Precursor-centric genome-mining approach for lasso peptide discovery

Utilization of lasso peptides for biodegradation of polycyclic aromatic hydrocarbons

Many microbial genes involved in degrading recalcitrant environmental contaminants such as polycyclic aromatic hydrocarbons (PAHs) have been identified and characterized. However, all molecular mechanisms required for PAH utilization have not yet been elucidated. In this work, we demonstrate the proposed involvement of lasso peptides in the utilization of the PAH phenanthrene in Sphingomonas BPH. Transpositional mutagenesis of Sphingomonas BPH with the miniTn5 transposon yielded 3 phenanthrene utilization deficient mutants, #257, #1778, and #1782. In mutant #1782, Tn5 had inserted into the large subunit of the naph/bph dioxygenase gene. In mutant #1778, Tn5 had inserted into the B2 protease gene of a lasso peptide cluster. This finding is the first report on the role of lasso peptides in PAH utilization. Our studies also demonstrate that interruption of the lasso peptide cluster resulted in a significant increase in the amount of biosurfactant produced in the presence of glucose when compared to the wild-type strain. Collectively, these results suggest that the mechanisms Sphingomonas BPH utilizes to degrade phenanthrene are far more complex than previously understood and that the #1778 mutant may be a good candidate for bioremediation when glucose is applied as an amendment due to its higher biosurfactant production.

The pearl jubilee of microcin J25: thirty years of research on an exceptional lasso peptide

Covering: 1992 up to 2023Since their discovery, lasso peptides went from peculiarities to be recognized as a major family of ribosomally synthesized and post-translationally modified peptide (RiPP) natural products that were shown to be spread throughout the bacterial kingdom. Microcin J25 was first described in 1992, making it one of the earliest known lasso peptides. No other lasso peptide has since then been studied to such an extent as microcin J25, yet, previous review articles merely skimmed over all the research done on this exceptional lasso peptide. Therefore, to commemorate the 30th anniversary of its first report, we give a comprehensive overview of all literature related to microcin J25. This review article spans the early work towards the discovery of microcin J25, its biosynthetic gene cluster, and the elucidation of its three-dimensional, threaded lasso structure. Furthermore, the current knowledge about the biosynthesis of microcin J25 and lasso peptides in general is summarized and a detailed overview is given on the biological activities associated with microcin J25, including means of self-immunity, uptake into target bacteria, inhibition of the Gram-negative RNA polymerase, and the effects of microcin J25 on mitochondria. The in vitro and in vivo models used to study the potential utility of microcin J25 in a (veterinary) medicine context are discussed and the efforts that went into employing the microcin J25 scaffold in bioengineering contexts are summed up.

Discovery and engineering of ribosomally synthesized and post-translationally modified peptide (RiPP) natural products

Ribosomally synthesized and post-translationally modified peptides (RiPPs) represent a diverse superfamily of natural products with immense potential for drug development. This review provides a concise overview of the recent advances in the discovery of RiPP natural products, focusing on rational strategies such as bioactivity guided screening, enzyme or precursor-based genome mining, and biosynthetic engineering. The challenges associated with activating silent biosynthetic gene clusters and the development of elaborate catalytic systems are also discussed. The logical frameworks emerging from these research studies offer valuable insights into RiPP biosynthesis and engineering, paving the way for broader pharmaceutic applications of these peptide natural products.

Antagonistic actions of Paucibacter aquatile B51 and its lasso peptide paucinodin toward cyanobacterial bloom-forming Microcystis aeruginosa PCC7806

Superior antagonistic activity against axenic Microcystis aeruginosa PCC7806 was observed with Paucibacter sp. B51 isolated from cyanobacterial bloom samples among 43 tested freshwater bacterial species. Complete genome sequencing, analyzing average nucleotide identity and digital DNA-DNA hybridization, designated the B51 strain as Paucibacter aquatile. Electron and fluorescence microscopic image analyses revealed the presence of the B51 strain in the vicinity of M. aeruginosa cells, which might provoke direct inhibition of the photosynthetic activity of the PCC7806 cells, leading to perturbation of cellular metabolisms and consequent cell death. Our speculation was supported by the findings that growth failure of the PCC7806 cells led to low pH conditions with fewer chlorophylls and down-regulation of photosystem genes (e.g., psbD and psaB) during their 48-h co-culture condition. Interestingly, the concentrated ethyl acetate extracts obtained from B51-grown supernatant exhibited a growth-inhibitory effect on PCC7806. The physical separation of both strains by a filter system led to no inhibitory activity of the B51 cells, suggesting that contact-mediated anti-cyanobacterial compounds might also be responsible for hampering the growth of the PCC7806 cells. Bioinformatic tools identified 12 gene clusters that possibly produce secondary metabolites, including a class II lasso peptide in the B51 genome. Further chemical analysis demonstrated anti-cyanobacterial activity from fractionated samples having a rubrivinodin-like lasso peptide, named paucinodin. Taken together, both contact-mediated inhibition of photosynthesis and the lasso peptide secretion of the B51 strain are responsible for the anti-cyanobacterial activity of P. aquatile B51.

Discovery, Characterization, and Bioactivity of the Achromonodins: Lasso Peptides Encoded by Achromobacter

Through genome mining efforts, two lasso peptide biosynthetic gene clusters (BGCs) within two different species of Achromobacter, a genus that contains pathogenic organisms that can infect patients with cystic fibrosis, were discovered. Using gene-refactored BGCs in E. coli, these lasso peptides, which were named achromonodin-1 and achromonodin-2, were heterologously expressed. Achromonodin-1 is naturally encoded by certain isolates from the sputum of patients with cystic fibrosis. The NMR structure of achromonodin-1 was determined, demonstrating that it is a threaded lasso peptide with a large loop and short tail structure, reminiscent of previously characterized lasso peptides that inhibit RNA polymerase (RNAP). Achromonodin-1 inhibits RNAP in vitro and has potent, focused activity toward Achromobacter pulmonis, another isolate from the sputum of a cystic fibrosis patient. These efforts expand the repertoire of antimicrobial lasso peptides and provide insights into how Achromobacter isolates from certain ecological niches interact with each other.

