Engineering of Streptomyces platensis MA7339 for overproduction of platencin and congeners

Development of platensimycin, platencin, and platensilin overproducers by biosynthetic pathway engineering and fermentation medium optimization

The platensimycin (PTM), platencin (PTN), and platensilin (PTL) family of natural products continues to inspire the discovery of new chemistry, enzymology, and medicine. Engineered production of this emerging family of natural products, however, remains laborious due to the lack of practical systems to manipulate their biosynthesis in the native-producing Streptomyces platensis species. Here we report solving this technology gap by implementing a CRISPR-Cas9 system in S. platensis CB00739 to develop an expedient method to manipulate the PTM, PTN, and PTL biosynthetic machinery in vivo. We showcase the utility of this technology by constructing designer recombinant strains S. platensis SB12051, SB12052, and SB12053, which, upon fermentation in the optimized PTM-MS medium, produced PTM, PTN, and PTL with the highest titers at 836 mg L-1, 791 mg L-1, and 40 mg L-1, respectively. Comparative analysis of these resultant recombinant strains also revealed distinct chemistries, catalyzed by PtmT1 and PtmT3, two diterpene synthases that nature has evolved for PTM, PTN, and PTL biosynthesis. The ΔptmR1/ΔptmT1/ΔptmT3 triple mutant strain S. platensis SB12054 could be envisaged as a platform strain to engineer diterpenoid biosynthesis by introducing varying ent-copalyl diphosphate-acting diterpene synthases, taking advantage of its clean metabolite background, ability to support diterpene biosynthesis in high titers, and the promiscuous tailoring biosynthetic machinery.

ONE-SENTENCE SUMMARY

Implementation of a CRISPR-Cas9 system in Streptomyces platensis CB00739 enabled the construction of a suite of designer recombinant strains for the overproduction of platensimycin, platencin, and platensilin, discovery of new diterpene synthase chemistries, and development of platform strains for future diterpenoid biosynthesis engineering.

Actinomycetes as Producers of Biologically Active Terpenoids: Current Trends and Patents

Terpenes and their derivatives (terpenoids and meroterpenoids, in particular) constitute the largest class of natural compounds, which have valuable biological activities and are promising therapeutic agents. The present review assesses the biosynthetic capabilities of actinomycetes to produce various terpene derivatives; reports the main methodological approaches to searching for new terpenes and their derivatives; identifies the most active terpene producers among actinomycetes; and describes the chemical diversity and biological properties of the obtained compounds. Among terpene derivatives isolated from actinomycetes, compounds with pronounced antifungal, antiviral, antitumor, anti-inflammatory, and other effects were determined. Actinomycete-produced terpenoids and meroterpenoids with high antimicrobial activity are of interest as a source of novel antibiotics effective against drug-resistant pathogenic bacteria. Most of the discovered terpene derivatives are produced by the genus Streptomyces; however, recent publications have reported terpene biosynthesis by members of the genera Actinomadura, Allokutzneria, Amycolatopsis, Kitasatosporia, Micromonospora, Nocardiopsis, Salinispora, Verrucosispora, etc. It should be noted that the use of genetically modified actinomycetes is an effective tool for studying and regulating terpenes, as well as increasing productivity of terpene biosynthesis in comparison with native producers. The review includes research articles on terpene biosynthesis by Actinomycetes between 2000 and 2022, and a patent analysis in this area shows current trends and actual research directions in this field.

Diterpenoids from Streptomyces: Structures, Biosyntheses and Bioactivities

Bacteria, especially Streptomyces spp., have been emerging as rich sources of natural diterpenoids with diverse structures and broad bioactivities. Here, we review diterpenoids biosynthesized by Streptomyces , with an emphasis on their structures, biosyntheses, and bioactivities. Although diterpenoids from Streptomyces are relatively rare compared to those from plants and fungi, their novel skeletons, biosyntheses and bioactivities present opportunities for discovering new drugs, enzyme mechanisms, and applications in bio-catalysis and metabolic pathway engineering.

