Recombinant thiopeptides containing noncanonical amino acids

Engineering of Recombinant Human Papillomavirus 16 L1 Protein for Incorporation with para-Azido-L-Phenylalanine

Human papillomavirus (HPV) L1 capsid protein were produced in several host systems, but few studies have focused on enhancing the properties of the L1 protein. In this study, we aimed to produce recombinant Human papillomavirus (HPV) L1 capsid protein containing para-azido-L-phenylalanine (pAzF) in Escherichia coli. First, we expressed the maltose-binding protein (MBP)-fused HPV16 L1, and 5 residues in HPV16 L1 protein were selected by the in silico modeling for amber codon substitution. Among the variants of the five locations, we identified a candidate that exhibited significant differences in expression with and without pAzF via genetic code expansion (GCE). The expressed recombinant MBP-HPV16L1 protein was confirmed for incorporation of pAzF and the formation of VLPs was tested in vitro.

Cellular Site-Specific Incorporation of Noncanonical Amino Acids in Synthetic Biology

Over the past two decades, genetic code expansion (GCE)-enabled methods for incorporating noncanonical amino acids (ncAAs) into proteins have significantly advanced the field of synthetic biology while also reaping substantial benefits from it. On one hand, they provide synthetic biologists with a powerful toolkit to enhance and diversify biological designs beyond natural constraints. Conversely, synthetic biology has not only propelled the development of ncAA incorporation through sophisticated tools and innovative strategies but also broadened its potential applications across various fields. This Review delves into the methodological advancements and primary applications of site-specific cellular incorporation of ncAAs in synthetic biology. The topics encompass expanding the genetic code through noncanonical codon addition, creating semiautonomous and autonomous organisms, designing regulatory elements, and manipulating and extending peptide natural product biosynthetic pathways. The Review concludes by examining the ongoing challenges and future prospects of GCE-enabled ncAA incorporation in synthetic biology and highlighting opportunities for further advancements in this rapidly evolving field.

Dipeptides Containing Pyrene and Modified Photochemically Reactive Tyrosine: Noncovalent and Covalent Binding to Polynucleotides

Dipeptides 1 and 2 were synthesized from unnatural amino acids containing pyrene as a fluorescent label and polynucleotide binding unit, and modified tyrosine as a photochemically reactive unit. Photophysical properties of the peptides were investigated by steady-state and time-resolved fluorescence. Both peptides are fluorescent (Φf = 0.3-0.4) and do not show a tendency to form pyrene excimers in the concentration range < 10-5 M, which is important for their application in the fluorescent labeling of polynucleotides. Furthermore, both peptides are photochemically reactive and undergo deamination delivering quinone methides (QMs) (ΦR = 0.01-0.02), as indicated from the preparative photomethanolysis study of the corresponding N-Boc protected derivatives 7 and 8. Both peptides form stable complexes with polynucleotides (log Ka > 6) by noncovalent interactions and similar affinities, binding to minor grooves, preferably to the AT reach regions. Peptide 2 with a longer spacer between the fluorophore and the photo-activable unit undergoes a more efficient deamination reaction, based on the comparison with the N-Boc protected derivatives. Upon light excitation of the complex 2·oligoAT10, the photo-generation of QM initiates the alkylation, which results in the fluorescent labeling of the oligonucleotide. This study demonstrated, as a proof of principle, that small molecules can combine dual forms of fluorescent labeling of polynucleotides, whereby initial addition of the dye rapidly forms a reversible high-affinity noncovalent complex with ds-DNA/RNA, which can be, upon irradiation by light, converted to the irreversible (covalent) form. Such a dual labeling ability of a dye could have many applications in biomedicinal sciences.

Protein Engineering in Ribosomally Synthesized and Post-translationally Modified Peptides (RiPPs)

Ribosomally synthesized and post-translationally modified peptides (RiPPs) make up a rapidly growing superfamily of natural products. RiPPs exhibit an extraordinary range of structures, but they all begin as gene-encoded precursor peptides that are linear chains of amino acids produced by ribosomes. Given the gene-encoded nature of RiPP precursor peptides, the toolbox of protein engineering can be directly applied to these precursors. This Perspective will discuss examples of site-directed mutagenesis, noncanonical amino acid mutagenesis, and the construction and screening of combinatorial libraries as applied to RiPPs. These studies have led to important insights into the biosynthesis and bioactivity of RiPPs and the reengineering of RiPPs for entirely new functions.

Incorporation of Non-Canonical Amino Acids into Antimicrobial Peptides: Advances, Challenges, and Perspectives

The emergence of antimicrobial resistance is a global health concern and calls for the development of novel antibiotic agents. Antimicrobial peptides seem to be promising candidates due to their diverse sources, mechanisms of action, and physicochemical characteristics, as well as the relatively low emergence of resistance. The incorporation of noncanonical amino acids into antimicrobial peptides could effectively improve their physicochemical and pharmacological diversity. Recently, various antimicrobial peptides variants with improved or novel properties have been produced by the incorporation of single and multiple distinct noncanonical amino acids. In this review, we summarize strategies for the incorporation of noncanonical amino acids into antimicrobial peptides, as well as their features and suitabilities. Recent applications of noncanonical amino acid incorporation into antimicrobial peptides are also presented. Finally, we discuss the related challenges and prospects.