Exploring the Diverse Landscape of Biaryl-Containing Peptides Generated by Cytochrome P450 Macrocyclases

Cytochrome P450 enzymes (P450s) catalyze diverse oxidative cross-coupling reactions between aromatic substrates in the natural product biosynthesis. Specifically, P450s install distinct biaryl macrocyclic linkages in three families of ribosomally synthesized and post-translationally modified peptides (RiPPs). However, the chemical diversity of biaryl-containing macrocyclic RiPPs remains largely unexplored. Here, we demonstrate that P450s have the capability to generate diverse biaryl linkages on RiPPs, collectively named "cyptides". Homology-based genome mining for P450 macrocyclases revealed 19 novel groups of homologous biosynthetic gene clusters (BGCs) with distinct aromatic residue patterns in the precursor peptides. Using the P450-modified precursor peptides heterologously coexpressed with corresponding P450s in Escherichia coli, we determined the NMR structures of three novel biaryl-containing peptides─the enzymatic products, roseovertin (1), rubrin (2), and lapparbin (3)─and confirmed the formation of three unprecedented or rare biaryl linkages: Trp C-7'-to-His N-τ in 1, Trp C-7'-to-Tyr C-6 in 2, and Tyr C-6-to-Trp N-1' in 3. Biochemical characterization indicated that certain P450s in these pathways have a relaxed substrate specificity. Overall, our studies suggest that P450 macrocyclases have evolved to create diverse biaryl linkages in RiPPs, promoting the exploration of a broader chemical space for biaryl-containing peptides encoded in bacterial genomes.

Genome Mining and Discovery of Imiditides, a Family of RiPPs with a Class-Defining Aspartimide Modification

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a large and diverse class of natural products of ribosomal origin. In the past decade, various sophisticated machine-learning-based software packages have been established to discover novel RiPPs that do not resemble the known families. Here, we show that tailoring enzymes that cluster with various RiPP families can serve as effective bioinformatic seeds, providing a complementary approach for novel RiPP discovery. Leveraging the fact that O-methyltransferases homologous to protein isoaspartyl methyltransferases (PIMTs) are associated with lasso peptide, graspetide, and lanthipeptide biosynthetic gene clusters (BGCs), we utilized a C-terminal motif unique to RiPP-associated O-methyltransferases as the search query to discover a novel family of RiPPs, the imiditides. Our genome-mining algorithm reveals a total of 670 imiditide BGCs, distributed across Gram-positive bacterial genomes. In addition, we demonstrate the heterologous production of the founding member of the imiditide family, mNmaAM, encoded in the genome of Nonomuraea maritima. In contrast to other RiPP-associated PIMTs that recognize constrained peptides as substrates, the PIMT homologue in the mNmaAM BGC, NmaM, methylates a specific Asp residue on the linear precursor peptide, NmaA. The methyl ester is then turned into an aspartimide spontaneously. Substrate specificity is achieved by extensive charge-charge interactions between the precursor NmaA and the modifying enzyme NmaM suggested by both experiments and an AlphaFold model prediction. Our study shows that PIMT-mediated aspartimide formation is an emerging backbone modification strategy in the biosynthesis of multiple RiPP families.

Genome Mining and Discovery of Imiditides, a Novel Family of RiPPs with a Class-defining Aspartimide Modification

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a fascinating class of natural products of ribosomal origins. In the past decade, various sophisticated machine learning-based software packages have been established to discover novel RiPPs that do not resemble the known families. Instead, we argue that tailoring enzymes that cluster with various RiPP families can serve as effective bioinformatic seeds for novel RiPP discovery. Leveraging that O -methyltransferases homologous to protein isoaspartyl methyltransferases (PIMTs) are associated with lasso peptide, graspetide, and lanthipeptide biosynthetic gene clusters (BGCs), we utilized the C-terminal motif unique to RiPP-associated O -methyltransferases as the search query to discover a novel family of RiPPs, imiditides. Our genome-mining algorithm reveals a total of 670 imiditide BGCs, widely distributed in Gram-positive bacterial genomes. In addition, we demonstrate the heterologous production of the founding member of the imiditide family, mNmaA M , encoded in the genome of Nonomuraea maritima . In contrast to other RiPP associated PIMTs that recognize constrained peptides as substrates, the PIMT homolog in mNmaA M BGC, NmaM, methylates a specific Asp residue on the linear precursor peptide, NmaA. The methyl ester is then turned into an aspartimide spontaneously. The aspartimide moiety formed is unusually stable, leading to the accumulation of the aspartimidylated product in vivo . The substrate specificity is achieved by extensive charge-charge interactions between the precursor NmaA and the modifying enzyme NmaM suggested by both experimental validations as well as an AlphaFold model prediction. Our study suggests that PIMT-mediated aspartimide formation is an underappreciated backbone modification strategy in RiPP biosynthesis, compared to the well-studied backbone rigidification chemistries, such as thiazol(in)e and oxazol(in)e formations. Additionally, our findings suggest that aspartimide formation in Gram-positive bacterial proteomes are not limited to spontaneous protein aging and degradation.

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Cloacaenodin, an Antimicrobial Lasso Peptide with Activity against Enterobacter

Using genome mining and heterologous expression, we report the discovery and production of a new antimicrobial lasso peptide from species related to the Enterobacter cloacae complex. Using NMR and mass spectrometric analysis, we show that this lasso peptide, named cloacaenodin, employs a threaded lasso fold which imparts proteolytic resistance that its unthreaded counterpart lacks. Cloacaenodin has selective, low micromolar, antimicrobial activity against species related to the E. cloacae complex, including species implicated in nosocomial infections and against clinical isolates of carbapenem-resistant Enterobacterales. We further used site-directed mutagenesis to probe the importance of specific residues to the peptide's biosynthesis, stability, and bioactivity.

Unusual Post-Translational Modifications in the Biosynthesis of Lasso Peptides

Lasso peptides are a subclass of ribosomally synthesized and post-translationally modified peptides (RiPPs) and feature the threaded, lariat knot-like topology. The basic post-translational modifications (PTMs) of lasso peptide contain two steps, including the leader peptide removal of the ribosome-derived linear precursor peptide by an ATP-dependent cysteine protease, and the macrolactam cyclization by an ATP-dependent macrolactam synthetase. Recently, advanced bioinformatic tools combined with genome mining have paved the way to uncover a rapidly growing number of lasso peptides as well as a series of PTMs other than the general class-defining processes. Despite abundant reviews focusing on lasso peptide discoveries, structures, properties, and physiological functionalities, few summaries concerned their unique PTMs. In this review, we summarized all the unique PTMs of lasso peptides uncovered to date, shedding light on the related investigations in the future.

Triculamin: An Unusual Lasso Peptide with Potent Antimycobacterial Activity

Lasso peptides are ribosomally synthesized and post-translationally modified peptides (RiPPs) produced by microorganisms. Here we show that the two natural products triculamin and alboverticillin, originally isolated in 1967 and 1958, respectively, with potent and specific activity against mycobacteria are in fact the same lasso peptide. We solved the structure using 2D NMR spectroscopy and expanded on the previously reported bioactivity. Through genome sequencing, we identify the responsible biosynthetic gene clusters, which curiously revealed that, unlike any known lasso peptides, their precursor peptides appear to have a follower instead of a leader peptide.

