Type II non-ribosomal peptide synthetase proteins: structure, mechanism, and protein-protein interactions

Bacillus cereus sensu lato antimicrobial arsenal: An overview

The Bacillus cereus group contains genetically closed bacteria displaying a variety of phenotypic features and lifestyles. The group is mainly known through the properties of three major species: the entomopathogen Bacillus thuringiensis, the animal and human pathogen Bacillus anthracis and the foodborne opportunistic strains of B. cereus sensu stricto. Yet, the actual diversity of the group is far broader and includes multiple lifestyles. Another less-appreciated aspect of B. cereus members lies within their antimicrobial potential which deserves consideration in the context of growing emergence of resistance to antibiotics and pesticides, and makes it crucial to find new sources of antimicrobial molecules. This review presents the state of knowledge on the known antimicrobial compounds of the B. cereus group members, which are grouped according to their chemical features and biosynthetic pathways. The objective is to provide a comprehensive review of the antimicrobial range exhibited by this group of bacteria, underscoring the interest in its potent biocontrol arsenal and encouraging further research in this regard.

Protein features fusion using attributed network embedding for predicting protein-protein interaction

BACKGROUND

Protein-protein interactions (PPIs) hold significant importance in biology, with precise PPI prediction as a pivotal factor in comprehending cellular processes and facilitating drug design. However, experimental determination of PPIs is laborious, time-consuming, and often constrained by technical limitations.

METHODS

We introduce a new node representation method based on initial information fusion, called FFANE, which amalgamates PPI networks and protein sequence data to enhance the precision of PPIs' prediction. A Gaussian kernel similarity matrix is initially established by leveraging protein structural resemblances. Concurrently, protein sequence similarities are gauged using the Levenshtein distance, enabling the capture of diverse protein attributes. Subsequently, to construct an initial information matrix, these two feature matrices are merged by employing weighted fusion to achieve an organic amalgamation of structural and sequence details. To gain a more profound understanding of the amalgamated features, a Stacked Autoencoder (SAE) is employed for encoding learning, thereby yielding more representative feature representations. Ultimately, classification models are trained to predict PPIs by using the well-learned fusion feature.

RESULTS

When employing 5-fold cross-validation experiments on SVM, our proposed method achieved average accuracies of 94.28%, 97.69%, and 84.05% in terms of Saccharomyces cerevisiae, Homo sapiens, and Helicobacter pylori datasets, respectively.

CONCLUSION

Experimental findings across various authentic datasets validate the efficacy and superiority of this fusion feature representation approach, underscoring its potential value in bioinformatics.

Characterization and Statistical Optimization of Enterobatin Synthesized by Escherichia coli OQ866153

Microorganisms produce siderophores, which are secondary metabolites with a high affinity for iron. Siderophores have received significant attention due to their diverse applications in ecological and clinical research. In this study, siderophores production by Escherichia coli OQ866153 was optimized using two-stage statistical approach involving Plackett-Burman design (PBD) and response surface methodology (RSM) using central composite design (CCD). Out of 23 variables, succinate, tryptophan, Na2HPO4, CaCl2, agitation, and KH2PO4 were found to have the most significant effect on siderophores production in the first optimization stage with the highest SU% of 43.67%. In the second stage, RSM using CCD was utilized, and the optimal conditions were determined to be 0.3 g/l succinate, 0 g/l tryptophan, 6 g/l Na2HPO4, 0.1 g/l CaCl2, 150 RPM agitation, and 0.6 g/l KH2PO4, resulting in a maximum siderophore units (SU%) of 89.13%. The model was significant, as indicated by the model f-value of 314.14 (p-value = 0.0004) and coefficient of determination R2 of 0.9950. During validation experiments, the obtained maximum SU% was increased up to 87.1472%, which was two times as the value obtained under ordinary conditions (46.62%). The produced siderophores were purified and characterized using 1H, 13C NMR, IR spectroscopy. The obtained results indicated that the compound was enterobactin and entABCDEF genes were further detected in Escherichia coli OQ866153 extracted DNA. To our knowledge, this is the first report of statistical optimization for enterobactin synthesis by an E. coli strain isolated from a clinical source in Egypt.

A dissected non-ribosomal peptide synthetase maintains activity

Non-ribosomal peptide synthetases (NRPSs) generate chemically complex compounds and their modular architecture suggests that changing their domain organization can predictably alter their products. Ebony, a small three-domain NRPS, catalyzes the formation of β-alanine containing amides from biogenic amines. To examine the necessity of interdomain interactions, we modeled and docked domains of Ebony to reveal potential interfaces between them. Testing the same domain combinations in vitro showed that 8 % of activity was preserved after Ebony was dissected into a di-domain and a detached C-terminal domain, suggesting that sufficient interaction was maintained after dissection. Our work creates a model to identify domain interfaces necessary for catalysis, an important step toward utilizing Ebony as a combinatorial engineering platform for novel amides.

