Expanded Genetic Code Technologies for Incorporating Modified Lysine at Multiple Sites

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.

Biosynthesis, Engineering, and Delivery of Selenoproteins

Selenocysteine (Sec) was discovered as the 21st genetically encoded amino acid. In nature, site-directed incorporation of Sec into proteins requires specialized biosynthesis and recoding machinery that evolved distinctly in bacteria compared to archaea and eukaryotes. Many organisms, including higher plants and most fungi, lack the Sec-decoding trait. We review the discovery of Sec and its role in redox enzymes that are essential to human health and important targets in disease. We highlight recent genetic code expansion efforts to engineer site-directed incorporation of Sec in bacteria and yeast. We also review methods to produce selenoproteins with 21 or more amino acids and approaches to delivering recombinant selenoproteins to mammalian cells as new applications for selenoproteins in synthetic biology.

Crystal Structure of Pyrrolysyl-tRNA Synthetase from a Methanogenic Archaeon ISO4-G1 and Its Structure-Based Engineering for Highly-Productive Cell-Free Genetic Code Expansion with Non-Canonical Amino Acids

Pairs of pyrrolysyl-tRNA synthetase (PylRS) and tRNAPyl from Methanosarcina mazei and Methanosarcina barkeri are widely used for site-specific incorporations of non-canonical amino acids into proteins (genetic code expansion). Previously, we achieved full productivity of cell-free protein synthesis for bulky non-canonical amino acids, including Nε-((((E)-cyclooct-2-en-1-yl)oxy)carbonyl)-L-lysine (TCO*Lys), by using Methanomethylophilus alvus PylRS with structure-based mutations in and around the amino acid binding pocket (first-layer and second-layer mutations, respectively). Recently, the PylRS·tRNAPyl pair from a methanogenic archaeon ISO4-G1 was used for genetic code expansion. In the present study, we determined the crystal structure of the methanogenic archaeon ISO4-G1 PylRS (ISO4-G1 PylRS) and compared it with those of structure-known PylRSs. Based on the ISO4-G1 PylRS structure, we attempted the site-specific incorporation of Nε-(p-ethynylbenzyloxycarbonyl)-L-lysine (pEtZLys) into proteins, but it was much less efficient than that of TCO*Lys with M. alvus PylRS mutants. Thus, the first-layer mutations (Y125A and M128L) of ISO4-G1 PylRS, with no additional second-layer mutations, increased the protein productivity with pEtZLys up to 57 ± 8% of that with TCO*Lys at high enzyme concentrations in the cell-free protein synthesis.

Update of the Pyrrolysyl-tRNA Synthetase/tRNAPyl Pair and Derivatives for Genetic Code Expansion

The cotranslational incorporation of pyrrolysine (Pyl), the 22nd proteinogenic amino acid, into proteins in response to the UAG stop codon represents an outstanding example of natural genetic code expansion. Genetic encoding of Pyl is conducted by the pyrrolysyl-tRNA synthetase (PylRS) and its cognate tRNA, tRNAPyl. Owing to the high tolerance of PylRS toward diverse amino acid substrates and great orthogonality in various model organisms, the PylRS/tRNAPyl-derived pairs are ideal for genetic code expansion to insert noncanonical amino acids (ncAAs) into proteins of interest. Since the discovery of cellular components involved in the biosynthesis and genetic encoding of Pyl, synthetic biologists have been enthusiastic about engineering PylRS/tRNAPyl-derived pairs to rewrite the genetic code of living cells. Recently, considerable progress has been made in understanding the molecular phylogeny, biochemical properties, and structural features of the PylRS/tRNAPyl pair, guiding its further engineering and optimization. In this review, we cover the basic and updated knowledge of the PylRS/tRNAPyl pair's unique characteristics that make it an outstanding tool for reprogramming the genetic code. In addition, we summarize the recent efforts to create efficient and (mutually) orthogonal PylRS/tRNAPyl-derived pairs for incorporation of diverse ncAAs by genome mining, rational design, and advanced directed evolution methods.

