Expanding the amino acid repertoire of ribosomal polypeptide synthesis via the artificial division of codon boxes

Engineering tRNAs for the Ribosomal Translation of Non-proteinogenic Monomers

Ribosome-dependent protein biosynthesis is an essential cellular process mediated by transfer RNAs (tRNAs). Generally, ribosomally synthesized proteins are limited to the 22 proteinogenic amino acids (pAAs: 20 l-α-amino acids present in the standard genetic code, selenocysteine, and pyrrolysine). However, engineering tRNAs for the ribosomal incorporation of non-proteinogenic monomers (npMs) as building blocks has led to the creation of unique polypeptides with broad applications in cellular biology, material science, spectroscopy, and pharmaceuticals. Ribosomal polymerization of these engineered polypeptides presents a variety of challenges for biochemists, as translation efficiency and fidelity is often insufficient when employing npMs. In this Review, we will focus on the methodologies for engineering tRNAs to overcome these issues and explore recent advances both in vitro and in vivo. These efforts include increasing orthogonality, recruiting essential translation factors, and creation of expanded genetic codes. After our review on the biochemical optimizations of tRNAs, we provide examples of their use in genetic code manipulation, with a focus on the in vitro discovery of bioactive macrocyclic peptides containing npMs. Finally, an analysis of the current state of tRNA engineering is presented, along with existing challenges and future perspectives for the field.

Cheminformatics-Guided Cell-Free Exploration of Peptide Natural Products

There have been significant advances in the flexibility and power of in vitro cell-free translation systems. The increasing ability to incorporate noncanonical amino acids and complement translation with recombinant enzymes has enabled cell-free production of peptide-based natural products (NPs) and NP-like molecules. We anticipate that many more such compounds and analogs might be accessed in this way. To assess the peptide NP space that is directly accessible to current cell-free technologies, we developed a peptide parsing algorithm that breaks down peptide NPs into building blocks based on ribosomal translation logic. Using the resultant data set, we broadly analyze the biophysical properties of these privileged compounds and perform a retrobiosynthetic analysis to predict which peptide NPs could be directly synthesized in augmented cell-free translation reactions. We then tested these predictions by preparing a library of highly modified peptide NPs. Two macrocyclases, PatG and PCY1, were used to effect the head-to-tail macrocyclization of candidate NPs. This retrobiosynthetic analysis identified a collection of high-priority building blocks that are enriched throughout peptide NPs, yet they had not previously been tested in cell-free translation. To expand the cell-free toolbox into this space, we established, optimized, and characterized the flexizyme-enabled ribosomal incorporation of piperazic acids. Overall, these results demonstrate the feasibility of cell-free translation for peptide NP total synthesis while expanding the limits of the technology. This work provides a novel computational tool for exploration of peptide NP chemical space, that could be expanded in the future to allow design of ribosomal biosynthetic pathways for NPs and NP-like molecules.

De Novo Single-Stranded RNA-Binding Peptides Discovered by Codon-Restricted mRNA Display

RNA-binding proteins participate in diverse cellular processes, including DNA repair, post-transcriptional modification, and cancer progression through their interactions with RNAs, making them attractive for biotechnological applications. While nature provides an array of naturally occurring RNA-binding proteins, developing de novo RNA-binding peptides remains challenging. In particular, tailoring peptides to target single-stranded RNA with low complexity is difficult due to the inherent structural flexibility of RNA molecules. Here, we developed a codon-restricted mRNA display and identified multiple de novo peptides from a peptide library that bind to poly(C) and poly(A) RNA with KDs ranging from micromolar to submicromolar concentrations. One of the newly identified peptides is capable of binding to the cytosine-rich sequences of the oncogenic Cdk6 3'UTR RNA and MYU lncRNA, with affinity comparable to that of the endogenous binding protein. Hence, we present a novel platform for discovering de novo single-stranded RNA-binding peptides that offer promising avenues for regulating RNA functions.

Extensive breaking of genetic code degeneracy with non-canonical amino acids

Genetic code expansion (GCE) offers many exciting opportunities for the creation of synthetic organisms and for drug discovery methods that utilize in vitro translation. One type of GCE, sense codon reassignment (SCR), focuses on breaking the degeneracy of the 61 sense codons which encode for only 20 amino acids. SCR has great potential for genetic code expansion, but extensive SCR is limited by the post-transcriptional modifications on tRNAs and wobble reading of these tRNAs by the ribosome. To better understand codon-tRNA pairing, here we develop an assay to evaluate the ability of aminoacyl-tRNAs to compete with each other for a given codon. We then show that hyperaccurate ribosome mutants demonstrate reduced wobble reading, and when paired with unmodified tRNAs lead to extensive and predictable SCR. Together, we encode seven distinct amino acids across nine codons spanning just two codon boxes, thereby demonstrating that the genetic code hosts far more re-assignable space than previously expected, opening the door to extensive genetic code engineering.