A small step to discover candidate biological control agents from preexisting bioresources by using novel nonribosomal peptide synthetases hidden in activated sludge metagenomes

Biological control agents (BCAs), beneficial organisms that reduce the incidence or severity of plant disease, have been expected to be alternatives to replace chemical pesticides worldwide. To date, BCAs have been screened by culture-dependent methods from various environments. However, previously unknown BCA candidates may be buried and overlooked because this approach preferentially selects only easy-to-culture microbial lineages. To overcome this limitation, as a small-scale test case, we attempted to explore novel BCA candidates by employing the shotgun metagenomic information of the activated sludge (AS) microbiome, which is thought to contain unutilized biological resources. We first performed genome-resolved metagenomics for AS taken from a municipal sewage treatment plant and obtained 97 nonribosomal peptide synthetase (NRPS)/polyketide synthase (PKS)-related gene sequences from 43 metagenomic assembled bins, most of which were assigned to the phyla Proteobacteria and Myxococcota. Furthermore, these NRPS/PKS-related genes are predicted to be novel because they were genetically dissimilar to known NRPS/PKS gene clusters. Of these, the condensation domain of the syringomycin-related NRPS gene cluster was detected in Rhodoferax- and Rhodocyclaceae-related bins, and its homolog was found in previously reported AS metagenomes as well as the genomes of three strains available from the microbial culture collections, implying their potential BCA ability. Then, we tested the antimicrobial activity of these strains against phytopathogenic fungi to investigate the potential ability of BCA by in vitro cultivation and successfully confirmed the actual antifungal activity of three strains harboring a possibly novel NRPS gene cluster. Our findings provide a possible strategy for discovering novel BCAs buried in the environment using genome-resolved metagenomics.

Structural advances toward understanding the catalytic activity and conformational dynamics of modular nonribosomal peptide synthetases

Covering: up to fall 2022.Nonribosomal peptide synthetases (NRPSs) are a family of modular, multidomain enzymes that catalyze the biosynthesis of important peptide natural products, including antibiotics, siderophores, and molecules with other biological activity. The NRPS architecture involves an assembly line strategy that tethers amino acid building blocks and the growing peptides to integrated carrier protein domains that migrate between different catalytic domains for peptide bond formation and other chemical modifications. Examination of the structures of individual domains and larger multidomain proteins has identified conserved conformational states within a single module that are adopted by NRPS modules to carry out a coordinated biosynthetic strategy that is shared by diverse systems. In contrast, interactions between modules are much more dynamic and do not yet suggest conserved conformational states between modules. Here we describe the structures of NRPS protein domains and modules and discuss the implications for future natural product discovery.

Interface Engineering of Carrier-Protein-Dependent Metabolic Pathways

Carrier-protein-dependent metabolic pathways biosynthesize fatty acids, polyketides, and non-ribosomal peptides, producing metabolites with important pharmaceutical, environmental, and industrial properties. Recent findings demonstrate that these pathways rely on selective communication mechanisms involving protein-protein interactions (PPIs) that guide enzyme reactivity and timing. While rational design of these PPIs could enable pathway design and modification, this goal remains a challenge due to the complex nature of protein interfaces. Computational methods offer an encouraging avenue, though many score functions fail to predict experimental observables, leading to low success rates. Here, we improve upon the Rosetta score function, leveraging experimental data through iterative rounds of computational prediction and mutagenesis, to design a hybrid fatty acid-non-ribosomal peptide initiation pathway. By increasing the weight of the electrostatic score term, the computational protocol proved to be more predictive, requiring fewer rounds of iteration to identify mutants with high in vitro activity. This allowed efficient design of new PPIs between a non-ribosomal peptide synthetase adenylation domain, PltF, and a fatty acid synthase acyl carrier protein, AcpP, as validated by activity and structural studies. This method provides a promising platform for customized pathway design, establishing a standard for carrier-protein-dependent pathway engineering through PPI optimization.

Charting the Lipopeptidome of Nonpathogenic Pseudomonas

A major source of pseudomonad-specialized metabolites is the nonribosomal peptide synthetases (NRPSs) assembling siderophores and lipopeptides. Cyclic lipopeptides (CLPs) of the Mycin and Peptin families are frequently associated with, but not restricted to, phytopathogenic species. We conducted an in silico analysis of the NRPSs encoded by lipopeptide biosynthetic gene clusters in nonpathogenic Pseudomonas genomes, covering 13 chemically diversified families. This global assessment of lipopeptide production capacity revealed it to be confined to the Pseudomonas fluorescens lineage, with most strains synthesizing a single type of CLP. Whereas certain lipopeptide families are specific for a taxonomic subgroup, others are found in distant groups. NRPS activation domain-guided peptide predictions enabled reliable family assignments, including identification of novel members. Focusing on the two most abundant lipopeptide families (Viscosin and Amphisin), a portion of their uncharted diversity was mapped, including characterization of two novel Amphisin family members (nepenthesin and oakridgin). Using NMR fingerprint matching, known Viscosin-family lipopeptides were identified in 15 (type) species spread across different taxonomic groups. A bifurcate genomic organization predominates among Viscosin-family producers and typifies Xantholysin-, Entolysin-, and Poaeamide-family producers but most families feature a single NRPS gene cluster embedded between cognate regulator and transporter genes. The strong correlation observed between NRPS system phylogeny and rpoD-based taxonomic affiliation indicates that much of the structural diversity is linked to speciation, providing few indications of horizontal gene transfer. The grouping of most NRPS systems in four superfamilies based on activation domain homology suggests extensive module dynamics driven by domain deletions, duplications, and exchanges. IMPORTANCE Pseudomonas species are prominent producers of lipopeptides that support proliferation in a multitude of environments and foster varied lifestyles. By genome mining of biosynthetic gene clusters (BGCs) with lipopeptide-specific organization, we mapped the global Pseudomonas lipopeptidome and linked its staggering diversity to taxonomy of the producers, belonging to different groups within the major Pseudomonas fluorescens lineage. Activation domain phylogeny of newly mined lipopeptide synthetases combined with previously characterized enzymes enabled assignment of predicted BGC products to specific lipopeptide families. In addition, novel peptide sequences were detected, showing the value of substrate specificity analysis for prioritization of BGCs for further characterization. NMR fingerprint matching proved an excellent tool to unequivocally identify multiple lipopeptides bioinformatically assigned to the Viscosin family, by far the most abundant one in Pseudomonas and with stereochemistry of all its current members elucidated. In-depth analysis of activation domains provided insight into mechanisms driving lipopeptide structural diversification.