One-Pot Chemoenzymatic Synthesis of Microviridin Analogs Containing Functional Tags

Microviridins are a prominent family of ribosomally synthesized and posttranslationally modified peptides (RiPPs) featuring characteristic lactone and lactam rings. Their unusual cage-like architecture renders them highly potent serine protease inhibitors of which individual variants specifically inhibit different types of proteases of pharmacological interest. While posttranslational modifications are key for the stability and bioactivity of RiPPs, additional attractive properties can be introduced by functional tags. To date - although highly desirable - no method has been reported to incorporate functional tags in microviridin scaffolds or the overarching class of graspetides. In this study, a chemoenzymatic in vitro platform is used to introduce functional tags in various microviridin variants yielding biotinylated, dansylated or propargylated congeners. This straightforward approach paves the way for customized protease inhibitors with built-in functionalities that can help to unravel the still elusive ecological roles and targets of this remarkable class of compounds and to foster applications based on protease inhibition.

Accessing Diverse Pyridine-Based Macrocyclic Peptides by a Two-Site Recognition Pathway

Macrocyclic peptides are sought-after molecular scaffolds for drug discovery, and new methods to access diverse libraries are of increasing interest. Here, we report the enzymatic synthesis of pyridine-based macrocyclic peptides (pyritides) from linear precursor peptides. Pyritides are a recently described class of ribosomally synthesized and post-translationally modified peptides (RiPPs) and are related to the long-known thiopeptide natural products. RiPP precursors typically contain an N-terminal leader region that is physically engaged by the biosynthetic proteins that catalyze modification of the C-terminal core region of the precursor peptide. We demonstrate that pyritide-forming enzymes recognize both the leader region and a C-terminal tripeptide motif, with each contributing to site-selective substrate modification. Substitutions in the core region were well-tolerated and facilitated the generation of a wide range of pyritide analogues, with variations in macrocycle sequence and size. A combination of the pyritide biosynthetic pathway with azole-forming enzymes was utilized to generate a thiazole-containing pyritide (historically known as a thiopeptide) with no similarity in sequence and macrocycle size to the naturally encoded pyritides. The broad substrate scope of the pyritide biosynthetic enzymes serves as a future platform for macrocyclic peptide lead discovery and optimization.

Natural and engineered precision antibiotics in the context of resistance

Antibiotics are essential weapons in our fight against infectious disease, yet the consequences of broad-spectrum antibiotic use on microbiome stability and pathogen resistance are prompting investigations into more selective alternatives. Echoing the advent of precision medicine in oncology, precision antibiotics with focused activities are emerging as a means of addressing infections without damaging microbiomes or incentivizing resistance. Historically, antibiotic design principles have been gleaned from Nature, and reinvestigation of overlooked antibacterials is now providing scaffolds and targets for the design of pathogen-specific drugs. In this perspective, we summarize the biosynthetic and antibacterial mechanisms used to access these activities, and discuss how such strategies may be co-opted through engineering approaches to afford precision antibiotics.

Reprogramming the Biosynthesis of Precursor Peptide to Create a Selenazole-Containing Nosiheptide Analogue

Nosiheptide (NOS), a potent bactericidal thiopeptide, belongs to a class of natural products produced by ribosomal synthesis and post-translational modifications, and its biosynthetic pathway has largely been elucidated. However, the central trithiazolylpyridine structure of NOS remains inaccessible to structural changes. Here we report the creation of a NOS analogue containing a unique selenazole ring by the construction of an artificial system in Streptomyces actuosus ATCC25421, where the genes responsible for the biosynthesis of selenoprotein from Escherichia coli and the biosynthetic gene cluster of NOS were rationally integrated to produce a selenazole-containing analogue of NOS. The thiazole at the fifth position in NOS was specifically replaced by a selenazole to afford the first selenazole-containing "unnatural" natural product. The present strategy is useful for structural manipulation of various RiPP natural products.

Making Sense of "Nonsense" and More: Challenges and Opportunities in the Genetic Code Expansion, in the World of tRNA Modifications

The evolutional development of the RNA translation process that leads to protein synthesis based on naturally occurring amino acids has its continuation via synthetic biology, the so-called rational bioengineering. Genetic code expansion (GCE) explores beyond the natural translational processes to further enhance the structural properties and augment the functionality of a wide range of proteins. Prokaryotic and eukaryotic ribosomal machinery have been proven to accept engineered tRNAs from orthogonal organisms to efficiently incorporate noncanonical amino acids (ncAAs) with rationally designed side chains. These side chains can be reactive or functional groups, which can be extensively utilized in biochemical, biophysical, and cellular studies. Genetic code extension offers the contingency of introducing more than one ncAA into protein through frameshift suppression, multi-site-specific incorporation of ncAAs, thereby increasing the vast number of possible applications. However, different mediating factors reduce the yield and efficiency of ncAA incorporation into synthetic proteins. In this review, we comment on the recent advancements in genetic code expansion to signify the relevance of systems biology in improving ncAA incorporation efficiency. We discuss the emerging impact of tRNA modifications and metabolism in protein design. We also provide examples of the latest successful accomplishments in synthetic protein therapeutics and show how codon expansion has been employed in various scientific and biotechnological applications.