Ribozyme synthesis of both L- and D- amino acid oligos

The ribosome is responsible for assembling proteins using 20 naturally occurring L-handed amino acids. However, incorporating non-natural amino acids into a protein is a challenging process needs improvement. In this study, we report a new possible approach to creating nonnatural peptides using ribozymes inspired by the peptidyl transfer center. These RNA scaffolds, which are approximately 100 nucleotides in length, bind to RNase T1 truncated tRNA-like chimeras and bring them into close proximity to facilitate peptide ligation. We used single-molecule fluorescence resonance energy transfer (smFRET) to show close distances between RNA-RNA, tRNALys-tRNALys, and RNA-tRNALys pairs, which strongly suggests that the mechanism of peptide ligation is due to the proximity of the substrate through dimerization of the enzymes. Mass spectrometry analysis confirmed the detection of oligopeptides from four amino acids, including L-Lysine, D-Lysine, L-Phenylalanine, and D-Phenylalanine. These results indicate that ribozymes have greater flexibility in accommodating nonnatural amino acids. Our findings pave the way for potentially new avenues in the synthesis of nonnatural peptides, beyond the limitations of ribosomal peptide synthesis and other existing methods.

In Vitro Transcription/Translation-Coupled DNA Replication through Partial Regeneration of 20 Aminoacyl-tRNA Synthetases

The in vitro reconstruction of life-like self-reproducing systems is a major challenge in in vitro synthetic biology. Self-reproduction requires regeneration of all molecules involved in DNA replication, transcription, and translation. This study demonstrated the continuous DNA replication and partial regeneration of major translation factors, 20 aminoacyl-tRNA synthetases (aaRS), in a reconstituted transcription/translation system (PURE system) for the first time. First, we replicated each DNA that encodes one of the 20 aaRSs through aaRS expression from the DNA by serial transfer experiments. Thereafter, we successively increased the number of aaRS genes and achieved simultaneous, continuous replication of DNA that encodes all 20 aaRSs, which comprised approximately half the number of protein factors in the PURE system, except for ribosomes, by employing dialyzed reaction and sequence optimization. This study provides a step-by-step methodology for continuous DNA replication with an increasing number of self-regenerative genes toward self-reproducing artificial systems.

Cell-free Biosynthesis of Peptidomimetics

A wide variety of peptidomimetics (peptide analogs) possessing innovative biological functions have been brought forth as therapeutic candidates through cell-free protein synthesis (CFPS) systems. A key feature of these peptidomimetic drugs is the use of non-canonical amino acid building blocks with diverse biochemical properties that expand functional diversity. Here, we summarize recent technologies leveraging CFPS platforms to expand the reach of peptidomimetics drugs. We also offer perspectives on engineering the translational machinery that may open new opportunities for expanding genetically encoded chemistry to transform drug discovery practice beyond traditional boundaries.

Programmable Synthesis of Biobased Materials Using Cell-Free Systems

Motivated by the intricate mechanisms underlying biomolecule syntheses in cells that chemistry is currently unable to mimic, researchers have harnessed biological systems for manufacturing novel materials. Cell-free systems (CFSs) utilizing the bioactivity of transcriptional and translational machineries in vitro are excellent tools that allow supplementation of exogenous materials for production of innovative materials beyond the capability of natural biological systems. Herein, recent studies that have advanced the ability to expand the scope of biobased materials using CFS are summarized and approaches enabling the production of high-value materials, prototyping of genetic parts and modules, and biofunctionalization are discussed. By extending the reach of chemical and enzymatic reactions complementary to cellular materials, CFSs provide new opportunities at the interface of materials science and synthetic biology.

Expansion of the genetic code through reassignment of redundant sense codons using fully modified tRNA

Breaking codon degeneracy for the introduction of non-canonical amino acids offers many opportunities in synthetic biology. Yet, despite the existence of 64 codons, the code has only been expanded to 25 amino acids in vitro. A limiting factor could be the over-reliance on synthetic tRNAs which lack the post-transcriptional modifications that improve translational fidelity. To determine whether modified, wild-type tRNA could improve sense codon reassignment, we developed a new fluorous method for tRNA capture and applied it to the isolation of roughly half of the Escherichia coli tRNA isoacceptors. We then performed codon competition experiments between the five captured wild-type leucyl-tRNAs and their synthetic counterparts, revealing a strong preference for wild-type tRNA in an in vitro translation system. Finally, we compared the ability of wild-type and synthetic leucyl-tRNA to break the degeneracy of the leucine codon box, showing that only captured wild-type tRNAs are discriminated with enough fidelity to accurately split the leucine codon box for the encoding of three separate amino acids. Wild-type tRNAs are therefore enabling reagents for maximizing the reassignment potential of the genetic code.

Effects of DNA template preparation on variability in cell-free protein production

DNA templates for protein production remain an unexplored source of variability in the performance of cell-free expression (CFE) systems. To characterize this variability, we investigated the effects of two common DNA extraction methodologies, a postprocessing step and manual versus automated preparation on protein production using CFE. We assess the concentration of the DNA template, the quality of the DNA template in terms of physical damage and the quality of the DNA solution in terms of purity resulting from eight DNA preparation workflows. We measure the variance in protein titer and rate of protein production in CFE reactions associated with the biological replicate of the DNA template, the technical replicate DNA solution prepared with the same workflow and the measurement replicate of nominally identical CFE reactions. We offer practical guidance for preparing and characterizing DNA templates to achieve acceptable variability in CFE performance.