Cell-Free Mutant Analysis Combined with Structure Prediction of a Lasso Peptide Biosynthetic Protease B2

Biochemical and structural analyses of purified proteins are essential for the understanding of their properties. However, many proteins are unstable and difficult to purify, hindering their characterization. The B2 proteins of the lasso peptide biosynthetic pathways are cysteine proteases that cleave precursor peptides during the maturation process. The B2 proteins are poorly soluble, and no experimentally solved structures are available. Here, we performed a rapid semicomprehensive mutational analysis of the B2 protein from the thermophilic actinobacterium, Thermobifida fusca (FusB2), using a cell-free transcription/translation system, and compared the results with the structure prediction by AlphaFold2. Analysis of 34 FusB2 mutants with substitutions of hydrophobic residues confirmed the accuracy of the predicted structure, and revealed a hydrophobic patch on the protein surface, which likely serves as the binding site of the partner protein, FusB1. Our results suggest that the combination of rapid cell-free mutant analyses with precise structure predictions can greatly accelerate structure-function research of proteins for which no structures are available.

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.

Synergy between machine learning and natural products cheminformatics: Application to the lead discovery of anthraquinone derivatives

Cheminformatics utilizing machine learning (ML) techniques have opened up a new horizon in drug discovery. This is owing to vast chemical space expansion with rocketing numbers of expected hits and lead compounds that match druggable macromolecular targets, in particular from natural compounds. Due to the natural products' (NP) structural complexity, uniqueness, and diversity, they could occupy a bigger space in pharmaceuticals, allowing the industry to pursue more selective leads in the nanomolar range of binding affinity. ML is an essential part of each step of the drug design pipeline, such as target prediction, compound library preparation, and lead optimization. Notably, molecular mechanic and dynamic simulations, induced docking, and free energy perturbations are essential in predicting best binding poses, binding free energy values, and molecular mechanics force fields. Those applications have leveraged from artificial intelligence (AI), which decreases the computational costs required for such costly simulations. This review aimed to describe chemical space and compound libraries related to NPs. High-throughput screening utilized for fractionating NPs and high-throughput virtual screening and their strategies, and significance, are reviewed. Particular emphasis was given to AI approaches, ML tools, algorithms, and techniques, especially in drug discovery of macrocyclic compounds and approaches in computer-aided and ML-based drug discovery. Anthraquinone derivatives were discussed as a source of new lead compounds that can be developed using ML tools for diverse medicinal uses such as cancer, infectious diseases, and metabolic disorders. Furthermore, the power of principal component analysis in understanding relevant protein conformations, and molecular modeling of protein-ligand interaction were also presented. Apart from being a concise reference for cheminformatics, this review is a useful text to understand the application of ML-based algorithms to molecular dynamics simulation and in silico absorption, distribution, metabolism, excretion, and toxicity prediction.

Microcins reveal natural mechanisms of bacterial manipulation to inform therapeutic development

Microcins are an understudied and poorly characterized class of antimicrobial peptides. Despite the existence of only 15 examples, all identified from the Enterobacteriaceae, microcins display diversity in sequence, structure, target cell uptake, cytotoxic mechanism of action and target specificity. Collectively, these features describe some of the unique means nature has contrived for molecules to cross the 'impermeable' barrier of the Gram-negative bacterial outer membrane and inflict cytotoxic effects. Microcins appear to be widely dispersed among different species and in different environments, where they function in regulating microbial communities in diverse ways, including through competition. Growing evidence suggests that microcins may be adapted for therapeutic uses such as antimicrobial drugs, microbiome modulators or facilitators of peptide uptake into cells. Advancing our biological, ecological and biochemical understanding of the roles of microcins in bacterial interactions, and learning how to regulate and modify microcin activity, is essential to enable such therapeutic applications.

Discovery and Characterization of Rubrinodin Provide Clues into the Evolution of Lasso Peptides

Lasso peptides are unique natural products that comprise a class of ribosomally synthesized and post-translationally modified peptides. Their defining three-dimensional structure is a lariat knot, in which the C-terminal tail is threaded through a macrolactam ring formed between the N-terminal amino group and an Asp or Glu side chain (i.e., an isopeptide bond). Recent genome mining strategies have revealed various types of lasso peptide biosynthetic gene clusters and have thus redefined the known chemical space of lasso peptides. To date, over 20 different types of these gene clusters have been discovered, including several different clades from Proteobacteria. Despite the diverse architectures of these gene clusters, which may or may not encode various tailoring enzymes, most currently known lasso peptides are synthesized by two discrete clades defined by the presence of an ATP-binding cassette transporter or its absence and (sometimes) concurrent appearance of an isopeptidase, raising questions about their evolutionary history. Herein, we discovered and characterized the lasso peptide rubrinodin, which is assembled by a gene cluster encoding both an ATP-binding cassette transporter and an isopeptidase. Our bioinformatics analyses of this and other representative cluster types provided new clues into the evolutionary history of lasso peptides. Furthermore, our structural and biochemical investigations of rubrinodin permitted the conversion of this thermolabile lasso peptide into a more thermostable scaffold.

Combined thermal and carboxypeptidase Y stability assays for probing the threaded fold of lasso peptides

Lasso peptides are natural products belonging to the superfamily of ribosomally synthesized and post-translationally modified peptides (RiPPs). The defining characteristic of lasso peptides is their threaded structure, which is reminiscent of a lariat knot. When working with lasso peptides, it is therefore of major importance to understand and evidence their threaded folds. While the full elucidation of their three-dimensional structures via NMR spectroscopy or crystallization remains the gold standard, these methods are time-consuming, require large quantities of highly pure lasso peptides, and therefore might not always be applicable. Instead, the unique properties of lasso peptides in context of their behavior at elevated temperatures and toward carboxypeptidase Y treatment can be leveraged as a tool to investigate and evidence the threaded lasso fold using only minute amounts of compound that does not need to be purified first. This chapter will provide insights into the thermal stability properties of lasso peptides and their behavior when treated with carboxypeptidase Y in comparison to a branched-cyclic peptide with the same amino acid sequence. Furthermore, it will be described in detail how to set up a combined thermal and carboxypeptidase Y stability assay and how to analyze its outcomes.

Toward the Heterologous Biosynthesis of Plant Natural Products: Gene Discovery and Characterization

Plant natural products (PNPs) represent a vast and diverse group of natural products, which have wide applications such as emulsifiers in cosmetics, sweeteners in foods, and active ingredients in medicines. Large-scale production of certain PNPs (e.g., artemisinin, taxol) has been implemented by reconstruction of biosynthetic pathways in heterologous hosts. However, unknown biosynthetic pathways greatly restrict wide applications of heterologous production of PNPs of interest. With the rapid development of sequencing and multiomics analysis technologies, huge amounts of omics data, i.e., genomics, transcriptomics, and proteomics, have been deposited in public databases, which is a precious resource for identification of the unknown biosynthetic pathway of PNPs. Herein, we have enumerated the approaches which have been widely used to screen candidate genes involved in the biosynthesis of PNPs of interest. We also discuss recent developments in the characterization of putative genes and elucidation of the complete biosynthetic pathway in heterologous hosts.

