F-O-G Ring Formation in Glycopeptide Antibiotic Biosynthesis is Catalysed by OxyE

Biocatalytic role of cytochrome P450s to produce antibiotics: A review

Cytochrome P450s belong to a family of heme-binding monooxygenases, which catalyze regio- and stereospecific functionalisation of C-H, C-C, and C-N bonds, including heteroatom oxidation, oxidative C-C bond cleavages, and nitrene transfer. P450s are considered useful biocatalysts for the production of pharmaceutical products, fine chemicals, and bioremediating agents. Despite having tremendous biotechnological potential, being heme-monooxygenases, P450s require either autologous or heterologous redox partner(s) to perform chemical transformations. Randomly distributed P450s throughout a bacterial genome and devoid of particular redox partners in natural products biosynthetic gene clusters (BGCs) showed an extra challenge to reveal their pharmaceutical potential. However, continuous efforts have been made to understand their involvement in antibiotic biosynthesis and their modification, and this review focused on such BGCs. Here, particularly, we have discussed the role of P450s involved in the production of macrolides and aminocoumarin antibiotics, nonribosomal peptide (NRPSs) antibiotics, ribosomally synthesized and post-translationally modified peptide (RiPPs) antibiotics, and others. Several reactions catalyzed by P450s, as well as the role of their redox partners involved in the BGCs of various antibiotics and their derivatives, have been primarily addressed in this review, which would be useful in further exploration of P450s for the biosynthesis of new therapeutics.

Exploring the Flexibility of the Glycopeptide Antibiotic Crosslinking Cascade for Extended Peptide Backbones

The glycopeptide antibiotics (GPAs) are a clinically approved class of antimicrobial agents that classically function through the inhibition of bacterial cell-wall biosynthesis by sequestration of the precursor lipid II. The oxidative crosslinking of the core peptide by cytochrome P450 (Oxy) enzymes during GPA biosynthesis is both essential to their function and the source of their synthetic challenge. Thus, understanding the activity and selectivity of these Oxy enzymes is of key importance for the future engineering of this important compound class. Recent reports of GPAs that display an alternative mode of action and a wider range of core peptide structures compared to classic lipid II-binding GPAs raises the question of the tolerance of Oxy enzymes for larger changes in their peptide substrates. In this work, we explore the ability of Oxy enzymes from the biosynthesis pathways of lipid II-binding GPAs to accept altered peptide substrates based on a vancomycin template. Our results show that Oxy enzymes are more tolerant of changes at the N terminus of their substrates, whilst C-terminal extension of the peptide substrates is deleterious to the activity of all Oxy enzymes. Thus, future studies should prioritise the study of Oxy enzymes from atypical GPA biosynthesis pathways bearing C-terminal peptide extension to increase the substrate scope of these important cyclisation enzymes.

Epoxinnamide: An Epoxy Cinnamoyl-Containing Nonribosomal Peptide from an Intertidal Mudflat-Derived Streptomyces sp

Cinnamoyl-containing nonribosomal peptides (CCNPs) form a unique family of actinobacterial secondary metabolites and display various biological activities. A new CCNP named epoxinnamide (1) was discovered from intertidal mudflat-derived Streptomyces sp. OID44. The structure of 1 was determined by the analysis of one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR) data along with a mass spectrum. The absolute configuration of 1 was assigned by the combination of advanced Marfey’s method, ³J_HH and rotating-frame overhauser effect spectroscopy (ROESY) analysis, DP4 calculation, and genomic analysis. The putative biosynthetic pathway of epoxinnamide (1) was identified through the whole-genome sequencing of Streptomyces sp. OID44. In particular, the thioesterase domain in the nonribosomal peptide synthetase (NRPS) biosynthetic gene cluster was proposed as a bifunctional enzyme, which catalyzes both epimerization and macrocyclization. Epoxinnamide (1) induced quinone reductase (QR) activity in murine Hepa-1c1c7 cells by 1.6-fold at 5 μM. It also exhibited effective antiangiogenesis activity in human umbilical vein endothelial cells (IC₅₀ = 13.4 μM).

