Genetically programmed cell-based synthesis of non-natural peptide and depsipeptide macrocycles

Expanding the genetic Code: Strategies for noncanonical amino acid incorporation in biopolymer

Codon expansion has become a powerful tool for overcoming the limitations of the standard genetic code system, which restricts the building block of proteins to 20 canonical amino acids. The incorporation of non-canonical amino acids (ncAAs) into proteins presents a significant opportunity to expand their functional diversity. The precise incorporation of ncAAs in vivo requires an orthogonal tRNA/aminoacyl-tRNA synthetase pair and a blank codon to assign them. Studies have focused on the biosynthesis of proteins with novel chemical properties alongside biotechnological advancements in codon expansion research. The three principal strategies for codon expansion are: stop codon utilization, quadruplet codon generation, and sense codon compression. Although using stop codons as blank codons remains an effective approach, the need for additional blank codons has expanded research into quadruplet codons and sense-codon compression. This review presents an overview of each strategy by integrating recent advances in research. We discuss the advantages and limitations of these approaches, as well as the challenges encountered. Subsequently, we propose potential solutions to enhance the efficiency and fidelity of ncAA incorporation. The insights presented in this review provide perspectives for future research and facilitate the advancement of codon expansion and its applications in biotechnology.

Direct and quantitative analysis of tRNA acylation using intact tRNA liquid chromatography-mass spectrometry

Aminoacyl-tRNA synthetases (aaRSs) provide an essential functional link between an mRNA sequence and the protein it encodes. aaRS enzymes catalyze a two-step chemical reaction that acylates specific tRNAs with a cognate α-amino acid. In addition to their role in translation, acylated tRNAs contribute to non-ribosomal natural product biosynthesis and are implicated in multiple human diseases. In synthetic biology, the acylation of tRNAs with a non-canonical α-amino acid or, more recently, a non-α-amino acid monomer is a critical first step in the incorporation of these monomers into proteins, where they can be used for fundamental and applied science. These endeavors all demand an understanding of aaRS activity and specificity. Here, we describe a liquid chromatography-mass spectrometry assay that directly monitors aaRS activity by detecting the intact acyl-tRNA product. After a simple tRNA acylation reaction workup, acyl- and non-acyl-tRNA molecules are resolved by using ion-pairing reverse-phase chromatography, and their exact masses are determined by using high-resolution time-of-flight mass spectrometry. Our assay is fast and simple, quantifies reaction yields as low as 0.23% and can also be used on tRNAs acylated with flexizyme to detect products that are undetectable by using standard techniques. The protocol requires basic expertise in molecular biology, liquid chromatography-mass spectrometry and RNase-free techniques. This protocol takes ≥5 h to complete, depending on the number of samples.

Automated orthogonal tRNA generation

The ability to generate orthogonal, active tRNAs-central to genetic code expansion and reprogramming-is still fundamentally limited. In this study, we developed Chi-T, a method for the de novo generation of orthogonal tRNAs. Chi-T segments millions of isoacceptor tRNA sequences into parts and then assembles chimeric tRNAs from these parts. Chi-T fixes the parts, containing identity elements, and combinatorially varies all other parts to generate chimeric sequences. Chi-T also filters the variable parts and chimeric sequences to minimize host identity elements. We show here that experimentally characterized orthogonal tRNAs are more likely to have predicted minimum free energy cloverleaf structures, and Chi-T filters for sequences with a predicted cloverleaf structure. We report RS-ID for the identification of synthetases that may acylate the tRNAs generated by Chi-T. We computationally identified new orthogonal tRNAs and engineered an orthogonal pair generated by Chi-T/RS-ID to direct non-canonical amino acid incorporation, in response to both amber codons and sense codons, with an efficiency similar to benchmark genetic code expansion systems.

Monitoring monomer-specific acyl-tRNA levels in cells with PARTI

We describe a new assay that reports directly on the acylation state of a user-chosen transfer RNA (tRNA) in cells. We call this assay 3-Prime Adenosine-Retaining Aminoacyl-tRNA Isolation (PARTI). It relies on high-resolution mass spectrometry identification of the acyl-adenosine species released upon RNase A cleavage of isolated cellular tRNA. Here we develop the PARTI workflow and apply it to understand three recent observations related to the cellular incorporation of non-α-amino acid monomers into protein: (i) the origins of the apparent selectivity of translation with respect to β2-hydroxy acid enantiomers; (ii) the activity of PylRS variants for benzyl derivatives of malonic acid; and (iii) the apparent inability of N-Me amino acids to function as ribosome substrates in living cells. Using the PARTI assay, we also provide direct evidence for the cellular production of 2',3'-diacylated tRNA in certain cases. The ease and simplicity of the PARTI workflow should benefit ongoing efforts to study and improve the cellular incorporation of non-α-amino acid monomers into proteins.