A Bifunctional Leader Peptidase/ABC Transporter Protein Is Involved in the Maturation of the Lasso Peptide Cochonodin I from Streptococcus suis

Lasso peptides are members of the natural product superfamily of ribosomally synthesized and post-translationally modified peptides (RiPPs). Here, we describe the first lasso peptide originating from a biosynthetic gene cluster belonging to a unique lasso peptide subclade defined by the presence of a bifunctional protein harboring both a leader peptidase (B2) and an ABC transporter (D) domain. Bioinformatic analysis revealed that these clusters also encode homologues of the NisR/NisK regulatory system and the NisF/NisE/NisG immunity factors, which are usually associated with the clusters of antimicrobial class I lanthipeptides, such as nisin, another distinct RiPP subfamily. The cluster enabling the heterologous production of the lasso peptide cochonodin I in E. coli originated from Streptococcus suis LSS65, and the threaded structure of cochonodin I was evidenced through extensive MS/MS analysis and stability assays. It was shown that the ABC transporter domain from SsuB2/D is not essential for lasso peptide maturation. By extensive genome mining dedicated exclusively to other lasso peptide biosynthetic gene clusters featuring bifunctional B2/D proteins, it was furthermore revealed that many bacteria associated with human or animal microbiota hold the biosynthetic potential to produce cochonodin-like lasso peptides, implying that these natural products might play roles in human and animal health.

Rational generation of lasso peptides based on biosynthetic gene mutations and site-selective chemical modifications

Lasso peptides are a unique family of natural products whose structures feature a specific threaded fold, which confers these peptides the resistance to thermal and proteolytic degradation. This stability gives lasso peptides excellent pharmacokinetic properties, which together with their diverse reported bioactivities have garnered extensive attention because of their drug development potential. Notably, the threaded fold has proven quite inaccessible by chemical synthesis, which has hindered efficient generation of structurally diverse lasso peptides. We herein report the discovery of a new lasso peptide stlassin (1) by gene activation based on a Streptomyces heterologous expression system. Site-directed mutagenesis on the precursor peptide-encoding gene is carried out systematically, generating 17 stlassin derivatives (2–17 and 21) with residue-replacements at specific positions of 1. The solution NMR structures of 1, 3, 4, 14 and 16 are determined, supporting structural comparisons that ultimately enabled the rational production of disulfide bond-containing derivatives 18 and 19, whose structures do not belong to any of the four classes currently used to classify lasso peptides. Several site-selective chemical modifications are first applied on 16 and 21, efficiently generating new derivatives (20, 22–27) whose structures bear various decorations beyond the peptidyl monotonicity. The high production yields of these stlassin derivatives facilitate biological assays, which show that 1, 4, 16, 20, 21 and 24 possess antagonistic activities against the binding of lipopolysaccharides to toll-like receptor 4 (TLR4). These results demonstrate proof-of-concept for the combined mutational/chemical generation of lasso peptide libraries to support drug lead development.

A new class II lasso peptide stlassin (1) was discovered and stlassin derivatives (2–27) were rationally generated by biosynthetic gene mutations and site-selective chemical modifications, expanding the structural diversity of lasso peptides.

Discovery of the Class I Antimicrobial Lasso Peptide Arcumycin

Lasso peptides are a structurally diverse superfamily of conformationally constrained peptide natural products, of which a subset exhibits broad antimicrobial activity. Although advances in bioinformatics have increased our knowledge of strains harboring the biosynthetic machinery for lasso peptide production, relating peptide sequence to bioactivity remains a continuous challenge. To this end, genome mining investigation of Actinobacteria-produced antimicrobial lasso peptides was performed to correlate predicted structure with antibiotic activity. Bioinformatic evaluation revealed eight putative novel class I lasso peptide sequences. Fermentation of one of these hits, Streptomyces NRRL F-5639, resulted in the production of a novel class I lasso peptide, arcumycin. Arcumycin exhibited antibiotic activity against Gram-positive bacteria including Bacillus subtilis (4 μg/mL), Staphylococcus aureus (8 μg/mL), and Micrococcus luteus (8 μg/mL). Arcumycin treatment of B. subtilis liaI-β-gal promoter fusion reporter strain resulted in upregulation of the liaRS system by the promoter liaI, indicating arcumycin interferes with lipid II biosynthesis. Cumulatively, the results illustrate the relationship between phylogenetically related lasso peptides and their bioactivity as validated through the isolation, structural determination, and evaluation of bioactivity of the novel class I antimicrobial lasso peptide arcumycin.

Cellulonodin-2 and Lihuanodin: Lasso Peptides with an Aspartimide Post-Translational Modification

Lasso peptides are a family of ribosomally synthesized and post-translationally modified peptides (RiPPs) defined by their threaded structure. Besides the class-defining isopeptide bond, other post-translational modifications (PTMs) that further tailor lasso peptides have been previously reported. Using genome mining tools, we identified a subset of lasso peptide biosynthetic gene clusters (BGCs) that are colocalized with genes encoding protein l-isoaspartyl methyltransferase (PIMT) homologues. PIMTs have an important role in protein repair, restoring isoaspartate residues formed from asparagine deamidation to aspartate. Here we report a new function for PIMT enzymes in the post-translational modification of lasso peptides. The PIMTs associated with lasso peptide BGCs first methylate an l-aspartate side chain found within the ring of the lasso peptide. The methyl ester is then converted into a stable aspartimide moiety, endowing the lasso peptide ring with rigidity relative to its unmodified counterpart. We describe the heterologous expression and structural characterization of two examples of aspartimide-modified lasso peptides from thermophilic Gram-positive bacteria. The lasso peptide cellulonodin-2 is encoded in the genome of actinobacterium Thermobifida cellulosilytica, while lihuanodin is encoded in the genome of firmicute Lihuaxuella thermophila. Additional genome mining revealed PIMT-containing lasso peptide BGCs in 48 organisms. In addition to heterologous expression, we have reconstituted PIMT-mediated aspartimide formation in vitro, showing that lasso peptide-associated PIMTs transfer methyl groups very rapidly as compared to canonical PIMTs. Furthermore, in stark contrast to other characterized lasso peptide PTMs, the methyltransferase functions only on lassoed substrates.

Mechanisms of action of ribosomally synthesized and posttranslationally modified peptides (RiPPs)

Natural products remain a critical source of medicines and drug leads. One of the most rapidly growing superclasses of natural products is RiPPs: ribosomally synthesized and posttranslationally modified peptides. RiPPs have rich and diverse bioactivities. This review highlights examples of the molecular mechanisms of action that underly those bioactivities. Particular emphasis is placed on RiPP/target interactions for which there is structural information. This detailed mechanism of action work is critical toward the development of RiPPs as therapeutics and can also be used to prioritize hits in RiPP genome mining studies.