Bacterial terpenome

Covering: up to mid-2020 Terpenoids, also called isoprenoids, are the largest and most structurally diverse family of natural products. Found in all domains of life, there are over 80 000 known compounds. The majority of characterized terpenoids, which include some of the most well known, pharmaceutically relevant, and commercially valuable natural products, are produced by plants and fungi. Comparatively, terpenoids of bacterial origin are rare. This is counter-intuitive to the fact that recent microbial genomics revealed that almost all bacteria have the biosynthetic potential to create the C5 building blocks necessary for terpenoid biosynthesis. In this review, we catalogue terpenoids produced by bacteria. We collected 1062 natural products, consisting of both primary and secondary metabolites, and classified them into two major families and 55 distinct subfamilies. To highlight the structural and chemical space of bacterial terpenoids, we discuss their structures, biosynthesis, and biological activities. Although the bacterial terpenome is relatively small, it presents a fascinating dichotomy for future research. Similarities between bacterial and non-bacterial terpenoids and their biosynthetic pathways provides alternative model systems for detailed characterization while the abundance of novel skeletons, biosynthetic pathways, and bioactivies presents new opportunities for drug discovery, genome mining, and enzymology.

Leveraging a large microbial strain collection for natural product discovery

Throughout history, natural products have significantly contributed to the discovery of novel chemistry, drug leads, and tool molecules to probe and address complex challenges in biology and medicine. Recent microbial genome sequencing efforts have uncovered many microbial biosynthetic gene clusters without an associated natural product. This means that the natural products isolated to date do not fully reflect the biosynthetic potential of microbial strains. This observation has rejuvenated the natural product community and inspired a return to microbial strain collections. Mining large microbial strain collections with the most current technologies in genome sequencing, bioinformatics, and high-throughput screening techniques presents new opportunities in natural product discovery. In this review, we report on the newly expanded microbial strain collection at The Scripps Research Institute, which represents one of the largest and most diverse strain collections in the world. Two complementary approaches, i.e. structure-centric and function-centric, are presented here to showcase how to leverage a large microbial strain collection for natural product discovery and to address challenges and harness opportunities for future efforts. Highlighted examples include the discovery of alternative producers of known natural products with superior growth characteristics and high titers, novel analogs of privileged scaffolds, novel natural products, and new activities of known and new natural products. We anticipate that this large microbial strain collection will facilitate the discovery of new natural products for many applications.

Challenges and opportunities for natural product discovery, production, and engineering in native producers versus heterologous hosts

Recent advances and emerging technologies for metabolic pathway engineering and synthetic biology have transformed the field of natural product discovery, production, and engineering. Despite these advancements, there remain many challenges in understanding how biosynthetic gene clusters are silenced or activated, including changes in the transcription of key biosynthetic and regulatory genes. This knowledge gap is highlighted by the success and failed attempts of manipulating regulatory genes within biosynthetic gene clusters in both native producers and heterologous hosts. These complexities make the choice of native producers versus heterologous hosts, fermentation medium, and supply of precursors crucial factors in achieving the production of the target natural products and engineering designer analogs. Nature continues to serve as inspiration for filling the knowledge gaps and developing new research strategies. By exploiting the evolutionary power of nature, alternative producers, with the desired genetic amenability and higher titers of the target natural products, and new strains, harboring gene clusters that encode evolutionary optimized congeners of the targeted natural product scaffolds, can be discovered. These newly identified strains can serve as an outstanding biotechnology platform for the engineered production of sufficient quantities of the target natural products and their analogs, enabling biosynthetic studies and potential therapeutic applications. These challenges and opportunities are showcased herein using fredericamycin, iso-migrastatin, platencin and platensimycin, the enediynes of C-1027, tiancimycin, and yangpumicin, and the leinamycin family of natural products.

Biosynthesis of thiocarboxylic acid-containing natural products

Thiocarboxylic acid-containing natural products are rare and their biosynthesis and biological significance remain unknown. Thioplatensimycin (thioPTM) and thioplatencin (thioPTN), thiocarboxylic acid congeners of the antibacterial natural products platensimycin (PTM) and platencin (PTN), were recently discovered. Here we report the biosynthetic origin of the thiocarboxylic acid moiety in thioPTM and thioPTN. We identify a thioacid cassette encoding two proteins, PtmA3 and PtmU4, responsible for carboxylate activation by coenzyme A and sulfur transfer, respectively. ThioPTM and thioPTN bind tightly to β-ketoacyl-ACP synthase II (FabF) and retain strong antibacterial activities. Density functional theory calculations of binding and solvation free energies suggest thioPTM and thioPTN bind to FabF more favorably than PTM and PTN. Additionally, thioacid cassettes are prevalent in the genomes of bacteria, implicating that thiocarboxylic acid-containing natural products are underappreciated. These results suggest that thiocarboxylic acid, as an alternative pharmacophore, and thiocarboxylic acid-containing natural products may be considered for future drug discovery.

