Combinatorial biosynthesis of RiPPs: docking with marine life

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

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

An Autocatalytic Peptide Cyclase Improves Fidelity and Yield of Circular Peptides In Vivo and In Vitro

Peptide cyclization improves conformational rigidity, providing favorable pharmacological properties, such as proteolytic resistance, target specificity, and membrane permeability. Thus, many synthetic and biosynthetic peptide circularization strategies have been developed. PatG and related natural macrocyclases process diverse peptide sequences, generating millions of cyclic derivatives. However, the application of these cyclases is limited by low yields and the potential presence of unwanted intermediates. Here, we designed a covalently fused G macrocyclase with substrates that efficiently and spontaneously release cyclic peptides. To increase the fidelity of synthesis, we developed an orthogonal control mechanism enabling precision synthesis in Escherichia coli. As a result, a library comprising 4.8 million cyclic derivatives was constructed, producing an estimated 2.6 million distinct cyclic peptides with an improved yield and fidelity.

Uncovering the diversity and distribution of biosynthetic gene clusters of prochlorosins and other putative RiPPs in marine Synechococcus strains

IMPORTANCE

Genome mining studies have revealed the remarkable combinatorial diversity of ribosomally synthesized and post-translationally modified peptides (RiPPs) in marine bacteria, including prochlorosins. However, mining strategies also prove valuable in investigating the genomic landscape of associated genes within biosynthetic gene cluster (BGC) specific to targeted RiPPs of interest. Our study contributes to the enrichment of knowledge regarding prochlorosin diversity. It offers insights into potential mechanisms involved in their biosynthesis and modification, such as hyper-modification, which may give rise to active lantibiotics. Additionally, our study uncovers putative novel promiscuous post-translational enzymes, thereby expanding the chemical space explored within the Synechococcus genus. Moreover, this research extends the applications of mining techniques beyond the discovery of new RiPP-like clusters, allowing for a deeper understanding of genomics and diversity. Furthermore, it holds the potential to reveal previously unknown functions within the intriguing RiPP families, particularly in the case of prochlorosins.

D. Recent Advancements in Bottromycin Biosynthesis

Bottromycin is a structurally complex cyclic peptidic compound isolated from Streptomyces bottropensis and related organisms and belongs to the RiPP family of natural products (ribosomally synthesized and post-translationally modified peptides). It exhibits potent antibacterial properties against gram-positive pathogens (including drug resistant strains such as MRSA, MIC 1 μg/mL and VRE, MIC 0.5 μg/mL) and mycoplasma. Bottromycin blocks the binding of the aminoacyl-tRNA to the A-site on the 50S ribosome and hence inhibits protein synthesis. Bottromycins contain structurally diverse post-translational modifications (PTMs) on a small peptide (GPVVVFDC) including a unique macrocyclic amidine, rare β-methylation, terminal thiazole heterocycle, oxidative decarboxylation, and Asp epimerization, among others. It exhibits a precursor peptide organization with a C-terminal follower peptide and a N-terminal core peptide. There are several new studies reported recently which gave detailed insights into the bottromycin biosynthesis pathway. This Account highlights the current advancements in understanding the biosynthetic pathway of bottromycin focusing mainly on the biochemically and structurally characterized enzymes and intricate details of the peptide–protein biophysical interactions. These studies have provided a strong foundation for conducting combinatorial biosynthesis and synthetic biological studies to create novel bottromycin variants for therapeutic applications.

1 Introduction

2 Biosynthetic Pathway for Bottromycin

3 Enzymology of Bottromycin Biosynthesis

3.1 Cleavage of Methionine (BotP)

3.2 Radical SAM Methyltransferases (BotRMT1, BotRMT2, BotRMT3)

3.3 ATP-Dependent YcaO Enzymes

3.3.1 Thiazoline Formation by BotC

3.3.2 Macrolactamidine Formation by BotCD

3.4 Follower Peptide Hydrolysis (BotAH)

3.5 Aspartate Epimerization (BotH)

3.6 Oxidative Decarboxylation (BotCYP)

3.7 O-Methyltransferase (BotOMT)

4 Heterologous Bottromycin Production and Analogue Preparation

5 Summary and Outlook

Integrated Metabolomic-Genomic Workflows Accelerate Microbial Natural Product Discovery