Transfer RNA Synthesis-Coupled Translation and DNA Replication in a Reconstituted Transcription/Translation System

Transfer RNAs (tRNAs) are key molecules involved in translation. In vitro synthesis of tRNAs and their coupled translation are important challenges in the construction of a self-regenerative molecular system. Here, we first purified EF-Tu and ribosome components in a reconstituted translation system of Escherichia coli to remove residual tRNAs. Next, we expressed 15 types of tRNAs in the repurified translation system and performed translation of the reporter luciferase gene depending on the expression. Furthermore, we demonstrated DNA replication through expression of a tRNA encoded by DNA, mimicking information processing within the cell. Our findings highlight the feasibility of an in vitro self-reproductive system, in which tRNAs can be synthesized from replicating DNA.

Biochemistry of Aminoacyl tRNA Synthetase and tRNAs and Their Engineering for Cell-Free and Synthetic Cell Applications

Cell-free biology is increasingly utilized for engineering biological systems, incorporating novel functionality, and circumventing many of the complications associated with cells. The central dogma describes the information flow in biology consisting of transcription and translation steps to decode genetic information. Aminoacyl tRNA synthetases (AARSs) and tRNAs are key components involved in translation and thus protein synthesis. This review provides information on AARSs and tRNA biochemistry, their role in the translation process, summarizes progress in cell-free engineering of tRNAs and AARSs, and discusses prospects and challenges lying ahead in cell-free engineering.

In Vitro Genetic Code Reprogramming for the Expansion of Usable Noncanonical Amino Acids

Genetic code reprogramming has enabled us to ribosomally incorporate various nonproteinogenic amino acids (npAAs) into peptides in vitro. The repertoire of usable npAAs has been expanded to include not only l-α-amino acids with noncanonical sidechains but also those with noncanonical backbones. Despite successful single incorporation of npAAs, multiple and consecutive incorporations often suffer from low efficiency or are even unsuccessful. To overcome this stumbling block, engineering approaches have been used to modify ribosomes, EF-Tu, and tRNAs. Here, we provide an overview of these in vitro methods that are aimed at optimal expansion of the npAA repertoire and their applications for the development of de novo bioactive peptides containing various npAAs.

Opportunities for Expanding Encoded Chemical Diversification and Improving Hit Enrichment in mRNA-Displayed Peptide Libraries

DNA-encoded small-molecule libraries and mRNA displayed peptide libraries both use numerically large pools of oligonucleotide-tagged molecules to identify potential hits for protein targets. They differ dramatically, however, in the 'drug-likeness' of the molecules that each can be used to discover. We give here an overview of the two techniques, comparing some advantages and disadvantages of each, and suggest areas where particularly mRNA display can benefit from adopting advances developed with DNA-encoded small molecule libraries. We outline cases where chemical modification of the peptide library has already been used in mRNA display, and survey opportunities to expand this using examples from DNA-encoded small molecule libraries. We also propose potential opportunities for encoding such reactions within the mRNA/cDNA tag of an mRNA-displayed peptide library to allow a more diversity-oriented approach to library modification. Finally, we outline alternate approaches for enriching target-binding hits from a pooled and tagged library, and close by detailing several examples of how an adjusted mRNA-display based approach could be used to discover new 'drug-like' modified small peptides.

Genetic Code Engineering by Natural and Unnatural Base Pair Systems for the Site-Specific Incorporation of Non-Standard Amino Acids Into Proteins

Amino acid sequences of proteins are encoded in nucleic acids composed of four letters, A, G, C, and T(U). However, this four-letter alphabet coding system limits further functionalities of proteins by the twenty letters of amino acids. If we expand the genetic code or develop alternative codes, we could create novel biological systems and biotechnologies by the site-specific incorporation of non-standard amino acids (or unnatural amino acids, unAAs) into proteins. To this end, new codons and their complementary anticodons are required for unAAs. In this review, we introduce the current status of methods to incorporate new amino acids into proteins by in vitro and in vivo translation systems, by focusing on the creation of new codon-anticodon interactions, including unnatural base pair systems for genetic alphabet expansion.

Codon-Reduced Protein Synthesis With Manipulating tRNA Components in Cell-Free System

Manipulating transfer RNAs (tRNAs) for emancipating sense codons to simplify genetic codons in a cell-free protein synthesis (CFPS) system can offer more flexibility and controllability. Here, we provide an overview of the tRNA complement protein synthesis system construction in the tRNA-depleted Protein synthesis Using purified Recombinant Elements (PURE) system or S30 extract. These designed polypeptide coding sequences reduce the genetic codon and contain only a single tRNA corresponding to a single amino acid in this presented system. Strategies for removing tRNAs from cell lysates and synthesizing tRNAs in vivo/vitro are summarized and discussed in detail. Furthermore, we point out the trend toward a minimized genetic codon for reducing codon redundancy by manipulating tRNAs in the different proteins. It is hoped that the tRNA complement protein synthesis system can facilitate the construction of minimal cells and expand the biomedical application scope of synthetic biology.

Genome-wide association study reveals the genetic basis of growth trait in yellow catfish with sexual size dimorphism

Sexual size dimorphism has been widely observed in a large number of animals including fish species. Genome-wide association study (GWAS) is a powerful tool to dissect the genetic basis of complex traits, whereas the sex-differences in the genomics of animal complex traits have been ignored in the GWAS analysis. Yellow catfish (Pelteobagrus fulvidraco) is an important aquaculture fish in China with significant sexual size dimorphism. In this study, GWAS was conducted to identify candidate SNPs and genes related to body length (BL) and body weight (BW) in 125 female yellow catfish from a breeding population. In total, one BL-related SNP and three BW-related SNPs were identified to be significantly associated with the traits. Besides, one of these SNPs (Chr15:19195072) was shared in both the BW and BL traits in female yellow catfish, which was further validated in 185 male individuals and located on the exon of stat5b gene. Transgenic yellow catfish and zebrafish that expressed yellow catfish stat5b showed increased growth rate and reduction of sexual size dimorphism. These results not only reveal the genetic basis of growth trait and sexual size dimorphism in fish species, but also provide useful information for the marker-assisted breeding in yellow catfish.

