Mutational analysis of posttranslational heterocycle biosynthesis in the gyrase inhibitor microcin B17: distance dependence from propeptide and tolerance for substitution in a GSCG cyclizable sequence

The Microbial Toxin Microcin B17: Prospects for the Development of New Antibacterial Agents

Microcin B17 (MccB17) is an antibacterial peptide produced by strains of Escherichia coli harboring the plasmid-borne mccB17 operon. MccB17 possesses many notable features. It is able to stabilize the transient DNA gyrase–DNA cleavage complex, a very efficient mode of action shared with the highly successful fluoroquinolone drugs. MccB17 stabilizes this complex by a distinct mechanism making it potentially valuable in the fight against bacterial antibiotic resistance. MccB17 was the first compound discovered from the thiazole/oxazole-modified microcins family and the linear azole-containing peptides; these ribosomal peptides are post-translationally modified to convert serine and cysteine residues into oxazole and thiazole rings. These chemical moieties are found in many other bioactive compounds like the vitamin thiamine, the anti-cancer drug bleomycin, the antibacterial sulfathiazole and the antiviral nitazoxanide. Therefore, the biosynthetic machinery that produces these azole rings is noteworthy as a general method to create bioactive compounds. Our knowledge of MccB17 now extends to many aspects of antibacterial–bacteria interactions: production, transport, interaction with its target, and resistance mechanisms; this knowledge has wide potential applicability. After a long time with limited progress on MccB17, recent publications have addressed critical aspects of MccB17 biosynthesis as well as an explosion in the discovery of new related compounds in the thiazole/oxazole-modified microcins/linear azole-containing peptides family. It is therefore timely to summarize the evidence gathered over more than 40 years about this still enigmatic molecule and place it in the wider context of antibacterials.

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Microcin B17 (MccB17) is a microbial toxin with a unique mode of action.

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MccB17 stabilizes the gyrase–DNA cleavage complex; it is a potential substitute for fluoroquinolones.

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The structures of microcin synthase and TldD/E have given key insight into its biosynthesis.

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A variety of modified McB17s have been generated and characterized.

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MccB17 has been implicated in IBD.

Thiopeptide Pyridine Synthase TbtD Catalyzes an Intermolecular Formal Aza-Diels-Alder Reaction

Thiopeptide pyridine synthases catalyze a multistep reaction involving a unique and nonspontaneous intramolecular aza-[4 + 2] cycloaddition between two dehydroalanines to forge a trisubstituted pyridine core. We discovered that the in vitro activity of pyridine synthases from the thiocillin and thiomuracin pathways are significantly enhanced by general base catalysis and that this broadly expands the enzymes substrate tolerance. Remarkably, TbtD is competent to perform an intermolecular cyclization in addition to its cognate intramolecular reaction, underscoring its versatility as a biocatalyst. These data provide evidence that pyridine synthases use a two-site substrate recognition model to engage and process their substrates.

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

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

Cyclisation mechanisms in the biosynthesis of ribosomally synthesised and post-translationally modified peptides

Ribosomally synthesised and post-translationally modified peptides (RiPPs) are a large class of natural products that are remarkably chemically diverse given an intrinsic requirement to be assembled from proteinogenic amino acids. The vast chemical space occupied by RiPPs means that they possess a wide variety of biological activities, and the class includes antibiotics, co-factors, signalling molecules, anticancer and anti-HIV compounds, and toxins. A considerable amount of RiPP chemical diversity is generated from cyclisation reactions, and the current mechanistic understanding of these reactions will be discussed here. These cyclisations involve a diverse array of chemical reactions, including 1,4-nucleophilic additions, [4 + 2] cycloadditions, ATP-dependent heterocyclisation to form thiazolines or oxazolines, and radical-mediated reactions between unactivated carbons. Future prospects for RiPP pathway discovery and characterisation will also be highlighted.