The pairing of analytical chemistry with genomic techniques represents a new wave in natural product chemistry. With an increase in the availability of sequencing and assembly of microbial genomes, interrogation into the biosynthetic capability of producers with valuable secondary metabolites is possible. However, without the development of robust, accessible, and medium to high throughput tools, the bottleneck in pairing metabolic potential and compound isolation will continue. Several innovative approaches have proven useful in the nascent stages of microbial genome-informed drug discovery. Here, we consider a number of these approaches which have led to prioritization of strain targets and have mitigated rediscovery rates. Likewise, we discuss integration of principles of comparative evolutionary studies and retrobiosynthetic predictions to better understand biosynthetic mechanistic details and link genome sequence to structure. Lastly, we discuss advances in engineering, chemistry, and molecular networking and other computational approaches that are accelerating progress in the field of omic-informed natural product drug discovery. Together, these strategies enhance the synergy between cutting edge omics, chemical characterization, and computational technologies that pitch the discovery of natural products with pharmaceutical and other potential applications to the crest of the wave where progress is ripe for rapid advances.

A Silent Biosynthetic Gene Cluster from a Methanotrophic Bacterium Potentiates Discovery of a Substrate Promiscuous Proteusin Cyclodehydratase

Natural product-encoding biosynthetic gene clusters (BGCs) within microbial genomes far outnumber the known natural products; chemical products from such BGCs remain cryptic. These silent BGCs hold promise not only for the elaboration of new natural products but also for the discovery of useful biosynthetic enzymes. Here, we describe a genome mining strategy targeted toward the discovery of substrate promiscuous natural product biosynthetic enzymes. In the genome of the methanotrophic bacterium Methylovulum psychrotolerans Sph1^T, we discover a transcriptionally silent natural product BGC that encoded numerous ribosomally synthesized and post-translationally modified peptide (RiPP) natural products. These cryptic RiPP natural products were accessed using heterologous expression of the substrate peptide and biosynthetic enzyme-encoded genes. In line with our genome mining strategy, the RiPP biosynthetic enzymes in this BGC were found to be substrate promiscuous, which allowed us to use them in a combinatorial fashion with a similarly substrate-tolerant cyanobactin biosynthetic enzyme to introduce head-to-tail macrocyclization in the proteusin family of RiPP natural products.

Control of Nucleophile Chemoselectivity in Cyanobactin YcaO Heterocyclases PatD and TruD

Members of the YcaO superfamily are among the most common post-translational modification enzymes in natural product biosynthesis, with wide usage in biotechnology and synthetic biology applications. Here, we use domain-swapped chimeras and discovered unstructured regions in cyanobactin YcaOs that guide interactions with the substrates, governing access to interior amino acids in the substrates and explaining the chemoselectivity between PatD and TruD. These results define how the cyanobactin heterocyclases modify exceptionally sequence diverse substrates, yet with a high degree of positional and nucleophile selectivity.

Comprehensive Mutational Analysis of the Lasso Peptide Klebsidin

Antibiotic resistance is a growing threat to public health, making the development of antibiotics of critical importance. One promising class of potential new antibiotics are ribosomally synthesized and post-translationally modified peptides (RiPPs), which include klebsidin, a lasso peptide from Klebsiella pneumoniae that inhibits certain bacterial RNA polymerases. We develop a high-throughput assay based on growth inhibition of Escherichia coli to analyze the mutational tolerance of klebsidin. We transform a library of klebsidin variants into E. coli and use next-generation DNA sequencing to count the frequency of each variant before and after its expression, thereby generating functional scores for 320 of 361 single amino acid changes. We identify multiple positions in the macrocyclic ring and the C-terminal tail region of klebsidin that are intolerant to mutation, as well as positions in the loop region that are highly tolerant to mutation. Characterization of selected peptide variants scored as active reveals that each adopts a threaded lasso conformation; active loop variants applied extracellularly as peptides slow the growth of E. coli and K. pneumoniae. We generate an E. coli strain with a mutation in RNA polymerase that confers resistance to klebsidin and similarly carry out a selection with the klebsidin library. We identify a single variant, klebsidin F9Y, that maintains activity against the resistant E. coli when expressed intracellularly. This finding supports the utility of this method and suggests that comprehensive mutational analysis of lasso peptides can identify unique and potentially improved variants.