Compilation of the Antimicrobial Compounds Produced by Burkholderia Sensu Stricto

Due to the increase in multidrug-resistant microorganisms, the investigation of novel or more efficient antimicrobial compounds is essential. The World Health Organization issued a list of priority multidrug-resistant bacteria whose eradication will require new antibiotics. Among them, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae are in the "critical" (most urgent) category. As a result, major investigations are ongoing worldwide to discover new antimicrobial compounds. Burkholderia, specifically Burkholderia sensu stricto, is recognized as an antimicrobial-producing group of species. Highly dissimilar compounds are among the molecules produced by this genus, such as those that are unique to a particular strain (like compound CF66I produced by Burkholderia cepacia CF-66) or antimicrobials found in a number of species, e.g., phenazines or ornibactins. The compounds produced by Burkholderia include N-containing heterocycles, volatile organic compounds, polyenes, polyynes, siderophores, macrolides, bacteriocins, quinolones, and other not classified antimicrobials. Some of them might be candidates not only for antimicrobials for both bacteria and fungi, but also as anticancer or antitumor agents. Therefore, in this review, the wide range of antimicrobial compounds produced by Burkholderia is explored, focusing especially on those compounds that were tested in vitro for antimicrobial activity. In addition, information was gathered regarding novel compounds discovered by genome-guided approaches.

Using NMR Titration Experiments to Study E. coli FAS-II- and AcpP-Mediated Protein-Protein Interactions

Acyl carrier proteins (ACPs) are central to many primary and secondary metabolic pathways. In E. coli fatty acid biosynthesis (FAB), the central ACP, AcpP, transports intermediates to a suite of partner proteins (PP) for iterative modification and elongation. The regulatory protein-protein interactions that occur between AcpP and the PP in FAB are poorly understood due to the dynamic and transient nature of these interactions. Solution-state NMR spectroscopy can reveal information at the atomic level through experiments such as the 2D heteronuclear single quantum coherence (HSQC). The following protocol describes NMR HSQC titration experiments that can elucidate biomolecular recognition events.

Amino acid (acyl carrier protein) ligase-associated biosynthetic gene clusters reveal unexplored biosynthetic potential

This study aimed to evaluate the postulated cellular function of a novel family of amino acid (acyl carrier protein) ligases (AALs) in natural product biosynthesis. Here, we analyzed the manually curated, putative, aal-associated natural product biosynthetic gene clusters (NP BGCs) using two computational platforms for NP prediction, antiSMASH-BiG-SCAPE-CORASON and DeepBGC. The detected BGCs included a diversity of type I polyketide/nonribosomal peptide (PKS/NRPS) hybrid BGCs, exemplified by the guadinomine BGC, which suggested a dedicated function of AALs in the biosynthesis of rare (2S)-aminomalonyl-ACP extension units. Besides modular PKS/NRPSs and NRPSs, AAL-associated BGCs were predicted to assemble arylpolyenes, ladderane lipids, phosphonates, aminoglycosides, β-lactones, and thioamides of both nonribosomal and ribosomal origins. Additionally, we revealed a frequent association of AALs with putative, seldom observed transglutaminase-like and BtrH-like transferases of the cysteine protease superfamily, which may form larger families of ACP-dependent amide bond catalysts used in NP synthesis. Our results disclosed an exceptional chemical novelty and biosynthetic potential of the AAL-associated BGCs in NP biosynthesis. The presented in silico evidence supports the initial hypothesis and provides an important foundation for future experimental studies aimed at discovering novel pharmaceutically relevant active compounds.

Cross-Linking of the Nonribosomal Peptide Synthetase Adenylation Domain with a Carrier Protein Using a Pantetheine-Type Probe

Adenylation domains (A-domains) are responsible for the selective incorporation of carboxylic acid substrates in the biosynthesis of nonribosomal peptides and related natural products. The A-domain transfers an acyl substrate onto its cognate carrier protein (CP). The proper interactions between an A-domain and the cognate CP are important for functional substrate transfer. To stabilize the transient interactions sufficiently for structural analysis of A-domain-CP complex, vinylsulfonamide adenosine inhibitors have been traditionally used as molecular probes. Recently, we have developed an alternative strategy using a synthetic pantetheine-type probe that enables site-specific cross-linking between an A-domain and a CP. In this chapter, we describe the laboratory protocols for this cross-linking reaction.