Genetic encoding of isobutyryl-, isovaleryl-, and β-hydroxybutryl-lysine in E. coli

Here we report the synthesis and genetic encoding of the lysine post translational modifications, β-hydroxybutyryl-lysine, isobutyryl-lysine and isovaleryl-lysine. The ability to obtain a homogenous protein samples with site-specific incorporation of these acylated lysine residues can serve as a powerful tool to study the biological role of lysine post translational modifications.

Ancestral archaea expanded the genetic code with pyrrolysine

The pyrrolysyl-tRNA synthetase (PylRS) facilitates the cotranslational installation of the 22nd amino acid pyrrolysine. Owing to its tolerance for diverse amino acid substrates, and its orthogonality in multiple organisms, PylRS has emerged as a major route to install noncanonical amino acids into proteins in living cells. Recently, a novel class of PylRS enzymes was identified in a subset of methanogenic archaea. Enzymes within this class (ΔPylSn) lack the N-terminal tRNA-binding domain that is widely conserved amongst PylRS enzymes, yet remain active and orthogonal in bacteria and eukaryotes. In this study, we use biochemical and in vivo UAG-readthrough assays to characterize the aminoacylation efficiency and substrate spectrum of a ΔPylSn class PylRS from the archaeon Candidatus Methanomethylophilus alvus. We show that, compared with the full-length enzyme from Methanosarcina mazei, the Ca. M. alvus PylRS displays reduced aminoacylation efficiency but an expanded amino acid substrate spectrum. To gain insight into the evolution of ΔPylSn enzymes, we performed molecular phylogeny using 156 PylRS and 105 pyrrolysine tRNA (tRNAPyl) sequences from diverse archaea and bacteria. This analysis suggests that the PylRS•tRNAPyl pair diverged before the evolution of the three domains of life, placing an early limit on the evolution of the Pyl-decoding trait. Furthermore, our results document the coevolutionary history of PylRS and tRNAPyl and reveal the emergence of tRNAPyl sequences with unique A73 and U73 discriminator bases. The orthogonality of these tRNAPyl species with the more common G73-containing tRNAPyl will enable future efforts to engineer PylRS systems for further genetic code expansion.

Elucidation of binding preferences of YEATS domains to site-specific acetylated nucleosome core particles

Acetylated lysine residues (Kac) in histones are recognized by epigenetic reader proteins, such as Yaf9, ENL, AF9, Taf14, and Sas5 (YEATS) domain-containing proteins. Human YEATS domains bind to the acetylated N-terminal tail of histone H3; however, their Kac-binding preferences at the level of the nucleosome are unknown. Through genetic code reprogramming, here, we established a nucleosome core particle (NCP) array containing histones that were acetylated at specific residues and used it to compare the Kac-binding preferences of human YEATS domains. We found that AF9-YEATS showed basal binding to the unmodified NCP and that it bound stronger to the NCP containing a single acetylation at one of K4, K9, K14, or K27 of H3, or to histone H4 multi-acetylated between K5 and K16. Crystal structures of AF9-YEATS in complex with an H4 peptide diacetylated either at K5/K8 or K8/K12 revealed that the aromatic cage of the YEATS domain recognized the acetylated K8 residue. Interestingly, E57 and D103 of AF9, both located outside of the aromatic cage, were shown to interact with acetylated K5 and K12 of H4, respectively, consistent with the increase in AF9-YEATS binding to the H4K8-acetylated NCP upon additional acetylation at K5 or K12. Finally, we show that a mutation of E57 to alanine in AF9-YEATS reduced the binding affinity for H4 multiacetylated NCPs containing H4K5ac. Our data suggest that the Kac-binding affinity of AF9-YEATS increases additively with the number of Kac in the histone tail.