Multi-omics Comparative Analysis of Streptomyces Mutants Obtained by Iterative Atmosphere and Room-Temperature Plasma Mutagenesis

Sponges, the most primitive multicellular animals, contain a large number of unique microbial communities. Sponge-associated microorganisms, particularly actinomyces, have the potential to produce diverse active natural products. However, a large number of silent secondary metabolic gene clusters have failed to be revived under laboratory culture conditions. In this study, iterative atmospheric room-temperature plasma. (ARTP) mutagenesis coupled with multi-omics conjoint analysis was adopted to activate the inactive wild Streptomyces strain. The desirable exposure time employed in this study was 75 s to obtain the appropriate lethality rate (94%) and mutation positive rate (40.94%). After three iterations of ARTP mutagenesis, the proportion of mutants exhibiting antibacterial activities significantly increased by 75%. Transcriptome analysis further demonstrated that the differential gene expression levels of encoding type I lasso peptide aborycin had a significant upward trend in active mutants compared with wild-type strains, which was confirmed by LC-MS results with a relative molecular mass of 1082.43 ([M + 2H]²⁺ at m/z = 2164.86). Moreover, metabolome comparative analysis of the mutant and wild-type strains showed that four spectra or mass peaks presented obvious differences in terms of the total ion count or extracting ion current profiles with each peak corresponding to a specific compound exhibiting moderate antibacterial activity against Gram-positive indicators. Taken together, our data suggest that the ARTP treatment method coupled with multi-omics profiling analysis could be used to estimate the valid active molecules of metabolites from microbial crudes without requiring a time-consuming isolation process.

Class IV Lasso Peptides Synergistically Induce Proliferation of Cancer Cells and Sensitize Them to Doxorubicin

Heterologous expression of a biosynthesis gene cluster from Amycolatopsis sp. resulted in the discovery of two unique class IV lasso peptides, felipeptins A1 and A2. A mixture of felipeptins stimulated proliferation of cancer cells, while having no such effect on the normal cells. Detailed investigation revealed, that pre-treatment of cancer cells with a mixture of felipeptins resulted in downregulation of the tumor suppressor Rb, making the cancer cells to proliferate faster. Pre-treatment with felipeptins made cancer cells considerably more sensitive to the anticancer agent doxorubicin and re-sensitized doxorubicin-resistant cells to this drug. Structural characterization and binding experiments showed an interaction between felipeptins resulting in complex formation, which explains their synergistic effect. This discovery may open an alternative avenue in cancer treatment, helping to eliminate quiescent cells that often lead to cancer relapse.

Bacteriocins to Thwart Bacterial Resistance in Gram Negative Bacteria

An overuse of antibiotics both in human and animal health and as growth promoters in farming practices has increased the prevalence of antibiotic resistance in bacteria. Antibiotic resistant and multi-resistant bacteria are now considered a major and increasing threat by national health agencies, making the need for novel strategies to fight bugs and super bugs a first priority. In particular, Gram-negative bacteria are responsible for a high proportion of nosocomial infections attributable for a large part to Enterobacteriaceae, such as pathogenic Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. To cope with their highly competitive environments, bacteria have evolved various adaptive strategies, among which the production of narrow spectrum antimicrobial peptides called bacteriocins and specifically microcins in Gram-negative bacteria. They are produced as precursor peptides that further undergo proteolytic cleavage and in many cases more or less complex posttranslational modifications, which contribute to improve their stability and efficiency. Many have a high stability in the gastrointestinal tract where they can target a single pathogen whilst only slightly perturbing the gut microbiota. Several microcins and antibiotics can bind to similar bacterial receptors and use similar pathways to cross the double-membrane of Gram-negative bacteria and reach their intracellular targets, which they also can share. Consequently, bacteria may use common mechanisms of resistance against microcins and antibiotics. This review describes both unmodified and modified microcins [lasso peptides, siderophore peptides, nucleotide peptides, linear azole(in)e-containing peptides], highlighting their potential as weapons to thwart bacterial resistance in Gram-negative pathogens and discusses the possibility of cross-resistance and co-resistance occurrence between antibiotics and microcins in Gram-negative bacteria.

How to harness biosynthetic gene clusters of lasso peptides

Lasso peptides produced by bacteria have a very unique cyclic structure (“lasso” structure) and are resistant to protease. To date, a number of lasso peptides have been isolated from proteobacteria and actinobacteria. Many lasso peptides exhibit various biological activities, such as antibacterial activity, and are expected to have various applications. Based on study of genome mining, large numbers of biosynthetic gene cluster of lasso peptides are revealed to distribute over genomes of proteobacteria and actinobacteria. However, the biosynthetic gene clusters are cryptic in most cases. Therefore, the combination of genome mining and heterologous production is efficient method for the production of lasso peptides. To utilize lasso peptide as fine chemical, there have been several attempts to add new function to lasso peptide by genetic engineering. Currently, a more efficient lasso peptide production system is being developed to harness cryptic biosynthetic gene clusters of lasso peptide. In this review, the overview of lasso peptide study is discussed.

Lasso Peptides: Heterologous Production and Potential Medical Application

Lasso peptides are natural products found in bacteria. They belong to a specific family of ribosomally-synthesized and posttranslationally-modified peptides with an unusual lasso structure. Lasso peptides possess remarkable thermal and proteolytic stability and various biological activities, such as antimicrobial activity, enzyme inhibition, receptor blocking, anticancer properties and HIV antagonism. They have promising potential therapeutic effects on gastrointestinal diseases, tuberculosis, Alzheimer’s disease, cardiovascular disease, fungal infections and cancer. Lasso peptides with high stability have been shown to be good carriers for other bioactive peptides. These make them attractive candidates for pharmaceutical research. This review aimed to describe the strategies used for the heterologous production of lasso peptides. Also, it indicated their therapeutical potential and their capacity to use as an efficient scaffold for epitope grafting.

GLN: a method to reveal unique properties of lasso type topology in proteins

Geometry and topology are the main factors that determine the functional properties of proteins. In this work, we show how to use the Gauss linking integral (GLN) in the form of a matrix diagram-for a pair of a loop and a tail-to study both the geometry and topology of proteins with closed loops e.g. lassos. We show that the GLN method is a significantly faster technique to detect entanglement in lasso proteins in comparison with other methods. Based on the GLN technique, we conduct comprehensive analysis of all proteins deposited in the PDB and compare it to the statistical properties of the polymers. We show how high and low GLN values correlate with the internal exibility of proteins, and how the GLN in the form of a matrix diagram can be used to study folding and unfolding routes. Finally, we discuss how the GLN method can be applied to study entanglement between two structures none of which are closed loops. Since this approach is much faster than other linking invariants, the next step will be evaluation of lassos in much longer molecules such as RNA or loops in a single chromosome.