Thioplatensimycin (thioPTM) and thioplatencin (thioPTN) are recently discovered thiocarboxylic acid congeners of the antibacterial compounds PTM and PTN. Here, the authors identify a thioacid cassette encoding PtmA3 and PtmU4 that are responsible for carboxylate activation and sulfur transfer, respectively.

Strain improvement by combined UV mutagenesis and ribosome engineering and subsequent fermentation optimization for enhanced 6'-deoxy-bleomycin Z production

The bleomycins (BLMs) are important clinical drugs extensively used in combination chemotherapy for the treatment of various cancers. Dose-dependent lung toxicity and the development of drug resistance have restricted their wide applications. 6'-Deoxy-BLM Z, a recently engineered BLM analogue with improved antitumor activity, has the potential to be developed into the next-generation BLM anticancer drug. However, its low titer in the recombinant strain Streptomyces flavoviridis SB9026 has hampered current efforts, which require sufficient compound, to pursue preclinical studies and subsequent clinical development. Here, we report the strain improvement by combined UV mutagenesis and ribosome engineering, as well as the fermentation optimization, for enhanced 6'-deoxy-BLM production. A high producer, named S. flavoviridis G-4F12, was successfully isolated, producing 6'-deoxy-BLM at above 70 mg/L under the optimized fermentation conditions, representing a sevenfold increase in comparison with that of the original producer. These findings demonstrated the effectiveness of combined empirical breeding methods in strain improvement and set the stage for sustainable production of 6'-deoxy-BLM via pilot-scale microbial fermentation.

In vivo instability of platensimycin and platencin: Synthesis and biological evaluation of urea- and carbamate-platensimycin

Platensimycin (PTM) and platencin (PTN), two natural products and promising drug leads that target bacterial and mammalian fatty acid synthases, are known to have unfavorable pharmacokinetic properties. It is not clear, however, what the metabolic fates of PTM and PTN are and no efforts have been reported to address this key roadblock in the development of these compounds as viable drug options. Here we describe the pharmacokinetics of PTM and PTN, and reveal rapid renal clearance as the primary metabolic liability with three additional sites of chemical liability: (i) amide hydrolysis, (ii) glucuronidation, and (iii) oxidation. We determined that hydrolysis is a viable clearance mechanism in vivo and synthesized two PTM analogues to address in vivo hydrolysis. Urea- and carbamate-PTM analogues showed no detectable hydrolysis in vivo, at the expense of antibacterial activity, with no further improvement in systemic exposure. The antibacterial sulfur-containing analogues PTM D1 and PTM ML14 showed significant decreases in renal clearance. These studies set the stage for continued generation of PTM and PTN analogues in an effort to improve their pharmacokinetics while retaining or improving their biological activities.

A Facile Semi-Synthetic Approach towards Halogen-Substituted Aminobenzoic Acid Analogues of Platensimycin

Platensimycin (PTM), produced by several strains of Streptomyces platensis, is a promising drug lead for infectious diseases and diabetes. The recent pilot-scale production of PTM from S. platensis SB12026 has set the stage for the facile semi-synthesis of a focused library of PTM analogues. In this study, gram-quantity of platensic acid (PTMA) was prepared by the sulfuric acid-catalyzed ethanolysis of PTM, followed by a mild hydrolysis in aqueous lithium hydroxide. Three PTMA esters were also obtained in near quantitative yields in a single step, suggesting a facile route to make PTMA aliphatic esters. 1-[Bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate (HATU)-catalyzed coupling of PTMA and 33 aminobenzoates resulted in the synthesis of 28 substituted aminobenzoate analogues of PTM, among which 26 of them were reported for the first time. Several of the PTM analogues showed weak antibacterial activity against methicillin-resistant Staphylococcus aureus. Our study supported the potential utility to integrate natural product biosynthetic and semi-synthetic approaches for structure diversification.

