles the materials necessary for transcription and translation and recreates the in vitro protein synthesis environment by escaping a physical living boundary. The cell extract plays an essential role in this in vitro format. Here, we propose a practical protocol and method for Escherichia coli-derived cell extract preparation and optimization, which can be easily applied to both commercially available and genomically engineered E. coli strains. The protocol includes: (1) The preparation step for cell growth and harvest, (2) the thorough step-by-step procedures for E. coli cell extract preparation including the cell wash and lysis, centrifugation, runoff reaction, and dialysis, (3) the preparation for the CFPS reaction components and, (4) the quantification of cell extract and cell-free synthesized protein. We anticipate that the protocol in this research will provide a simple preparation and optimization procedure of a highly active E. coli cell extract.

Oligonucleotide-mediated tRNA sequestration enables one-pot sense codon reassignment in vitro

Sense codon reassignment to unnatural amino acids (uAAs) represents a powerful approach for introducing novel properties into polypeptides. The main obstacle to this approach is competition between the native isoacceptor tRNA(s) and orthogonal tRNA(s) for the reassigned codon. While several chromatographic and enzymatic procedures for selective deactivation of tRNA isoacceptors in cell-free translation systems exist, they are complex and not scalable. We designed a set of tRNA antisense oligonucleotides composed of either deoxy-, ribo- or 2′-O-methyl ribonucleotides and tested their ability to efficiently complex tRNAs of choice. Methylated oligonucleotides targeting sequence between the anticodon and variable loop of tRNA^SerGCU displayed subnanomolar binding affinity with slow dissociation kinetics. Such oligonucleotides efficiently and selectively sequestered native tRNA^SerGCU directly in translation-competent Escherichia coli S30 lysate, thereby, abrogating its translational activity and liberating the AGU/AGC codons. Expression of eGFP protein from the template harboring a single reassignable AGU codon in tRNA^SerGCU-depleted E. coli lysate allowed its homogeneous modification with n-propargyl-l-lysine or p-azido-l-phenylalanine. The strategy developed here is generic, as demonstrated by sequestration of tRNA^ArgCCU isoacceptor in E. coli translation system. Furthermore, this method is likely to be species-independent and was successfully applied to the eukaryotic Leishmania tarentolae in vitro translation system. This approach represents a new direction in genetic code reassignment with numerous practical applications.

The Role of Orthogonality in Genetic Code Expansion

The genetic code defines how information in the genome is translated into protein. Aside from a handful of isolated exceptions, this code is universal. Researchers have developed techniques to artificially expand the genetic code, repurposing codons and translational machinery to incorporate nonstandard amino acids (nsAAs) into proteins. A key challenge for robust genetic code expansion is orthogonality; the engineered machinery used to introduce nsAAs into proteins must co-exist with native translation and gene expression without cross-reactivity or pleiotropy. The issue of orthogonality manifests at several levels, including those of codons, ribosomes, aminoacyl-tRNA synthetases, tRNAs, and elongation factors. In this concept paper, we describe advances in genome recoding, translational engineering and associated challenges rooted in establishing orthogonality needed to expand the genetic code.

Advances and Challenges in Cell-Free Incorporation of Unnatural Amino Acids Into Proteins

Incorporation of unnatural amino acids (UNAAs) into proteins currently is an active biological research area for various fundamental and applied science. In this context, cell-free synthetic biology (CFSB) has been developed and recognized as a robust testing and biomanufacturing platform for highly efficient UNAA incorporation. It enables the orchestration of unnatural biological machinery toward an exclusive user-defined objective of unnatural protein synthesis. This review aims to overview the principles of cell-free unnatural protein synthesis (CFUPS) systems, their advantages, different UNAA incorporation approaches, and recent achievements. These have catalyzed cutting-edge research and diverse emerging applications. Especially, present challenges and future trends are focused and discussed. With the development of CFSB and the fusion with other advanced next-generation technologies, CFUPS systems would explicitly deliver their values for biopharmaceutical applications.

Incorporation of non-standard amino acids into proteins: challenges, recent achievements, and emerging applications

The natural genetic code only allows for 20 standard amino acids in protein translation, but genetic code reprogramming enables the incorporation of non-standard amino acids (NSAAs). Proteins containing NSAAs provide enhanced or novel properties and open diverse applications. With increased attention to the recent advancements in synthetic biology, various improved and novel methods have been developed to incorporate single and multiple distinct NSAAs into proteins. However, various challenges remain in regard to NSAA incorporation, such as low yield and misincorporation. In this review, we summarize the recent efforts to improve NSAA incorporation by utilizing orthogonal translational system optimization, cell-free protein synthesis, genomically recoded organisms, artificial codon boxes, quadruplet codons, and orthogonal ribosomes, before closing with a discussion of the emerging applications of NSAA incorporation.