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

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

Recent Advances and Perspectives on Expanding the Chemical Diversity of Lasso Peptides

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a growing family of natural products that exhibit a range of structures and bioactivities. Initially assembled from the twenty proteinogenic amino acids in a ribosome-dependent manner, RiPPs assume their peculiar bioactive structures through various post-translational modifications. The essential modifications representative of each subfamily of RiPP are performed on a precursor peptide by the so-called processing enzymes; however, various tailoring enzymes can also embellish the precursor peptide or processed peptide with additional functional groups. Lasso peptides are an interesting subfamily of RiPPs characterized by their unique lariat knot-like structure, wherein the C-terminal tail is inserted through a macrolactam ring fused by an isopeptide bond between the N-terminal amino group and an acidic side chain. Until recently, relatively few lasso peptides were found to be tailored with extra functional groups. Nevertheless, the development of new routes to diversify lasso peptides and thus introduce novel or enhanced biological, medicinally relevant, or catalytic properties is appealing. In this review, we highlight several strategies through which lasso peptides have been successfully modified and provide a brief overview of the latest findings on the tailoring of these peptides. We also propose future directions for lasso peptide tailoring as well as potential applications for these peptides in hybrid catalyst design.

Designing efficient genetic code expansion in Bacillus subtilis to gain biological insights

Bacillus subtilis is a model gram-positive bacterium, commonly used to explore questions across bacterial cell biology and for industrial uses. To enable greater understanding and control of proteins in B. subtilis, here we report broad and efficient genetic code expansion in B. subtilis by incorporating 20 distinct non-standard amino acids within proteins using 3 different families of genetic code expansion systems and two choices of codons. We use these systems to achieve click-labelling, photo-crosslinking, and translational titration. These tools allow us to demonstrate differences between E. coli and B. subtilis stop codon suppression, validate a predicted protein-protein binding interface, and begin to interrogate properties underlying bacterial cytokinesis by precisely modulating cell division dynamics in vivo. We expect that the establishment of this simple and easily accessible chemical biology system in B. subtilis will help uncover an abundance of biological insights and aid genetic code expansion in other organisms.

Thiocillin and micrococcin exploit the ferrioxamine receptor of Pseudomonas aeruginosa for uptake

BACKGROUND

Thiopeptides are a class of antibiotics that are active against Gram-positive bacteria and inhibit translation. They were considered inactive against Gram-negative bacteria due to their inability to cross the outer membrane. However, we discovered previously that a member of this class, thiostrepton (TS), has activity against Pseudomonas aeruginosa and Acinetobacter baumannii under iron-limiting conditions. TS hijacks the pyoverdine siderophore receptors of P. aeruginosa to cross the outer membrane and synergizes with iron chelators.

OBJECTIVES

To test other thiopeptides for antimicrobial activity against P. aeruginosa and determine their mechanism of uptake, action and spectrum of activity.

METHODS

Eight thiopeptides were screened in chequerboard assays against a mutant of P. aeruginosa PA14 lacking both pyoverdine receptors. Thiopeptides that retain activity against a pyoverdine receptor-null mutant may use alternative siderophore receptors for entry. Susceptibility testing against siderophore receptor mutants was used to determine thiopeptide mechanism of uptake.

RESULTS

The thiopeptides thiocillin (TC) and micrococcin (MC) use the ferrioxamine siderophore receptor (FoxA) for uptake and inhibit the growth of P. aeruginosa at low micromolar concentrations. The activity of TC required the TonB-ExbBD system used to energize siderophore uptake. TC acted through its canonical mechanism of action of translation inhibition.

CONCLUSIONS

Multiple thiopeptides have antimicrobial activity against P. aeruginosa, countering the historical assumption that they cannot cross the outer membrane. These results demonstrate the potential for thiopeptides to act as antipseudomonal antibiotics.

New developments in RiPP discovery, enzymology and engineering

Covering: up to June 2020Ribosomally-synthesized and post-translationally modified peptides (RiPPs) are a large group of natural products. A community-driven review in 2013 described the emerging commonalities in the biosynthesis of RiPPs and the opportunities they offered for bioengineering and genome mining. Since then, the field has seen tremendous advances in understanding of the mechanisms by which nature assembles these compounds, in engineering their biosynthetic machinery for a wide range of applications, and in the discovery of entirely new RiPP families using bioinformatic tools developed specifically for this compound class. The First International Conference on RiPPs was held in 2019, and the meeting participants assembled the current review describing new developments since 2013. The review discusses the new classes of RiPPs that have been discovered, the advances in our understanding of the installation of both primary and secondary post-translational modifications, and the mechanisms by which the enzymes recognize the leader peptides in their substrates. In addition, genome mining tools used for RiPP discovery are discussed as well as various strategies for RiPP engineering. An outlook section presents directions for future research.