Antibacterial sulfur-containing platensimycin and platencin congeners from Streptomyces platensis SB12029

The platensimycin (PTM) and platencin (PTN) class of natural products are promising drug leads that target bacterial and mammalian fatty acid synthases. Natural congeners and synthetic analogues of PTM and PTN have been instrumental in determining their structure-activity relationships, with only a few analogues retaining the potencies of PTM and PTN. Here we describe the identification and isolation of two new sulfur-containing PTM congeners (3 and 5) from the engineered dual PTM-PTN overproducing Streptomyces platensis SB12029. Structure elucidation of platensimycin D1 (5), a sulfur-containing PTM pseudo-dimer, revealed the existence of its presumptive thioacid precursor (3). The unstable thioacid 3 was isolated and confirmed by structural characterization of its permethylated product (6). LC-MS analysis of crude extracts of SB12029 confirmed the presence of the thioacid analogue of PTN (4). The minimum inhibitory concentration (MIC) was determined for 5 revealing retention of the strong antibacterial activity of PTM.

Platensimycin and platencin: Inspirations for chemistry, biology, enzymology, and medicine

Natural products have served as the main source of drugs and drug leads, and natural products produced by microorganisms are one of the most prevalent sources of clinical antibiotics. Their unparalleled structural and chemical diversities provide a basis to investigate fundamental biological processes while providing access to a tremendous amount of chemical space. There is a pressing need for novel antibiotics with new mode of actions to combat the growing challenge of multidrug resistant pathogens. This review begins with the pioneering discovery and biological activities of platensimycin (PTM) and platencin (PTN), two antibacterial natural products isolated from Streptomyces platensis. The elucidation of their unique biochemical mode of action, structure-activity relationships, and pharmacokinetics is presented to highlight key aspects of their biological activities. It then presents an overview of how microbial genomics has impacted the field of PTM and PTN and revealed paradigm-shifting discoveries in terpenoid biosynthesis, fatty acid metabolism, and antibiotic and antidiabetic therapies. It concludes with a discussion covering the future perspectives of PTM and PTN in regard to natural products discovery, bacterial diterpenoid biosynthesis, and the pharmaceutical promise of PTM and PTN as antibiotics and for the treatment of metabolic disorders. PTM and PTN have inspired new discoveries in chemistry, biology, enzymology, and medicine and will undoubtedly continue to do so.

Diterpenoids from Streptomyces sp. SN194 and Their Antifungal Activity against Botrytis cinerea

Botrytis cinerea is a serious phytopathogen affecting a wide range of crops around the world. Many fungicides targeting Botrytis cinerea have failed due to the pathogen's genetic plasticity. In an effort to search for new fungicides from natural products, two new diterpenoids, named chloroxaloterpin A, 1, and B, 2, were isolated from culture broth of Streptomyces sp. SN194 along with four known diterpenoids, viguiepinol, 3, and oxaloterpins C-E, 4-6. Their structures were elucidated based on extensive MS, NMR, and X-ray crystallography analyses. Both the [(2-chlorophenyl)amino]carbonyl carbanic acyl group in 1 and the 2-[(2-chlorophenyl)amino]-2-oxo-acetyl group in 2 are discovered in natural products for the first time. All six compounds were tested against Botrytis cinerea, and chloroxaloterpin A, 1, and B, 2, demonstrated strong inhibitory activity against spore germination with EC₅₀ of 4.40 and 4.96 μg/mL, respectively.