Renewed interests in the discovery of bioactive actinomycete metabolites driven by emerging technologies

Actinomycetes are prolific sources of bioactive molecules. Traditional workflows including bacterial isolation, fermentation, metabolite identification and structure elucidation have resulted in high rates of natural product rediscovery in recent years. Recent advancements in multi-omics techniques have uncovered cryptic gene clusters within the genomes of actinomycetes, potentially introducing vast resources for the investigation of bioactive molecules. While developments in culture techniques have allowed for the fermentation of difficult-to-culture actinomycetes, high-throughput metabolite screening has offered plenary tools to accelerate hits discovery. A variety of new bioactive molecules have been isolated from actinomycetes of unique environmental origins, such as endophytic and symbiotic actinomycetes. Synthetic biology and genome mining have also emerged as new frontiers for the discovery of bioactive molecules. This review covers the highlights of recent developments in actinomycete-derived natural product drug discovery.

Heterologous characterization of mechercharmycin A biosynthesis reveals alternative insights into post-translational modifications for RiPPs

Mechercharmycin A (MCM-A) is a marine natural product belonging to a family of polyazole cyclopeptides with remarkable bioactivities and unique structures. Identification, heterologous expression, and genetic characterizations of the MCM biosynthetic gene cluster in Bacillus subtilis revealed that it is a ribosomally synthesized and post-translationally modified peptide (RiPP) possessing complex with distinctive modifications. Based on this heterologous expression system, two MCM analogs with comparable antitumor activity are generated by engineering the biosynthetic pathway. Combinatorial co-production of a precursor peptide with different modifying enzymes in Escherichia coli identifies a different timing of modifications, showing that a tRNA^Glu-dependent highly regioselective dehydration is the first modification step, followed by polyazole formation through heterocyclization and dehydrogenation in an N- to C-terminal direction. Therefore, a rational biosynthetic pathway of MCMs is proposed, which unveils a subfamily of azol(in)e-containing RiPPs and sets the stage for further investigations of the enzymatic mechanism and synthetic biology.

Chlorinated metabolites from Streptomyces sp. highlight the role of biosynthetic mosaics and superclusters in the evolution of chemical diversity

LCMS-guided screening of a library of biosynthetically talented bacteria and fungi identified Streptomyces sp. MST- as a prolific producer of chlorinated metabolites. We isolated and characterised six new and nine reported compounds from MST-, belonging to three discrete classes - the depsipeptide svetamycins, the indolocarbazole borregomycins and the aromatic polyketide anthrabenzoxocinones. Following genome sequencing of MST-, we describe, for the first time, the svetamycin biosynthetic gene cluster (sve), its mosaic structure and its relationship to several distantly related gene clusters. Our analysis of the sve cluster suggested that the reported stereostructures of the svetamycins may be incorrect. This was confirmed by single-crystal X-ray diffraction analysis, allowing us to formally revise the absolute configurations of svetamycins A-G. We also show that the borregomycins and anthrabenzoxocinones are encoded by a single supercluster (bab) implicating superclusters as potential nucleation points for the evolution of biosynthetic gene clusters. These clusters highlight how individual enzymes and functional subclusters can be co-opted during the formation of biosynthetic gene clusters, providing a rare insight into the poorly understood mechanisms underpinning the evolution of chemical diversity.

Heterologous production of new protease inhibitory peptide marinostatin E

Bicyclic peptides, marinostatins, are protease inhibitors derived from the marine bacterium Algicola sagamiensis. The biosynthetic gene cluster of marinostatin was previously identified, although no heterologous production was reported. In this report, the biosynthetic gene cluster of marinostatin (mstA and mstB) was cloned into the expression vector pET-41a(+). As a result of the coexpression experiment, a new analogous peptide named marinostatin E was successfully produced using Escherichia coli BL21(DE3). The structure of marinostatin E was determined by a combination of chemical treatments and tandem mass spectrometry experiments. Marinostatin E exhibited inhibitory activities against chymotrypsin and subtilisin with an IC50 of 4.0 and 39.6 μm, respectively.