Recent advancements in enzyme engineering via site-specific incorporation of unnatural amino acids

With increased attention to excellent biocatalysts, evolving methods based on nature or unnatural amino acid (UAAs) mutagenesis have become an important part of enzyme engineering. The emergence of powerful method through expanding the genetic code allows to incorporate UAAs with unique chemical functionalities into proteins, endowing proteins with more structural and functional features. To date, over 200 diverse UAAs have been incorporated site-specifically into proteins via this methodology and many of them have been widely exploited in the field of enzyme engineering, making this genetic code expansion approach possible to be a promising tool for modulating the properties of enzymes. In this context, we focus on how this robust method to specifically incorporate UAAs into proteins and summarize their applications in enzyme engineering for tuning and expanding the functional properties of enzymes. Meanwhile, we aim to discuss how the benefits can be achieved by using the genetically encoded UAAs. We hope that this method will become an integral part of the field of enzyme engineering in the future.

Quantification of the effect of site-specific histone acetylation on chromatin transcription rate

Eukaryotic transcription is epigenetically regulated by chromatin structure and post-translational modifications (PTMs). For example, lysine acetylation in histone H4 is correlated with activation of RNA polymerase I-, II- and III-driven transcription from chromatin templates, which requires prior chromatin remodeling. However, quantitative understanding of the contribution of particular PTM states to the sequential steps of eukaryotic transcription has been hampered partially because reconstitution of a chromatin template with designed PTMs is difficult. In this study, we reconstituted a di-nucleosome with site-specifically acetylated or unmodified histone H4, which contained two copies of the Xenopus somatic 5S rRNA gene with addition of a unique sequence detectable by hybridization-assisted fluorescence correlation spectroscopy. Using a Xenopus oocyte nuclear extract, we analyzed the time course of accumulation of nascent 5S rRNA-derived transcripts generated on chromatin templates in vitro. Our mathematically described kinetic model and fitting analysis revealed that tetra-acetylation of histone H4 at K5/K8/K12/K16 increases the rate of transcriptionally competent chromatin formation ∼3-fold in comparison with the absence of acetylation. We provide a kinetic model for quantitative evaluation of the contribution of epigenetic modifications to chromatin transcription.

Fully Productive Cell-Free Genetic Code Expansion by Structure-Based Engineering of Methanomethylophilus alvus Pyrrolysyl-tRNA Synthetase

Pyrrolysyl-tRNA synthetase (PylRS)/tRNA^Pyl pairs from Methanosarcina mazei and Methanosarcina barkeri are widely used for site-specific incorporations of non-canonical amino acids into proteins (genetic code expansion). In this study, we achieved the full productivity of cell-free protein synthesis for difficult, bulky non-canonical amino acids, such as N^ε-((((E)-cyclooct-2-en-1-yl)oxy)carbonyl)-l-lysine (TCO*Lys), by using Methanomethylophilus alvus PylRS. First, based on the crystal structure of M. alvus PylRS, the productivities for various non-canonical amino acids were greatly increased by rational engineering of the amino acid-binding pocket. The productivities were further enhanced by using a much higher concentration of PylRS over that of M. mazei PylRS, or by mutating the outer layer of the amino acid-binding pocket. Thus, we achieved full productivity even for TCO*Lys. The quantity and quality of the cell-free-produced antibody fragment containing TCO*Lys were drastically improved. These results demonstrate the importance of full productivity for the expanded genetic code.

The physical basis and practical consequences of biological promiscuity

Proteins interact with metabolites, nucleic acids, and other proteins to orchestrate the myriad catalytic, structural and regulatory functions that support life from the simplest microbes to the most complex multicellular organisms. These molecular interactions are often exquisitely specific, but never perfectly so. Adventitious "promiscuous" interactions are ubiquitous due to the thousands of macromolecules and small molecules crowded together in cells. Such interactions may perturb protein function at the molecular level, but as long as they do not compromise organismal fitness, they will not be removed by natural selection. Although promiscuous interactions are physiologically irrelevant, they are important because they can provide a vast reservoir of potential functions that can provide the starting point for evolution of new functions, both in nature and in the laboratory.