One-pot synthesis of azoline-containing peptides in a cell-free translation system integrated with a posttranslational cyclodehydratase

Azoline moieties in the backbones of peptidic natural products are important structural motifs that contribute to diverse bioactivities. Some azoline-containing peptides (Az-peptides) are produced from ribosomally synthesized precursor peptides, in which cysteine, serine, and threonine residues are converted to their corresponding azolines by posttranslational modification through a cyclodehydratase. We have devised an in vitro biosynthesis system of Az-peptides, referred to as the FIT-PatD (flexible in vitro translation) system, by the integration of a cell-free translation system with the posttranslational cyclodehydratase PatD. This system enabled the "one-pot" synthesis of a wide variety of Az-peptide derivatives expressed from synthetic DNA templates. The FIT-PatD system also facilitated mutagenesis studies on a wide array of precursor peptide sequences, unveiling unique in vitro substrate tolerance of PatD.

Engineering unnatural variants of plantazolicin through codon reprogramming

Plantazolicin (PZN) is a polyheterocyclic natural product derived from a ribosomal peptide that harbors remarkable antibiotic selectivity for the causative agent of anthrax, Bacillus anthracis. To simultaneously establish the structure-activity relationship of PZN and the substrate tolerance of the biosynthetic pathway, an Escherichia coli expression strain was engineered to heterologously produce PZN analogues. Variant PZN precursor genes were produced by site-directed mutagenesis and later screened by mass spectrometry to assess post-translational modification and export by E. coli. From a screen of 72 precursor peptides, 29 PZN variants were detected. This analogue collection provided insight into the selectivity of the post-translational modifying enzymes and established the boundaries of the natural biosynthetic pathway. Unlike other studied thiazole/oxazole-modified microcins, the biosynthetic machinery appeared to be finely tuned toward the production of PZN, such that the cognate enzymes did not process even other naturally occurring sequences from similar biosynthetic clusters. The modifying enzymes were exquisitely selective, installing heterocycles only at predefined positions within the precursor peptides while leaving neighboring residues unmodified. Nearly all substitutions at positions normally harboring heterocycles prevented maturation of a PZN variant, though some exceptions were successfully produced lacking a heterocycle at the penultimate residue. No variants containing additional heterocycles were detected, although several peptide sequences yielded multiple PZN variants as a result of varying oxidation states of select residues. Eleven PZN variants were produced in sufficient quantity to facilitate purification and assessment of their antibacterial activity, providing insight into the structure-activity relationship of PZN.

Structure of microcin B-like compounds produced by Pseudomonas syringae and species specificity of their antibacterial action

Escherichia coli microcin B (Ec-McB) is a posttranslationally modified antibacterial peptide containing multiple oxazole and thiazole heterocycles and targeting the DNA gyrase. We have found operons homologous to the Ec-McB biosynthesis-immunity operon mcb in recently sequenced genomes of several pathovars of the plant pathogen Pseudomonas syringae, and we produced two variants of P. syringae microcin B (Ps-McB) in E. coli by heterologous expression. Like Ec-McB, both versions of Ps-McB target the DNA gyrase, but unlike Ec-McB, they are active against various species of the Pseudomonas genus, including human pathogen P. aeruginosa. Through analysis of Ec-McB/Ps-McB chimeras, we demonstrate that three centrally located unmodified amino acids of Ps-McB are sufficient to determine activity against Pseudomonas, likely by allowing specific recognition by a transport system that remains to be identified. The results open the way for construction of McB-based antibacterial molecules with extended spectra of biological activity.

Ribosomally synthesized and post-translationally modified peptide natural products: new insights into the role of leader and core peptides during biosynthesis

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a major class of natural products with a high degree of structural diversity and a wide variety of bioactivities. Understanding the biosynthetic machinery of these RiPPs will benefit the discovery and development of new molecules with potential pharmaceutical applications. In this Concept article, we discuss the features of the biosynthetic pathways to different RiPP classes, and propose mechanisms regarding recognition of the precursor peptide by the post-translational modification enzymes. We propose that the leader peptides function as allosteric regulators that bind the active form of the biosynthetic enzymes in a conformational selection process. We also speculate how enzymes that generate polycyclic products of defined topologies may have been selected for during evolution.