Titer improvement and pilot-scale production of platensimycin from Streptomyces platensis SB12026

Platensimycin (PTM) and platencin (PTN), isolated from several strains of Streptomyces platensis are potent antibiotics against multi-drug resistant bacteria. PTM was also shown to have antidiabetic and antisteatotic activities in mouse models. Through a novel genome-mining method, we have recently identified six PTM and PTN dual-producing strains, and generated several mutants with improved production of PTM or PTN by inactivating the pathway-specific transcriptional repressor gene ptmR1. Among them, S. platensis SB12026 gave the highest titer of 310 mg/L for PTM. In this study, we now report titer improvement by medium and fermentation optimization and pilot-scale production and isolation of PTM from SB12026. The fermentation medium optimization was achieved by manipulating the carbon and nitrogen sources, as well as the inorganic salts. The highest titer of 1560 mg/L PTM was obtained in 15-L fermentors, using a formulated medium mainly containing soluble starch, soybean flour, morpholinepropanesulfonic acid sodium salt and CaCO3. In addition, a polyamide chromatographic step was applied to facilitate the purification and 45.14 g of PTM was successfully obtained from a 60 L scale fermentation. These results would speed up the future development of PTM as human medicine.

A comprehensive review of glycosylated bacterial natural products

A systematic analysis of all naturally-occurring glycosylated bacterial secondary metabolites reported in the scientific literature up through early 2013 is presented. This comprehensive analysis of 15 940 bacterial natural products revealed 3426 glycosides containing 344 distinct appended carbohydrates and highlights a range of unique opportunities for future biosynthetic study and glycodiversification efforts.

A genetically amenable platensimycin- and platencin-overproducer as a platform for biosynthetic explorations: a showcase of PtmO4, a long-chain acyl-CoA dehydrogenase

Platensimycin (PTM) and platencin (PTN) are members of a new class of promising drug leads that target bacterial and mammalian fatty acid synthases. We previously cloned and sequenced the PTM and PTN gene clusters, discovered six additional PTM-PTN dual producing strains, and demonstrated the dramatic overproduction of PTM and PTN by inactivating the pathway-specific regulators ptmR1 or ptnR1 in five different strains. Our ability to utilize these PTM-PTN dual overproducing strains was limited by their lack of genetic amenability. Here we report the construction of Streptomyces platensis SB12029, a genetically amenable, in-frame ΔptmR1 dual PTM-PTN overproducing strain. To highlight the potential of this strain for future PTM and PTN biosynthetic studies, we created the ΔptmR1 ΔptmO4 double mutant S. platensis SB12030. Fourteen PTM and PTN congeners, ten of which were new, were isolated from SB12030, shedding new insights into PTM and PTN biosynthesis. PtmO4, a long-chain acyl-CoA dehydrogenase, is strongly implicated to catalyze β-oxidation of the diterpenoid intermediates into the PTM and PTN scaffolds. SB12029 sets the stage for future biosynthetic and bioengineering studies of the PTM and PTN family of natural products.

Mycemycins A-E, New Dibenzoxazepinones Isolated from Two Different Streptomycetes

Five new dibenzoxazepinone derivatives, mycemycins A–E (1–5), were isolated from the ethanol extracts of mycelia of two different streptomycetes. 1 and 2 were isolated from an acidic red soil-derived strain, Streptomyces sp. FXJ1.235, and 3–5 from a gntR gene-disrupted deep-sea strain named Streptomyces olivaceus FXJ8.012Δ1741. The structures of mycemycins were elucidated by a combination of spectroscopic analyses, including 1D- and 2D-NMR techniques.

Strain prioritization for natural product discovery by a high-throughput real-time PCR method

Natural products offer unmatched chemical and structural diversity compared to other small-molecule libraries, but traditional natural product discovery programs are not sustainable, demanding too much time, effort, and resources. Here we report a strain prioritization method for natural product discovery. Central to the method is the application of real-time PCR, targeting genes characteristic to the biosynthetic machinery of natural products with distinct scaffolds in a high-throughput format. The practicality and effectiveness of the method were showcased by prioritizing 1911 actinomycete strains for diterpenoid discovery. A total of 488 potential diterpenoid producers were identified, among which six were confirmed as platensimycin and platencin dual producers and one as a viguiepinol and oxaloterpin producer. While the method as described is most appropriate to prioritize strains for discovering specific natural products, variations of this method should be applicable to the discovery of other classes of natural products. Applications of genome sequencing and genome mining to the high-priority strains could essentially eliminate the chance elements from traditional discovery programs and fundamentally change how natural products are discovered.