Structural Studies of Modular Nonribosomal Peptide Synthetases

The non-ribosomal peptide synthetases (NRPSs) are a family of modular enzymes involved in the production of peptide natural products. Not restricted by the constraints of ribosomal peptide and protein production, the NRPSs are able to incorporate unusual amino acids and other suitable building blocks into the final product. The NRPSs operate with an assembly line strategy in which peptide intermediates are covalently tethered to a peptidyl carrier protein and transported to different catalytic domains for the multiple steps in the biosynthesis. Often the carrier and catalytic domains are joined into a single large multidomain protein. This chapter serves to introduce the NRPS enzymes, using the nocardicin NRPS system as an example that highlights many common features to NRPS biochemistry. We then describe recent advances in the structural biology of NRPSs focusing on large multidomain structures that have been determined.

Essential Role of Loop Dynamics in Type II NRPS Biomolecular Recognition

Non-ribosomal peptides play a critical role in the clinic as therapeutic agents. To access more chemically diverse therapeutics, non-ribosomal peptide synthetases (NRPSs) have been targeted for engineering through combinatorial biosynthesis; however, this has been met with limited success in part due to the lack of proper protein-protein interactions between non-cognate proteins. Herein, we report our use of chemical biology to enable X-ray crystallography, molecular dynamics (MD) simulations, and biochemical studies to elucidate binding specificities between peptidyl carrier proteins (PCPs) and adenylation (A) domains. Specifically, we determined X-ray crystal structures of a type II PCP crosslinked to its cognate A domain, PigG and PigI, and of PigG crosslinked to a non-cognate PigI homologue, PltF. The crosslinked PCP-A domain structures possess large protein-protein interfaces that predominantly feature hydrophobic interactions, with specific electrostatic interactions that orient the substrate for active site delivery. MD simulations of the PCP-A domain complexes and unbound PCP structures provide a dynamical evaluation of the transient interactions formed at PCP-A domain interfaces, which confirm the previously hypothesized role of a PCP loop as a crucial recognition element. Finally, we demonstrate that the interfacial interactions at the PCP loop 1 region can be modified to control PCP binding specificity through gain-of-function mutations. This work suggests that loop conformational preferences and dynamism account for improved shape complementary in the PCP-A domain interactions. Ultimately, these studies show how crystallographic, biochemical, and computational methods can be used to rationally re-engineer NRPSs for non-cognate interactions.

Identification of protein–protein interaction associated functions based on gene ontology and KEGG pathway

Protein–protein interactions (PPIs) are extremely important for gaining mechanistic insights into the functional organization of the proteome. The resolution of PPI functions can help in the identification of novel diagnostic and therapeutic targets with medical utility, thus facilitating the development of new medications. However, the traditional methods for resolving PPI functions are mainly experimental methods, such as co-immunoprecipitation, pull-down assays, cross-linking, label transfer, and far-Western blot analysis, that are not only expensive but also time-consuming. In this study, we constructed an integrated feature selection scheme for the large-scale selection of the relevant functions of PPIs by using the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotations of PPI participants. First, we encoded the proteins in each PPI with their gene ontologies and KEGG pathways. Then, the encoded protein features were refined as features of both positive and negative PPIs. Subsequently, Boruta was used for the initial filtering of features to obtain 5684 features. Three feature ranking algorithms, namely, least absolute shrinkage and selection operator, light gradient boosting machine, and max-relevance and min-redundancy, were applied to evaluate feature importance. Finally, the top-ranked features derived from multiple datasets were comprehensively evaluated, and the intersection of results mined by three feature ranking algorithms was taken to identify the features with high correlation with PPIs. Some functional terms were identified in our study, including cytokine–cytokine receptor interaction (hsa04060), intrinsic component of membrane (GO:0031224), and protein-binding biological process (GO:0005515). Our newly proposed integrated computational approach offers a novel perspective of the large-scale mining of biological functions linked to PPI.

A Nonfunctional Halogenase Masquerades as an Aromatizing Dehydratase in Biosynthesis of Pyrrolic Polyketides by Type I Polyketide Synthases

The bacterial modular type I polyketide synthases (PKSs) typically furnish nonaromatic lactone and lactam natural products. Here, by the complete in vitro enzymatic production of the polyketide antibiotic pyoluteorin, we describe the biosynthetic mechanism for the construction of an aromatic resorcylic ring by a type I PKS. We find that the pyoluteorin type I PKS does not produce an aromatic product, rather furnishing an alicyclic dihydrophloroglucinol that is later enzymatically dehydrated and aromatized. The aromatizing dehydratase is encoded in the pyoluteorin biosynthetic gene cluster (BGC), and its presence is conserved in other BGCs encoding production of pyrrolic polyketides. Sequence similarity and mutational analysis demonstrates that the overall structure and position of the active site for the aromatizing dehydratase is shared with flavin-dependent halogenases albeit with a loss in ability to perform redox catalysis. We demonstrate that the post-PKS dehydrative aromatization is critical for the antibiotic activity of pyoluteorin.