Fragments of the bacterial toxin microcin B17 as gyrase poisons

Fluoroquinolones are very important drugs in the clinical antibacterial arsenal; their success is principally due to their mode of action: the stabilisation of a gyrase-DNA intermediate (the cleavage complex), which triggers a chain of events leading to cell death. Microcin B17 (MccB17) is a modified peptide bacterial toxin that acts by a similar mode of action, but is unfortunately unsuitable as a therapeutic drug. However, its structure and mechanism could inspire the design of new antibacterial compounds that are needed to circumvent the rise in bacterial resistance to current antibiotics. Here we describe the investigation of the structural features responsible for MccB17 activity and the identification of fragments of the toxin that retain the ability to stabilise the cleavage complex.

YcaO domains use ATP to activate amide backbones during peptide cyclodehydrations

Thiazole/oxazole-modified microcins (TOMMs) encompass a recently defined class of ribosomally synthesized natural products with a diverse set of biological activities. Although TOMM biosynthesis has been investigated for over a decade, the mechanism of heterocycle formation by the synthetase enzymes remains poorly understood. Using substrate analogs and isotopic labeling, we demonstrate that ATP is used to directly phosphorylate the peptide amide backbone during TOMM heterocycle formation. Moreover, we present what is to our knowledge the first experimental evidence that the D-protein component of the heterocycle-forming synthetase (YcaO/domain of unknown function 181 family member), formerly annotated as a docking protein involved in complex formation and regulation, is able to perform the ATP-dependent cyclodehydration reaction in the absence of the other TOMM biosynthetic proteins. Together, these data reveal the role of ATP in the biosynthesis of azole and azoline heterocycles in ribosomal natural products and prompt a reclassification of the enzymes involved in their installation.

Three ring posttranslational circuses: insertion of oxazoles, thiazoles, and pyridines into protein-derived frameworks

Nitrogen heterocycles are the key functional and structural elements in both RNA and DNA, in half a dozen of the most important coenzymes, and in many synthetic drug scaffolds. On the other hand, only 3 of 20 proteinogenic amino acids have nitrogen heterocycles: proline, histidine, and tryptophan. This inventory can be augmented in microbial proteins by posttranslational modifications downstream of leader peptide regions that convert up to 10 serine, threonine, and cysteine residues, side chains and peptide backbones, into oxazoles, thiazoles, and pyridine rings. Subsequent proteolysis releases these heterocyclic scaffolds in both linear and macrocyclic frameworks as bioactive small molecules.

Discovery, biosynthesis, and engineering of lantipeptides

Aided by genome-mining strategies, knowledge of the prevalence and diversity of ribosomally synthesized natural products (RNPs) is rapidly increasing. Among these are the lantipeptides, posttranslationally modified peptides containing characteristic thioether cross-links imperative for bioactivity and stability. Though this family was once thought to be a limited class of antimicrobial compounds produced by gram-positive bacteria, new insights have revealed a much larger diversity of activity, structure, biosynthetic machinery, and producing organisms than previously appreciated. Detailed investigation of the enzymes responsible for installing the posttranslational modifications has resulted in improved in vivo and in vitro engineering systems focusing on enhancement of the therapeutic potential of these compounds. Although dozens of new lantipeptides have been isolated in recent years, bioinformatic analyses indicate that many hundreds more await discovery owing to the widespread frequency of lantipeptide biosynthetic machinery in bacterial genomes.