The Application of Cell-Free Protein Synthesis in Genetic Code Expansion for Post-translational Modifications

The translation system is a sophisticated machinery that synthesizes proteins from 20 canonical amino acids. Recently, the repertoire of such composition has been expanded by the introduction of non-canonical amino acids (ncAAs) with the genetic code expansion strategy, which provides proteins with designed properties and structures for protein studies and engineering. Although the genetic code expansion strategy has been mostly implemented by using living cells as the host, a number of limits such as poor cellular uptake or solubility of specific ncAA substrates and the toxicity of target proteins have hindered the production of certain ncAA-modified proteins. To overcome those challenges, cell-free protein synthesis (CFPS) has been applied as it allows the precise control of reaction components. Several approaches have been recently developed to increase the purity and efficiency of ncAA incorporation in CFPS. Here, we summarized recent development of CFPS with an emphasis on its applications in generating site-specific protein post-translational modifications by the genetic code expansion strategy.

Cell-Free Expression and Photo-Crosslinking of the Human Neuropeptide Y2 Receptor

G protein-coupled receptors (GPCRs) represent a large family of different proteins, which are involved in physiological processes throughout the entire body. Furthermore, they represent important drug targets. For rational drug design, it is important to get further insights into the binding mode of endogenous ligands as well as of therapeutic agents at the respective target receptors. However, structural investigations usually require homogenous, solubilized and functional receptors, which is still challenging. Cell-free expression methods have emerged in the last years and many different proteins are successfully expressed, including hydrophobic membrane proteins like GPCRs. In this work, an Escherichia coli based cell-free expression system was used to express the neuropeptide Y₂ receptor (Y₂R) for structural investigations. This GPCR was expressed in two different variants, a C-terminal enhanced green fluorescent fusion protein and a cysteine deficient variant. In order to obtain soluble receptors, the expression was performed in the presence of mild detergents, either Brij-35 or Brij-58, which led to high amounts of soluble receptor. Furthermore, the influence of temperature, pH value and additives on protein expression and solubilization was tested. For functional and structural investigations, the receptors were expressed at 37°C, pH 7.4 in the presence of 1 mM oxidized and 5 mM reduced glutathione. The expressed receptors were purified by ligand affinity chromatography and functionality of Y₂R_cysteine_deficient was verified by a homogenous binding assay. Finally, photo-crosslinking studies were performed between cell-free expressed Y₂R_cysteine_deficient and a neuropeptide Y (NPY) analog bearing the photoactive, unnatural amino acid p-benzoyl-phenylalanine at position 27 and biotin at position 22 for purification. After enzymatic digestion, fragments of crosslinked receptor were identified by mass spectrometry. Our findings demonstrate that, in contrast to Y₁R, NPY position 27 remains flexible when bound to Y₂R. These results are in agreement with the suggested binding mode of NPY at Y₂R.

Efficient Incorporation of Unnatural Amino Acids into Proteins with a Robust Cell-Free System

Unnatural proteins are crucial biomacromolecules and have been widely applied in fundamental science, novel biopolymer materials, enzymes, and therapeutics. Cell-free protein synthesis (CFPS) system can serve as a robust platform to synthesize unnatural proteins by highly effective site-specific incorporation of unnatural amino acids (UNAAs), without the limitations of cell membrane permeability and the toxicity of unnatural components. Here, we describe a quick and simple method to synthesize unnatural proteins in CFPS system based on Escherichia coli crude extract, with unnatural orthogonal aminoacyl-tRNA synthetase and suppressor tRNA evolved from Methanocaldococcus jannaschii. The superfolder green fluorescent protein (sfGFP) and p-propargyloxyphenylalanine (pPaF) were used as the model protein and UNAA. The synthesis of unnatural sfGFPs was characterized by microplate spectrophotometer, affinity chromatography, and liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). This protocol provides a detailed procedure guiding how to use the powerful CFPS system to synthesize unnatural proteins on demand.

Cell-Free Protein Synthesis Using S30 Extracts from Escherichia coli RFzero Strains for Efficient Incorporation of Non-Natural Amino Acids into Proteins

Cell-free protein synthesis is useful for synthesizing difficult targets. The site-specific incorporation of non-natural amino acids into proteins is a powerful protein engineering method. In this study, we optimized the protocol for cell extract preparation from the Escherichia coli strain RFzero-iy, which is engineered to lack release factor 1 (RF-1). The BL21(DE3)-based RFzero-iy strain exhibited quite high cell-free protein productivity, and thus we established the protocols for its cell culture and extract preparation. In the presence of 3-iodo-l-tyrosine (IY), cell-free protein synthesis using the RFzero-iy-based S30 extract translated the UAG codon to IY at various sites with a high translation efficiency of >90%. In the absence of IY, the RFzero-iy-based cell-free system did not translate UAG to any amino acid, leaving UAG unassigned. Actually, UAG was readily reassigned to various non-natural amino acids, by supplementing them with their specific aminoacyl-tRNA synthetase variants (and their specific tRNAs) into the system. The high incorporation rate of our RFzero-iy-based cell-free system enables the incorporation of a variety of non-natural amino acids into multiple sites of proteins. The present strategy to create the RFzero strain is rapid, and thus promising for RF-1 deletions of various E. coli strains genomically engineered for specific requirements.