Best Practices for DNA Template Preparation Toward Improved Reproducibility in Cell-Free Protein Production

Performance variability is a common challenge in cell-free protein production and hinders a wider adoption of these systems for both research and biomanufacturing. While the inherent stochasticity and complexity of biology likely contributes to variability, other systematic factors may also play a role, including the source and preparation of the cell extract, the composition of the supplemental reaction buffer, the facility at which experiments are conducted, and the human operator (Cole et al. ACS Synth Biol 8:2080-2091, 2019). Variability in protein production could also arise from differences in the DNA template-specifically the amount of functional DNA added to a cell-free reaction and the quality of the DNA preparation in terms of contaminants and strand breakage. Here, we present protocols and suggest best practices optimized for DNA template preparation and quantitation for cell-free systems toward reducing variability in cell-free protein production.

Directed Evolution in Drops: Molecular Aspects and Applications

The process of optimizing the properties of biological molecules is paramount for many industrial and medical applications. Directed evolution is a powerful technique for modifying and improving biomolecules such as proteins or nucleic acids (DNA or RNA). Mimicking the mechanism of natural evolution, one can enhance a desired property by applying a suitable selection pressure and sorting improved variants. Droplet-based microfluidic systems offer a high-throughput solution to this approach by helping to overcome the limiting screening steps and allowing the analysis of variants within increasingly complex libraries. Here, we review cases where successful evolution of biomolecules was achieved using droplet-based microfluidics, focusing on the molecular processes involved and the incorporation of microfluidics to the workflow. We highlight the advantages and limitations of these microfluidic systems compared to low-throughput methods and show how the integration of these systems into directed evolution workflows can open new avenues to discover or improve biomolecules according to user-defined conditions.

The RaPID Platform for the Discovery of Pseudo-Natural Macrocyclic Peptides

Although macrocyclic peptides bearing exotic building blocks have proven their utility as pharmaceuticals, the sources of macrocyclic peptide drugs have been largely limited to mimetics of native peptides or natural product peptides. However, the recent emergence of technologies for discovering de novo bioactive peptides has led to their reconceptualization as a promising therapeutic modality. For the construction and screening of libraries of such macrocyclic peptides, our group has devised a platform to conduct affinity-based selection of massive libraries (>10¹² unique sequences) of in vitro expressed macrocyclic peptides, which is referred to as the random nonstandard peptides integrated discovery (RaPID) system. The RaPID system integrates genetic code reprogramming using the FIT (flexible in vitro translation) system, which is largely facilitated by flexizymes (flexible tRNA-aminoacylating ribozymes), with mRNA display technology.We have demonstrated that the RaPID system enables rapid discovery of various de novo pseudo-natural peptide ligands for protein targets of interest. Many examples discussed in this Account prove that thioether-closed macrocyclic peptides (teMPs) obtained by the RaPID system generally exhibit remarkably high affinity and specificity, thereby potently inhibiting or activating a specific function(s) of the target. Moreover, such teMPs are used for a wide range of biochemical applications, for example, as crystallization chaperones for intractable transmembrane proteins and for in vivo recognition of specific cell types. Furthermore, recent studies demonstrate that some teMPs exhibit pharmacological activities in animal models and that even intracellular proteins can be inhibited by teMPs, illustrating the potential of this class of peptides as drug leads.Besides the ring-closing thioether linkage in the teMPs, genetic code reprogramming by the FIT system allows for incorporation of a variety of other exotic building blocks. For instance, diverse nonproteinogenic amino acids, hydroxy acids (ester linkage), amino carbothioic acid (thioamide linkage), and abiotic foldamer units have been successfully incorporated into ribosomally synthesized peptides. Despite such enormous successes in the conventional FIT system, multiple or consecutive incorporation of highly exotic amino acids, such as d- and β-amino acids, is yet challenging, and particularly the synthesis of peptides bearing non-carbonyl backbone structures remains a demanding task. To upgrade the RaPID system to the next generation, we have engaged in intensive manipulation of the FIT system to expand the structural diversity of peptides accessible by our in vitro biosynthesis strategy. Semilogical engineering of tRNA body sequences led to a new suppressor tRNA (tRNA^Pro1E2) capable of effectively recruiting translation factors, particularly EF-Tu and EF-P. The use of tRNA^Pro1E2 in the FIT system allows for not only single but also consecutive and multiple elongation of exotic amino acids, such as d-, β-, and γ-amino acids as well as aminobenzoic acids. Moreover, the integration of the FIT system with various chemical or enzymatic posttranslational modifications enables us to expand the range of accessible backbone structures to non-carbonyl moieties prominent in natural products and peptidomimetics. In such systems, FIT-expressed peptides undergo multistep backbone conversions in a one-pot manner to yield designer peptides composed of modified backbones such as azolines, azoles, and ring-closing pyridines. Our current research endeavors focus on applying such in vitro biosynthesis systems for the discovery of bioactive de novo pseudo-natural products.