Nyuzenamide C, an Antiangiogenic Epoxy Cinnamic Acid-Containing Bicyclic Peptide from a Riverine Streptomyces sp

A new nonribosomal peptide, nyuzenamide C (1), was discovered from riverine sediment-derived Streptomyces sp. DM14. Comprehensive analysis of the spectroscopic data of nyuzenamide C (1) revealed that 1 has a bicyclic backbone composed of six common amino acid residues (Asn, Leu, Pro, Gly, Val, and Thr) and four nonproteinogenic amino acid units, including hydroxyglycine, β-hydroxyphenylalanine, p-hydroxyphenylglycine, and 3,β-dihydroxytyrosine, along with 1,2-epoxypropyl cinnamic acid. The absolute configuration of 1 was proposed by J-based configuration analysis, the advanced Marfey's method, quantum mechanics-based DP4 calculations, and bioinformatic analysis of its nonribosomal peptide synthetase biosynthetic gene cluster. Nyuzenamide C (1) displayed antiangiogenic activity in human umbilical vein endothelial cells and induced quinone reductase in murine Hepa-1c1c7 cells.

The Cytochrome P450 OxyA from the Kistamicin Biosynthesis Cyclization Cascade is Highly Sensitive to Oxidative Damage

Cytochrome P450 enzymes (P450s) are a superfamily of monooxygenases that utilize a cysteine thiolate–ligated heme moiety to perform a wide range of demanding oxidative transformations. Given the oxidative power of the active intermediate formed within P450s during their active cycle, it is remarkable that these enzymes can avoid auto-oxidation and retain the axial cysteine ligand in the deprotonated—and thus highly acidic—thiolate form. While little is known about the process of heme incorporation during P450 folding, there is an overwhelming preference for one heme orientation within the P450 active site. Indeed, very few structures to date contain an alternate heme orientation, of which two are OxyA homologs from glycopeptide antibiotic (GPA) biosynthesis. Given the apparent preference for the unusual heme orientation shown by OxyA enzymes, we investigated the OxyA homolog from kistamicin biosynthesis (OxyA_kis), which is an atypical GPA. We determined that OxyA_kis is highly sensitive to oxidative damage by peroxide, with both UV and EPR measurements showing rapid bleaching of the heme signal. We determined the structure of OxyA_kis and found a mixed population of heme orientations present in this enzyme. Our analysis further revealed the possible modification of the heme moiety, which was only present in samples where the alternate heme orientation was present in the protein. These results suggest that the typical heme orientation in cytochrome P450s can help prevent potential damage to the heme—and hence deactivation of the enzyme—during P450 catalysis. It also suggests that some P450 enzymes involved in GPA biosynthesis may be especially prone to oxidative damage due to the heme orientation found in their active sites.

Nonribosomal Peptide Synthesis Definitely Working Out of the Rules

Nonribosomal peptides are microbial secondary metabolites exhibiting a tremendous structural diversity and a broad range of biological activities useful in the medical and agro-ecological fields. They are built up by huge multimodular enzymes called nonribosomal peptide synthetases. These synthetases are organized in modules constituted of adenylation, thiolation, and condensation core domains. As such, each module governs, according to the collinearity rule, the incorporation of a monomer within the growing peptide. The release of the peptide from the assembly chain is finally performed by a terminal core thioesterase domain. Secondary domains with modifying catalytic activities such as epimerization or methylation are sometimes included in the assembly lines as supplementary domains. This assembly line structure is analyzed by bioinformatics tools to predict the sequence and structure of the final peptides according to the sequence of the corresponding synthetases. However, a constantly expanding literature unravels new examples of nonribosomal synthetases exhibiting very rare domains and noncanonical organizations of domains and modules, leading to several amazing strategies developed by microorganisms to synthesize nonribosomal peptides. In this review, through several examples, we aim at highlighting these noncanonical pathways in order for the readers to perceive their complexity.

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.