Protein-protein interface analysis of the non-ribosomal peptide synthetase peptidyl carrier protein and enzymatic domains

Non-ribosomal peptide synthetases (NRPSs) are attractive targets for biosynthetic pathway engineering due to their modular architecture and the therapeutic relevance of their products. With catalysis mediated by specific protein-protein interactions formed between the peptidyl carrier protein (PCP) and its partner enzymes, NRPS enzymology and control remains fertile ground for discovery. This review focuses on the recent efforts within structural biology by compiling high-resolution structural data that shed light into the various protein-protein interfaces formed between the PCP and its partner enzymes, including the phosphopantetheinyl transferase (PPTase), adenylation (A) domain, condensation (C) domain, thioesterase (TE) domain and other tailoring enzymes within the synthetase. Integrating our understanding of how the PCP recognizes partner proteins with the potential to use directed evolution and combinatorial biosynthetic methods will enhance future efforts in discovery and production of new bioactive compounds.

The α/β hydrolase AzpM catalyzes dipeptide synthesis in alazopeptin biosynthesis using two molecules of carrier protein-tethered amino acid

During the biosynthesis of alazopeptin, a tripeptide composed of two molecules of 6-diazo-5-oxo-L-norleucine (DON) and one of alanine, the α/β hydrolase AzpM synthesizes the DON-DON dipeptide using DON tethered to the carrier protein AzpF (DON-AzpF). However, whether AzpM catalyzes the condensation of DON-AzpF with DON or DON-AzpF remains unclear. Here, to distinguish between these two condensation possibilities, the reaction catalyzed by AzpM was examined in vitro using a DON analog, azaserine (AZS). We found that AzpM catalyzed the condensation between AZS-AzpF and DON-AzpF, but not between AZS-AzpF and DON. Possible reaction intermediates, DON-DON-AzpF and AZS-AZS-AzpF, were also detected during AzpM-catalyzed dipeptide formation from DON-AzpF and AZS-AzpF, respectively. From these results, we concluded that AzpM catalyzed the condensation of the two molecules of DON-AzpF and subsequent hydrolysis to produce DON-DON. Thus, AzpM is an unprecedented α/β hydrolase that catalyzes dipeptide synthesis from two molecules of a carrier protein-tethered amino acid.

Specificity and genetic polymorphism in the Vfm quorum sensing system of plant pathogenic bacteria of the genus Dickeya

The Vfm quorum sensing (QS) system is preponderant for the virulence of different species of the bacterial genus Dickeya. The vfm gene cluster encodes 26 genes involved in the production, sensing or transduction of the QS signal. To date, the Vfm QS signal has escaped detection by analytical chemistry methods. However, we report here a strain‐specific polymorphism in the biosynthesis genes vfmO and vfmP, which is predicted to be related to the production of different analogues of the QS signal. Consequently, the Vfm communication could be impossible between strains possessing different variants of the genes vfmO/P. We constructed three Vfm QS biosensor strains possessing different vfmO/P variants and compared these biosensors for their responses to samples prepared from 34 Dickeya strains possessing different vfmO/P variants. A pattern of specificity was demonstrated, providing evidence that the polymorphism in the genes vfmO/P determines the biosynthesis of different analogues of the QS signal. Unexpectedly, this vfmO/P‐dependent pattern of specificity is linked to a polymorphism in the ABC transporter gene vfmG, suggesting an adaptation of the putative permease VfmG to specifically bind different analogues of the QS signal. Accordingly, we discuss the possible involvement of VfmG as co‐sensor of the Vfm two‐component regulatory system.

Ribosome-independent peptide biosynthesis: the challenge of a unifying nomenclature

The first machineries for non-ribosomal peptide (NRP) biosynthesis were uncovered over 50 years ago, and the dissection of these megasynthetases set the stage for the nomenclature system that has been used ever since. Although the number of exceptions to the canonical biosynthetic pathways has surged in the intervening years, the NRP synthetase (NRPS) classification system has remained relatively unchanged. This has led to the exclusion of many biosynthetic pathways whose biosynthetic machineries violate the classical rules for NRP assembly, and ultimately to a rupture in the field of NRP biosynthesis. In an attempt to unify the classification of NRP pathways and to facilitate the communication within the research field, we propose a revised framework for grouping ribosome-independent peptide biosynthetic pathways based on recognizable commonalities in their biosynthetic logic. Importantly, the framework can be further refined as needed.

Recent advances in biocatalysis of nitrogen-containing heterocycles

Nitrogen-containing heterocycles (N-heterocycles) are ubiquitous in both organisms and pharmaceutical products. Biocatalysts are providing green approaches for synthesizing various N-heterocycles under mild reaction conditions. This review summarizes the recent advances in the biocatalysis of N-heterocycles through the discovery and engineering of natural N-heterocycle synthetic pathway, and the design of artificial synthetic routes, with an emphasis on biocatalysts applied in retrosynthetic design for preparing complex N-heterocycles. Furthermore, this review discusses the future prospects and challenges of biocatalysts involved in the synthesis of N-heterocycles.