Expanding the genetic code: In vivo approaches for incorporating non-proteinogenic monomers

The application of genetic code expansion has enabled the incorporation of non-canonical amino acids (ncAAs) into proteins, introducing novel functional groups and significantly broadening the scope of protein engineering. Over the past decade, this approach has extended beyond ncAAs to include non-proteinogenic monomers (npMs), such as β-amino acids and hydroxy acids. In vivo incorporation of these monomers requires maintaining orthogonality between endogenous and engineered aminoacyl-tRNA synthetase (aaRS)/tRNA pairs while optimizing the use of the translational machinery. This review introduces the fundamental principles of genetic code expansion and highlights the development of orthogonal aaRS/tRNA pairs and ribosomal engineering to incorporate npMs. Despite these advancements, challenges remain in engineering aaRS/tRNA pairs to accommodate npMs, especially monomers that differ significantly from L-α-amino acids due to their incompatibility with existing translational machinery. This review also introduces recent methodologies that allow aaRSs to recognize and aminoacylate npMs without reliance on the ribosomal translation system, thereby unlocking new possibilities for synthesizing biopolymers with chemically diverse monomers.

Self-assembling Depsipeptides on Aggregation-Induced Emission Luminogens: A New Way to Create Programmable Nanovesicles and Soft Nanocarriers

We introduce the proof of concept of a new methodology to produce robust hollow nanovesicles stable in water or mixtures of water and organic solvents. The bottom-up produced nanovesicles are formed by the self-assembly of depsipeptide chains of natural origin combined with new aggregation-induced emission luminogens that function as constitutional vesicle-forming moieties and fluorescent indicators of the structure of the nanovesicle. The newly formed nanovesicles are robust enough to be used to carry large molecules such as physiological peptides without losing their structural characteristics, acting as programmable nanocarrier systems within living cells as Trojan horse systems, constituting a new approach to active transport and nanoencapsulation.

Genetic Code Expansion: Recent Developments and Emerging Applications

The concept of genetic code expansion (GCE) has revolutionized the field of chemical and synthetic biology, enabling the site-specific incorporation of noncanonical amino acids (ncAAs) into proteins, thus opening new avenues in research and applications across biology and medicine. In this review, we cover the principles of GCE, including the optimization of the aminoacyl-tRNA synthetase (aaRS)/tRNA system and the advancements in translation system engineering. Notable developments include the refinement of aaRS/tRNA pairs, enhancements in screening methods, and the biosynthesis of noncanonical amino acids. The applications of GCE technology span from synthetic biology, where it facilitates gene expression regulation and protein engineering, to medicine, with promising approaches in drug development, vaccine production, and gene editing. The review concludes with a perspective on the future of GCE, underscoring its potential to further expand the toolkit of biology and medicine. Through this comprehensive review, we aim to provide a detailed overview of the current state of GCE technology, its challenges, opportunities, and the frontier it represents in the expansion of the genetic code for novel biological research and therapeutic applications.

Predicting the polyspecificity of aminoacyl-tRNA synthetase for non-canonical amino acids using molecular dynamics simulation and MM/PBSA

With the advancement of genetic code expansion, the field is progressing towards incorporating multiple non-canonical amino acids (ncAAs). The specificity of aminoacyl-tRNA synthetases (aaRSs) towards ncAAs is a critical factor, as engineered aaRSs frequently show polyspecificity, complicating the precise incorporation of multiple ncAAs. To address this challenge, predicting binding affinity can be beneficial. In this study, we expressed sfGFP using an orthogonal aaRS/tRNA pair with 4-Azido-L-phenylalanine (AzF) and another four different ncAAs. The experimental results showed specificity with O-Methyl-L-tyrosine as well as AzF, and these results were compared with computational predictions. We constructed a mutant aaRS structure specific for AzF through homology modelling and conducted docking studies with tyrosine and five ncAAs, followed by molecular dynamics simulations. The binding affinity was calculated using the molecular mechanics/Poisson-Boltzmann surface area, focusing on nonpolar solvation terms. While the analysis is based on the incorporation of limited number of ncAAs, the cavity and dispersion term method showed consistency with experimental data, highlighting its potential utility compared to the surface area term method. These findings enhance understanding of the ncAA specificity of aaRS in relation to computer simulations and energy calculations, which can be utilized to rationally design or predict the specificity of aaRS.

Recent advances in peptide macrocyclization strategies

Recently, owing to their special spatial structures, peptide-based macrocycles have shown tremendous promise and aroused great interest in multidisciplinary research ranging from potent antibiotics against resistant strains to functional biomaterials with novel properties. Besides traditional monocyclic peptides, many fascinating polycyclic and remarkable higher-order cyclic, spherical and cylindric peptidic systems have come into the limelight owing to breakthroughs in various chemical (e.g., native chemical ligation and transition metal catalysis), biological (e.g., post-translational enzymatic modification and genetic code reprogramming), and supramolecular (e.g., mechanically interlocked, metal-directed folding and self-assembly via noncovalent interactions) macrocyclization strategies developed in recent decades. In this tutorial review, diverse state-of-the-art macrocyclization methodologies and techniques for peptides and peptidomimetics are surveyed and discussed, with insights into their practical advantages and intrinsic limitations. Finally, the synthetic-technical aspects, current unresolved challenges, and outlook of this field are discussed.