Understanding the Glycopeptide Antibiotic Crosslinking Cascade: In Vitro Approaches Reveal the Details of a Complex Biosynthesis Pathway

The glycopeptide antibiotics (GPAs) are a fascinating example of complex natural product biosynthesis, with the nonribosomal synthesis of the peptide core coupled to a cytochrome P450-mediated cyclisation cascade that crosslinks aromatic side chains within this peptide. Given that the challenges associated with the synthesis of GPAs stems from their highly crosslinked structure, there is great interest in understanding how biosynthesis accomplishes this challenging set of transformations. In this regard, the use of in vitro experiments has delivered important insights into this process, including the identification of the unique role of the X-domain as a platform for P450 recruitment. In this minireview, we present an analysis of the results of in vitro studies into the GPA cyclisation cascade that have demonstrated both the tolerances and limitations of this process for modified substrates, and in turn developed rules for the future reengineering of this important antibiotic class.

Understanding the early stages of peptide formation during the biosynthesis of teicoplanin and related glycopeptide antibiotics

The biosynthesis of the glycopeptide antibiotics (GPAs) demonstrates the exceptional ability of nonribosomal peptide (NRP) synthesis to generate diverse and complex structures from an expanded array of amino acid precursors. Whilst the heptapeptide cores of GPAs share a conserved C terminus, including the aromatic residues involved cross-linking and that are essential for the antibiotic activity of GPAs, most structural diversity is found within the N terminus of the peptide. Furthermore, the origin of the (D)-stereochemistry of residue 1 of all GPAs is currently unclear, despite its importance for antibiotic activity. Given these important features, we have now reconstituted modules (M) 1-4 of the NRP synthetase (NRPS) assembly lines that synthesise the clinically relevant type IV GPA teicoplanin and the related compound A40926. Our results show that important roles in amino acid modification during the NRPS-mediated biosynthesis of GPAs can be ascribed to the actions of condensation domains present within these modules, including the incorporation of (D)-amino acids at position 1 of the peptide. Our results also indicate that hybrid NRPS assembly lines can be generated in a facile manner by mixing NRPS proteins from different systems and that uncoupling of peptide formation due to different rates of activity seen for NRPS modules can be controlled by varying the ratio of NRPS modules. Taken together, this indicates that NRPS assembly lines function as dynamic peptide assembly lines and not static megaenzyme complexes, which has significant implications for biosynthetic redesign of these important biosynthetic systems.

Teicoplanin biosynthesis: unraveling the interplay of structural, regulatory, and resistance genes

Teicoplanin (Tcp) is a clinically relevant glycopeptide antibiotic (GPA) that is produced by the actinobacterium Actinoplanes teichomyceticus. Tcp is a front-line therapy for treating severe infections caused by multidrug-resistant Gram-positive pathogens in adults and infants. In this review, we provide a detailed overview of how Tcp is produced by A. teichomyceticus by describing Tcp biosynthesis, regulation, and resistance. We summarize the knowledge gained from in vivo and in vitro studies to provide an integrated model of teicoplanin biosynthesis. Then, we discuss genetic and nutritional factors that contribute to the regulation of teicoplanin biosynthesis, focusing on those that have been successfully applied for improving teicoplanin production. A current view on teicoplanin self-resistance mechanisms in A. teichomyceticus is given, and we compare the Tcp biosynthetic gene cluster with other glycopeptide gene clusters from actinoplanetes and from unidentified isolates/metagenomics samples. Finally, we provide an outlook for further directions in studying Tcp biosynthesis and regulation.

Structure-Activity Relationships of cyclo(l-Tyrosyl-l-tyrosine) Derivatives Binding to Mycobacterium tuberculosis CYP121: Iodinated Analogues Promote Shift to High-Spin Adduct

A series of analogues of cyclo(l-tyrosyl-l-tyrosine), the substrate of the Mycobacterium tuberculosis enzyme CYP121, have been synthesized and analyzed by UV-vis and electron paramagnetic resonance spectroscopy and by X-ray crystallography. The introduction of iodine substituents onto cyclo(l-tyrosyl-l-tyrosine) results in sub-μM binding affinity for the CYP121 enzyme and a complete shift to the high-spin state of the heme Fe^III. The introduction of halogens that are able to interact with heme groups is thus a feasible approach to the development of next-generation, tight binding inhibitors of the CYP121 enzyme, in the search for novel antitubercular compounds.