A Selection of Macrocyclic Peptides That Bind STING From an mRNA-Display Library With Split Degenerate Codons

Recent improvements in mRNA display have enabled the selection of peptides that incorporate non‐natural amino acids, thus expanding the chemical diversity of macrocycles beyond what is accessible in nature. Such libraries have incorporated non‐natural amino acids at the expense of natural amino acids by reassigning their codons. Here we report an alternative approach to expanded amino‐acid diversity that preserves all 19 natural amino acids (no methionine) and adds 6 non‐natural amino acids, resulting in the highest sequence complexity reported to date. We have applied mRNA display to this 25‐letter library to select functional macrocycles that bind human STING, a protein involved in immunoregulation. The resulting STING‐binding peptides include a 9‐mer macrocycle with a dissociation constant (K_D) of 3.4 nM, which blocks binding of cGAMP to STING and induces STING dimerization. This approach is generalizable to expanding the amino‐acid alphabet in a library beyond 25 building blocks.

Incorporation of backbone modifications in mRNA-displayable peptides

Here we comprehensively summarize the most recent efforts in our research team, aiming at installing N-methyl and azole backbones into peptides expressed in translation. The genetic code reprogramming using the Flexible In-vitro Translation system (FIT system) has proven to be the most reliable and versatile approach for ribosomally installing various exotic amino acids. However, it had been yet difficult in translating diverse kinds of multiple and consecutive sequences of N-methyl amino acids (^MeAAs). We have recently reported that a semi-rational fine tuning of ^MeAA-tRNA affinities for EF-Tu by altering tRNA T-stem sequence achieves efficient delivery of ^MeAA-tRNAs to the ribosome. Indeed, this approach has made it possible to express N-methyl-peptides containing multiple ^MeAAs with a remarkably high fidelity. Another interesting backbone modification in peptides is azole moieties often found in natural products, but they are explicitly installed by post-translational modifying enzymes. We have recently devised a method to bypass such enzymatic processes where a bromovinyl group-containing amino acid is incorporated into the peptide by genetic code reprogramming and then chemically converted to an azole group via an intramolecular heterocyclization reaction. These methods will grant more drug-like properties to peptides than ordinary peptides in terms of protease resistance and cell membrane permeability. Particularly when they can be integrated with in vitro mRNA display, such as the RaPID system, the discovery of de novo bioactive peptides can be realized.

Evolution of KIPPIS as a versatile platform for evaluating intracellularly functional peptide aptamers

Chimeric proteins have been widely used to evaluate intracellular protein–protein interactions (PPIs) in living cells with various readouts. By combining an interleukin-3-dependent murine cells and chimeric proteins containing a receptor tyrosine kinase c-kit, we previously established a c-kit-based PPI screening (KIPPIS) system to evaluate and select protein binders. In the KIPPIS components, proteins of interest are connected with a chemically inducible helper module and the intracellular domain of the growth-signaling receptor c-kit, which detects PPIs based on cell proliferation as a readout. In this system, proteins of interest can be incorporated into chimeric proteins without any scaffold proteins, which would be advantageous for evaluating interaction between small peptides/domains. To prove this superiority, we apply KIPPIS to 6 peptide aptamer–polypeptide pairs, which are derived from endogenous, synthetic, and viral proteins. Consequently, all of the 6 peptide aptamer–polypeptide interactions are successfully detected by cell proliferation. The detection sensitivity can be modulated in a helper ligand-dependent manner. The assay results of KIPPIS correlate with the activation levels of Src, which is located downstream of c-kit-mediated signal transduction. Control experiments reveal that KIPPIS clearly discriminates interacting aptamers from non-interacting ones. Thus, KIPPIS proves to be a versatile platform for evaluating the binding properties of peptide aptamers.

Engineering molecular translation systems

Molecular translation systems provide a genetically encoded framework for protein synthesis, which is essential for all life. Engineering these systems to incorporate non-canonical amino acids (ncAAs) into peptides and proteins has opened many exciting opportunities in chemical and synthetic biology. Here, we review recent advances that are transforming our ability to engineer molecular translation systems. In cell-based systems, new processes to synthesize recoded genomes, tether ribosomal subunits, and engineer orthogonality with high-throughput workflows have emerged. In cell-free systems, adoption of flexizyme technology and cell-free ribosome synthesis and evolution platforms are expanding the limits of chemistry at the ribosome's RNA-based active site. Looking forward, innovations will deepen understanding of molecular translation and provide a path to polymers with previously unimaginable structures and functions.