Recent advances in the expanding genetic code

For over a billion years, the central dogma of biology has been limited largely to 20 canonical amino acids with relatively simple functionalities. The ability to rationally add new building blocks to the genetic code has enabled the site-specific incorporation of hundreds of noncanonical amino acids (ncAAs) with novel properties into proteins in living organisms. Recent technological advances have enabled high level mammalian expression of proteins containing ncAAs, the use of unique codons to direct ncAA incorporation, extension of this methodology to a range of eukaryotic organisms, and the ability to encode building blocks beyond α-amino acids. These ncAAs have been used to study and control proteins in their native cellular context and to engineer enzymes and biotherapeutics with improved or novel properties. Herein we discuss recent developments in the field and potential future research directions.

Genetic Code Expansion History and Modern Innovations

The genetic code is the foundation for all life. With few exceptions, the translation of nucleic acid messages into proteins follows conserved rules, which are defined by codons that specify each of the 20 proteinogenic amino acids. For decades, leading research groups have developed a catalogue of innovative approaches to extend nature's amino acid repertoire to include one or more noncanonical building blocks in a single protein. In this review, we summarize advances in the history of in vitro and in vivo genetic code expansion, and highlight recent innovations that increase the scope of biochemically accessible monomers and codons. We further summarize state-of-the-art knowledge in engineered cellular translation, as well as alterations to regulatory mechanisms that improve overall genetic code expansion. Finally, we distill existing limitations of these technologies into must-have improvements for the next generation of technologies, and speculate on future strategies that may be capable of overcoming current gaps in knowledge.

Engineering Pyrrolysine Systems for Genetic Code Expansion and Reprogramming

Over the past 16 years, genetic code expansion and reprogramming in living organisms has been transformed by advances that leverage the unique properties of pyrrolysyl-tRNA synthetase (PylRS)/tRNAPyl pairs. Here we summarize the discovery of the pyrrolysine system and describe the unique properties of PylRS/tRNAPyl pairs that provide a foundation for their transformational role in genetic code expansion and reprogramming. We describe the development of genetic code expansion, from E. coli to all domains of life, using PylRS/tRNAPyl pairs, and the development of systems that biosynthesize and incorporate ncAAs using pyl systems. We review applications that have been uniquely enabled by the development of PylRS/tRNAPyl pairs for incorporating new noncanonical amino acids (ncAAs), and strategies for engineering PylRS/tRNAPyl pairs to add noncanonical monomers, beyond α-L-amino acids, to the genetic code of living organisms. We review rapid progress in the discovery and scalable generation of mutually orthogonal PylRS/tRNAPyl pairs that can be directed to incorporate diverse ncAAs in response to diverse codons, and we review strategies for incorporating multiple distinct ncAAs into proteins using mutually orthogonal PylRS/tRNAPyl pairs. Finally, we review recent advances in the encoded cellular synthesis of noncanonical polymers and macrocycles and discuss future developments for PylRS/tRNAPyl pairs.

Cracking the Code: Reprogramming the Genetic Script in Prokaryotes and Eukaryotes to Harness the Power of Noncanonical Amino Acids

Over 500 natural and synthetic amino acids have been genetically encoded in the last two decades. Incorporating these noncanonical amino acids into proteins enables many powerful applications, ranging from basic research to biotechnology, materials science, and medicine. However, major challenges remain to unleash the full potential of genetic code expansion across disciplines. Here, we provide an overview of diverse genetic code expansion methodologies and systems and their final applications in prokaryotes and eukaryotes, represented by Escherichia coli and mammalian cells as the main workhorse model systems. We highlight the power of how new technologies can be first established in simple and then transferred to more complex systems. For example, whole-genome engineering provides an excellent platform in bacteria for enabling transcript-specific genetic code expansion without off-targets in the transcriptome. In contrast, the complexity of a eukaryotic cell poses challenges that require entirely new approaches, such as striving toward establishing novel base pairs or generating orthogonally translating organelles within living cells. We connect the milestones in expanding the genetic code of living cells for encoding novel chemical functionalities to the most recent scientific discoveries, from optimizing the physicochemical properties of noncanonical amino acids to the technological advancements for their in vivo incorporation. This journey offers a glimpse into the promising developments in the years to come.

Advanced and Safe Synthetic Microbial Chassis with Orthogonal Translation System Integration

Through the use of CRISPR-assisted transposition, we have engineered a safe Escherichia coli chassis that integrates an orthogonal translation system (OTS) directly into the chromosome. This approach circumvents the limitations and genetic instability associated with conventional plasmid vectors. Precision in genome modification is crucial for the top-down creation of synthetic cells, especially in the orthogonalization of vital cellular processes, such as metabolism and protein translation. Here, we targeted multiple loci in the E. coli chromosome to integrate the OTS simultaneously, creating a synthetic auxotrophic chassis with an altered genetic code to establish a reliable, robust, and safe synthetic protein producer. Our OTS-integrated chassis enabled the site-specific incorporation of m-oNB-Dopa through in-frame amber stop codon readthrough. This allowed for the expression of advanced underwater bioglues containing Dopa-Lysine motifs, which are crucial for wound healing and tissue regeneration. Additionally, we have enhanced the expression process by incorporating scaffold-stabilizing fluoroprolines into bioglues, utilizing our chassis, which has been modified through metabolic engineering (i.e., by introducing proline auxotrophy). We also engineered a synthetic auxotroph reliant on caged Dopa, creating a genetic barrier (genetic firewall) between the synthetic cells and their surroundings, thereby boosting their stability and safety.