Eurothiocin A and B, sulfur-containing benzofurans from a soft coral-derived fungus Eurotium rubrum SH-823

Two new sulfur-containing benzofuran derivatives, eurothiocin A and B (1 and 2), along with five known compounds, zinniol (3), butyrolactone I (4), aspernolide D (5), vermistatin (6), and methoxyvermistatin (7), were isolated from the cultures of Eurotium rubrum SH-823, a fungus obtained from a Sarcophyton sp. soft coral collected from the South China Sea. The new compounds (1 and 2) share a methyl thiolester moiety, which is quite rare among natural secondary metabolites. The structures of these metabolites were assigned on the basis of detailed spectroscopic analysis. The absolute configurations of 1 and 2 were determined by comparison of the experimental and calculated electronic circular dichroism (ECD) data. Compounds 1 and 2 exhibited more potent inhibitory effects against α-glucosidase activity than the clinical α-glucosidase inhibitor acarbose. Further mechanistic analysis showed that both of them exhibited competitive inhibition characteristics.

Mechanisms of self-resistance in the platensimycin- and platencin-producing Streptomyces platensis MA7327 and MA7339 strains

Platensimycin (PTM) and platencin (PTN) are potent inhibitors of bacterial fatty acid synthases and have emerged as promising antibacterial drug leads. We previously characterized the PTM and PTN biosynthetic machineries in the Streptomyces platensis producers. We now identify two mechanisms for PTM and PTN resistance in the S. platensis producers-the ptmP3 or ptnP3 gene within the PTM-PTN or PTN biosynthetic cluster and the fabF gene within the fatty acid synthase locus. PtmP3/PtnP3 and FabF confer PTM and PTN resistance by target replacement and target modification, respectively. PtmP3/PtnP3 also represents an unprecedented mechanism for fatty acid biosynthesis in which FabH and FabF are functionally replaced by a single condensing enzyme. These findings challenge the current paradigm for fatty acid biosynthesis and should be considered in future development of effective therapeutics targeting fatty acid synthase.

Expression of the platencin biosynthetic gene cluster in heterologous hosts yielding new platencin congeners

Platensimycin (PTM) and platencin (PTN) are potent and selective inhibitors of bacterial and mammalian fatty acid synthases and have emerged as promising drug leads for both antibacterial and antidiabetic therapies. We have previously cloned and sequenced the PTM-PTN dual biosynthetic gene cluster from Streptomyces platensis MA7327 and the PTN biosynthetic gene cluster from S. platensis MA7339, the latter of which is composed of 31 genes encoding PTN biosynthesis, regulation, and resistance. We have also demonstrated that PTM or PTN production can be significantly improved upon inactivation of the pathway-specific regulator ptmR1 or ptnR1 in S. platensis MA7327 or MA7339, respectively. We now report engineered production of PTN and congeners in a heterologous Streptomyces host. Expression constructs containing the ptn biosynthetic gene cluster were engineered from SuperCos 1 library clones and introduced into five model Streptomyces hosts, and PTN production was achieved in Streptomyces lividans K4-114. Inactivation of ptnR1 was crucial for expression of the ptn biosynthetic gene cluster, thereby PTN production, in S. lividans K4-114. Six PTN congeners, five of which were new, were also isolated from the recombinant strain S. lividans SB12606, revealing new insights into PTN biosynthesis. Production of PTN in a model Streptomyces host provides new opportunities to apply combinatorial biosynthetic strategies to the PTN biosynthetic machinery for structural diversity.

Bacterial diterpene synthases: new opportunities for mechanistic enzymology and engineered biosynthesis

Diterpenoid biosynthesis has been extensively studied in plants and fungi, yet cloning and engineering diterpenoid pathways in these organisms remain challenging. Bacteria are emerging as prolific producers of diterpenoid natural products, and bacterial diterpene synthases are poised to make significant contributions to our understanding of terpenoid biosynthesis. Here we will first survey diterpenoid natural products of bacterial origin and briefly review their biosynthesis with emphasis on diterpene synthases (DTSs) that channel geranylgeranyl diphosphate to various diterpenoid scaffolds. We will then highlight differences of DTSs of bacterial and higher organism origins and discuss the challenges in discovering novel bacterial DTSs. We will conclude by discussing new opportunities for DTS mechanistic enzymology and applications of bacterial DTS in biocatalysis and metabolic pathway engineering.