Uniform affinity-tuning of N-methyl-aminoacyl-tRNAs to EF-Tu enhances their multiple incorporation

In ribosomal translation, the accommodation of aminoacyl-tRNAs into the ribosome is mediated by elongation factor thermo unstable (EF-Tu). The structures of proteinogenic aminoacyl-tRNAs (pAA-tRNAs) are fine-tuned to have uniform binding affinities to EF-Tu in order that all proteinogenic amino acids can be incorporated into the nascent peptide chain with similar efficiencies. Although genetic code reprogramming has enabled the incorporation of non-proteinogenic amino acids (npAAs) into the nascent peptide chain, the incorporation of some npAAs, such as N-methyl-amino acids (^MeAAs), is less efficient, especially when ^MeAAs frequently and/or consecutively appear in a peptide sequence. Such poor incorporation efficiencies can be attributed to inadequate affinities of ^MeAA-tRNAs to EF-Tu. Taking advantage of flexizymes, here we have experimentally verified that the affinities of ^MeAA-tRNAs to EF-Tu are indeed weaker than those of pAA-tRNAs. Since the T-stem of tRNA plays a major role in interacting with EF-Tu, we have engineered the T-stem sequence to tune the affinity of ^MeAA-tRNAs to EF-Tu. The uniform affinity-tuning of the individual pairs has successfully enhanced the incorporation of ^MeAAs, achieving the incorporation of nine distinct ^MeAAs into both linear and thioether-macrocyclic peptide scaffolds.

Some theoretical aspects of reprogramming the standard genetic code

Reprogramming of the standard genetic code to include non-canonical amino acids (ncAAs) opens new prospects for medicine, industry, and biotechnology. There are several methods of code engineering, which allow us for storing new genetic information in DNA sequences and producing proteins with new properties. Here, we provided a theoretical background for the optimal genetic code expansion, which may find application in the experimental design of the genetic code. We assumed that the expanded genetic code includes both canonical and non-canonical information stored in 64 classical codons. What is more, the new coding system is robust to point mutations and minimizes the possibility of reversion from the new to old information. In order to find such codes, we applied graph theory to analyze the properties of optimal codon sets. We presented the formal procedure in finding the optimal codes with various number of vacant codons that could be assigned to new amino acids. Finally, we discussed the optimal number of the newly incorporated ncAAs and also the optimal size of codon groups that can be assigned to ncAAs.

Transfer RNAs: diversity in form and function

As the adaptor that decodes mRNA sequence into protein, the basic aspects of tRNA structure and function are central to all studies of biology. Yet the complexities of their properties and cellular roles go beyond the view of tRNAs as static participants in protein synthesis. Detailed analyses through more than 60 years of study have revealed tRNAs to be a fascinatingly diverse group of molecules in form and function, impacting cell biology, physiology, disease and synthetic biology. This review analyzes tRNA structure, biosynthesis and function, and includes topics that demonstrate their diversity and growing importance.

Expanding the Chemical Diversity of Genetically Encoded Libraries

The power of ribosomes has increasingly been harnessed for the synthesis and selection of molecular libraries. Technologies, such as phage display, yeast display, and mRNA display, effectively couple genotype to phenotype for the molecular evolution of high affinity epitopes for many therapeutic targets. Genetic code expansion is central to the success of these technologies, allowing researchers to surpass the intrinsic capabilities of the ribosome and access new, genetically encoded materials for these selections. Here, we review techniques for the chemical expansion of genetically encoded libraries, their abilities and limits, and opportunities for further development. Importantly, we also discuss methods and metrics used to assess the efficiency of modification and library diversity with these new techniques.

Combating Antimicrobial Resistance With New-To-Nature Lanthipeptides Created by Genetic Code Expansion

Due to the rapid emergence of multi-resistant bacterial strains in recent decades, the commercially available effective antibiotics are becoming increasingly limited. On the other hand, widespread antimicrobial peptides (AMPs) such as the lantibiotic nisin has been used worldwide for more than 40 years without the appearance of significant bacterial resistance. Lantibiotics are ribosomally synthesized antimicrobials generated by posttranslational modifications. Their biotechnological production is of particular interest to redesign natural scaffolds improving their pharmaceutical properties, which has great potential for therapeutic use in human medicine and other areas. However, conventional protein engineering methods are limited to 20 canonical amino acids prescribed by the genetic code. Therefore, the expansion of the genetic code as the most advanced approach in Synthetic Biology allows the addition of new amino acid building blocks (non-canonical amino acids, ncAAs) during protein translation. We now have solid proof-of-principle evidence that bioexpression with these novel building blocks enabled lantibiotics with chemical properties transcending those produced by natural evolution. The unique scaffolds with novel structural and functional properties are the result of this bioengineering. Here we will critically examine and evaluate the use of the expanded genetic code and its alternatives in lantibiotics research over the last 7 years. We anticipate that Synthetic Biology, using engineered lantibiotics and even more complex scaffolds will be a promising tool to address an urgent problem of antibiotic resistance, especially in a class of multi-drug resistant microbes known as superbugs.

Using Genetic Code Expansion for Protein Biochemical Studies

Protein identification has gone beyond simply using protein/peptide tags and labeling canonical amino acids. Genetic code expansion has allowed residue- or site-specific incorporation of non-canonical amino acids into proteins. By taking advantage of the unique properties of non-canonical amino acids, we can identify spatiotemporal-specific protein states within living cells. Insertion of more than one non-canonical amino acid allows for selective labeling that can aid in the identification of weak or transient protein-protein interactions. This review will discuss recent studies applying genetic code expansion for protein labeling and identifying protein-protein interactions and offer considerations for future work in expanding genetic code expansion methods.