Efficient genetic code expansion without host genome modifications

Supplementing translation with noncanonical amino acids (ncAAs) can yield protein sequences with new-to-nature functions but existing ncAA incorporation strategies suffer from low efficiency and context dependence. We uncover codon usage as a previously unrecognized contributor to efficient genetic code expansion using non-native codons. Relying only on conventional Escherichia coli strains with native ribosomes, we develop a plasmid-based codon compression strategy that minimizes context dependence and improves ncAA incorporation at quadruplet codons. We confirm that this strategy is compatible with all known genetic code expansion resources, which allowed us to identify 12 mutually orthogonal transfer RNA (tRNA)-synthetase pairs. Enabled by these findings, we evolved and optimized five tRNA-synthetase pairs to incorporate a broad repertoire of ncAAs at orthogonal quadruplet codons. Lastly, we extend these resources to an in vivo biosynthesis platform that can readily create >100 new-to-nature peptide macrocycles bearing up to three unique ncAAs. Our approach will accelerate innovations in multiplexed genetic code expansion and the discovery of chemically diverse biomolecules.

Evolution of Pyrrolysyl-tRNA Synthetase: From Methanogenesis to Genetic Code Expansion

Over 20 years ago, the pyrrolysine encoding translation system was discovered in specific archaea. Our Review provides an overview of how the once obscure pyrrolysyl-tRNA synthetase (PylRS) tRNA pair, originally responsible for accurately translating enzymes crucial in methanogenic metabolic pathways, laid the foundation for the burgeoning field of genetic code expansion. Our primary focus is the discussion of how to successfully engineer the PylRS to recognize new substrates and exhibit higher in vivo activity. We have compiled a comprehensive list of ncAAs incorporable with the PylRS system. Additionally, we also summarize recent successful applications of the PylRS system in creating innovative therapeutic solutions, such as new antibody-drug conjugates, advancements in vaccine modalities, and the potential production of new antimicrobials.

Reduction-to-synthesis: the dominant approach to genome-scale synthetic biology

Advances in systems and synthetic biology have propelled the construction of reduced bacterial genomes. Genome reduction was initially focused on exploring properties of minimal genomes, but more recently it has been deployed as an engineering strategy to enhance strain performance. This review provides the latest updates on reduced genomes, focusing on dual-track approaches of top-down reduction and bottom-up synthesis for their construction. Using cases from studies that are based on established industrial workhorse strains, we discuss the construction of a series of synthetic phenotypes that are candidates for biotechnological applications. Finally, we address the possible uses of reduced genomes for biotechnological applications and the needed future research directions that may ultimately lead to the total synthesis of rationally designed genomes.

Crafting Unnatural Peptide Macrocycles via Rh(III)-Catalyzed Carboamidation

Contemporary developments in the field of peptide macrocyclization methodology are imperative for enabling the advance of drug design in medicinal chemistry. This report discloses a Rh(III)-catalyzed macrocyclization via carboamidation, reacting acryloyl-peptide-dioxazolone precursors and arylboronic acids to form complex cyclic peptides with concomitant incorporation of noncanonical α-amino acids. The diverse and modular technology allows for expedient access to a wide variety of cyclic peptides from 4 to 15 amino acids in size and features simultaneous formation of unnatural phenylalanine and tyrosine derivatives with up to >20:1 diastereoselectivity. The reaction showcases an expansive substrate scope with 45 examples and is compatible with the majority of standard protected amino acids used in Fmoc-solid phase peptide synthesis. The methodology is applied to the synthesis of multiple peptidomimetic macrocyclic analogs, including derivatives of cyclosomatostatin and gramicidin S.

β-Amino Acids Reduce Ternary Complex Stability and Alter the Translation Elongation Mechanism

Templated synthesis of proteins containing non-natural amino acids (nnAAs) promises to expand the chemical space available to biological therapeutics and materials, but existing technologies are still limiting. Addressing these limitations requires a deeper understanding of the mechanism of protein synthesis and how it is perturbed by nnAAs. Here we examine the impact of nnAAs on the formation and ribosome utilization of the central elongation substrate: the ternary complex of native, aminoacylated tRNA, thermally unstable elongation factor, and GTP. By performing ensemble and single-molecule fluorescence resonance energy transfer measurements, we reveal that both the (R)- and (S)-β2 isomers of phenylalanine (Phe) disrupt ternary complex formation to levels below in vitro detection limits, while (R)- and (S)-β3-Phe reduce ternary complex stability by 1 order of magnitude. Consistent with these findings, (R)- and (S)-β2-Phe-charged tRNAs were not utilized by the ribosome, while (R)- and (S)-β3-Phe stereoisomers were utilized inefficiently. (R)-β3-Phe but not (S)-β3-Phe also exhibited order of magnitude defects in the rate of translocation after mRNA decoding. We conclude from these findings that non-natural amino acids can negatively impact the translation mechanism on multiple fronts and that the bottlenecks for improvement must include the consideration of the efficiency and stability of ternary complex formation.