Heterologous expression of cryptic biosynthetic gene cluster from Streptomyces prunicolor yields novel bicyclic peptide prunipeptin

Recently, ω-ester-containing peptides (OEPs) were indicated to be a class of ribosomally synthesized and post-translationally modified peptides. Based on genome mining, new biosynthetic gene cluster of OEPs was found in the genome sequence of actinobacterium Streptomyces prunicolor. The biosynthetic gene cluster contained just two genes including precursor peptide (pruA) and ATP-grasp ligase (pruB) coding genes. Heterologous co-expression of the two genes was accomplished using expression vector pET-41a(+) in Escherichia coli. As a result, new OEP named prunipeptin was produced by this system. By site-directed mutagenesis experiment, a variant peptide prunipeptin 15HW was obtained. The bridging pattern of prunipeptin 15HW was determined by combination of chemical cleavage and MS experiments. Prunipeptin 15HW possessed bicyclic structure with an ester bond and an isopeptide bond. The ATP-grasp ligase PruB was indicated to catalyze the two different intramolecular bonds.

Heterologous production of new lasso peptide koreensin based on genome mining

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

Minimal lactazole scaffold for in vitro thiopeptide bioengineering

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

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

Genome Mining Reveals High Topological Diversity of ω-Ester-Containing Peptides and Divergent Evolution of ATP-Grasp Macrocyclases

ω-Ester-containing peptides (OEPs) are a family of ribosomally synthesized and post-translationally modified peptides (RiPPs) containing intramolecular ω-ester or ω-amide bonds. Although their distinct side-to-side connections may create considerable topological diversity of multicyclic peptides, it is largely unknown how diverse ring patterns have been developed in nature. Here, using genome mining of biosynthetic enzymes of OEPs, we identified genes encoding nine new groups of putative OEPs with novel core consensus sequences, disclosing a total of ∼1500 candidate OEPs in 12 groups. Connectivity analysis revealed that OEPs from three different groups contain novel tricyclic structures, one of which has a distinct biosynthetic pathway where a single ATP-grasp enzyme produces both ω-ester and ω-amide linkages. Analysis of the enzyme cross-reactivity showed that, while enzymes are promiscuous to nonconserved regions of the core peptide, they have high specificity to the cognate core consensus sequence, suggesting that the enzyme-core pair has coevolved to create a unique ring topology within the same group and has sufficiently diversified across different groups. Collectively, our results demonstrate that the diverse ring topologies, in addition to diverse sequences, have been developed in nature with multiple ω-ester or ω-amide linkages in the OEP family of RiPPs.

Evolutionary dynamics of natural product biosynthesis in bacteria

Covering: 2008 up to 2019The forces of biochemical adaptive evolution operate at the level of genes, manifesting in complex phenotypes and the global biodiversity of proteins and metabolites. While evolutionary histories have been deciphered for some other complex traits, the origins of natural product biosynthesis largely remain a mystery. This fundamental knowledge gap is surprising given the many decades of research probing the genetic, chemical, and biophysical mechanisms of bacterial natural product biosynthesis. Recently, evolutionary thinking has begun to permeate this otherwise mechanistically dominated field. Natural products are now sometimes referred to as 'specialized' rather than 'secondary' metabolites, reinforcing the importance of their biological and ecological functions. Here, we review known evolutionary mechanisms underlying the overwhelming chemical diversity of bacterial secondary metabolism, focusing on enzyme promiscuity and the evolution of enzymatic domains that enable metabolic traits. We discuss the mechanisms that drive the assembly of natural product biosynthetic gene clusters and propose formal definitions for 'specialized' and 'secondary' metabolism. We further explore how biosynthetic gene clusters evolve to synthesize related molecular species, and in turn how the biological and ecological roles that emerge from metabolic diversity are acted on by selection. Finally, we reconcile chemical, functional, and genetic data into an evolutionary model, the dynamic chemical matrix evolutionary hypothesis, in which the relationships between chemical distance, biomolecular activity, and relative fitness shape adaptive landscapes.