An Amino Acid-Swapped Genetic Code

Preventing the escape of hazardous genes from genetically modified organisms (GMOs) into the environment is one of the most important issues in biotechnology research. Various strategies were developed to create "genetic firewalls" that prevent the leakage of GMOs; however, they were not specially designed to prevent the escape of genes. To address this issue, we developed amino acid (AA)-swapped genetic codes orthogonal to the standard genetic code, namely SL (Ser and Leu were swapped) and SLA genetic codes (Ser, Leu, and Ala were swapped). From mRNAs encoded by the AA-swapped genetic codes, functional proteins were only synthesized in translation systems featuring the corresponding genetic codes. These results clearly demonstrated the orthogonality of the AA-swapped genetic codes against the standard genetic code and their potential to function as "genetic firewalls for genes". Furthermore, we propose "a codon-bypass strategy" to develop a GMO with an AA-swapped genetic code.

Deciphering protein post-translational modifications using chemical biology tools

Proteins carry out a wide variety of catalytic, regulatory, signalling and structural functions in living systems. Following their assembly on ribosomes and throughout their lifetimes, most eukaryotic proteins are modified by post-translational modifications; small functional groups and complex biomolecules are conjugated to amino acid side chains or termini, and the protein backbone is cleaved, spliced or cyclized, to name just a few examples. These modifications modulate protein activity, structure, location and interactions, and, thereby, control many core biological processes. Aberrant post-translational modifications are markers of cellular stress or malfunction and are implicated in several diseases. Therefore, gaining an understanding of which proteins are modified, at which sites and the resulting biological consequences is an important but complex challenge requiring interdisciplinary approaches. One of the key challenges is accessing precisely modified proteins to assign functional consequences to specific modifications. Chemical biologists have developed a versatile set of tools for accessing specifically modified proteins by applying robust chemistries to biological molecules and developing strategies for synthesizing and ligating proteins. This Review provides an overview of these tools, with selected recent examples of how they have been applied to decipher the roles of a variety of protein post-translational modifications. Relative advantages and disadvantages of each of the techniques are discussed, highlighting examples where they are used in combination and have the potential to address new frontiers in understanding complex biological processes.

Noncanonical Amino Acids in Synthetic Biosafety and Post-translational Modification Studies

The incorporation of noncanonical amino acids (ncAAs) has been extensively studied because of its broad applicability. In the past decades, various in vitro and in vivo ncAA incorporation approaches have been developed to generate synthetic recombinant proteins. Herein, we discuss the methodologies for ncAA incorporation, and their use in diverse research areas, such as in synthetic biosafety and for studies of post-translational modifications.

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.

Ribosome-mediated polymerization of long chain carbon and cyclic amino acids into peptides in vitro

Ribosome-mediated polymerization of backbone-extended monomers into polypeptides is challenging due to their poor compatibility with the translation apparatus, which evolved to use α-L-amino acids. Moreover, mechanisms to acylate (or charge) these monomers to transfer RNAs (tRNAs) to make aminoacyl-tRNA substrates is a bottleneck. Here, we rationally design non-canonical amino acid analogs with extended carbon chains (γ-, δ-, ε-, and ζ-) or cyclic structures (cyclobutane, cyclopentane, and cyclohexane) to improve tRNA charging. We then demonstrate site-specific incorporation of these non-canonical, backbone-extended monomers at the N- and C- terminus of peptides using wild-type and engineered ribosomes. This work expands the scope of ribosome-mediated polymerization, setting the stage for new medicines and materials.

Backbone extended monomers are poorly compatible with the natural ribosomes, impeding their polymerization into polypeptides. Here the authors design non-canonical amino acid analogs with cyclic structures or extended carbon chains and used an engineered ribosome to improve tRNA-charging and incorporation into peptides.

Reconstituted cell-free protein synthesis using in vitro transcribed tRNAs

Entire reconstitution of tRNAs for active protein production in a cell-free system brings flexibility into the genetic code engineering. It can also contribute to the field of cell-free synthetic biology, which aims to construct self-replicable artificial cells. Herein, we developed a system equipped only with in vitro transcribed tRNA (iVTtRNA) based on a reconstituted cell-free protein synthesis (PURE) system. The developed system, consisting of 21 iVTtRNAs without nucleotide modifications, is able to synthesize active proteins according to the redesigned genetic code. Manipulation of iVTtRNA composition in the system enabled genetic code rewriting. Introduction of modified nucleotides into specific iVTtRNAs demonstrated to be effective for both protein yield and decoding fidelity, where the production yield of DHFR reached about 40% of the reaction with native tRNA at 30°C. The developed system will prove useful for studying decoding processes, and may be employed in genetic code and protein engineering applications.

Keita Hibi et al. develop a system to reconstitute cell-free protein synthesis using only in vitro transcribed tRNA (iVTtRNAs). They use 21 iVTtRNAs with and without nucleotide modifications to successfully synthesize functional proteins with about 40% production yield. Their system will be useful to study gene and protein engineering.

Methodologies for Backbone Macrocyclic Peptide Synthesis Compatible With Screening Technologies

Backbone macrocyclic structures are often found in diverse bioactive peptides and contribute to greater conformational rigidity, peptidase resistance, and potential membrane permeability compared to their linear counterparts. Therefore, such peptide scaffolds are an attractive platform for drug-discovery endeavors. Recent advances in synthetic methods for backbone macrocyclic peptides have enabled the discovery of novel peptide drug candidates against diverse targets. Here, we overview recent technical advancements in the synthetic methods including 1) enzymatic synthesis, 2) chemical synthesis, 3) split-intein circular ligation of peptides and proteins (SICLOPPS), and 4) in vitro translation system combined with genetic code reprogramming. We also discuss screening methodologies compatible with those synthetic methodologies, such as one-beads one-compound (OBOC) screening compatible with the synthetic method 2, cell-based assay compatible with 3, limiting-dilution PCR and mRNA display compatible with 4.