A Translation-Independent Directed Evolution Strategy to Engineer Aminoacyl-tRNA Synthetases

Using directed evolution, aminoacyl-tRNA synthetases (aaRSs) have been engineered to incorporate numerous noncanonical amino acids (ncAAs). Until now, the selection of such novel aaRS mutants has relied on the expression of a selectable reporter protein. However, such translation-dependent selections are incompatible with exotic monomers that are suboptimal substrates for the ribosome. A two-step solution is needed to overcome this limitation: (A) engineering an aaRS to charge the exotic monomer, without ribosomal translation; (B) subsequent engineering of the ribosome to accept the resulting acyl-tRNA for translation. Here, we report a platform for aaRS engineering that directly selects tRNA-acylation without ribosomal translation (START). In START, each distinct aaRS mutant is correlated to a cognate tRNA containing a unique sequence barcode. Acylation by an active aaRS mutant protects the corresponding barcode-containing tRNAs from oxidative treatment designed to damage the 3'-terminus of the uncharged tRNAs. Sequencing of these surviving barcode-containing tRNAs is then used to reveal the identity of the aaRS mutants that acylated the correlated tRNA sequences. The efficacy of START was demonstrated by identifying novel mutants of the Methanomethylophilus alvus pyrrolysyl-tRNA synthetase from a naïve library that enables incorporation of ncAAs into proteins in living cells.

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.

Incorporation of Multiple β2-Hydroxy Acids into a Protein In Vivo Using an Orthogonal Aminoacyl-tRNA Synthetase

The programmed synthesis of sequence-defined biomaterials whose monomer backbones diverge from those of canonical α-amino acids represents the next frontier in protein and biomaterial evolution. Such next-generation molecules provide otherwise nonexistent opportunities to develop improved biologic therapies, bioremediation tools, and biodegradable plastic-like materials. One monomer family of particular interest for biomaterials includes β-hydroxy acids. Many natural products contain isolated β-hydroxy acid monomers, and polymers of β-hydroxy acids (β-esters) are found in polyhydroxyalkanoate (PHA) polyesters under development as bioplastics and drug encapsulation/delivery systems. Here we report that β2-hydroxy acids possessing both (R) and (S) absolute configuration are substrates for pyrrolysyl-tRNA synthetase (PylRS) enzymes in vitro and that (S)-β2-hydroxy acids are substrates in cellulo. Using the orthogonal MaPylRS/MatRNAPyl synthetase/tRNA pair, in conjunction with wild-type E. coli ribosomes and EF-Tu, we report the cellular synthesis of model proteins containing two (S)-β2-hydroxy acid residues at internal positions. Metadynamics simulations provide a rationale for the observed preference for the (S)-β2-hydroxy acid and provide mechanistic insights that inform future engineering efforts. As far as we know, this finding represents the first example of an orthogonal synthetase that acylates tRNA with a β2-hydroxy acid substrate and the first example of a protein hetero-oligomer containing multiple expanded-backbone monomers produced in cellulo.

Diversification of Phage-Displayed Peptide Libraries with Noncanonical Amino Acid Mutagenesis and Chemical Modification

Sitting on the interface between biologics and small molecules, peptides represent an emerging class of therapeutics. Numerous techniques have been developed in the past 30 years to take advantage of biological methods to generate and screen peptide libraries for the identification of therapeutic compounds, with phage display being one of the most accessible techniques. Although traditional phage display can generate billions of peptides simultaneously, it is limited to expression of canonical amino acids. Recently, several groups have successfully undergone efforts to apply genetic code expansion to introduce noncanonical amino acids (ncAAs) with novel reactivities and chemistries into phage-displayed peptide libraries. In addition to biological methods, several different chemical approaches have also been used to install noncanonical motifs into phage libraries. This review focuses on these recent advances that have taken advantage of both biological and chemical means for diversification of phage libraries with ncAAs.