Selectivity, directionality, and promiscuity in peptide processing from a Bacillus sp. Al Hakam cyclodehydratase

The thiazole/oxazole-modified microcins (TOMMs) represent a burgeoning class of ribosomal natural products decorated with thiazoles and (methyl)oxazoles originating from cysteines, serines, and threonines. The ribosomal nature of TOMMs allows for the generation of derivative products from mutations in the amino acid sequence of the precursor peptide, which ultimately manifest in differing structures and, sometimes, biological functions. Employing a TOMM system for the purpose of creating new structures and functions via combinatorial biosynthesis requires processing machinery that can tolerate highly variable substrates. In this study, TOMM enzymatic promiscuity was assessed using a currently uncharacterized cluster in Bacillus sp. Al Hakam. As determined by Fourier transform tandem mass spectrometry (FT-MS/MS), azole rings were formed in both a regio- and chemoselective fashion. Cognate and noncognate precursor peptides were modified in an overall C- to N-terminal directionality, which to date is unique among characterized ribosomal natural products. Studies focused on the inherent promiscuity of the biosynthetic machinery elucidated a modest bias for glycine at the preceding (-1) position and a remarkable flexibility in the following (+1) position, even allowing for the incorporation of charged amino acids and bisheterocyclization. Two unnatural substrates were utilized as the conclusive test of substrate flexibility, of which both were processed in a predictable fashion. A greater understanding of substrate processing and enzymatic tolerance toward unnatural substrates will prove beneficial when designing combinatorial libraries to screen for artificial TOMMs that exhibit desired activities.

Linking chemistry and genetics in the growing cyanobactin natural products family

Ribosomal peptide natural products are ubiquitous, yet relatively few tools exist to predict structures and clone new pathways. Cyanobactin ribosomal peptides are found in ~30% of all cyanobacteria, but the connection between gene sequence and structure was not defined, limiting the rapid identification of new compounds and pathways. Here, we report discovery of four orphan cyanobactin gene clusters by genome mining and an additional pathway by targeted cloning, which represented a tyrosine O-prenylating biosynthetic pathway. Genome mining enabled discovery of five cyanobactins, including peptide natural products from Spirulina supplements. A phylogenetic model defined four cyanobactin genotypes, which explain the synthesis of multiple cyanobactin structural classes and help direct pathway cloning and structure prediction efforts. These strategies were applied to DNA isolated from a mixed cyanobacterial bloom containing cyanobactins.

Thiazole/oxazole-modified microcins: complex natural products from ribosomal templates

With billions of years of evolution under its belt, Nature has been expanding and optimizing its biosynthetic capabilities. Chemically complex secondary metabolites continue to challenge and inspire today's most talented synthetic chemists. A brief glance at these natural products, especially the substantial structural variation within a class of compounds, clearly demonstrates that Nature has long played the role of medicinal chemist. The recent explosion in genome sequencing has expanded our appreciation of natural product space and the vastness of uncharted territory that remains. One small corner of natural product chemical space is occupied by the recently dubbed thiazole/oxazole-modified microcins (TOMMs), which are ribosomally produced peptides with posttranslationally installed heterocycles derived from cysteine, serine and threonine residues. As with other classes of natural products, the genetic capacity to synthesize TOMMs has been widely disseminated among bacteria. Over the evolutionary timescale, Nature has tested countless random mutations and selected for gain of function in TOMM biosynthetic gene clusters, yielding several privileged molecular scaffolds. Today, this burgeoning class of natural products encompasses a structurally and functionally diverse set of molecules (i.e. microcin B17, cyanobactins, and thiopeptides). TOMMs presumably provide their producers with an ecological advantage. This advantage can include chemical weapons wielded in the battle for nutrients, disease-promoting virulence factors, or compounds presumably beneficial for symbiosis. Despite this plethora of functions, many TOMMs await experimental interrogation. This review will focus on the biosynthesis and natural combinatorial diversity of the TOMM family.

Total synthesis of microcin B17 via a fragment condensation approach

The total synthesis of the 43 amino acid antibacterial peptide Microcin B17 (MccB17) is described. The natural product was synthesized via a convergent approach from a heterocycle-derived peptide and peptide thioester fragments prepared via Fmoc-strategy solid phase peptide synthesis (SPPS). Final assembly was achieved in an efficient manner using two Ag(I)-assisted peptide ligation reactions to afford MccB17 in excellent overall yield.