In silico Screening Unveil the Great Potential of Ruminal Bacteria Synthesizing Lasso Peptides

Studies of rumen microbial ecology suggest that the capacity to produce antimicrobial peptides could be a useful trait in species competing for ecological niches in the ruminal ecosystem. However, little is known about the synthesis of lasso peptides by ruminal microorganisms. Here we analyzed the distribution and diversity of lasso peptide gene clusters in 425 bacterial genomes from the rumen ecosystem. Genome mining was performed using antiSMASH 5, BAGEL4, and a database of well-known precursor sequences. The genomic context of the biosynthetic clusters was investigated to identify putative lasA genes and protein sequences from enzymes of the biosynthetic machinery were evaluated to identify conserved motifs. Metatranscriptome analysis evaluated the expression of the biosynthetic genes in the rumen microbiome. Several incomplete (n = 23) and complete (n = 11) putative lasso peptide clusters were detected in the genomes of ruminal bacteria. The complete gene clusters were exclusively found within the phylum Firmicutes, mainly (48%) in strains of the genus Butyrivibrio. The analysis of the genetic organization of complete putative lasso peptide clusters revealed the presence of co-occurring genes, including kinases (85%), transcriptional regulators (49%), and glycosyltransferases (36%). Moreover, a conserved pattern of cluster organization was detected between strains of the same genus/species. The maturation enzymes LasB, LasC, and LasD showed regions highly conserved, including the presence of a transglutaminase core in LasB, an asparagine synthetase domain in LasC, and an ABC-type transporter system in LasD. Phylogenetic trees of the essential biosynthetic proteins revealed that sequences split into monophyletic groups according to their shared single common ancestor. Metatranscriptome analyses indicated the expression of the lasso peptides biosynthetic genes within the active rumen microbiota. Overall, our in silico screening allowed the discovery of novel biosynthetic gene clusters in the genomes of ruminal bacteria and revealed several strains with the genetic potential to synthesize lasso peptides, suggesting that the ruminal microbiota represents a potential source of these promising peptides.

Challenges and advances in genome mining of ribosomally synthesized and post-translationally modified peptides (RiPPs)

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a class of cyclic or linear peptidic natural products with remarkable structural and functional diversity. Recent advances in genomics and synthetic biology, are facilitating us to discover a large number of new ribosomal natural products, including lanthipeptides, lasso peptides, sactipeptides, thiopeptides, microviridins, cyanobactins, linear thiazole/oxazole-containing peptides and so on. In this review, we summarize bioinformatic strategies that have been developed to identify and prioritize biosynthetic gene clusters (BGCs) encoding RiPPs, and the genome mining-guided discovery of novel RiPPs. We also prospectively provide a vision of what genomics-guided discovery of RiPPs may look like in the future, especially the discovery of RiPPs from dominant but uncultivated microbes, which will be promoted by the combinational use of synthetic biology and metagenome mining strategies.

Heterologous production of new lasso peptide koreensin based on genome mining

Lasso peptides are a class of ribosomally biosynthesized and posttranslationally modified peptides with a knot structure as a common motif. Based on a genome search, a new biosynthetic gene cluster of lasso peptide was found in the genome of the proteobacterium Sphingomonas koreensis. Interestingly, the amino acid sequence of the precursor peptide gene includes two cell adhesion motif sequences (KGD and DGR). Heterologous production of the new lasso peptide was performed using the cryptic biosynthetic gene cluster of S. koreensis. As a result, a new lasso peptide named koreensin was produced by the gene expression system in the host strain Sphingomonas subterranea. The structure of koreensin was determined by NMR and ESI-MS analysis. The three-dimensional structure of koreensin was obtained based on an NOE experiment and the coupling constants. A variant peptide (koreensin-RGD), which had RGD instead of KGD, was produced by heterologous production with site-directed mutagenesis experiment. Koreensin and koreensin-RGD did not show cell adhesion inhibitory activity, although the molecules possessed cell adhesion motifs. The possible presence of a salt bridge between the motifs in koreensin was indicated, and it may prevent the cell adhesion motif from functioning.

Peptides derived from Kex2-processed repeat proteins are widely distributed and highly diverse in the Fungi kingdom

BACKGROUND

Recently, a gene cluster responsible for biosynthesis of ustiloxin in Aspergillus flavus was identified as the first case of a ribosomally synthesized and post-translationally modified peptide (RiPP) synthetic pathway in Ascomycota. RiPPs are biosynthesized from precursor peptides, which are processed to produce the RiPP backbone (core peptides) for further modifications such as methylation and cyclization. Ustiloxin precursor peptide has two distinctive features: a signal peptide for translocation into the endoplasmic reticulum and highly repeated core sequences cleaved by Kex2 protease in the Golgi apparatus. On the basis of these characteristics, the ustiloxin-type RiPP precursor peptides or Kex2-processed repeat proteins (KEPs) in strains belonging to the Fungi kingdom were computationally surveyed, in order to investigate the distribution and putative functions of KEPs in fungal ecology.

RESULTS

In total, 7878 KEPs were detected in 1345 of 1461 strains belonging to 8 phyla. The average number of KEPs per strain was 5.25 in Ascomycota and 5.30 in Basidiomycota, but only 1.35 in the class Saccharomycetes (Ascomycota) and 1.00 in the class Tremellomycetes (Basidiomycota). The KEPs were classified into 838 types and 2560 stand-alone ones, which had no homologs. Nearly 200 types were distributed in more than one genus, and 14 types in more than one phylum. These types included yeast α-mating factors and fungal pheromones. Genes for 22% KEPs were accompanied by genes for DUF3328-domain-containing proteins, which are indispensable for cyclization of the core peptides. DUF3328-domain-containing protein genes were located at an average distance of 3.09 genes from KEP genes. Genes for almost all (with three exceptions) KEPs annotated as yeast α-mating factors or fungal pheromones were not accompanied by DUF3328-domain-containing protein genes.

CONCLUSION

KEPs are widely distributed in the Fungi kingdom, but their repeated sequences are highly diverse. From these results and some examples, a hypothesis was raised that KEPs initially evolved as unmodified linear peptides (e.g., mating factors), and then those that adopted a modified cyclic form emerged (e.g., toxins) to utilize their strong bioactivity against predators and competitive microorganisms.

Genome mining strategies for ribosomally synthesised and post-translationally modified peptides

Genome mining is a computational method for the automatic detection and annotation of biosynthetic gene clusters (BGCs) from genomic data. This approach has been increasingly utilised in natural product (NP) discovery due to the large amount of sequencing data that is now available. Ribosomally synthesised and post-translationally modified peptides (RiPPs) are a class of structurally complex NP with diverse bioactivities. RiPPs have recently been shown to occupy a much larger expanse of genomic and chemical space than previously appreciated, indicating that annotation of RiPP BGCs in genomes may have been overlooked in the past. This review provides an overview of the genome mining tools that have been specifically developed to aid in the discovery of RiPP BGCs, which have been built from an increasing knowledgebase of RiPP structures and biosynthesis. Given these recent advances, the application of targeted genome mining has great potential to accelerate the discovery of important molecules such as antimicrobial and anticancer agents whilst increasing our understanding about how these compounds are biosynthesised in nature.