A System for the Evolution of Protein-Protein Interaction Inducers

Molecules that induce interactions between proteins, often referred to as "molecular glues", are increasingly recognized as important therapeutic modalities and as entry points for rewiring cellular signaling networks. Here, we report a new PACE-based method to rapidly select and evolve molecules that mediate interactions between otherwise noninteracting proteins: rapid evolution of protein-protein interaction glues (rePPI-G). By leveraging proximity-dependent split RNA polymerase-based biosensors, we developed E. coli-based detection and selection systems that drive gene expression outputs only when interactions between target proteins are induced. We then validated the system using engineered bivalent molecular glues, showing that rePPI-G robustly selects for molecules that induce the target interaction. Proof-of-concept evolutions demonstrated that rePPI-G reduces the "hook effect" of the engineered molecular glues, due at least in part to tuning the interaction affinities of each individual component of the bifunctional molecule. Altogether, this work validates rePPI-G as a continuous, phage-based evolutionary technology for optimizing molecular glues, providing a strategy for developing molecules that reprogram protein-protein interactions.

Recent advances in the biosynthesis strategies of nitrogen heterocyclic natural products

Covering: 2015 to 2020Nitrogen heterocyclic natural products (NHNPs) are primary or secondary metabolites containing nitrogen heterocyclic (N-heterocyclic) skeletons. Due to the existence of the N-heterocyclic structure, NHNPs exhibit various bioactivities such as anticancer and antibacterial, which makes them widely used in medicines, pesticides, and food additives. However, the low content of these NHNPs in native organisms severely restricts their commercial application. Although a variety of NHNPs have been produced through extraction or chemical synthesis strategies, these methods suffer from several problems. The development of biotechnology provides new options for the production of NHNPs. This review introduces the recent progress of two strategies for the biosynthesis of NHNPs: enzymatic biosynthesis and microbial cell factory. In the enzymatic biosynthesis part, the recent progress in the mining of enzymes that synthesize N-heterocyclic skeletons (e.g., pyrrole, piperidine, diketopiperazine, and isoquinoline), the engineering of tailoring enzymes, and enzyme cascades constructed to synthesize NHNPs are discussed. In the microbial cell factory part, with tropane alkaloids (TAs) and tetrahydroisoquinoline (THIQ) alkaloids as the representative compounds, the strategies of unraveling unknown natural biosynthesis pathways of NHNPs in plants are summarized, and various metabolic engineering strategies to enhance their production in microbes are introduced. Ultimately, future perspectives for accelerating the biosynthesis of NHNPs are discussed.

The inherent flexibility of type I non-ribosomal peptide synthetase multienzymes drives their catalytic activities

Non-ribosomal peptide synthetases (NRPSs) are multienzymes that produce complex natural metabolites with many applications in medicine and agriculture. They are composed of numerous catalytic domains that elongate and chemically modify amino acid substrates or derivatives and of non-catalytic carrier protein domains that can tether and shuttle the growing products to the different catalytic domains. The intrinsic flexibility of NRPSs permits conformational rearrangements that are required to allow interactions between catalytic and carrier protein domains. Their large size coupled to this flexibility renders these multi-domain proteins very challenging for structural characterization. Here, we summarize recent studies that offer structural views of multi-domain NRPSs in various catalytically relevant conformations, thus providing an increased comprehension of their catalytic cycle. A better structural understanding of these multienzymes provides novel perspectives for their re-engineering to synthesize new bioactive metabolites.

Salicylic Acid Biosynthesis and Metabolism: A Divergent Pathway for Plants and Bacteria

Salicylic acid (SA) is an active secondary metabolite that occurs in bacteria, fungi, and plants. SA and its derivatives (collectively called salicylates) are synthesized from chorismate (derived from shikimate pathway). SA is considered an important phytohormone that regulates various aspects of plant growth, environmental stress, and defense responses against pathogens. Besides plants, a large number of bacterial species, such as Pseudomonas, Bacillus, Azospirillum, Salmonella, Achromobacter, Vibrio, Yersinia, and Mycobacteria, have been reported to synthesize salicylates through the NRPS/PKS biosynthetic gene clusters. This bacterial salicylate production is often linked to the biosynthesis of small ferric-ion-chelating molecules, salicyl-derived siderophores (known as catecholate) under iron-limited conditions. Although bacteria possess entirely different biosynthetic pathways from plants, they share one common biosynthetic enzyme, isochorismate synthase, which converts chorismate to isochorismate, a common precursor for synthesizing SA. Additionally, SA in plants and bacteria can undergo several modifications to carry out their specific functions. In this review, we will systematically focus on the plant and bacterial salicylate biosynthesis and its metabolism.

An Unexpected Split-Merge Pathway in the Assembly of the Symmetric Nonribosomal Peptide Antibiotic Closthioamide

Closthioamide (CTA) is a symmetric nonribosomal peptide (NRP) comprised of two diaminopropane-linked polythioamidated monomers. CTA is biosynthesized by Ruminiclostridium cellulolyticum via an atypical NRP synthetase (NRPS)-independent biosynthetic pathway. Although the logic for monomer assembly was recently elucidated, the strategy for the biosynthesis and incorporation of the diamine linker remained a mystery. By means of genome editing, synthesis, and in vitro biochemical assays, we demonstrate that the final steps in CTA maturation proceed through a surprising split-merge pathway involving the dual use of a thiotemplated intermediate. This pathway includes the first examples of an aldo-keto reductase catalyzing the reductive release of a thiotemplated product, and of a transthioamidating transglutaminase. In addition to clarifying the remaining steps in CTA assembly, our data shed light on largely unexplored pathways for NRPS-independent peptide biosynthesis.