Exploration and genome mining of natural products from marine Streptomyces

Marine Streptomyces sp. are an important source of bioactive compounds owing to their unique habitats and metabolic pathways. Whole-genome sequencing and bioinformatics analyses have shown that the potential of synthesizing secondary metabolites from marine-derived Streptomyces has been substantially underestimated. Genome mining is an integrated strategy used to discover natural products based on gene cluster sequences and biosynthetic pathways. Its emergence has greatly enhanced the discovery of natural compounds from marine Streptomyces, thereby yielding a large number of bioactive molecules with novel structures and potent activities. In this review, we briefly summarize the current applications of genome mining in marine Streptomyces, such as bioinformatics-based optimization of culture conditions, ribosome engineering, control of regulatory networks, heterologous expression of biosynthetic gene cluster, and combinatorial biosynthesis of natural compounds. Furthermore, we discuss the factors hindering the utilization of marine-derived natural products and conclude with the prospects for this technique.

Heterologous production of coryneazolicin in Escherichia coli

Coryneazolicin is a plantazolicin family peptide, belonging to linear azole-containing peptides (LAPs). Although coryneazolicin was previously synthesized by in vitro experiments, its biological activity has not been evaluated. In this report, the heterologous production of coryneazolicin was accomplished to obtain enough coryneazolicin for biological activity tests. The structure of coryneazolicin was confirmed by ESI-MS and NMR analyses. The biological activity tests indicated that coryneazolicin possessed potent antibacterial activity and cytotoxicity. Although antibacterial activity of plantazolicin was previously reported, cytotoxicity was newly found in coryneazolicin among plantazolicin type peptides. In addition, we revealed that coryneazolicin induced apoptosis on HCT116 and HOS cancer cell lines.

Isolation and structure determination of a new lasso peptide specialicin based on genome mining

Based on genome mining, a new lasso peptide specialicin was isolated from the extract of Streptomyces specialis. The structure of specialicin was established by ESI-MS and NMR analyses to be a lasso peptide with the length of 21 amino acids, containing an isopeptide bond and two disulfide bonds in the molecule. The stereochemistries of the constituent amino acids except for Trp were determined to be L and the stereochemistry of Trp at C-terminus was determined to be D. Three dimensional structure of specialicin was determined based on NOE experimental data, which indicated that specialicin possessed the similar conformational structure with siamycin I. Specialicin showed the antibacterial activity against Micrococcus luteus and the moderate anti-HIV activity against HIV-1 NL4-3. The biosynthetic gene cluster of specialicin was proposed from the genome sequence data of S. specialis.

Heterologous production of a new lasso peptide brevunsin in Sphingomonas subterranea

A shuttle vector pHSG396Sp was constructed to perform gene expression using Sphingomonas subterranea as a host. A new lasso peptide biosynthetic gene cluster, derived from Brevundimonas diminuta, was amplified by PCR and integrated to afford a expression vector pHSG396Sp-12697L. The new lasso peptide brevunsin was successfully produced by S. subterranea, harboring the expression vector, with a high production yield (10.2 mg from 1 L culture). The chemical structure of brevunsin was established by NMR and MS/MS experiments. Based on the information obtained from the NOE experiment, the three-dimensional structure of brevunsin was determined, which indicated that brevunsin possessed a typical lasso structure. This expression vector system provides a new heterologous production method for unexplored lasso peptides that are encoded by bacterial genomes.

Ascidian Toxins with Potential for Drug Development

Ascidians (tunicates) are invertebrate chordates, and prolific producers of a wide variety of biologically active secondary metabolites from cyclic peptides to aromatic alkaloids. Several of these compounds have properties which make them candidates for potential new drugs to treat diseases such as cancer. Many of these natural products are not produced by the ascidians themselves, rather by their associated symbionts. This review will focus mainly on the mechanism of action of important classes of cytotoxic molecules isolated from ascidians. These toxins affect DNA transcription, protein translation, drug efflux pumps, signaling pathways and the cytoskeleton. Two ascidian compounds have already found applications in the treatment of cancer and others are being investigated for their potential in cancer, neurodegenerative and other diseases.

The Biochemistry and Structural Biology of Cyanobactin Pathways: Enabling Combinatorial Biosynthesis

Cyanobactin biosynthetic enzymes have exceptional versatility in the synthesis of natural and unnatural products. Cyanobactins are ribosomally synthesized and posttranslationally modified peptides synthesized by multistep pathways involving a broad suite of enzymes, including heterocyclases/cyclodehydratases, macrocyclases, proteases, prenyltransferases, methyltransferases, and others. Here, we describe the enzymology and structural biology of cyanobactin biosynthetic enzymes, aiming at the twin goals of understanding biochemical mechanisms and biosynthetic plasticity. We highlight how this common suite of enzymes may be utilized to generate a large array or structurally and chemically diverse compounds.