The Molecular Architecture of Unnatural Amino Acid Translation Systems

Expanding the genetic code using non-canonical amino acids has revolutionized synthetic biology. In a recent issue of Cell Chemical Biology, Yanagisawa et al. (2019) expanded our knowledge of orthogonal translation systems by presenting a tour de force structural characterization of multiple genetic code expansion systems based on pyrrolysyl-tRNA synthetase.

Plasticity and Constraints of tRNA Aminoacylation Define Directed Evolution of Aminoacyl-tRNA Synthetases

Genetic incorporation of noncanonical amino acids (ncAAs) has become a powerful tool to enhance existing functions or introduce new ones into proteins through expanded chemistry. This technology relies on the process of nonsense suppression, which is made possible by directing aminoacyl-tRNA synthetases (aaRSs) to attach an ncAA onto a cognate suppressor tRNA. However, different mechanisms govern aaRS specificity toward its natural amino acid (AA) substrate and hinder the engineering of aaRSs for applications beyond the incorporation of a single l-α-AA. Directed evolution of aaRSs therefore faces two interlinked challenges: the removal of the affinity for cognate AA and improvement of ncAA acylation. Here we review aspects of AA recognition that directly influence the feasibility and success of aaRS engineering toward d- and β-AAs incorporation into proteins in vivo. Emerging directed evolution methods are described and evaluated on the basis of aaRS active site plasticity and its inherent constraints.

Structural Basis for Genetic-Code Expansion with Bulky Lysine Derivatives by an Engineered Pyrrolysyl-tRNA Synthetase

Pyrrolysyl-tRNA synthetase (PylRS) and tRNA^Pyl have been extensively used for genetic-code expansion. A Methanosarcina mazei PylRS mutant bearing the Y306A and Y384F mutations (PylRS(Y306A/Y384F)) encodes various bulky non-natural lysine derivatives by UAG. In this study, we examined how PylRS(Y306A/Y384F) recognizes many amino acids. Among 17 non-natural lysine derivatives, N^ɛ-(benzyloxycarbonyl)lysine (ZLys) and 10 ortho/meta/para-substituted ZLys derivatives were efficiently ligated to tRNA^Pyl and were incorporated into proteins by PylRS(Y306A/Y384F). We determined crystal structures of 14 non-natural lysine derivatives bound to the PylRS(Y306A/Y384F) catalytic fragment. The meta- and para-substituted ZLys derivatives are snugly accommodated in the productive mode. In contrast, ZLys and the unsubstituted or ortho-substituted ZLys derivatives exhibited an alternative binding mode in addition to the productive mode. PylRS(Y306A/Y384F) displayed a high aminoacylation rate for ZLys, indicating that the double-binding mode minimally affects aminoacylation. These precise substrate recognition mechanisms by PylRS(Y306A/Y384F) may facilitate the structure-based design of novel non-natural amino acids.

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

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

Upgrading aminoacyl-tRNA synthetases for genetic code expansion

Synthesis of proteins with non-canonical amino acids via genetic code expansion is at the forefront of synthetic biology. Progress in this field has enabled site-specific incorporation of over 200 chemically and structurally diverse amino acids into proteins in an increasing number of organisms. This has been facilitated by our ability to repurpose aminoacyl-tRNA synthetases to attach non-canonical amino acids to engineered tRNAs. Current efforts in the field focus on overcoming existing limitations to the simultaneous incorporation of multiple non-canonical amino acids or amino acids that differ from the l-α-amino acid structure (e.g. d-amino acid or β-amino acid). Here, we summarize the progress and challenges in developing more selective and efficient aminoacyl-tRNA synthetases for genetic code expansion.