Combinatorial biosynthesis for the generation of new-to-nature peptide antimicrobials

Natural peptide products are a valuable source of important therapeutic agents, including antibiotics, antivirals and crop protection agents. Aided by an increased understanding of structure-activity relationships of these complex molecules and the biosynthetic machineries that produce them, it has become possible to re-engineer complete machineries and biosynthetic pathways to create novel products with improved pharmacological properties or modified structures to combat antimicrobial resistance. In this review, we will address the progress that has been made using non-ribosomally produced peptides and ribosomally synthesized and post-translationally modified peptides as scaffolds for designed biosynthetic pathways or combinatorial synthesis for the creation of novel peptide antimicrobials.

Beyond peptide bond formation: the versatile role of condensation domains in natural product biosynthesis

Condensation domains perform highly diverse functions during natural product biosynthesis and are capable of generating remarkable chemical diversity.

Food-Poisoning Bacteria Employ a Citrate Synthase and a Type II NRPS To Synthesize Bolaamphiphilic Lipopeptide Antibiotics*

Mining the genome of the food-spoiling bacterium Burkholderia gladioli pv. cocovenenans revealed five nonribosomal peptide synthetase (NRPS) gene clusters, including an orphan gene locus (bol). Gene inactivation and metabolic profiling linked the bol gene cluster to novel bolaamphiphilic lipopeptides with antimycobacterial activity. A combination of chemical analysis and bioinformatics elucidated the structures of bolagladin A and B, lipocyclopeptides featuring an unusual dehydro-β-alanine enamide linker fused to an unprecedented tricarboxylic fatty acid tail. Through a series of targeted gene deletions, we proved the involvement of a designated citrate synthase (CS), priming ketosynthases III (KS III), a type II NRPS, including a novel desaturase for enamide formation, and a multimodular NRPS in generating the cyclopeptide. Network analyses revealed the evolutionary origin of the CS and identified cryptic CS/NRPS gene loci in various bacterial genomes.

Lipopeptide families at the interface between pathogenic and beneficial Pseudomonas-plant interactions

Lipopeptides (LPs) are a prominent class of molecules among the steadily growing spectrum of specialized metabolites retrieved from Pseudomonas, in particular soil-dwelling and plant-associated isolates. Among the multiple LP families, pioneering research focussed on phytotoxic and antimicrobial cyclic lipopeptides (CLPs) of the ubiquitous plant pathogen Pseudomonas syringae (syringomycin and syringopeptin). Their non-ribosomal peptide synthetases (NRPSs) are embedded in biosynthetic gene clusters (BGCs) that are tightly co-clustered on a pathogenicity island. Other members of the P. syringae group (Pseudomonas cichorii) and some species of the Pseudomonas asplenii group and Pseudomonas fluorescens complex have adopted these biosynthetic strategies to co-produce their own mycin and peptin variants, in some strains supplemented with an analogue of the P. syringae linear LP (LLP), syringafactin. This capacity is not confined to phytopathogens but also occurs in some biocontrol strains, which indicates that these LP families not solely function as general virulence factors. We address this issue by scrutinizing the structural diversity and bioactivities of LPs from the mycin, peptin, and factin families in a phylogenetic and evolutionary perspective. BGC functional organization (including associated regulatory and transport genes) and NRPS modular architectures in known and candidate LP producers were assessed by genome mining.

Peptone from casein, an antagonist of nonribosomal peptide synthesis: a case study of pedopeptins produced by Pedobacter lusitanus NL19

Novel natural products are urgently needed to address the worldwide incidence of bacterial resistance to antibiotics. Extreme environments are a major source of novel compounds with unusual chemical structures. Pedobacter lusitanus NL19 is a new bacterial species that was isolated from one such environment and which produces compounds with potent activity against relevant microorganisms in the clinical, food, veterinary and aquaculture areas. The production of antimicrobials by P. lusitanus NL19 was identified in tryptic soy agar (TSA), but not in its equivalent broth (TSB). It was observed that in TSB medium a high concentration of casein peptone (PC) repressed the production of antibacterial compounds. HPLC, MS and MS/MS spectra with de novo sequencing revealed that the bioactivity of P. lusitanus NL19 was due to the production of pedopeptins. Hence, biosynthesis of pedopeptins is inhibited by high concentrations of PC in the broth medium. Furthermore, a nonribosomal peptide synthetase (NRPS) gene cluster was identified in the genome of NL19 encoding the biosynthesis of the peptides. qPCR analysis confirmed that the transcription of these genes is repressed in cells cultivated in high concentrations of PC. It is shown that pedopeptins are nonribosomal peptides with a broad-spectrum activity, including against Gram-positive and Gram-negative bacteria and yeasts.