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.

Basic principles of the genetic code extension

Compounds including non-canonical amino acids (ncAAs) or other artificially designed molecules can find a lot of applications in medicine, industry and biotechnology. They can be produced thanks to the modification or extension of the standard genetic code (SGC). Such peptides or proteins including the ncAAs can be constantly delivered in a stable way by organisms with the customized genetic code. Among several methods of engineering the code, using non-canonical base pairs is especially promising, because it enables generating many new codons, which can be used to encode any new amino acid. Since even one pair of new bases can extend the SGC up to 216 codons generated by a six-letter nucleotide alphabet, the extension of the SGC can be achieved in many ways. Here, we proposed a stepwise procedure of the SGC extension with one pair of non-canonical bases to minimize the consequences of point mutations. We reported relationships between codons in the framework of graph theory. All 216 codons were represented as nodes of the graph, whereas its edges were induced by all possible single nucleotide mutations occurring between codons. Therefore, every set of canonical and newly added codons induces a specific subgraph. We characterized the properties of the induced subgraphs generated by selected sets of codons. Thanks to that, we were able to describe a procedure for incremental addition of the set of meaningful codons up to the full coding system consisting of three pairs of bases. The procedure of gradual extension of the SGC makes the whole system robust to changing genetic information due to mutations and is compatible with the views assuming that codons and amino acids were added successively to the primordial SGC, which evolved minimizing harmful consequences of mutations or mistranslations of encoded proteins.

Expanding the limits of the second genetic code with ribozymes

The site-specific incorporation of noncanonical monomers into polypeptides through genetic code reprogramming permits synthesis of bio-based products that extend beyond natural limits. To better enable such efforts, flexizymes (transfer RNA (tRNA) synthetase-like ribozymes that recognize synthetic leaving groups) have been used to expand the scope of chemical substrates for ribosome-directed polymerization. The development of design rules for flexizyme-catalyzed acylation should allow scalable and rational expansion of genetic code reprogramming. Here we report the systematic synthesis of 37 substrates based on 4 chemically diverse scaffolds (phenylalanine, benzoic acid, heteroaromatic, and aliphatic monomers) with different electronic and steric factors. Of these substrates, 32 were acylated onto tRNA and incorporated into peptides by in vitro translation. Based on the design rules derived from this expanded alphabet, we successfully predicted the acylation of 6 additional monomers that could uniquely be incorporated into peptides and direct N-terminal incorporation of an aldehyde group for orthogonal bioconjugation reactions.

Programmable On-Chip Artificial Cell Producing Post-Translationally Modified Ubiquitinated Protein

In nature, intracellular microcompartments have evolved to allow the simultaneous execution of tightly regulated complex processes within a controlled environment. This architecture serves as the blueprint for the construction of a wide array of artificial cells. However, such systems are inadequate in their ability to confine and sequentially control multiple central dogma activities (transcription, translation, and post-translational modifications) resulting in a limited production of complex biomolecules. Here, an artificial cell-on-a-chip comprising hierarchical compartments allowing the processing and transport of products from transcription, translation, and post-translational modifications through connecting channels is designed and fabricated. This platform generates a tightly controlled system, yielding directly a purified modified protein, with the potential to produce proteoform of choice. Using this platform, the full ubiquitinated form of the Parkinson's disease-associated α-synuclein is generated starting from DNA, in a single device. By bringing together all central dogma activities in a single controllable platform, this approach will open up new possibilities for the synthesis of complex targets, will allow to decipher diverse molecular mechanisms in health and disease and to engineer protein-based materials and pharmaceutical agents.

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.

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.

Osmolyte-Enhanced Protein Synthesis Activity of a Reconstituted Translation System

Molecular crowding is receiving great attention in cell-free synthetic biology because molecular crowding is a critical feature of natural cell discrimination from artificial cells. Further, it has significant and generic influences on biomolecular functions. Although there are reports on how the macromolecular crowder reagents affect cell-free systems such as transcription and translation, the second class of molecular crowder reagents with low molecular weight, osmolyte, was much less studied in cell-free systems. In the present study, we focused on trimethylamine- N-oxide (TMAO) and betaine, methylamine osmolytes, and investigated the effectiveness of these osmolytes on gene expression activity of reconstituted cell-free protein synthesis. The gene expression activity of the fluorescent proteins Venus and tdTomato and the enzymes β-galactosidase and dihydrofolate reductase were tested. At 37 °C, 0.4 M TMAO showed the highest enhancement of translational activity by a factor of 1.6-3.8, regardless of protein type. In contrast, betaine showed only a moderate effect that was limited to fluorescent proteins. Excess amounts of osmolytes suppressed gene expression activity. An mRNA-start assay and SDS-PAGE quantitative analysis provided firm evidence that TMAO enhances the translation process, instead of transcription, folding, or the maturation of fluorescent proteins. Interestingly, at 26 °C, TMAO and betaine showed the highest enhancement of protein synthesis activity at lower concentrations than at 37 °C. These findings provide implications on how osmolytes assist translation in natural cells. Further, they provide guidelines for modulation of protein synthesis activity in artificial cells through osmolyte addition.