β-amino acids reduce ternary complex stability and alter the translation elongation mechanism

Templated synthesis of proteins containing non-natural amino acids (nnAAs) promises to vastly expand the chemical space available to biological therapeutics and materials. Existing technologies limit the identity and number of nnAAs than can be incorporated into a given protein. Addressing these bottlenecks requires deeper understanding of the mechanism of messenger RNA (mRNA) templated protein synthesis and how this mechanism is perturbed by nnAAs. Here we examine the impact of both monomer backbone and side chain on formation and ribosome-utilization of the central protein synthesis substate: the ternary complex of native, aminoacylated transfer RNA (aa-tRNA), thermally unstable elongation factor (EF-Tu), and GTP. By performing ensemble and single-molecule fluorescence resonance energy transfer (FRET) measurements, we reveal the dramatic effect of monomer backbone on ternary complex formation and protein synthesis. Both the (R) and (S)-β2 isomers of Phe disrupt ternary complex formation to levels below in vitro detection limits, while (R)- and (S)-β3-Phe reduce ternary complex stability by approximately one order of magnitude. Consistent with these findings, (R)- and (S)-β2-Phe-charged tRNAs were not utilized by the ribosome, while (R)- and (S)-β3-Phe stereoisomers were utilized inefficiently. The reduced affinities of both species for EF-Tu ostensibly bypassed the proofreading stage of mRNA decoding. (R)-β3-Phe but not (S)-β3-Phe also exhibited order of magnitude defects in the rate of substrate translocation after mRNA decoding, in line with defects in peptide bond formation that have been observed for D-α-Phe. We conclude from these findings that non-natural amino acids can negatively impact the translation mechanism on multiple fronts and that the bottlenecks for improvement must include consideration of the efficiency and stability of ternary complex formation.

Adding α,α-disubstituted and β-linked monomers to the genetic code of an organism

The genetic code of living cells has been reprogrammed to enable the site-specific incorporation of hundreds of non-canonical amino acids into proteins, and the encoded synthesis of non-canonical polymers and macrocyclic peptides and depsipeptides1-3. Current methods for engineering orthogonal aminoacyl-tRNA synthetases to acylate new monomers, as required for the expansion and reprogramming of the genetic code, rely on translational readouts and therefore require the monomers to be ribosomal substrates4-6. Orthogonal synthetases cannot be evolved to acylate orthogonal tRNAs with non-canonical monomers (ncMs) that are poor ribosomal substrates, and ribosomes cannot be evolved to polymerize ncMs that cannot be acylated onto orthogonal tRNAs-this co-dependence creates an evolutionary deadlock that has essentially restricted the scope of translation in living cells to α-L-amino acids and closely related hydroxy acids. Here we break this deadlock by developing tRNA display, which enables direct, rapid and scalable selection for orthogonal synthetases that selectively acylate their cognate orthogonal tRNAs with ncMs in Escherichia coli, independent of whether the ncMs are ribosomal substrates. Using tRNA display, we directly select orthogonal synthetases that specifically acylate their cognate orthogonal tRNA with eight non-canonical amino acids and eight ncMs, including several β-amino acids, α,α-disubstituted-amino acids and β-hydroxy acids. We build on these advances to demonstrate the genetically encoded, site-specific cellular incorporation of β-amino acids and α,α-disubstituted amino acids into a protein, and thereby expand the chemical scope of the genetic code to new classes of monomers.

An evolved pyrrolysyl-tRNA synthetase with polysubstrate specificity expands the toolbox for engineering enzymes with incorporation of noncanonical amino acids

Aminoacyl-tRNA synthetase (aaRS) is a core component for genetic code expansion (GCE), a powerful technique that enables the incorporation of noncanonical amino acids (ncAAs) into a protein. The aaRS with polyspecificity can be exploited in incorporating additional ncAAs into a protein without the evolution of new, orthogonal aaRS/tRNA pair, which hence provides a useful tool for probing the enzyme mechanism or expanding protein function. A variant (N346A/C348A) of pyrrolysyl-tRNA synthetase from Methanosarcina mazei (MmPylRS) exhibited a wide substrate scope of accepting over 40 phenylalanine derivatives. However, for most of the substrates, the incorporation efficiency was low. Here, a MbPylRS (N311A/C313A) variant was constructed that showed higher ncAA incorporation efficiency than its homologous MmPylRS (N346A/C348A). Next, N-terminal of MbPylRS (N311A/C313A) was engineered by a greedy combination of single variants identified previously, resulting in an IPE (N311A/C313A/V31I/T56P/A100E) variant with significantly improved activity against various ncAAs. Activity of IPE was then tested toward 43 novel ncAAs, and 16 of them were identified to be accepted by the variant. The variant hence could incorporate nearly 60 ncAAs in total into proteins. With the utility of this variant, eight various ncAAs were then incorporated into a lanthanide-dependent alcohol dehydrogenase PedH. Incorporation of phenyllactic acid improved the catalytic efficiency of PedH toward methanol by 1.8-fold, indicating the role of modifying protein main chain in enzyme engineering. Incorporation of O-tert-Butyl-L-tyrosine modified the enantioselectivity of PedH by influencing the interactions between substrate and protein. Enzymatic characterization and molecular dynamics simulations revealed the mechanism of ncAAs affecting PedH catalysis. This study provides a PylRS variant with high activity and substrate promiscuity, which increases the utility of GCE in enzyme mechanism illustration and engineering.