Unrivalled diversity: the many roles and reactions of bacterial cytochromes P450 in secondary metabolism

Covering: 2000 up to 2018 The cytochromes P450 (P450s) are a superfamily of heme-containing monooxygenases that perform diverse catalytic roles in many species, including bacteria. The P450 superfamily is widely known for the hydroxylation of unactivated C-H bonds, but the diversity of reactions that P450s can perform vastly exceeds this undoubtedly impressive chemical transformation. Within bacteria, P450s play important roles in many biosynthetic and biodegradative processes that span a wide range of secondary metabolite pathways and present diverse chemical transformations. In this review, we aim to provide an overview of the range of chemical transformations that P450 enzymes can catalyse within bacterial secondary metabolism, with the intention to provide an important resource to aid in understanding of the potential roles of P450 enzymes within newly identified bacterial biosynthetic pathways.

The many faces and important roles of protein-protein interactions during non-ribosomal peptide synthesis

Covering: up to July 2018 Non-ribosomal peptide synthetase (NRPS) machineries are complex, multi-domain proteins that are responsible for the biosynthesis of many important, peptide-derived compounds. By decoupling peptide synthesis from the ribosome, NRPS assembly lines are able to access a significant pool of amino acid monomers for peptide synthesis. This is combined with a modular protein architecture that allows for great variation in stereochemistry, peptide length, cyclisation state and further modifications. The architecture of NRPS assembly lines relies upon a repetitive set of catalytic domains, which are organised into modules responsible for amino acid incorporation. Central to NRPS-mediated biosynthesis is the carrier protein (CP) domain, to which all intermediates following initial monomer activation are bound during peptide synthesis up until the final handover to the thioesterase domain that cleaves the mature peptide from the NRPS. This mechanism makes understanding the protein-protein interactions that occur between different NRPS domains during peptide biosynthesis of crucial importance to understanding overall NRPS function. This endeavour is also highly challenging due to the inherent flexibility and dynamics of NRPS systems. In this review, we present the current state of understanding of the protein-protein interactions that govern NRPS-mediated biosynthesis, with a focus on insights gained from structural studies relating to CP domain interactions within these impressive peptide assembly lines.

Kistamicin biosynthesis reveals the biosynthetic requirements for production of highly crosslinked glycopeptide antibiotics

Kistamicin is a divergent member of the glycopeptide antibiotics, a structurally complex class of important, clinically relevant antibiotics often used as the last resort against resistant bacteria. The extensively crosslinked structure of these antibiotics that is essential for their activity makes their chemical synthesis highly challenging and limits their production to bacterial fermentation. Kistamicin contains three crosslinks, including an unusual 15-membered A-O-B ring, despite the presence of only two Cytochrome P450 Oxy enzymes thought to catalyse formation of such crosslinks within the biosynthetic gene cluster. In this study, we characterise the kistamicin cyclisation pathway, showing that the two Oxy enzymes are responsible for these crosslinks within kistamicin and that they function through interactions with the X-domain, unique to glycopeptide antibiotic biosynthesis. We also show that the kistamicin OxyC enzyme is a promiscuous biocatalyst, able to install multiple crosslinks into peptides containing phenolic amino acids.

Kistamicin is a structurally divergent glycopeptide antibiotic (GPA) that contains a unique 15-membered A-O-B ring. Here, the authors obtained a crystal structure of the kistamicin OxyA/X-domain complex and analysed the cyclisation cascade leading to the formation of the A-O-B ring.