Precursor-Guided Mining of Marine Sponge Metabolomes Lends Insight into Biosynthesis of Pyrrole-Imidazole Alkaloids

Pyrrole-imidazole alkaloids are natural products isolated from marine sponges, holobiont metazoans that are associated with symbiotic microbiomes. Pyrrole-imidazole alkaloids have attracted attention due to their chemical complexity and their favorable pharmacological properties. However, insights into how these molecules are biosynthesized within the sponge holobionts are scarce. Here, we provide a multiomic profiling of the microbiome and metabolomic architectures of three sponge genera that are prolific producers of pyrrole-imidazole alkaloids. Using a retrobiosynthetic scheme as a guide, we mine the metabolomes of these sponges to detect intermediates in pyrrole-imidazole alkaloid biosynthesis. Our findings reveal that the nonproteinogenic amino acid homoarginine is a critical branch point that connects primary metabolite lysine to the production of pyrrole-imidazole alkaloids. These insights are derived from the polar metabolomes of these sponges which additionally reveal the presence of zwitterionic betaines that may serve important ecological roles in marine habitats. We also establish that metabolomic richness does not correlate with microbial diversity of the sponge holobiont for neither the polar nor the nonpolar metabolomes. Our findings now provide the biochemical foundation for genomic interrogation of the sponge holobiont to establish biogenetic routes for pyrrole-imidazole alkaloid production.

Structural Characterization of Complex of Adenylation Domain and Carrier Protein by Using Pantetheine Cross-Linking Probe

Adenylation domains (A-domains) are responsible for selective incorporation of carboxylic acid substrates in the biosynthesis of various natural products. Each A-domain must recognize a cognate carrier protein (CP) for functional substrate transfer. The transient interactions between an A-domain and CP have been investigated by using acyl vinylsulfonamide adenosine inhibitors as probes to determine the structures of several A-domain-CP complexes. However, this strategy requires a specific vinylsulfonamide inhibitor that contains an acyl group corresponding to the substrate specificity of a target A-domain in every case. Here, we report an alternative strategy for structural characterization of A-domain-CP complexes. We used a bromoacetamide pantetheine cross-linking probe in combination with a Cys mutation to trap the standalone A-domain-CP complex involved in macrolactam polyketide biosynthesis through a covalent linkage, allowing the determination of the complex structure. This strategy facilitates the structural determination of A-domain-CP complexes.

Repurposing Modular Polyketide Synthases and Non-ribosomal Peptide Synthetases for Novel Chemical Biosynthesis

In nature, various enzymes govern diverse biochemical reactions through their specific three-dimensional structures, which have been harnessed to produce many useful bioactive compounds including clinical agents and commodity chemicals. Polyketide synthases (PKSs) and non-ribosomal peptide synthetases (NRPSs) are particularly unique multifunctional enzymes that display modular organization. Individual modules incorporate their own specific substrates and collaborate to assemble complex polyketides or non-ribosomal polypeptides in a linear fashion. Due to the modular properties of PKSs and NRPSs, they have been attractive rational engineering targets for novel chemical production through the predictable modification of each moiety of the complex chemical through engineering of the cognate module. Thus, individual reactions of each module could be separated as a retro-biosynthetic biopart and repurposed to new biosynthetic pathways for the production of biofuels or commodity chemicals. Despite these potentials, repurposing attempts have often failed owing to impaired catalytic activity or the production of unintended products due to incompatible protein–protein interactions between the modules and structural perturbation of the enzyme. Recent advances in the structural, computational, and synthetic tools provide more opportunities for successful repurposing. In this review, we focused on the representative strategies and examples for the repurposing of modular PKSs and NRPSs, along with their advantages and current limitations. Thereafter, synthetic biology tools and perspectives were suggested for potential further advancement, including the rational and large-scale high-throughput approaches. Ultimately, the potential diverse reactions from modular PKSs and NRPSs would be leveraged to expand the reservoir of useful chemicals.

Revised Structure of Anthelvencin A and Characterization of the Anthelvencin Biosynthetic Gene Cluster

Anthelvencins A and B are pyrrolamide metabolites produced by Streptomyces venezuelae ATCC 14583 and 14585. Isolated in 1965, they were reported to exhibit anthelmintic and moderate antibacterial activities. In this study, we revise the structure of anthelvencin A and identify a third anthelvencin metabolite, bearing two N-methylated pyrrole groups, which we named anthelvencin C. We sequenced the genome of S. venezuelae ATCC 14583 and identified a gene cluster predicted to direct the biosynthesis of anthelvencins. Functional analysis of this gene cluster confirmed its involvement in anthelvencin biosynthesis and allowed us to propose a biosynthetic pathway for anthelvencins. In addition to a nonribosomal peptide synthetase (NRPS), the assembly of anthelvencins involves an enzyme from the ATP-grasp ligase family, Ant23. We propose that Ant23 uses a PCP-loaded 4-aminopyrrole-2-carboxylate as substrate. As observed for the biosynthesis of the other pyrrolamides congocidine (produced by Streptomyces ambofaciens ATCC 25877) and distamycin (produced by Streptomyces netropsis DSM 40846), the NRPS assembling anthelvencins is composed of stand-alone domains only. Such NRPSs, sometimes called type II NRPSs, are less studied than the classical multimodular NRPSs. Yet, they constitute an interesting model to study protein-protein interactions in NRPSs and are good candidates for combinatorial biosynthesis approaches.

Dynamic visualization of type II peptidyl carrier protein recognition in pyoluteorin biosynthesis

Using a covalent chemical probe and X-ray crystallography coupled to nuclear magnetic resonance data, we elucidated the dynamic molecular basis of protein recognition between the carrier protein and adenylation domain in pyoluteorin biosynthesis. These findings reveal a unique binding mode, which contrasts previously solved carrier protein and partner protein interfaces.

The interface interactions of a type II peptidyl carrier protein and partner enzyme are observed to be unique and dynamic.