The biosynthesis of indolocarbazoles in a heterologous E. coli host

Chemistry and biology of specialized metabolites produced by Actinomadura

Covering: up to the end of 2022In recent years rare Actinobacteria have become increasingly recognised as a rich source of novel bioactive metabolites. Actinomadura are Gram-positive bacteria that occupy a wide range of ecological niches. This review highlights about 230 secondary metabolites produced by Actinomadura spp., reported until the end of 2022, including their bioactivities and selected biosynthetic pathways. Notably, the bioactive compounds produced by Actinomadura spp. demonstrate a wide range of activities, including antimicrobial, antitumor and anticoccidial effects, highlighting their potential in various fields.

The modular pYT vector series employed for chromosomal gene integration and expression to produce carbazoles and glycolipids in P. putida

The expression of biosynthetic genes in bacterial hosts can enable access to high-value compounds, for which appropriate molecular genetic tools are essential. Therefore, we developed a toolbox of modular vectors, which facilitate chromosomal gene integration and expression in Pseudomonas putida KT2440. To this end, we designed an integrative sequence, allowing customisation regarding the modes of integration (random, at attTn7, or into the 16S rRNA gene), promoters, antibiotic resistance markers as well as fluorescent proteins and enzymes as transcription reporters. We thus established a toolbox of vectors carrying integrative sequences, designated as pYT series, of which we present 27 ready-to-use variants along with a set of strains equipped with unique 'landing pads' for directing a pYT interposon into one specific copy of the 16S rRNA gene. We used genes of the well-described violacein biosynthesis as reporter to showcase random Tn5-based chromosomal integration leading to constitutive expression and production of violacein and deoxyviolacein. Deoxyviolacein was likewise produced after gene integration into the 16S rRNA gene of rrn operons. Integration in the attTn7 site was used to characterise the suitability of different inducible promoters and successive strain development for the metabolically challenging production of mono-rhamnolipids. Finally, to establish arcyriaflavin A production in P. putida for the first time, we compared different integration and expression modes, revealing integration at attTn7 and expression with NagR/PnagAa to be most suitable. In summary, the new toolbox can be utilised for the rapid generation of various types of P. putida expression and production strains.

OleD Loki as a Catalyst for Hydroxamate Glycosylation

Herein we describe the ability of the permissive glycosyltransferase (GT) OleD Loki to convert a diverse set of >15 histone deacetylase (HDAC) inhibitors (HDACis) into their corresponding hydroxamate glycosyl esters. Representative glycosyl esters were subsequently evaluated in assays for cancer cell line cytotoxicity, chemical and enzymatic stability, and axolotl embryo tail regeneration. Computational substrate docking models were predictive of enzyme-catalyzed turnover and suggest certain HDACis may form unproductive, potentially inhibitory, complexes with GTs.

Leloir Glycosyltransferases in Applied Biocatalysis: A Multidisciplinary Approach

Enzymes are nature’s catalyst of choice for the highly selective and efficient coupling of carbohydrates. Enzymatic sugar coupling is a competitive technology for industrial glycosylation reactions, since chemical synthetic routes require extensive use of laborious protection group manipulations and often lack regio- and stereoselectivity. The application of Leloir glycosyltransferases has received considerable attention in recent years and offers excellent control over the reactivity and selectivity of glycosylation reactions with unprotected carbohydrates, paving the way for previously inaccessible synthetic routes. The development of nucleotide recycling cascades has allowed for the efficient production and reuse of nucleotide sugar donors in robust one-pot multi-enzyme glycosylation cascades. In this way, large glycans and glycoconjugates with complex stereochemistry can be constructed. With recent advances, LeLoir glycosyltransferases are close to being applied industrially in multi-enzyme, programmable cascade glycosylations.

The antitumor antibiotic rebeccamycin-challenges and advanced approaches in production processes

Rebeccamycin is an antibiotic and antitumor substance isolated from the filamentous bacterium Lentzea aerocolonigenes. After its discovery, investigations of rebeccamycin focused on elucidating its structure, biological activity, and biosynthetic pathway. For potential medical application, a sufficient drug supply has to be ensured, meaning that the production process of rebeccamycin plays a major role. In addition to the natural production of rebeccamycin in L. aerocolonigenes, where the complex cell morphology is an important factor for a sufficient production, rebeccamycin can also be heterologously produced or chemically synthesized. Each of these production processes has its own challenges, and first approaches to production often lead to low final product concentrations, which is why process optimizations are performed. This review provides an overview of the production of rebeccamycin and the different approaches used for rebeccamycin formation including process optimizations.

OleD Loki as a Catalyst for Tertiary Amine and Hydroxamate Glycosylation

We describe the ability of an engineered glycosyltransferase (OleD Loki) to catalyze the N-glycosylation of tertiary-amine-containing drugs and trichostatin hydroxamate glycosyl ester formation. As such, this study highlights the first bacterial model catalyst for tertiary-amine N-glycosylation and further expands the substrate scope and synthetic potential of engineered OleDs. In addition, this work could open the door to the discovery of similar capabilities among other permissive bacterial glycosyltransferases.

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.