Studying trans-acting enzymes that target carrier protein-bound amino acids during nonribosomal peptide synthesis

Nonribosomal peptide biosynthesis is a complex enzymatic assembly responsible for producing a great diversity of bioactive peptide natural products. Due to the recurring arrangement of catalytic domains within these machineries, great interest has been shown in reengineering these pathways to produce novel, designer peptide products. However, in order to realize such ambitions, it is first necessary to develop a comprehensive understanding of the selectivity, mechanisms, and structure of these complex enzymes, which in turn requires significant in vitro experiments. Within nonribosomal biosynthesis, some modifications are performed by enzymatic domains that are not linked to the main nonribosomal peptide synthetase but rather act in trans: these systems offer great potential for redesign, but in turn require detailed study. In this chapter, we present an overview of in vitro experiments that can be used to characterize examples of such trans-interacting enzymes from nonribosomal peptide biosynthesis: Cytochrome P450 monooxygenases and flavin-dependent halogenases.

The biosynthetic implications of late-stage condensation domain selectivity during glycopeptide antibiotic biosynthesis

Non-ribosomal peptide synthesis is a highly important biosynthetic pathway for the formation of many secondary metabolites of medical relevance. Due to the challenges associated with the chemical synthesis of many of the products of these assembly lines, understanding the activity and selectivity of non-ribosomal peptide synthetase (NRPS) machineries is an essential step towards the redesign of such machineries to produce new bioactive peptides. Whilst the selectivity of the adenylation domains responsible for amino acid activation during NRPS synthesis has been widely studied, the selectivity of the essential peptide bond forming domains - known as condensation domains - is not well understood. Here, we present the results of a combination of in vitro and in vivo investigations into the final condensation domain from the NRPS machinery that produces the glycopeptide antibiotics (GPAs). Our results show that this condensation domain is tolerant for a range of peptide substrates and even those with unnatural stereochemistry of the peptide C-terminus, which is in contrast to the widely ascribed role of these domains as a stereochemical gatekeeper during NRPS synthesis. Furthermore, we show that this condensation domain has a significant preference for linear peptide substrates over crosslinked peptides, which indicates that the GPA crosslinking cascade targets the heptapeptide bound to the final module of the NRPS machinery and reinforces the role of the unique GPA X-domain in this process. Finally, we demonstrate that the peptide bond forming activity of this condensation domain is coupled to the rate of amino acid activation performed by the subsequent adenylation domain. This is a significant result with implications for NRPS redesign, as it indicates that the rate of amino acid activation of modified adenylation domains must be maintained to prevent unwanted peptide hydrolysis from the NRPS due to a loss of the productive coupling of amino acid selection and peptide bond formation. Taken together, our results indicate that assessing condensation domain activity is a vital step in not only understanding the biosynthetic logic and timing of NRPS-mediated peptide assembly, but also the rules which redesign efforts must obey in order to successfully produce functional, modified NRPS assembly lines.

Developments in Glycopeptide Antibiotics

Glycopeptide antibiotics (GPAs) are a key weapon in the fight against drug resistant bacteria, with vancomycin still a mainstream therapy against serious Gram-positive infections more than 50 years after it was first introduced. New, more potent semisynthetic derivatives that have entered the clinic, such as dalbavancin and oritavancin, have superior pharmacokinetic and target engagement profiles that enable successful treatment of vancomycin-resistant infections. In the face of resistance development, with multidrug resistant (MDR) S. pneumoniae and methicillin-resistant Staphylococcus aureus (MRSA) together causing 20-fold more infections than all MDR Gram-negative infections combined, further improvements are desirable to ensure the Gram-positive armamentarium is adequately maintained for future generations. A range of modified glycopeptides has been generated in the past decade via total syntheses, semisynthetic modifications of natural products, or biological engineering. Several of these have undergone extensive characterization with demonstrated in vivo efficacy, good PK/PD profiles, and no reported preclinical toxicity; some may be suitable for formal preclinical development. The natural product monobactam, cephalosporin, and β-lactam antibiotics all spawned multiple generations of commercially and clinically successful semisynthetic derivatives. Similarly, next-generation glycopeptides are now technically well positioned to advance to the clinic, if sufficient funding and market support returns to antibiotic development.