Marine molecular machines: heterocyclization in cyanobactin biosynthesis

Natural products that contain amino-acid-derived (Cys, Ser, Thr) heterocycles are ubiquitous in nature, yet key aspects of their biosynthesis remain undefined. Cyanobactins are heterocyclic ribosomal peptide natural products from cyanobacteria, including symbiotic bacteria living with marine ascidians. In contrast to other ribosomal peptide heterocyclases that have been studied, the cyanobactin heterocyclase is a single protein that does not require an oxidase enzyme. Using this simplifying condition, we provide new evidence to support the hypothesis that these enzymes are molecular machines that use ATP in a product binding or orientation cycle. Further, we show that both protease inhibitors and ATP analogues inhibit heterocyclization and define the order of biochemical steps in the cyanobactin biosynthetic pathway. The cyanobactin pathway enzymes, PatD and TruD, are thiazoline and oxazoline synthetases.

Follow the leader: the use of leader peptides to guide natural product biosynthesis

The avalanche of genomic information in the past decade has revealed that natural product biosynthesis using the ribosomal machinery is much more widespread than originally anticipated. Nearly all of these compounds are crafted through post-translational modifications of a larger precursor peptide that often contains the marching orders for the biosynthetic enzymes. We review here the available information for how the peptide sequences in the precursors govern the post-translational tailoring processes for several classes of natural products. In addition, we highlight the great potential these leader peptide-directed biosynthetic systems offer for engineering conformationally restrained and pharmacophore-rich products with structural diversity that greatly expands the proteinogenic repertoire.

Ribosomal peptide natural products: bridging the ribosomal and nonribosomal worlds

Ribosomally synthesized bacterial natural products rival the nonribosomal peptides in their structural and functional diversity. The last decade has seen substantial progress in the identification and characterization of biosynthetic pathways leading to ribosomal peptide natural products with new and unusual structural motifs. In some of these cases, the motifs are similar to those found in nonribosomal peptides, and many are constructed by convergent or even paralogous enzymes. Here, we summarize the major structural and biosynthetic categories of ribosomally synthesized bacterial natural products and, where applicable, compare them to their homologs from nonribosomal biosynthesis.

Discovery of a widely distributed toxin biosynthetic gene cluster

Bacteriocins represent a large family of ribosomally produced peptide antibiotics. Here we describe the discovery of a widely conserved biosynthetic gene cluster for the synthesis of thiazole and oxazole heterocycles on ribosomally produced peptides. These clusters encode a toxin precursor and all necessary proteins for toxin maturation and export. Using the toxin precursor peptide and heterocycle-forming synthetase proteins from the human pathogen Streptococcus pyogenes, we demonstrate the in vitro reconstitution of streptolysin S activity. We provide evidence that the synthetase enzymes, as predicted from our bioinformatics analysis, introduce heterocycles onto precursor peptides, thereby providing molecular insight into the chemical structure of streptolysin S. Furthermore, our studies reveal that the synthetase exhibits relaxed substrate specificity and modifies toxin precursors from both related and distant species. Given our findings, it is likely that the discovery of similar peptidic toxins will rapidly expand to existing and emerging genomes.

Lantibiotics and microcins: polypeptides with unusual chemical diversity

Bacterial-derived antimicrobial polypeptides enjoy a large degree of structural and chemical diversity. Two well-studied examples of such polypeptides are the lanthionine-containing lantibiotics produced by a variety of Gram-positive bacteria, and their Gram-negative counterparts, the microcins. Both groups are produced as gene-encoded precursor peptides and undergo post-translational modification to generate the active moieties. Structure elucidation of novel lantibiotics and microcins has recently uncovered further novel structural and chemical features and, combined with the generation of analogue peptides by genetic manipulation, new insights into structure-function relationships have been gained. Furthermore, study of the mode of action of the lantibiotics nisin and mersacidin has revealed their use of a 'docking molecule' in the target cell to facilitate their biological activities. Meanwhile, in vitro studies with microcin B17 have helped to uncover the molecular mechanisms by which post-translational modification results in the formation of heterocyclic oxazole and thiazole rings. From a practical standpoint, both groups of polypeptides represent new lead structures for future development of antimicrobial agents, whilst the identification of the 'docking molecules' represents a step forward in the search for novel targets for future antibiosis.