Thiopeptides: antibiotics with unique chemical structures and diverse biological activities

Thiopeptides are a class of natural product antibiotics with diverse structures and functions. Their complex structures and biosynthesis have intrigued researchers since their discovery in 1948, but not a single thiopeptide has been approved for human use. This is mainly due to their poor solubility, challenging synthesis, and low bioavailability. This review summarizes the current research on the biosynthesis and biological activity of thiopeptide antibiotics since 2015. The focus of research since 2015 has been on uncovering biosynthetic routes, developing methods for total synthesis, and understanding the biological activity of thiopeptides. Overall, there is still much to learn about this family of molecules.

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.

Genetic incorporation of non-canonical amino acid photocrosslinkers in Neisseria meningitidis: New method provides insights into the physiological function of the function-unknown NMB1345 protein

Although whole-genome sequencing has provided novel insights into Neisseria meningitidis, many open reading frames have only been annotated as hypothetical proteins with unknown biological functions. Our previous genetic analyses revealed that the hypothetical protein, NMB1345, plays a crucial role in meningococcal infection in human brain microvascular endothelial cells; however, NMB1345 has no homology to any identified protein in databases and its physiological function could not be elucidated using pre-existing methods. Among the many biological technologies to examine transient protein-protein interaction in vivo, one of the developed methods is genetic code expansion with non-canonical amino acids (ncAAs) utilizing a pyrrolysyl-tRNA synthetase/tRNAPyl pair from Methanosarcina species: However, this method has never been applied to assign function-unknown proteins in pathogenic bacteria. In the present study, we developed a new method to genetically incorporate ncAAs-encoded photocrosslinking probes into N. meningitidis by utilizing a pyrrolysyl-tRNA synthetase/tRNAPyl pair and elucidated the biological function(s) of the NMB1345 protein. The results revealed that the NMB1345 protein directly interacts with PilE, a major component of meningococcal pili, and further physicochemical and genetic analyses showed that the interaction between the NMB1345 protein and PilE was important for both functional pilus formation and meningococcal infectious ability in N. meningitidis. The present study using this new methodology for N. meningitidis provides novel insights into meningococcal pathogenesis by assigning the function of a hypothetical protein.

Interception of the Bycroft-Gowland Intermediate in the Enzymatic Macrocyclization of Thiopeptides

Thiopeptides are a broad class of macrocyclic, heavily modified peptide natural products that are unified by the presence of a substituted, nitrogen-containing heterocycle core. Early work indicated that this core might be fashioned from two dehydroalanines by an enzyme-catalyzed aza-[4 + 2] cycloaddition to give a cyclic-hemiaminal intermediate. This common intermediate could then follow a reductive path toward a dehydropiperidine, as in the thiopeptide thiostrepton, or an aromatization path to yield the pyridine groups observed in many other thiopeptides. Although several of the enzymes proposed to perform this cycloaddition have been reconstituted, only pyridine products have been isolated and any hemiaminal intermediates have yet to be observed. Here, we identify the conditions and substrates that decouple the cycloaddition from subsequent steps and allow interception and characterization of this long hypothesized intermediate. Transition state modeling indicates that the key amide-iminol tautomerization is the major hurdle in an otherwise energetically favorable cycloaddition. An anionic model suggests that deprotonation and polarization of this amide bond by TbtD removes this barrier and provides a sufficient driving force for facile (stepwise) cycloaddition. This work provides evidence for a mechanistic link between disparate cyclases in thiopeptide biosynthesis.

Introduction to Thiopeptides: Biological Activity, Biosynthesis, and Strategies for Functional Reprogramming

Thiopeptides (also known as thiazolyl peptides) are structurally complex natural products with rich biological activities. Known for over 70 years for potent killing of Gram-positive bacteria, thiopeptides are experiencing a resurgence of interest in the last decade, primarily brought about by the genomic revolution of the 21st century. Every area of thiopeptide research-from elucidating their biological function and biosynthesis to expanding their structural diversity through genome mining-has made great strides in recent years. These advances lay the foundation for and inspire novel strategies for thiopeptide engineering. Accordingly, a number of diverse approaches are being actively pursued in the hope of developing the next generation of natural-product-inspired therapeutics. Here, we review the contemporary understanding of thiopeptide biological activities, biosynthetic pathways, and approaches to structural and functional reprogramming, with a special focus on the latter.