Site-specific photo-crosslinking/cleavage for protein-protein interface identification reveals oligomeric assembly of lysosomal-associated membrane protein type 2A in mammalian cells

Genetic code expansion enables site-specific photo-crosslinking by introducing photo-reactive non-canonical amino acids into proteins at defined positions during translation. This technology is widely used for analyzing protein-protein interactions and is applicable in mammalian cells. However, the identification of the crosslinked region still remains challenging. Here, we developed a new method to identify the crosslinked region by pre-installing a site-specific cleavage site, an α-hydroxy acid (Nε -allyloxycarbonyl-α-hydroxyl-l-lysine acid, AllocLys-OH), into the target protein. Alkaline treatment cleaves the crosslinked complex at the position of the α-hydroxy acid residue and thus helps to identify which side of the cleavage site, either closer to the N-terminus or C-terminus, the crosslinked site is located within the target protein. A series of AllocLys-OH introductions narrows down the crosslinked region. By applying this method, we identified the crosslinked regions in lysosomal-associated membrane protein type 2A (LAMP2A), a receptor of chaperone-mediated autophagy, in mammalian cells. The results suggested that at least two interfaces are involved in the homophilic interaction, which requires a trimeric or higher oligomeric assembly of adjacent LAMP2A molecules. Thus, the combination of site-specific crosslinking and site-specific cleavage promises to be useful for revealing binding interfaces and protein complex geometries.

Macrocyclization strategies for the total synthesis of cyclic depsipeptides

Cyclic depsipeptides are an important class of peptide natural products that are defined by the presence of ester and amide bonds within the macrocycle. The structural diversity of depsipeptides has required the development of a broad range of synthetic strategies to access these biologically active compounds. Solid phase peptide synthesis (SPPS) strategies have been an invaluable tool in their synthesis. The key aspect of their synthesis is the macrocyclization strategy. Three main strategies are used, solution phase macrolactamization of acyclic ester containing peptide, on-resin macrolactamization of a sidechain-anchored peptide, and the solution phase macrolactonization of a linear peptide. Additionally, biocatalysts have been used to produce these compounds in a regio- and chemo-selective manner. Each compound offers unique challenges, requiring careful synthetic design to avoid undesirable side reactivity or unwanted epimerization during the esterification and macrocyclizing steps. This focused review analyzes these three strategies for cyclic depsipeptide natural product total synthesis with selected examples from the literature between 2001-2023.

A Simple Screening and Optimization Bioprocess for Long-Chain Peptide Catalysts Applied to Asymmetric Aldol Reaction

Peptides have demonstrated their efficacy as catalysts in asymmetric aldol reactions. But the constraints inherent in chemical synthesis have imposed limitations on the viability of long-chain peptide catalysts. A noticeable dearth of tools has impeded the swift and effective screening of peptide catalysts using biological methods. To address this, we introduce a straightforward bioprocess for the screening of peptide catalysts for asymmetric aldol reactions. We synthesized several peptides through this method and obtained a 15-amino acid peptide. This peptide exhibited asymmetric aldol catalytic activity, achieving 77% ee in DMSO solvent and 63% ee with over an 80.8% yield in DMSO mixed with a pH 9.0 buffer solution. The successful application of our innovative approach not only represents an advancement but also paves the way for currently unexplored research avenues.

Expanding the substrate scope of pyrrolysyl-transfer RNA synthetase enzymes to include non-α-amino acids in vitro and in vivo

The absence of orthogonal aminoacyl-transfer RNA (tRNA) synthetases that accept non-L-α-amino acids is a primary bottleneck hindering the in vivo translation of sequence-defined hetero-oligomers and biomaterials. Here we report that pyrrolysyl-tRNA synthetase (PylRS) and certain PylRS variants accept α-hydroxy, α-thio and N-formyl-L-α-amino acids, as well as α-carboxy acid monomers that are precursors to polyketide natural products. These monomers are accommodated and accepted by the translation apparatus in vitro; those with reactive nucleophiles are incorporated into proteins in vivo. High-resolution structural analysis of the complex formed between one PylRS enzyme and a m-substituted 2-benzylmalonic acid derivative revealed an active site that discriminates prochiral carboxylates and accommodates the large size and distinct electrostatics of an α-carboxy substituent. This work emphasizes the potential of PylRS-derived enzymes for acylating tRNA with monomers whose α-substituent diverges substantially from the α-amine of proteinogenic amino acids. These enzymes or derivatives thereof could synergize with natural or evolved ribosomes and/or translation factors to generate diverse sequence-defined non-protein heteropolymers.