Cell-Free Protein Synthesis for Multiple Site-Specific Incorporation of Noncanonical Amino Acids Using Cell Extracts from RF-1 Deletion E. coli Strains

Cell-free protein synthesis (CFPS) is an effective method for the site-specific incorporations of noncanonical amino acids (ncAAs) into proteins. The nature of in vitro synthesis enables the use of experimental conditions that are toxic or reduce cellular uptake during in vivo site-specific incorporations of ncAAs. Using the Escherichia coli cell extract (S30) from the highly reproductive RF-1 deletion strains, B-60.∆A::Z and B-95.∆A, with orthogonal tRNA and aminoacyl-tRNA synthetase (aaRS) pairs from Methanosarcina mazei, we have developed CFPS methods for the highly productive and efficient multiple incorporation of ncAAs. In this chapter, we describe our methods for the preparation of the S30 and the orthogonal tRNA^Pyl and PylRS pair, and two CFPS protocols for ncAA incorporation.

Engineering the Genetic Code in Cells and Animals: Biological Considerations and Impacts

Expansion of the genetic code allows unnatural amino acids (Uaas) to be site-specifically incorporated into proteins in live biological systems, thus enabling novel properties selectively introduced into target proteins in vivo for basic biological studies and for engineering of novel biological functions. Orthogonal components including tRNA and aminoacyl-tRNA synthetase (aaRS) are expressed in live cells to decode a unique codon (often the amber stop codon UAG) as the desired Uaa. Initially developed in E. coli, this methodology has now been expanded in multiple eukaryotic cells and animals. In this Account, we focus on addressing various biological challenges for rewriting the genetic code, describing impacts of code expansion on cell physiology and discussing implications for fundamental studies of code evolution. Specifically, a general method using the type-3 polymerase III promoter was developed to efficiently express prokaryotic tRNAs as orthogonal tRNAs and a transfer strategy was devised to generate Uaa-specific aaRS for use in eukaryotic cells and animals. The aaRSs have been found to be highly amenable for engineering substrate specificity toward Uaas that are structurally far deviating from the native amino acid, dramatically increasing the stereochemical diversity of Uaas accessible. Preparation of the Uaa in ester or dipeptide format markedly increases the bioavailability of Uaas to cells and animals. Nonsense-mediated mRNA decay (NMD), an mRNA surveillance mechanism of eukaryotic cells, degrades mRNA containing a premature stop codon. Inhibition of NMD increases Uaa incorporation efficiency in yeast and Caenorhabditis elegans. In bacteria, release factor one (RF1) competes with the orthogonal tRNA for the amber stop codon to terminate protein translation, leading to low Uaa incorporation efficiency. Contradictory to the paradigm that RF1 is essential, it is discovered that RF1 is actually nonessential in E. coli. Knockout of RF1 dramatically increases Uaa incorporation efficiency and enables Uaa incorporation at multiple sites, making it feasible to use Uaa for directed evolution. Using these strategies, the genetic code has been effectively expanded in yeast, mammalian cells, stem cells, worms, fruit flies, zebrafish, and mice. It is also intriguing to find out that the legitimate UAG codons terminating endogenous genes are not efficiently suppressed by the orthogonal tRNA/aaRS in E. coli. Moreover, E. coli responds to amber suppression pressure promptly using transposon insertion to inactivate the introduced orthogonal aaRS. Persistent amber suppression evading transposon inactivation leads to global proteomic changes with a notable up-regulation of a previously uncharacterized protein YdiI, for which an unexpected function of expelling plasmids is discovered. Genome integration of the orthogonal tRNA/aaRS in mice results in minor changes in RNA transcripts but no significant physiological impairment. Lastly, the RF1 knockout E. coli strains afford a previously unavailable model organism for studying otherwise intractable questions on code evolution in real time in the laboratory. We expect that genetically encoding Uaas in live systems will continue to unfold new questions and directions for studying biology in vivo, investigating the code itself, and reprograming genomes for synthetic biology.