Substrate Recognition by the Class II Lanthipeptide Synthetase HalM2

Class II lanthipeptides belong to a diverse group of natural products known as ribosomally synthesized and post-translationally modified peptides (RiPPs). Most RiPP precursor peptides contain an N-terminal recognition sequence known as the leader peptide, which is typically recognized by biosynthetic enzymes that catalyze modifications on the C-terminal core peptide. For class II lanthipeptides, these are carried out by a bifunctional lanthipeptide synthetase (LanM) that catalyzes dehydration and cyclization reactions on peptidic substrates to generate thioether-containing, macrocyclic molecules. Some lanthipeptide synthetases are extraordinarily substrate tolerant, making them promising candidates for biotechnological applications such as combinatorial biosynthesis and cyclic peptide library construction. In this study, we characterized the mode of leader peptide recognition by HalM2, the lanthipeptide synthetase responsible for the production of the antimicrobial peptide haloduracin β. Using NMR spectroscopic techniques, in vitro binding assays, and enzyme activity assays, we identified substrate residues that are important for binding to HalM2 and for post-translational modification of the peptide substrates. Additionally, we provide evidence of the binding site on the enzyme using binding assays with truncated enzyme variants, hydrogen-deuterium exchange mass spectrometry, and photoaffinity labeling. Understanding the mechanism by which lanthipeptide synthetases recognize their substrate will facilitate their use in biotechnology, as well as further our general understanding of how RiPP enzymes recognize their substrates.

Minimal lactazole scaffold for in vitro thiopeptide bioengineering

Lactazole A is a cryptic thiopeptide from Streptomyces lactacystinaeus, encoded by a compact 9.8 kb biosynthetic gene cluster. Here, we establish a platform for in vitro biosynthesis of lactazole A, referred to as the FIT-Laz system, via a combination of the flexible in vitro translation (FIT) system with recombinantly produced lactazole biosynthetic enzymes. Systematic dissection of lactazole biosynthesis reveals remarkable substrate tolerance of the biosynthetic enzymes and leads to the development of the minimal lactazole scaffold, a construct requiring only 6 post-translational modifications for macrocyclization. Efficient assembly of such minimal thiopeptides with FIT-Laz opens access to diverse lactazole analogs with 10 consecutive mutations, 14- to 62-membered macrocycles, and 18 amino acid-long tail regions, as well as to hybrid thiopeptides containing non-proteinogenic amino acids. This work suggests that the minimal lactazole scaffold is amenable to extensive bioengineering and opens possibilities to explore untapped chemical space of thiopeptides.

Lactazole A is a thiopeptide from Streptomyces lactacystinaeus, encoded by a compact 9.8 kb biosynthetic gene cluster. Here, the authors show a platform for in vitro biosynthesis of lactazole A via a combination of a flexible in vitro translation system with recombinantly produced lactazole biosynthetic enzymes.

Genetic Code Expansion, Protein Expression, and Protein Functionalization in Bacillus subtilis

The site-specific chemical modification of proteins through incorporation of noncanonical amino acids enables diverse applications, such as imaging, probing, and expanding protein functions, as well as to precisely engineer therapeutics. Here we report a general strategy that allows the incorporation of noncanonical amino acids into target proteins using the amber suppression method and their efficient secretion in the biotechnological relevant expression host Bacillus subtilis. This facilitates efficient purification of target proteins directly from the supernatant, followed by their functionalization using click chemistry. We used this strategy to site-specifically introduce norbornene lysine into a single chain antibody and functionalize it with fluorophores for the detection of human target proteins.

Total synthesis of micrococcin P1 and thiocillin I enabled by Mo(vi) catalyst

Thiopeptides are a class of potent antibiotics with promising therapeutic potential. We developed a novel Mo(vi)-oxide/picolinic acid catalyst for the cyclodehydration of cysteine peptides to form thiazoline heterocycles. With this powerful tool in hand, we completed the total syntheses of two representative thiopeptide antibiotics: micrococcin P1 and thiocillin I. These two concise syntheses (15 steps, longest linear sequence) feature a C-H activation strategy to install the trisubstituted pyridine core and thiazole groups. The synthetic material displays promising antimicrobial properties measured against a series of Gram-positive bacteria.

Geninthiocins C and D from Streptomyces as 35-membered macrocyclic thiopeptides with modified tail moiety

Geninthiocin is a thiopeptide with 35-membered macrocyclic core moiety. It has potent anti-Gram-positive (G⁺) bacteria activity. Herein, we reported two new congeners (2-3) of geninthiocin (geninthiocin A, 1) from Streptomyces sp. CPCC 200267, and designated them as geninthiocins C and D, whose structures were determined by NMR. Geninthiocins A, C and D had the same 35-membered macrocyclic core moiety, but possessed a -Dha-Dha-NH₂, -Dha-Ala-NH₂, and -NH₂ tail, respectively. Besides, the Ala residue in geninthiocin C was determined as L- configuration by C₃ Marfey's method. In vitro assays indicated that geninthiocins C-D showed no antibacterial activity, in contrast to the potent anti-G⁺ bacteria activity displayed by geninthiocin A. Therefore, the -Dha-Dha-NH₂ tail of geninthiocin A played an important role in its potent activity against G⁺ bacteria.