Parallel lives of symbionts and hosts: chemical mutualism in marine animals

Covering: up to 2018 Symbiotic microbes interact with animals, often by producing natural products (specialized metabolites; secondary metabolites) that exert a biological role. A major goal is to determine which microbes produce biologically important compounds, a deceptively challenging task that often rests on correlative results, rather than hypothesis testing. Here, we examine the challenges and successes from the perspective of marine animal-bacterial mutualisms. These animals have historically provided a useful model because of their technical accessibility. By comparing biological systems, we suggest a common framework for establishing chemical interactions between animals and microbes.

Structural Insights into Thioether Bond Formation in the Biosynthesis of Sactipeptides

Sactipeptides are ribosomally synthesized peptides that contain a characteristic thioether bridge (sactionine bond) that is installed posttranslationally and is absolutely required for their antibiotic activity. Sactipeptide biosynthesis requires a unique family of radical SAM enzymes, which contain multiple [4Fe-4S] clusters, to form the requisite thioether bridge between a cysteine and the α-carbon of an opposing amino acid through radical-based chemistry. Here we present the structure of the sactionine bond-forming enzyme CteB, from Clostridium thermocellum ATCC 27405, with both SAM and an N-terminal fragment of its peptidyl-substrate at 2.04 Å resolution. CteB has the (β/α)6-TIM barrel fold that is characteristic of radical SAM enzymes, as well as a C-terminal SPASM domain that contains two auxiliary [4Fe-4S] clusters. Importantly, one [4Fe-4S] cluster in the SPASM domain exhibits an open coordination site in absence of peptide substrate, which is coordinated by a peptidyl-cysteine residue in the bound state. The crystal structure of CteB also reveals an accessory N-terminal domain that has high structural similarity to a recently discovered motif present in several enzymes that act on ribosomally synthesized and post-translationally modified peptides (RiPPs), known as a RiPP precursor peptide recognition element (RRE). This crystal structure is the first of a sactionine bond forming enzyme and sheds light on structures and mechanisms of other members of this class such as AlbA or ThnB.

Ribosomal Natural Products, Tailored To Fit

Ribosomally synthesized and Post-translationally modified Peptides (RiPPs) take advantage of the ribosomal translation machinery to generate linear peptides that are subsequently modified with heterocycles and/or macrocycles to impose three-dimensional structure and thwart degradation by proteases. Although RiPP precursors are limited to proteinogenic amino acids, post-translational modifications (PTMs) can alter the structure of individual amino acids and thereby improve the stability and biological activity of the molecule. These "tailoring modifications" often occur on amino acid side chains-for example, hydroxylation, methylation, halogenation, prenylation, and acylation-but can also take place within the backbone, as in epimerization, or can result in capping of the N- or C-terminus. At one extreme, these modifications can be essential to the activity of the RiPP, either as a compulsory step in reaching the final molecule or by imparting chemical functionality required for biological activity. At the other extreme, tailoring PTMs may have little effect on the activity in an in vitro setting-possibly because of test conditions that do not match the biological context in which the PTMs evolved. Establishing the molecular basis for the function of tailoring PTMs often requires a three-dimensional structure of the RiPP bound to its biological target. These structures have revealed roles for tailoring PTMs that include providing additional hydrogen bonds to targets, rigidifying the RiPP structure to reduce the entropic cost of binding, or altering the secondary structure of the peptide backbone. Bacterial RiPPs are particularly suited to structural characterization, as they are relatively easy to isolate from laboratory cultures or to produce in a heterologous host. The identification of new tailoring PTMs within bacteria is also facilitated by clustering of the genes encoding tailoring enzymes with those of the RiPP precursor and primary modification enzymes. In this Account, we describe the effects of tailoring PTMs on RiPP structure, their interactions with biological targets, and their influence on RiPP stability, with a focus on bacterial RiPP classes. We also discuss the enzymes that generate tailoring PTMs and highlight examples of and prospects for engineering of RiPPs.