Installation of Multiple Aryl Ether Crosslinks onto Non-Native Substrate Peptides by the Vancomycin OxyB

The biosynthesis of glycopeptide antibiotics (GPAs) has been an active area of research for decades. Nonetheless, insights into the activity of the cytochrome P450 enzymes required for installing the aromatic crosslinks, which form their cup-shaped topologies and render GPAs bioactive, have only recently emerged. Presently, little is known about the substrate scope and promiscuity of the P450 enzymes. Herein, we report that OxyB_van, the P450 enzyme that installs the first crosslink in vancomycin biosynthesis, is capable of catalyzing the formation of its conventional C-O-D bis-aryl ether bond in non-natural substrates and, furthermore, the formation of a second, novel linkage when D-Trp is incorporated at position 6. HR-MS/MS and isotope labeling studies indicate the second crosslink is formed between rings A and B, resulting in a novel GPA-type scaffold. OxyB is also capable of installing two crosslinks in kistamicin- and complestatin-like substrate peptides. These findings highlight the utility of OxyB_van in creating crosslinked GPA derivatives and provide clues regarding the unusual biosynthesis of kistamicin.

The Thioesterase Domain in Glycopeptide Antibiotic Biosynthesis Is Selective for Cross-Linked Aglycones

The biosynthesis of the glycopeptide antibiotics (GPAs)-which include teicoplanin and vancomycin-is a complex enzymatic process relying on the interplay of nonribosomal peptide synthesis and a cytochrome P450-mediated cyclization cascade. This unique cyclization cascade generates the highly cross-linked state of these nonribosomal peptides, which is crucial for their antimicrobial activity. Given that these essential oxidative transformations occur while the peptide remains bound to the terminal module of the nonribosomal peptide synthetase (NRPS) machinery, it is important to assess the selectivity of the terminal thioesterase (TE) domain and how this domain contributes to the maintenance of an efficient biosynthetic pathway while at the same time ensuring GPA maturation is completed. In this study, we report the in vitro characterization of the thioesterase domain from teicoplanin biosynthesis, the first GPA thioesterase to be characterized. Our results show that the activity of this TE domain relies on the presence of an unusual extended N-terminal linker region that appears to be unique to the NRPS machineries found in GPA biosynthesis. In addition, we show that the activity of this domain against carrier protein bound substrates is dramatically enhanced for mature GPA aglycones as opposed to linear peptides and partially cyclized intermediates. These results demonstrate how the interplay between NRPS and P450s during late stage GPA biosynthesis is not only maintained but also leads to the efficient production of mature GPA aglycones. Thus, GPA TE domains represent another impressive example of the ability of TE domains to act as logic gates during NRPS biosynthesis, ensuring that essential late-stage peptide modifications are completed before catalyzing the release of the mature, bioactive peptide product.

A route to diastereomerically pure phenylglycine thioester peptides: crucial intermediates for investigating glycopeptide antibiotic biosynthesis

Non-ribosomal peptides contain an array of amino acid building blocks that can present challenges for the synthesis of important intermediates. Here we report a route to incorporate phenylglycine residues in peptide thioesters without significant racemisation.

In Vitro Reconstitution of OxyA Enzymatic Activity Clarifies Late Steps in Vancomycin Biosynthesis

Studies on the biosynthesis of glycopeptide antibiotics have provided many insights into the strategies that Nature employs to build architecturally strained molecules. A key structural feature of vancomycin, the founding member of this class, is a set of three aromatic cross-links that are introduced via yet unknown mechanisms. Previous reports have identified three cytochrome P450 enzymes involved in this process and demonstrated enzymatic activity for OxyB, which installs the first aromatic cross-link. However, the activities of the remaining two P450 enzymes have not been recapitulated. Herein, we show that OxyA generates the second bis-aryl ether bond in vancomycin and that it exhibits strict substrate specificity toward the chlorinated, OxyB-cross-linked product. No OxyA product is detected with the unchlorinated substrate. Together with previous results, these data suggest that chlorination occurs after OxyB- but before OxyA-catalyzed cross-link formation. Our results have important implications for the chemo-enzymatic synthesis of vancomycin and its analogs.