Thinking Outside the Box-Novel Antibacterials To Tackle the Resistance Crisis

The public view on antibiotics as reliable medicines changed when reports about "resistant superbugs" appeared in the news. While reasons for this resistance development are easily spotted, solutions for re-establishing effective antibiotics are still in their infancy. This Review encompasses several aspects of the antibiotic development pipeline from very early strategies to mature drugs. An interdisciplinary overview is given of methods suitable for mining novel antibiotics and strategies discussed to unravel their modes of action. Select examples of antibiotics recently identified by using these platforms not only illustrate the efficiency of these measures, but also highlight promising clinical candidates with therapeutic potential. Furthermore, the concept of molecules that disarm pathogens by addressing gatekeepers of virulence will be covered. The Review concludes with an evaluation of antibacterials currently in clinical development. Overall, this Review aims to connect select innovative antimicrobial approaches to stimulate interdisciplinary partnerships between chemists from academia and industry.

Therapeutic applications of genetic code expansion

In nature, a limited, conservative set of amino acids are utilized to synthesize proteins. Genetic code expansion technique reassigns codons and incorporates noncanonical amino acids (ncAAs) through orthogonal aminoacyl-tRNA synthetase (aaRS)/tRNA pairs. The past decade has witnessed the rapid growth in diversity and scope for therapeutic applications of this technology. Here, we provided an update on the recent progress using genetic code expansion in the following areas: antibody-drug conjugates (ADCs), bispecific antibodies (BsAb), immunotherapies, long-lasting protein therapeutics, biosynthesized peptides, engineered viruses and cells, as well as other therapeutic related applications, where the technique was used to elucidate the mechanisms of biotherapeutics and drug targets.

Small Unnatural Amino Acid Carried Raman Tag for Molecular Imaging of Genetically Targeted Proteins

Raman has been implemented to image biological systems for decades. However, Raman microscopy along with Raman probes is restricted to image metabolites or a few intracellular organelles so far and lacks genetic specificity for imaging proteins of interest, which significantly hinders their application. Here, we report the Raman spectra-based protein imaging method, which incorporates a small phenyl ring enhanced Raman tag (total of ∼0.55 kDa) with a single unnatural amino acid (UAA) to genetically label specific proteins. We further demonstrate hyperspectral stimulated Raman scattering (SRS) imaging of the Histone3.3 protein in the nucleus, Sec61β protein in the endoplasmic reticulum of HeLa cells, and Huntingtin protein Htt74Q in mutant huntingtin-induced cells. Genetic encoding of a small, stable, sensitive, and narrow-band Raman tag took one key step forward to enable SRS or Raman imaging of specific proteins and could further facilitate quantitative Raman spectra-based supermultiplexing microscopy in the future.

Genetic Code Expansion in Rhodobacter sphaeroides to Incorporate Noncanonical Amino Acids into Photosynthetic Reaction Centers

Photosynthetic reaction centers (RCs) are the membrane proteins responsible for the initial charge separation steps central to photosynthesis. As a complex and spectroscopically complicated membrane protein, the RC (and other associated photosynthetic proteins) would benefit greatly from the insight offered by site-specifically encoded noncanonical amino acids in the form of probes and an increased chemical range in key amino acid analogues. Toward that goal, we developed a method to transfer amber codon suppression machinery developed for E. coli into the model bacterium needed to produce RCs, Rhodobacter sphaeroides. Plasmids were developed and optimized to incorporate 3-chlorotyrosine, 3-bromotyrosine, and 3-iodotyrosine into RCs. Multiple challenges involving yield and orthogonality were overcome to implement amber suppression in R. sphaeroides, providing insights into the hurdles that can be involved in host transfer of amber suppression systems from E. coli. In the process of verifying noncanonical amino acid incorporation, characterization of this membrane protein via mass spectrometry (which has been difficult previously) was substantially improved. Importantly, the ability to incorporate noncanonical amino acids in R. sphaeroides expands research capabilities in the photosynthetic field.

Playing with the Molecules of Life

Our understanding of the complex molecular processes of living organisms at the molecular level is growing exponentially. This knowledge, together with a powerful arsenal of tools for manipulating the structures of macromolecules, is allowing chemists to to harness and reprogram the cellular machinery in ways previously unimaged. Here we review one example in which the genetic code itself has been expanded with new building blocks that allow us to probe and manipulate the structures and functions of proteins with unprecedented precision.

Incorporation of Nonproteinogenic Amino Acids in Class I and II Lantibiotics

Lantibiotics are ribosomally synthesized and post-translationally modified peptide natural products that contain thioether cross-links formed by lanthionine and methyllanthionine residues. They exert potent antimicrobial activity against Gram-positive bacteria. We herein report production of analogues of two lantibiotics, lacticin 481 and nisin, that contain nonproteinogenic amino acids using two different strategies involving amber stop codon suppression technology. These methods complement recent alternative approaches to incorporate nonproteinogenic amino acids into lantibiotics.