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

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

Chimeric Leader Peptides for the Generation of Non-Natural Hybrid RiPP Products

Combining biosynthetic enzymes from multiple pathways is an attractive approach for producing molecules with desired structural features; however, progress has been hampered by the incompatibility of enzymes from unrelated pathways and intolerance toward alternative substrates. Ribosomally synthesized and posttranslationally modified peptides (RiPPs) are a diverse natural product class that employs a biosynthetic logic that is highly amenable to engineering new compounds. RiPP biosynthetic proteins modify their substrates by binding to a motif typically located in the N-terminal leader region of the precursor peptide. Here, we exploit this feature by designing leader peptides that enable recognition and processing by multiple enzymes from unrelated RiPP pathways. Using this broadly applicable strategy, a thiazoline-forming cyclodehydratase was combined with enzymes from the sactipeptide and lanthipeptide families to create new-to-nature hybrid RiPPs. We also provide insight into design features that enable control over the hybrid biosynthesis to optimize enzyme compatibility and establish a general platform for engineering additional hybrid RiPPs.

Enzymatic Carbon-Sulfur Bond Formation in Natural Product Biosynthesis

Sulfur plays a critical role for the development and maintenance of life on earth, which is reflected by the wealth of primary metabolites, macromolecules, and cofactors bearing this element. Whereas a large body of knowledge has existed for sulfur trafficking in primary metabolism, the secondary metabolism involving sulfur has long been neglected. Yet, diverse sulfur functionalities have a major impact on the biological activities of natural products. Recent research at the genetic, biochemical, and chemical levels has unearthed a broad range of enzymes, sulfur shuttles, and chemical mechanisms for generating carbon-sulfur bonds. This Review will give the first systematic overview on enzymes catalyzing the formation of organosulfur natural products.

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.

Enzymatic N- and C-Protection in Cyanobactin RiPP Natural Products

Recent innovations in peptide natural product biosynthesis reveal a surprising wealth of previously uncharacterized biochemical reactions that have potential applications in synthetic biology. Among these, the cyanobactins are noteworthy because these peptides are protected at their N- and C-termini by macrocyclization. Here, we use a novel bifunctional enzyme AgeMTPT to protect linear peptides by attaching prenyl and methyl groups at their free N- and C-termini. Using this peptide protectase in combination with other modular biosynthetic enzymes, we describe the total synthesis of the natural product aeruginosamide B and the biosynthesis of linear cyanobactin natural products. Our studies help to define the enzymatic mechanism of macrocyclization, providing evidence against the water exclusion hypothesis of transpeptidation and favoring the kinetic lability hypothesis.

Biosynthesis and molecular engineering of templated natural products

Bioactive small molecules that are produced by living organisms, often referred to as natural products (NPs), historically play a critical role in the context of both medicinal chemistry and chemical biology. How nature creates these chemical entities with stunning structural complexity and diversity using a limited range of simple substrates has not been fully understood. Focusing on two types of NPs that share a highly evolvable ‘template’-biosynthetic logic, we here provide specific examples to highlight the conceptual and technological leaps in NP biosynthesis and witness the area of progress since the beginning of the twenty-first century. The biosynthesis of polyketides, non-ribosomal peptides and their hybrids that share an assembly-line enzymology of modular multifunctional proteins exemplifies an extended ‘central dogma’ that correlates the genotype of catalysts with the chemotype of products; in parallel, post-translational modifications of ribosomally synthesized peptides involve a number of unusual biochemical mechanisms for molecular maturation. Understanding the biosynthetic processes of these templated NPs would largely facilitate the design, development and utilization of compatible biosynthetic machineries to address the challenge that often arises from structural complexity to the accessibility and efficiency of current chemical synthesis.

Directing Biosynthesis: Practical Supply of Natural and Unnatural Cyanobactins

The increasingly rapid accumulation of genomic information is revolutionizing natural products discovery. However, the translation of sequence data to chemical products remains a challenge. Here, we detail methods used to circumvent the supply problem of cyanobactin natural products, both by engineered synthesis in Escherichia coli and by using purified enzymes in vitro. Such methodologies exploit nature's strategies of combinatorial chemistry in the cyanobactin class of RiPP natural products. As a result, it is possible to synthesize a wide variety of natural and unnatural compounds.