Crystal structures reveal an elusive functional domain of pyrrolysyl-tRNA synthetase

Pyrrolysyl-tRNA synthetase (PylRS) is a major tool in genetic code expansion with non-canonical amino acids, yet understanding of its structure and activity is incomplete. Here we describe the crystal structure of the previously uncharacterized essential N-terminal domain of this unique enzyme in complex with tRNA^Pyl. This structure explains why PylRS remains orthogonal in a broad range of organisms, from bacteria to humans. The structure also illustrates why tRNA^Pyl recognition by PylRS is anticodon-independent; the anticodon does not contact the enzyme. Using standard microbiological culture equipment, we then established a new method for laboratory evolution – a non-continuous counterpart of the previously developed phage-assisted continuous evolution. With this method, we evolved novel PylRS variants with enhanced activity and amino acid specificity. We finally employed an evolved PylRS variant to determine its N-terminal domain structure and show how its mutations improve PylRS activity in the genetic encoding of a non-canonical amino acid.

Expanding the genetic code of Salmonella with non-canonical amino acids

The diversity of non-canonical amino acids (ncAAs) endows proteins with new features for a variety of biological studies and biotechnological applications. The genetic code expansion strategy, which co-translationally incorporates ncAAs into specific sites of target proteins, has been applied in many organisms. However, there have been only few studies on pathogens using genetic code expansion. Here, we introduce this technique into the human pathogen Salmonella by incorporating p-azido-phenylalanine, benzoyl-phenylalanine, acetyl-lysine, and phosphoserine into selected Salmonella proteins including a microcompartment shell protein (PduA), a type III secretion effector protein (SteA), and a metabolic enzyme (malate dehydrogenase), and demonstrate practical applications of genetic code expansion in protein labeling, photocrosslinking, and post-translational modification studies in Salmonella. This work will provide powerful tools for a wide range of studies on Salmonella.

Pyrrolysyl-tRNA synthetase, an aminoacyl-tRNA synthetase for genetic code expansion

Genetic code expansion (GCE) has become a central topic of synthetic biology. GCE relies on engineered aminoacyl-tRNA synthetases (aaRSs) and a cognate tRNA species to allow codon reassignment by co-translational insertion of non-canonical amino acids (ncAAs) into proteins. Introduction of such amino acids increases the chemical diversity of recombinant proteins endowing them with novel properties. Such proteins serve in sophisticated biochemical and biophysical studies both in vitro and in vivo, they may become unique biomaterials or therapeutic agents, and they afford metabolic dependence of genetically modified organisms for biocontainment purposes. In the Methanosarcinaceae the incorporation of the 22nd genetically encoded amino acid, pyrrolysine (Pyl), is facilitated by pyrrolysyl-tRNA synthetase (PylRS) and the cognate UAG-recognizing tRNAPyl. This unique aaRS•tRNA pair functions as an orthogonal translation system (OTS) in most model organisms. The facile directed evolution of the large PylRS active site to accommodate many ncAAs, and the enzyme's anticodon-blind specific recognition of the cognate tRNAPyl make this system highly amenable for GCE purposes. The remarkable polyspecificity of PylRS has been exploited to incorporate >100 different ncAAs into proteins. Here we review the Pyl-OT system and selected GCE applications to examine the properties of an effective OTS.

Molecular Evolution Directs Protein Translation Using Unnatural Amino Acids

Unnatural amino acids have in recent years established their importance in a wide range of fields, from pharmaceuticals to polymer science. Unnatural amino acids can increase the number of chemical groups within proteins and thus expand or enhance biological function. Our ability to utilize these important building blocks, however, has been limited by the inherent difficulty in incorporating these molecules into proteins. To address this challenge, researchers have examined how the canonical twenty amino acids are incorporated, regulated, and modified in nature. This review focuses on achievements and techniques used to engineer the ribosomal protein-translation machinery, including the introduction of orthogonal translation components, how directed evolution enhances the incorporation of unnatural amino acids, and the potential utility of ancient biomolecules for this process.