Platensimycin and platencin biosynthesis in Streptomyces platensis, showcasing discovery and characterization of novel bacterial diterpene synthases

Diterpenoid natural products cover a vast chemical diversity and include many medicinally and industrially relevant compounds. All diterpenoids derive from a common substrate, (E,E,E)-geranylgeranyl diphosphate, which is cyclized into one of many scaffolds by a diterpene synthase (DTS). While diterpene biosynthesis has been extensively studied in plants and fungi, bacteria are now recognized for their production of unique diterpenoids and are likely to harbor an underexplored reservoir of new DTSs. Bacterial diterpenoid biosynthesis can be exploited for the discovery of new natural products, a better mechanistic understanding of DTSs, and the rational engineering of whole metabolic pathways. This chapter describes methods and protocols for identification and characterization of bacterial DTSs, based on our recent work with the DTSs involved in platensimycin and platencin biosynthesis.

[Advances on actinomycetic terpenoid biosynthesis]

Terpenoids are the most diverse class of natural products. Recently, a series of terpenoids with novel structures have been isolated from actinomyces. Their biosynthetic gene clusters have been identified and characterized either by direct cloning or genomic mining, which promoted investigations of their biosynthetic pathways, as well as the key enzymatic mechanisms. This paper provides a brief overview of the major research published in the last five years.

Platensimycin and platencin: promising antibiotics for future application in human medicine

Platensimycin and platencin are novel antibiotics produced by Streptomyces platensis. They are potent and non-toxic natural products active against Gram-positive pathogens, including antibiotic-resistant strains and Mycobacterium tuberculosis. They were isolated using an intriguing target-based whole-cell antisense differential sensitivity assay as inhibitors of fatty acid biosynthesis of type II. This type of biosynthesis is not present in humans. Platensimycin inhibits the elongation-condensing enzyme FabF, whereas platencin inhibits both FabF and FabH. For these antibiotics to become successful drugs, their pharmacokinetics must be improved. They have too high a rate of clearance in the body, yielding a low degree of systematic exposure. They work well when administered by continuous infusion, but this is not a useful method of delivery to patients. The two antibiotics and many analogs have been prepared by chemical synthesis. Natural congeners have also been obtained from the producing actinomycete. However, none of these molecules are as active as platensimycin and platencin. Using tools of rational metabolic engineering, superior strains have been produced making hundreds of times more antibiotic than the natural strains.

Dedicated ent-kaurene and ent-atiserene synthases for platensimycin and platencin biosynthesis

Platensimycin (PTM) and platencin (PTN) are potent and selective inhibitors of bacterial and mammalian fatty acid synthases and have emerged as promising drug leads for both antibacterial and antidiabetic therapies. Comparative analysis of the PTM and PTN biosynthetic machineries in Streptomyces platensis MA7327 and MA7339 revealed that the divergence of PTM and PTN biosynthesis is controlled by dedicated ent-kaurene and ent-atiserene synthases, the latter of which represents a new pathway for diterpenoid biosynthesis. The PTM and PTN biosynthetic machineries provide a rare glimpse at how secondary metabolic pathway evolution increases natural product structural diversity and support the wisdom of applying combinatorial biosynthesis methods for the generation of novel PTM and/or PTN analogues, thereby facilitating drug development efforts based on these privileged natural product scaffolds.

Platensimycin and platencin congeners from Streptomyces platensis

Platensimycin (1a) and platencin (2) are inhibitors of FabF and FabF/H bacterial fatty acid synthase. The discovery of natural congeners is an approach that can render a better understanding of the structure-function relationships of complex natural products. The isolation and structure elucidation of nine new congeners (11-20) of platensimycin and platencin are described from a fermentation broth of Streptomyces platensis. These hydroxylated congeners are likely derived by cytochrome P450 oxidation of the terpenoid units post-cyclization. Polar groups in the terpenoid portion of the molecule produce negative interactions with the hydrophobic pocket of FabF, resulting in poor activities. However, the discovery of these compounds serves an important purpose, not only to understand structure-function relationships, which cannot be easily accessed by chemical modification, but also to provide access to compounds that could be used for structural identification/confirmation of the oxidative trace metabolites produced in vivo during animal experiments.