Genome Mining and Heterologous Expression Reveal Two Distinct Families of Lasso Peptides Highly Conserved in Endofungal Bacteria

Genome mining identified the fungal-bacterial endosymbiosis Rhizopus microsporus-Mycetohabitans (previously Burkholderia) rhizoxinica as a rich source of novel natural products. However, most of the predicted compounds have remained cryptic. In this study, we employed heterologous expression to isolate and characterize three ribosomally synthesized and post-translationally modified peptides with lariat topology (lasso peptides) from the endosymbiont M. rhizoxinica: burhizin-23, mycetohabin-16, and mycetohabin-15. Through coexpression experiments, it was shown that an orphan gene product results in mature mycetohabin-15, albeit encoded remotely from the core biosynthetic gene cluster. Comparative genomics revealed that mycetohabins are highly conserved among M. rhizoxinica and related endosymbiotic bacteria. Gene knockout and reinfection experiments indicated that the lasso peptides are not crucial for establishing symbiosis; instead, the peptides are exported into the environment during endosymbiosis. This is the first report on lasso peptides from endosymbiotic bacteria.

Discovery of Ubonodin, an Antimicrobial Lasso Peptide Active against Members of the Burkholderia cepacia Complex

We report the heterologous expression, structure, and antimicrobial activity of a lasso peptide, ubonodin, encoded in the genome of Burkholderia ubonensis. The topology of ubonodin is unprecedented amongst lasso peptides, with 18 of its 28 amino acids found in the mechanically bonded loop segment. Ubonodin inhibits RNA polymerase in vitro and has potent antimicrobial activity against several pathogenic members of the Burkholderia genus, most notably B. cepacia and B. multivorans, causative agents of lung infections in cystic fibrosis patients.

The manifold roles of microbial ribosomal peptide-based natural products in physiology and ecology

The ribosomally synthesized and posttranslationally modified peptides (RiPPs), also called ribosomal peptide natural products (RPNPs), form a growing superfamily of natural products that are produced by many different organisms and particularly by bacteria. They are derived from precursor polypeptides whose modification by various dedicated enzymes helps to establish a vast array of chemical motifs. RiPPs have attracted much interest as a source of potential therapeutic agents, and in particular as alternatives to conventional antibiotics to address the bacterial resistance crisis. However, their ecological roles in nature are poorly understood and explored. The present review describes major RiPP actors in competition within microbial communities, the main ecological and physiological functions currently evidenced for RiPPs, and the microbial ecosystems that are the sites for these functions. We envision that the study of RiPPs may lead to discoveries of new biological functions and highlight that a better knowledge of how bacterial RiPPs mediate inter-/intraspecies and interkingdom interactions will hold promise for devising alternative strategies in antibiotic development.

Pandonodin: A Proteobacterial Lasso Peptide with an Exceptionally Long C-Terminal Tail

Lasso peptides are a family of ribosomally synthesized and post-translationally modified peptides (RiPPs) defined by their threaded-ring topology. The N-terminus of the peptide forms an isopeptide bond with an aspartate or glutamate side chain to create a 7-9 amino acid (aa) macrocyclic ring through which the rest of the peptide is threaded. The result is a highly constrained three-dimensional structure. Even though they share a threaded-ring feature, characterized lasso peptides vary greatly in sequence and size, ranging from 14 to 26 aa. Using genome mining, we identified a new lasso peptide gene cluster with a predicted lasso peptide that is 33 aa long. Here we report the heterologous expression of this new peptide, pandonodin, its NMR structure, and its unusual biophysical properties. Pandonodin has a long, proteolytically resistant 18-residue tail of low sequence complexity, which limits its water solubility. Within this tail is a 6 aa disulfide-bonded macrocycle that serves as a steric lock to maintain the lasso structure. This disulfide bond is unusually stable, requiring both heat and high concentrations of reductants for cleavage. Finally, we also show that segments of the C-terminal tail of pandonodin can be replaced with arbitrary sequences, allowing for the construction of pandonodin-protein fusions.

Steric complementarity directs sequence promiscuous leader binding in RiPP biosynthesis

Enzymes that generate ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products have garnered significant interest, given their ability to produce large libraries of chemically diverse scaffolds. Such RiPP biosynthetic enzymes are predicted to bind their corresponding peptide substrates through sequence-specific recognition of the leader sequence, which is removed after the installation of posttranslational modifications on the core sequence. The conservation of the leader sequence within a given RiPP class, in otherwise disparate precursor peptides, further supports the notion that strict sequence specificity is necessary for leader peptide engagement. Here, we demonstrate that leader binding by a biosynthetic enzyme in the lasso peptide class of RiPPs is directed by a minimal number of hydrophobic interactions. Biochemical and structural data illustrate how a single leader-binding domain can engage sequence-divergent leader peptides using a conserved motif that facilitates hydrophobic packing. The presence of this simple motif in noncognate peptides results in low micromolar affinity binding by binding domains from several different lasso biosynthetic systems. We also demonstrate that these observations likely extend to other RiPP biosynthetic classes. The portability of the binding motif opens avenues for the engineering of semisynthetic hybrid RiPP products.

Bacterial production and direct functional screening of expanded molecular libraries for discovering inhibitors of protein aggregation

Protein misfolding and aggregation are associated with a many human disorders, including Alzheimer's and Parkinson's diseases. Toward increasing the effectiveness of early-stage drug discovery for these conditions, we report a bacterial platform that enables the biosynthesis of molecular libraries with expanded diversities and their direct functional screening for discovering protein aggregation inhibitors. We illustrate this approach by performing, what is to our knowledge, the largest functional screen of small-size molecular entities described to date. We generated a combinatorial library of ~200 million drug-like, cyclic peptides and rapidly screened it for aggregation inhibitors against the amyloid-β peptide (Aβ42), linked to Alzheimer's disease. Through this procedure, we identified more than 400 macrocyclic compounds that efficiently reduce Aβ42 aggregation and toxicity in vitro and in vivo. Finally, we applied a combination of deep sequencing and mutagenesis analyses to demonstrate how this system can rapidly determine structure-activity relationships and define consensus motifs required for bioactivity.

Genome mining for lasso peptides: past, present, and future

Over the course of roughly a decade, the lasso peptide field has been transformed. Whereas new compounds were discovered infrequently via activity-driven approaches, now, the vast majority of lasso peptide discovery is driven by genome-mining approaches. This paper starts with a historical overview of the first genome-mining approaches for lasso peptide discovery, and then covers new tools that have emerged. Several examples of novel lasso peptides that have been discovered via genome mining are presented as are examples of new enzymes found associated with lasso peptide gene clusters. Finally, this paper concludes with future directions and unsolved challenges in lasso peptide genome mining.