Expanding the Genetic Code of Lactococcus lactis and Escherichia coli to Incorporate Non-canonical Amino Acids for Production of Modified Lantibiotics

The incorporation of non-canonical amino acids (ncAAs) into ribosomally synthesized and post-translationally modified peptides, e.g., nisin from the Gram-positive bacterium Lactococcus lactis, bears great potential to expand the chemical space of various antimicrobials. The ncAA N_ε-Boc-L-lysine (BocK) was chosen for incorporation into nisin using the archaeal pyrrolysyl-tRNA synthetase–tRNA^Pyl pair to establish orthogonal translation in L. lactis for read-through of in-frame amber stop codons. In parallel, recombinant nisin production and orthogonal translation were combined in Escherichia coli cells. Both organisms synthesized bioactive nisin(BocK) variants. Screening of a nisin amber codon library revealed suitable sites for ncAA incorporation and two variants displayed high antimicrobial activity. Orthogonal translation in E. coli and L. lactis presents a promising tool to create new-to-nature nisin derivatives.

Site-Specific Incorporation of a Thioester Containing Amino Acid into Proteins

Here, we report the site-specific incorporation of a thioester containing noncanonical amino acid (ncAA) into recombinantly expressed proteins. Specifically, we genetically encoded a thioester-activated aspartic acid (ThioD) in bacteria in good yield and with high fidelity using an orthogonal nonsense suppressor tRNA/aminoacyl-tRNA synthetase (aaRS) pair. To demonstrate the utility of ThioD, we used native chemical ligation to label green fluorescent protein with a fluorophore in good yield.

Post-translational modifications involved in the biosynthesis of thiopeptide antibiotics

Thiopeptide antibiotics are a class of typical ribosomally synthesized and post-translationally modified peptides (RiPPs) with complex chemical structures that are difficult to construct via chemical synthesis. To date, more than 100 thiopeptides have been discovered, and most of these compounds exhibit remarkable biological activities, such as antibacterial, antitumor and immunosuppressive activities. Therefore, studies of the biosynthesis of thiopeptides can contribute to the development of new drug leads and facilitate the understanding of the complex post-translational modifications (PTMs) of peptides and/or proteins. Since the biosynthetic gene clusters of thiopeptides were first discovered in 2009, several research studies regarding the biochemistry and enzymology of thiopeptide biosyntheses have been reported, indicating that their characteristic framework is constructed via a cascade of common PTMs and that additional specific PTMs diversify the molecules. In this review, we primarily summarize recent advances in understanding the biosynthesis of thiopeptide antibiotics and propose some potential applications based on our insights into the biosynthetic logic and machinery.

Evolving the N-Terminal Domain of Pyrrolysyl-tRNA Synthetase for Improved Incorporation of Noncanonical Amino Acids

By evolving the N-terminal domain of Methanosarcina mazei pyrrolysyl-tRNA synthetase (PylRS) that directly interacts with tRNA^Pyl , a mutant clone displaying improved amber-suppression efficiency for the genetic incorporation of N^ϵ -(tert-butoxycarbonyl)-l-lysine threefold more than the wild type was identified. The identified mutations were R19H/H29R/T122S. Direct transfer of these mutations to two other PylRS mutants that were previously evolved for the genetic incorporation of N^ϵ -acetyl-l-lysine and N^ϵ -(4-azidobenzoxycarbonyl)-l-δ,ϵ-dehydrolysine also improved the incorporation efficiency of these two noncanonical amino acids. As the three identified mutations were found in the N-terminal domain of PylRS that was separated from its catalytic domain for charging tRNA^Pyl with a noncanonical amino acid, they could potentially be introduced to all other PylRS mutants to improve the incorporation efficiency of their corresponding noncanonical amino acids. Therefore, it represents a general strategy to optimize the pyrrolysine incorporation system-based noncanonical amino-acid mutagenesis.

Streptomyces albus: A New Cell Factory for Non-Canonical Amino Acids Incorporation into Ribosomally Synthesized Natural Products

The incorporation of noncanonical amino acids (ncAAs) with different side chains into a peptide is a promising technique for changing the functional properties of that peptide. Of particular interest is the incorporation of ncAAs into peptide-derived natural products to optimize their biophysical properties for medical and industrial applications. Here, we present the first instance of ncAA incorporation into the natural product cinnamycin in streptomycetes using the orthogonal pyrrolysyl-tRNA synthetase/tRNA^Pyl pair from Methanosarcina barkeri. This approach allows site-specific incorporation of ncAAs via the read-through of a stop codon by the suppressor tRNA^Pyl, which can carry different pyrrolysine analogues. Five new deoxycinnamycin derivatives were obtained with three distinct pyrrolysine analogues incorporated into diverse positions of the antibiotic. The combination of partial hydrolysis and MS/MS fragmentation analysis was used to verify the exact position of the incorporation events. The introduction of ncAAs into different positions of the peptide had opposite effects on the peptide's biological activity.