Is Research on "Synthetic Cells" Moving to the Next Level?

"Synthetic cells" research focuses on the construction of cell-like models by using solute-filled artificial microcompartments with a biomimetic structure. In recent years this bottom-up synthetic biology area has considerably progressed, and the field is currently experiencing a rapid expansion. Here we summarize some technical and theoretical aspects of synthetic cells based on gene expression and other enzymatic reactions inside liposomes, and comment on the most recent trends. Such a tour will be an occasion for asking whether times are ripe for a sort of qualitative jump toward novel SC prototypes: is research on "synthetic cells" moving to a next level?

Biosynthesis and biotechnological application of non-canonical amino acids: Complex and unclear

Compared with the better-studied canonical amino acids, the distribution, metabolism and functions of natural non-canonical amino acids remain relatively obscure. Natural non-canonical amino acids have been mainly discovered in plants as secondary metabolites that perform diversified physiological functions. Due to their specific characteristics, a broader range of natural and artificial non-canonical amino acids have recently been applied in the development of functional materials and pharmaceutical products. With the rapid development of advanced methods in biotechnology, non-canonical amino acids can be incorporated into peptides, proteins and enzymes to improve the function and performance relative to their natural counterparts. Therefore, biotechnological application of non-canonical amino acids in artificial bio-macromolecules follows the central goal of synthetic biology to: create novel life forms and functions. However, many of the non-canonical amino acids are synthesized via chemo- or semi-synthetic methods, and few non-canonical amino acids can be synthesized using natural in vivo pathways. Therefore, further research is needed to clarify the metabolic pathways and key enzymes of the non-canonical amino acids. This will lead to the discovery of more candidate non-canonical amino acids, especially for those that are derived from microorganisms and are naturally bio-compatible with chassis strains for in vivo biosynthesis. In this review, we summarize representative natural and artificial non-canonical amino acids, their known information regarding associated metabolic pathways, their characteristics and their practical applications. Moreover, this review summarizes current barriers in developing in vivo pathways for the synthesis of non-canonical amino acids, as well as other considerations, future trends and potential applications of non-canonical amino acids in advanced biotechnology.

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.

Cell-sized mechanosensitive and biosensing compartment programmed with DNA

The bottom-up construction of cell-sized compartments programmed with DNA that are capable of sensing the chemical and physical environment remains challenging in synthetic cell engineering. Here, we construct mechanosensitive liposomes with biosensing capability by expressing the E. coli channel MscL and a calcium biosensor using cell-free expression.

Cell-free protein synthesis from genomically recoded bacteria enables multisite incorporation of noncanonical amino acids

Cell-free protein synthesis has emerged as a powerful approach for expanding the range of genetically encoded chemistry into proteins. Unfortunately, efforts to site-specifically incorporate multiple non-canonical amino acids into proteins using crude extract-based cell-free systems have been limited by release factor 1 competition. Here we address this limitation by establishing a bacterial cell-free protein synthesis platform based on genomically recoded Escherichia coli lacking release factor 1. This platform was developed by exploiting multiplex genome engineering to enhance extract performance by functionally inactivating negative effectors. Our most productive cell extracts enabled synthesis of 1,780 ± 30 mg/L superfolder green fluorescent protein. Using an optimized platform, we demonstrated the ability to introduce 40 identical p-acetyl-l-phenylalanine residues site specifically into an elastin-like polypeptide with high accuracy of incorporation ( ≥ 98%) and yield (96 ± 3 mg/L). We expect this cell-free platform to facilitate fundamental understanding and enable manufacturing paradigms for proteins with new and diverse chemistries.

Cell-free protein synthesis allows for producing proteins without the need of a host organism, thus sparing the researcher experimental hassle. Here, the authors developed a cell-free synthesis method that enables incorporating non-standard amino acids in the product.

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.

Isothiocyanyl Alanine as a Synthetic Intermediate for the Synthesis of Thioureayl Alanines and Subsequent Aminotetrazolyl Alanines

The synthesis of unnatural amino acids with small side-chain functionalities usable for further transformations is highly demanding for the expansion of the genetic code and other possible biotechnological applications. To this end, we wanted to report the utility of an unexplored unnatural amino acid, isothiocyanyl alanine (^NCSAla = Ita), for the synthesis of another class of unnatural amino acids, thioureayl alanines (^TUAla = Tua). The synthesis of a third class of unnatural amino acids, amino tetrazolyl alanines (^ATzAla = Ata), in a very good yield was subsequently achieved utilizing thioureayl alanines. Thus, a variety of aliphatic- and aromatic-substituted thioureayl alanines and aromatic-substituted amino tetrazolyl alanines were successfully synthesized in good to excellent yields. The photophysical properties of three of the fluorescent unnatural amino acids from two classes were also studied and presented herein.