Intra- and inter-nucleosomal interactions of the histone H4 tail revealed with a human nucleosome core particle with genetically-incorporated H4 tetra-acetylation

Post-translational modifications (PTMs) of histones, such as lysine acetylation of the N-terminal tails, play crucial roles in controlling gene expression. Due to the difficulty in reconstituting site-specifically acetylated nucleosomes with crystallization quality, structural analyses of histone acetylation are currently performed using synthesized tail peptides. Through engineering of the genetic code, translation termination, and cell-free protein synthesis, we reconstituted human H4-mono- to tetra-acetylated nucleosome core particles (NCPs), and solved the crystal structures of the H4-K5/K8/K12/K16-tetra-acetylated NCP and unmodified NCP at 2.4 Å and 2.2 Å resolutions, respectively. The structure of the H4-tetra-acetylated NCP resembled that of the unmodified NCP, and the DNA wrapped the histone octamer as precisely as in the unmodified NCP. However, the B-factors were significantly increased for the peripheral DNAs near the N-terminal tail of the intra- or inter-nucleosomal H4. In contrast, the B-factors were negligibly affected by the H4 tetra-acetylation in histone core residues, including those composing the acidic patch, and at H4-R23, which interacts with the acidic patch of the neighboring NCP. The present study revealed that the H4 tetra-acetylation impairs NCP self-association by changing the interactions of the H4 tail with DNA, and is the first demonstration of crystallization quality NCPs reconstituted with genuine PTMs.

Probing the active site tryptophan of Staphylococcus aureus thioredoxin with an analog

Genetically encoded non-canonical amino acids are powerful tools of protein research and engineering; in particular they allow substitution of individual chemical groups or atoms in a protein of interest. One such amino acid is the tryptophan (Trp) analog 3-benzothienyl-l-alanine (Bta) with an imino-to-sulfur substitution in the five-membered ring. Unlike Trp, Bta is not capable of forming a hydrogen bond, but preserves other properties of a Trp residue. Here we present a pyrrolysyl-tRNA synthetase-derived, engineered enzyme BtaRS that enables efficient and site-specific Bta incorporation into proteins of interest in vivo. Furthermore, we report a 2.1 Å-resolution crystal structure of a BtaRS•Bta complex to show how BtaRS discriminates Bta from canonical amino acids, including Trp. To show utility in protein mutagenesis, we used BtaRS to introduce Bta to replace the Trp28 residue in the active site of Staphylococcus aureus thioredoxin. This experiment showed that not the hydrogen bond between residues Trp28 and Asp58, but the bulky aromatic side chain of Trp28 is important for active site maintenance. Collectively, our study provides a new and robust tool for checking the function of Trp in proteins.

Genetic code flexibility in microorganisms: novel mechanisms and impact on physiology

The genetic code, initially thought to be universal and immutable, is now known to contain many variations, including biased codon usage, codon reassignment, ambiguous decoding and recoding. As a result of recent advances in the areas of genome sequencing, biochemistry, bioinformatics and structural biology, our understanding of genetic code flexibility has advanced substantially in the past decade. In this Review, we highlight the prevalence, evolution and mechanistic basis of genetic code variations in microorganisms, and we discuss how this flexibility of the genetic code affects microbial physiology.

Polyspecific pyrrolysyl-tRNA synthetases from directed evolution

Pyrrolysyl-tRNA synthetase (PylRS) and its cognate tRNA(Pyl) have emerged as ideal translation components for genetic code innovation. Variants of the enzyme facilitate the incorporation >100 noncanonical amino acids (ncAAs) into proteins. PylRS variants were previously selected to acylate N(ε)-acetyl-Lys (AcK) onto tRNA(Pyl). Here, we examine an N(ε)-acetyl-lysyl-tRNA synthetase (AcKRS), which is polyspecific (i.e., active with a broad range of ncAAs) and 30-fold more efficient with Phe derivatives than it is with AcK. Structural and biochemical data reveal the molecular basis of polyspecificity in AcKRS and in a PylRS variant [iodo-phenylalanyl-tRNA synthetase (IFRS)] that displays both enhanced activity and substrate promiscuity over a chemical library of 313 ncAAs. IFRS, a product of directed evolution, has distinct binding modes for different ncAAs. These data indicate that in vivo selections do not produce optimally specific tRNA synthetases and suggest that translation fidelity will become an increasingly dominant factor in expanding the genetic code far beyond 20 amino acids.