Diversity of nature's assembly lines - recent discoveries in non-ribosomal peptide synthesis

The biosynthesis of complex natural products by non-ribosomal peptide synthetases (NRPSs) and the related polyketide synthases (PKSs) represents a major source of important bioactive compounds. These large, multi-domain machineries are able to produce a fascinating range of molecules due to the nature of their modular architectures, which allows natural products to be assembled and tailored in a modular, step-wise fashion. In recent years there has been significant progress in characterising the important domains and underlying mechanisms of non-ribosomal peptide synthesis. More significantly, several studies have uncovered important examples of novel activity in many NRPS domains. These discoveries not only greatly increase the structural diversity of the possible products of NRPS machineries but - possibly more importantly - they improve our understanding of what is a highly important, yet complex, biosynthetic apparatus. In this review, several recent examples of novel NRPS function will be introduced, which highlight the range of previously uncharacterised activities that have now been detected in the biosynthesis of important natural products by these mega-enzyme synthetases.

Nonomuraea sp. ATCC 55076 harbours the largest actinomycete chromosome to date and the kistamicin biosynthetic gene cluster

Glycopeptide antibiotics (GPAs) have served as potent clinical drugs and as an inspiration to chemists in various disciplines. Among known GPAs, complestatin, chloropeptin, and kistamicin are unique in that they contain an unusual indole-phenol crosslink. The mechanism of formation of this linkage is unknown, and to date, the biosynthetic gene cluster of only one GPA with an indole-phenol crosslink, that of complestatin, has been identified. Here, we report the genome sequence of the kistamicin producer Nonomuraea sp. ATCC 55076. We find that this strain harbours the largest actinobacterial chromosome to date, consisting of a single linear chromosome of ∼13.1 Mbp. AntiSMASH analysis shows that ∼32 biosynthetic gene clusters and ∼10% of the genome are devoted to production of secondary metabolites, which include 1,6-dihydroxyphenazine and nomuricin, a new anthraquinone-type pentacyclic compound that we report herein. The kistamicin gene cluster (kis) was identified bioinformatically. A unique feature of kis is that it contains two cytochrome P450 enzymes, which likely catalyze three crosslinking reactions. These findings set the stage for examining the biosynthesis of kistamicin and its unusual indole-phenol crosslink in the future.

P450 monooxygenase ComJ catalyses side chain phenolic cross-coupling during complestatin biosynthesis

P450 monooxygenase enzyme ComJ catalyzed biaryl ether bond formation with high efficiency and low stereoselectivity on selected complestatin-like peptide substrates.

SNaPe: a versatile method to generate multiplexed protein fusions using synthetic linker peptides for in vitro applications

Understanding the structure and function of protein complexes and multi-domain proteins is highly important in biology, although the in vitro characterization of these systems is often complicated by their size or the transient nature of protein/protein interactions. To assist in the characterization of such protein complexes, we have developed a modular approach to fusion protein generation that relies upon Sortase-mediated and Native chemical ligation using synthetic Peptide linkers (SNaPe) to link two separately expressed proteins. In this approach, we utilize two separate linking steps - sortase-mediated and native chemical ligation - together with a library of peptide linkers to generate libraries of fusion proteins. We have demonstrated the viability of SNaPe to generate libraries from fusion protein constructs taken from the biosynthetic enzymes responsible for late stage aglycone assembly during glycopeptide antibiotic biosynthesis. Crucially, SNaPe was able to generate fusion proteins that are inaccessible via direct expression of the fusion construct itself. This highlights the advantages of SNaPe to not only access fusion proteins that have been previously unavailable for biochemical and structural characterization but also to do so in a manner that enables the linker itself to be controlled as an experimental parameter of fusion protein generation. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.