Evidence of Cis/Trans-Isomerization at Pro7/Pro16 in the Lasso Peptide Microcin J25

Microcin J25 is a ribosomal synthesized and post-translationally modified peptide (RiPP) characterized by a mechanically interlocked topology called the lasso fold. This structure provides microcin J25 a potent antimicrobial activity resulting from internalization via the siderophore receptor FhuA and further inhibition of the RNA polymerase. In the present work, nuclear magnetic resonance (NMR) and trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) were used to investigate the lasso structure of microcin J25. NMR experiments showed that the lasso peptide microcin J25 can adopt conformational states where Pro16 can be found in the cis- and trans-orientations. The high-resolution mobility analysis, aided by site-directed mutagenesis ([P7A], [P16A], and [P7A/P16A] variants), demonstrated that microcin J25 can adopt cis/cis-, cis/trans-, trans/cis-, and trans/trans-conformations at the Pro7 and Pro16 peptide bonds. It was also shown that interconversion between the conformers can occur as a function of the starting solvent conditions and ion heating (collision-induced activation, CIA) despite the lasso topology. Complementary to NMR findings, the cis-conformations at Pro7 were assigned using TIMS-MS. This study highlights the analytical power of TIMS-MS and site-directed mutagenesis for the study of biological systems with large micro-heterogeneity as a way to further increase our understanding of the receptor-binding dynamics and biological activity.

The Lasso Peptide Siamycin-I Targets Lipid II at the Gram-Positive Cell Surface

Ribosomally synthesized post-translationally modified peptides (RiPPs) are a diverse class of biologically active molecules produced by many environmental bacteria. While thousands of these compounds have been identified, mostly through genome mining, a relatively small number has been investigated at the molecular level. One less understood class of RiPPs is the lasso peptides. These are 20-25 amino acid residue compounds bearing an N-terminal macrocyclic ring and a C-terminal tail that is threaded through the ring. We have carried out a detailed investigation on the mechanism of action of the siamycin-I lasso peptide. We demonstrate that siamycin-I interacts with lipid II, the central building block of the major cell wall component peptidoglycan, which is readily accessible on the outside of the cell. This interaction compromises cell wall biosynthesis in a manner that activates the liaI stress response. Additionally, resistance to siamycin-I can be brought about by mutations in the essential WalKR two-component system that causes thickening of the cell wall. Siamycin-I is the first lasso peptide that has been shown to inhibit cell wall biosynthesis.

Lassomycin and lariatin lasso peptides as suitable antibiotics for combating mycobacterial infections: current state of biosynthesis and perspectives for production

Lasso peptides are ribosomally synthesized and post-translationally modified natural products with a characteristic slipknot-like structure, which confers these peptides remarkable stability and diverse pharmacologically relevant bioactivities. Among all the reported lasso peptides, lassomycin and lariatins are unique lasso peptides that exhibit noticeable anti-tuberculosis (TB) activity. Due to the unique threaded structure and the unusual bactericidal mechanism toward Mycobacterium tuberculosis, these peptides have drawn considerable interest, not only in the field of total synthesis but also in several other fields including biosynthesis, bioengineering, and structure-activity studies. During the past few years, significant progress has been made in understanding the biosynthetic mechanism of these intriguing compounds, which has provided a solid foundation for future work. This review highlights recent achievements in the discovery, structure elucidation, biological activity, and the unique anti-TB mechanism of lasso peptides. Moreover, the discovery of their biosynthetic pathway has laid the foundation for combinatorial biosynthesis of their analogs, which provides new perspectives for the production of novel anti-TB lasso peptides.

Discovery and structure of the antimicrobial lasso peptide citrocin

We report the identification of citrocin, a 19-amino acid-long antimicrobial lasso peptide from the bacteria Citrobacter pasteurii and Citrobacter braakii We refactored the citrocin gene cluster and heterologously expressed it in Escherichia coli We determined citrocin's NMR structure in water and found that is reminiscent of that of microcin J25 (MccJ25), an RNA polymerase-inhibiting lasso peptide that hijacks the TonB-dependent transporter FhuA to gain entry into cells. Citrocin has moderate antimicrobial activity against E. coli and Citrobacter strains. We then performed an in vitro RNA polymerase (RNAP) inhibition assay using citrocin and microcin J25 against E. coli RNAP. Citrocin has a higher minimal inhibition concentration than microcin J25 does against E. coli but surprisingly is ∼100-fold more potent as an RNAP inhibitor. This suggests that citrocin uptake by E. coli is limited. We found that unlike MccJ25, citrocin's activity against E. coli relied on neither of the two proton motive force-linked systems, Ton and Tol-Pal, for transport across the outer membrane. The structure of citrocin contains a patch of positive charge consisting of Lys-5 and Arg-17. We performed mutagenesis on these residues and found that the R17Y construct was matured into a lasso peptide but no longer had activity, showing the importance of this side chain for antimicrobial activity. In summary, we heterologously expressed and structurally and biochemically characterized an antimicrobial lasso peptide, citrocin. Despite being similar to MccJ25 in sequence, citrocin has an altered activity profile and does not use the same outer-membrane transporter to enter susceptible cells.

Structural mechanism of transcription inhibition by lasso peptides microcin J25 and capistruin

Many bacteria produce antimicrobial peptides for survival under stressful conditions. Some of these antimicrobial peptides are lasso peptides, which have a unique lasso-like topology and have generated great interest as a result of their stability in harsh conditions and amenability to functional engineering. In this study, we determined crystal structures of two lasso peptides, microcin J25 and capistruin, bound to their natural enzymatic target, the bacterial RNA polymerase (RNAP). The structures define peptide inhibitor–RNAP interactions that are important for inhibition and provide detailed insight into how the peptides inhibit RNAP function. This work provides a structural basis to guide the design of more potent lasso peptide antimicrobial approaches.

We report crystal structures of the antibacterial lasso peptides microcin J25 (MccJ25) and capistruin (Cap) bound to their natural enzymatic target, the bacterial RNA polymerase (RNAP). Both peptides bind within the RNAP secondary channel, through which NTP substrates enter the RNAP active site, and sterically block trigger-loop folding, which is essential for efficient catalysis by the RNAP. MccJ25 binds deep within the secondary channel in a manner expected to interfere with NTP substrate binding, explaining the partial competitive mechanism of inhibition with respect to NTPs found previously [Mukhopadhyay J, Sineva E, Knight J, Levy RM, Ebright RH (2004) Mol Cell 14:739–751]. The Cap binding determinant on RNAP overlaps, but is not identical to, that of MccJ25. Cap binds further from the RNAP active site and does not sterically interfere with NTP binding, and we show that Cap inhibition is partially noncompetitive with respect to NTPs. This work lays the groundwork for structure determination of other lasso peptides that target the bacterial RNAP and provides a structural foundation to guide lasso peptide antimicrobial engineering approaches.

The role of protein–protein interactions in the biosynthesis of ribosomally synthesized and post-translationally modified peptides

This review covers the role of protein–protein complexes in the biosynthesis of selected ribosomally synthesized and post-translationally modified peptide (RiPP) classes.