Bio-inspired engineering of thiopeptide antibiotics advances the expansion of molecular diversity and utility

Thiopeptide antibiotics, which are a class of sulfur-rich and highly modified peptide natural products, exhibit a wide variety of important biological properties. These antibiotics are ribosomally synthesized and arise from post-translational modifications, exemplifying a process through which nature develops the structural complexity from Ser/Thr and Cys-rich precursor peptides. Following a brief review of the knowledge gained from nature in terms of the formation of a common thiopeptide scaffold and its specialization to individual members, we highlight the significance of bio-inspired engineering, which has greatly expanded the molecular diversity and utility of thiopeptide antibiotics regarding the search for clinically useful agents, investigation into new mechanisms of action and access to typically 'inaccessible' biosynthetic processes over the past two years.

Protein Crosslinking by Genetically Encoded Noncanonical Amino Acids with Reactive Aryl Carbamate Side Chains

The use of genetically encoded noncanonical amino acids (ncAAs) to construct crosslinks within or between proteins has emerged as a useful method to enhance protein stability, investigate protein-protein interactions, and improve the pharmacological properties of proteins. We report ncAAs with aryl carbamate side chains (PheK and FPheK) that can react with proximal nucleophilic residues to form intra- or intermolecular protein crosslinks. We evolved a pyrrolysyl-tRNA synthetase that incorporates site-specifically PheK and FPheK into proteins in both E. coli and mammalian cells. PheK and FPheK when incorporated into proteins showed good stability during protein expression and purification. FPheK reacted with adjacent Lys, Cys, and Tyr residues in thioredoxin in high yields. In addition, crosslinks could be formed between FPheK and Lys residue of two interacting proteins, including the heavy chain and light chain of an antibody Fab.

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

With advances in sequencing technology, uncharacterized proteins and domains of unknown function (DUFs) are rapidly accumulating in sequence databases and offer an opportunity to discover new protein chemistry and reaction mechanisms. The focus of this review, the formerly enigmatic YcaO superfamily (DUF181), has been found to catalyze a unique phosphorylation of a ribosomal peptide backbone amide upon attack by different nucleophiles. Established nucleophiles are the side chains of Cys, Ser, and Thr which gives rise to azoline/azole biosynthesis in ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products. However, much remains unknown about the potential for YcaO proteins to collaborate with other nucleophiles. Recent work suggests potential in forming thioamides, macroamidines, and possibly additional post-translational modifications. This review covers all knowledge through mid-2016 regarding the biosynthetic gene clusters (BGCs), natural products, functions, mechanisms, and applications of YcaO proteins and outlines likely future research directions for this protein superfamily.

Structure-Activity Relationship and Molecular Mechanics Reveal the Importance of Ring Entropy in the Biosynthesis and Activity of a Natural Product

Macrocycles are appealing drug candidates due to their high affinity, specificity, and favorable pharmacological properties. In this study, we explored the effects of chemical modifications to a natural product macrocycle upon its activity, 3D geometry, and conformational entropy. We chose thiocillin as a model system, a thiopeptide in the ribosomally encoded family of natural products that exhibits potent antimicrobial effects against Gram-positive bacteria. Since thiocillin is derived from a genetically encoded peptide scaffold, site-directed mutagenesis allows for rapid generation of analogues. To understand thiocillin's structure-activity relationship, we generated a site-saturation mutagenesis library covering each position along thiocillin's macrocyclic ring. We report the identification of eight unique compounds more potent than wild-type thiocillin, the best having an 8-fold improvement in potency. Computational modeling of thiocillin's macrocyclic structure revealed a striking requirement for a low-entropy macrocycle for activity. The populated ensembles of the active mutants showed a rigid structure with few adoptable conformations while inactive mutants showed a more flexible macrocycle which is unfavorable for binding. This finding highlights the importance of macrocyclization in combination with rigidifying post-translational modifications to achieve high-potency binding.

Prospects of In vivo Incorporation of Non-canonical Amino Acids for the Chemical Diversification of Antimicrobial Peptides

The incorporation of non-canonical amino acids (ncAA) is an elegant way for the chemical diversification of recombinantly produced antimicrobial peptides (AMPs). Residue- and site-specific installation methods in several bacterial production hosts hold great promise for the generation of new-to-nature AMPs, and can contribute to tackle the ongoing emergence of antibiotic resistance in pathogens. Especially from a pharmacological point of view, desirable improvements span pH and protease resistance, solubility, oral availability and circulation half-life. Although the primary focus of this report is on ribosomally synthesized and post-translationally modified peptides (RiPPs), we have included selected cases of peptides produced by solid phase peptide synthesis to comparatively show the potential and impact of ncAA introduction. Generally speaking, the introduction of ncAAs in recombinant AMPs delivers novel levels of chemical diversification. Cotranslationally incorporated, they can take part in AMP biogenesis either through direction interaction with elements of the post-translational modification (PTM) machinery or as untargeted sites with unique physicochemical properties and chemical handles for further modification. Together with genetic libraries, genome mining and processing by PTM machineries, ncAAs present not a mere addition to this process, but a highly diverse pool of building blocks to significantly broaden the chemical space of this valuable class of molecules. This perspective summarizes new developments of ncAA containing peptides. Challenges to be resolved in order to reach large-scale pharmaceutical production of these promising compounds and prospects for future developments are discussed.