Quintuply orthogonal pyrrolysyl-tRNA synthetase/tRNAPyl pairs

Mutually orthogonal aminoacyl transfer RNA synthetase/transfer RNA pairs provide a foundation for encoding non-canonical amino acids into proteins, and encoded non-canonical polymer and macrocycle synthesis. Here we discover quintuply orthogonal pyrrolysyl-tRNA synthetase (PylRS)/pyrrolysyl-tRNA (tRNAPyl) pairs. We discover empirical sequence identity thresholds for mutual orthogonality and use these for agglomerative clustering of PylRS and tRNAPyl sequences; this defines numerous sequence clusters, spanning five classes of PylRS/tRNAPyl pairs (the existing classes +N, A and B, and newly defined classes C and S). Most of the PylRS clusters belong to classes that were unexplored for orthogonal pair generation. By testing pairs from distinct clusters and classes, and pyrrolysyl-tRNAs with unusual structures, we resolve 80% of the pairwise specificities required to make quintuply orthogonal PylRS/tRNAPyl pairs; we control the remaining specificities by engineering and directed evolution. Overall, we create 924 mutually orthogonal PylRS/tRNAPyl pairs, 1,324 triply orthogonal pairs, 128 quadruply orthogonal pairs and 8 quintuply orthogonal pairs. These advances may provide a key foundation for encoded polymer synthesis.

Continuous synthesis of E. coli genome sections and Mb-scale human DNA assembly

Whole-genome synthesis provides a powerful approach for understanding and expanding organism function1-3. To build large genomes rapidly, scalably and in parallel, we need (1) methods for assembling megabases of DNA from shorter precursors and (2) strategies for rapidly and scalably replacing the genomic DNA of organisms with synthetic DNA. Here we develop bacterial artificial chromosome (BAC) stepwise insertion synthesis (BASIS)-a method for megabase-scale assembly of DNA in Escherichia coli episomes. We used BASIS to assemble 1.1 Mb of human DNA containing numerous exons, introns, repetitive sequences, G-quadruplexes, and long and short interspersed nuclear elements (LINEs and SINEs). BASIS provides a powerful platform for building synthetic genomes for diverse organisms. We also developed continuous genome synthesis (CGS)-a method for continuously replacing sequential 100 kb stretches of the E. coli genome with synthetic DNA; CGS minimizes crossovers1,4 between the synthetic DNA and the genome such that the output for each 100 kb replacement provides, without sequencing, the input for the next 100 kb replacement. Using CGS, we synthesized a 0.5 Mb section of the E. coli genome-a key intermediate in its total synthesis1-from five episomes in 10  days. By parallelizing CGS and combining it with rapid oligonucleotide synthesis and episome assembly5,6, along with rapid methods for compiling a single genome from strains bearing distinct synthetic genome sections1,7,8, we anticipate that it will be possible to synthesize entire E. coli genomes from functional designs in less than 2 months.

Atomistic simulations of the Escherichia coli ribosome provide selection criteria for translationally active substrates

As genetic code expansion advances beyond L-α-amino acids to backbone modifications and new polymerization chemistries, delineating what substrates the ribosome can accommodate remains a challenge. The Escherichia coli ribosome tolerates non-L-α-amino acids in vitro, but few structural insights that explain how are available, and the boundary conditions for efficient bond formation are so far unknown. Here we determine a high-resolution cryogenic electron microscopy structure of the E. coli ribosome containing α-amino acid monomers and use metadynamics simulations to define energy surface minima and understand incorporation efficiencies. Reactive monomers across diverse structural classes favour a conformational space where the aminoacyl-tRNA nucleophile is <4 Å from the peptidyl-tRNA carbonyl with a Bürgi-Dunitz angle of 76-115°. Monomers with free energy minima that fall outside this conformational space do not react efficiently. This insight should accelerate the in vivo and in vitro ribosomal synthesis of sequence-defined, non-peptide heterooligomers.

Aminobenzoic Acid Derivatives Obstruct Induced Fit in the Catalytic Center of the Ribosome

The Escherichia coli (E. coli) ribosome can incorporate a variety of non-l-α-amino acid monomers into polypeptide chains in vitro but with poor efficiency. Although these monomers span a diverse set of compounds, there exists no high-resolution structural information regarding their positioning within the catalytic center of the ribosome, the peptidyl transferase center (PTC). Thus, details regarding the mechanism of amide bond formation and the structural basis for differences and defects in incorporation efficiency remain unknown. Within a set of three aminobenzoic acid derivatives-3-aminopyridine-4-carboxylic acid (Apy), ortho-aminobenzoic acid (oABZ), and meta-aminobenzoic acid (mABZ)-the ribosome incorporates Apy into polypeptide chains with the highest efficiency, followed by oABZ and then mABZ, a trend that does not track with the nucleophilicity of the reactive amines. Here, we report high-resolution cryo-EM structures of the ribosome with each of these three aminobenzoic acid derivatives charged on tRNA bound in the aminoacyl-tRNA site (A-site). The structures reveal how the aromatic ring of each monomer sterically blocks the positioning of nucleotide U2506, thereby preventing rearrangement of nucleotide U2585 and the resulting induced fit in the PTC required for efficient amide bond formation. They also reveal disruptions to the bound water network that is believed to facilitate formation and breakdown of the tetrahedral intermediate. Together, the cryo-EM structures reported here provide a mechanistic rationale for differences in reactivity of aminobenzoic acid derivatives relative to l-α-amino acids and each other and identify stereochemical constraints on the size and geometry of non-monomers that can be accepted efficiently by wild-type ribosomes.