Computational analysis of the first biheterocyclization site of the antibiotic microcin B17

Microcin B 17 (MccB17) undergoes an enzyme catalyzed posttranslational modification to form four oxazole and four thiazole rings. Four of these rings form 4,2 - connected biheterocyclic functionalities. In this study, the hexapeptide sequence surrounding the first biheterocyclization site of microcin B17 was examined using computational calculations and database analysis to see if it was preorganized for cyclization in a manner similar to that found in the autocatalytic posttranslational cyclization of Green Fluorescent Protein (GFP). Attention was focused on the intermolecular distances between the sulfur and oxygen atoms of the cysteine and serine residues and the carbonyl carbons which they attack in the ring formation. Conformational searches located some low energy conformations that contained relatively short oxygen to carbonyl carbon distances, which indicated that the oxazole forming fragment in microcin B17 is preorganized for cyclization. However, the lack of any clear patterns for the sulfur to carbon distances show that the side-chain of cysteine does not adopt any low energy conformations that are geometrically preorganized for cyclization. The MccB17 synthetase enzyme complex which catalyzes the cyclization process therefore has both steric and electronic functions. The data obtained in this investigation is in agreement with empirical data which shows that biheterocyclization will only occur if the thiazole forms before the oxazole.

In vivo processing and antibiotic activity of microcin B17 analogs with varying ring content and altered bisheterocyclic sites

BACKGROUND

The Escherichia coli peptide antibiotic microcin B17 (MccB17) contains four oxazole and four thiazole rings, and inhibits DNA gyrase. The role of individual and tandem pairs of heterocycles in bioactivity has not been determined previously.

RESULTS

The two tandem 4,2-bisheterocycles in MccB17 were varied by expression of MccB17 or mutants containing altered sequences at Gly39-Ser40-Cys41 or Gly54-Cys55-Ser56. A mixture of five-nine-ring MccB17 isoforms were separated and quantitated for antibiotic potency. Mutagenesis of the thiazole-oxazole pair significantly affected antibiotic activity compared with the upstream oxazole-thiazole, which might stabilize partially cyclized intermediates against proteolysis.

CONCLUSIONS

Enzymatic heterocyclization in native MccB17 occurs distributively. Antibiotic activity correlates with the number of rings and is differentially sensitive to both the location and the identity of the 4,2-tandem heterocycle pairs in MccB17. Such tandem heterocycles might be useful pharmacophores in combinatorial libraries.

The cyclic structure of microcin J25, a 21-residue peptide antibiotic from Escherichia coli

Microcin J25 (MccJ25) is the single representative of the immunity group J of the microcin group of peptide antibiotics produced by Enterobacteriaceae. It induces bacterial filamentation in susceptible cells in a non-SOS-dependent pathway [R. A. Salomon and R. Farias (1992) J. Bacteriol. 174, 7428-7435]. MccJ25 was purified to homogeneity from the growth medium of a microcin-overproducing Escherichia coli strain by reverse-phase HPLC. Based on amino acid composition and absolute configuration determination, liquid secondary ion and electrospray mass spectrometry, extensive two-dimensional NMR, enzymatic and chemical degradations studies, the structure of MccJ25 was elucidated as a 21-residue peptide, cyclo(-Val1-Gly-Ile-Gly-Thr- Pro-Ile-Ser-Phe-Tyr-Gly-Gly-Gly-Ala-Gly-His-Val-Pro-Glu-Tyr-Phe21- ). Although MccJ25 showed high resistance to most of endoproteases, linearization by thermolysin occurred from cleavage at the Phe21-Val1 bond and led to a single peptide, MccJ25-L. While MccJ25 exhibited remarkable antibiotic activity towards Salmonella newport and several E. coli strains (minimal inhibitory concentrations ranging between 0.01 and 0.2 microgram.mL-1), the thermolysin-linearized microcin showed a dramatic decrease of the activity, indicating that the cyclic structure is essential for the MccJ25 biological properties. As MccJ25 is ribosomally synthesized as a larger peptide precursor endowed with an N-terminal extremity, the present study shows that removal of this extension and head-tail cyclization of the resulting propeptide are the only post-translational modifications involved in the maturation of MccJ25, that appears as the first cyclic microcin.

Kinetics and regioselectivity of peptide-to-heterocycle conversions by microcin B17 synthetase

BACKGROUND

The Escherichia coli peptide antibiotic microcin B17 (MccB17) contains four oxazole and four thiazole rings introduced post-translationally in the 69 amino acid McbA gene product, an MccB17 precursor, by the microcin B,C,D enzyme complex. Both monocyclic and 4,2-bis-heterocyclic moieties are generated. The enzymatic cyclization involves 14 of the last 43 amino acids of McbA and requires the presence of the first 26 amino acids that function as a specificity-conferring propeptide.

RESULTS

We have constructed maltose-binding protein (MBP)-McbA1-46 fusion proteins and have mutagenized the Gly39-Ser40-Cys41 (GSC) wild-type sequence to assess the regioselectivity and chemoselectivity of MccB17-synthetase-mediated heterocycle formation at the first two loci, residues 40 and 41 of McbA. Four single-site and four double-site substrates showed substantial differences in turnover as assessed by western assays, UV-visible spectroscopy and mass spectrometry. Cysteine-derived thiazoles form at a greater rate than serine-derived oxazoles. Formation of bis-heterocycles is sensitive both to composition and sequence context.

CONCLUSIONS

The E. coli McbB,C,D MccB17 synthetase is the first peptide heterocyclization enzyme to be characterized. This study reveals substantial regioselectivity and chemoselectivity (thiazole > oxazole) at the most amino-terminal bis-heterocyclization site of McbA. The heterocyclization of GSS and GCC mutants of McbA1-46 by MccB17 synthetase demonstrates that the complex can efficiently generate tandem bis-oxazoles and bis-thiazoles, moieties not found in MccB17 but present in natural products such as hennoxazole and bleomycin. The observations suggest a common enzymatic mechanism for the formation of peptide-derived heterocyclic natural products.

Role of the microcin B17 propeptide in substrate recognition: solution structure and mutational analysis of McbA1-26

BACKGROUND

The peptide antibiotic microcin B17 (MccB17) contains oxazole and thiazole heterocycles formed by the post-translational modification of four cysteine and four serine residues. An amino-terminal propeptide targets the 69 amino acid precursor of MccB17 (preproMccB17) to the heterocyclization enzyme MccB17 synthetase. The mode of synthetase recognition has been unclear, because there has been limited structural information available on the MccB17 propeptide to date.

RESULTS

The solution structure of the MccB17 propeptide (McbA1-26), determined using nuclear magnetic resonance, reveals that McbA1-26 is an amphipathic alpha helix. Mutational analysis of 13 propeptide residues showed that Phe8 and Leu12 are essential residues for MccB17 synthetase recognition. A domain of the propeptide was putatively identified as the region that interacts with the synthetase.

CONCLUSIONS

MccB17 synthetase recognizes key hydrophobic residues within a helical propeptide, allowing the selective heterocyclization of downstream cysteine and serine residues in preproMccB17. The determination of the solution structure of the propeptide should facilitate the investigation of other functions of the propeptide, including a potential role in antibiotic secretion.