Characterization of Early Enzymes Involved in TDP-Aminodideoxypentose Biosynthesis en Route to Indolocarbazole AT2433

The characterization of TDP-α-D-glucose dehydrogenase (AtmS8), TDP-α-D-glucuronic acid decarboxylase (AtmS9), and TDP-4-keto-α-D-xylose 2,3-dehydratase (AtmS14), involved in Actinomadura melliaura AT2433 aminodideoxypentose biosynthesis, is reported. This study provides the first biochemical evidence that both deoxypentose and deoxyhexose biosynthetic pathways share common strategies for sugar 2,3-dehydration/reduction and implicates the sugar nucleotide base specificity of AtmS14 as a potential mechanism for sugar nucleotide commitment to secondary metabolism. In addition, a re-evaluation of the AtmS9 homologue involved in calicheamicin aminodeoxypentose biosynthesis (CalS9) reveals that CalS9 catalyzes UDP-4-keto-α-D-xylose as the predominant product, rather than UDP-α-D-xylose as previously reported. Cumulatively, this work provides additional fundamental insights regarding the biosynthesis of novel pentoses attached to complex bacterial secondary metabolites.

Cytotoxic Indolocarbazoles from Actinomadura melliaura ATCC 39691

Actinomadura melliaura ATCC 39691, a strain isolated from a soil sample collected in Bristol Cove, California, is a known producer of the disaccharide-substituted AT2433 indolocarbazoles (6-9). Reinvestigation of this strain using new media conditions led to >40-fold improvement in the production of previously reported AT2433 metabolites and the isolation and structure elucidation of the four new analogues, AT2433-A3, A4, A5, and B3 (1-4). The availability of this broader set of compounds enabled a subsequent small antibacterial/fungal/cancer SAR study that revealed disaccharyl substitution, N-6 methylation, and C-11 chlorination as key modulators of bioactivity. The slightly improved anticancer potency of the newly reported N-6-desmethyl 1 (compared to 6) contrasts extensive SAR of monoglycosylated rebeccamycin-type topoisomerase I inhibitors where N-6 alkylation has contributed to improved potency and ADME. Complete 2D NMR assignments for the known metabolite BMY-41219 (5) and (13)C NMR spectroscopic data for the known analogue AT2433-B1 (7) are also provided for the first time.

Structural characterization of AtmS13, a putative sugar aminotransferase involved in indolocarbazole AT2433 aminopentose biosynthesis

AT2433 from Actinomadura melliaura is an indolocarbazole antitumor antibiotic structurally distinguished by its unique aminodideoxypentose-containing disaccharide moiety. The corresponding sugar nucleotide-based biosynthetic pathway for this unusual sugar derives from comparative genomics where AtmS13 has been suggested as the contributing sugar aminotransferase (SAT). Determination of the AtmS13 X-ray structure at 1.50-Å resolution reveals it as a member of the aspartate aminotransferase fold type I (AAT-I). Structural comparisons of AtmS13 with homologous SATs that act upon similar substrates implicate potential active site residues that contribute to distinctions in sugar C5 (hexose vs. pentose) and/or sugar C2 (deoxy vs. hydroxyl) substrate specificity.

Facile chemoenzymatic strategies for the synthesis and utilization of S-adenosyl-(L)-methionine analogues

A chemoenzymatic platform for the synthesis of S-adenosyl-L-methionine (SAM) analogues compatible with downstream SAM-utilizing enzymes is reported. Forty-four non-native S/Se-alkylated Met analogues were synthesized and applied to probing the substrate specificity of five diverse methionine adenosyltransferases (MATs). Human MAT II was among the most permissive of the MATs analyzed and enabled the chemoenzymatic synthesis of 29 non-native SAM analogues. As a proof of concept for the feasibility of natural product "alkylrandomization", a small set of differentially-alkylated indolocarbazole analogues was generated by using a coupled hMAT2-RebM system (RebM is the sugar C4'-O-methyltransferase that is involved in rebeccamycin biosynthesis). The ability to couple SAM synthesis and utilization in a single vessel circumvents issues associated with the rapid decomposition of SAM analogues and thereby opens the door for the further interrogation of a wide range of SAM utilizing enzymes.

Discovery and synthetic refactoring of tryptophan dimer gene clusters from the environment

Here we investigate bacterial tryptophan dimer (TD) biosynthesis by probing environmental DNA (eDNA) libraries for chromopyrrolic acid (CPA) synthase genes. Functional and bioinformatics analyses of TD clusters indicate that CPA synthase gene sequences diverge in concert with the functional output of their respective clusters, making this gene a powerful tool for guiding the discovery of novel TDs from the environment. Twelve unprecedented TD biosynthetic gene clusters that can be arranged into five groups (A-E) based on their ability to generate distinct TD core substructures were recovered from eDNA libraries. Four of these groups contain clusters from both cultured and culture independent studies, while the remaining group consists entirely of eDNA-derived clusters. The complete synthetic refactoring of a representative gene cluster from the latter eDNA specific group led to the characterization of the erdasporines, cytotoxins with a novel carboxy-indolocarbazole TD substructure. Analysis of CPA synthase genes in crude eDNA suggests the presence of additional TD gene clusters in soil environments.

Expression of biosynthetic gene clusters in heterologous hosts for natural product production and combinatorial biosynthesis

Expression of biosynthetic gene clusters in heterologous hosts for natural product production and combinatorial biosynthesis is playing an increasingly important role in natural product-based drug discovery and development programmes. This review highlights the requirements and challenges associated with this conceptually simple strategy of using surrogate hosts for the production of natural products in good yields and for the generation of novel analogues by combinatorial biosynthesis methods, taking advantage of the recombinant DNA technologies and tools available in the model hosts. Specific topics addressed include: i) the mobilisation of biosynthetic gene clusters using different vector systems; ii) the selection of suitable model heterologous hosts; iii) the requirement of post-translational protein modifications and precursor supply within the model hosts; iv) the influence of promoters and pathway regulators; and v) the choice of suitable fermentation conditions. Lastly, the use of heterologous expression in combinatorial biosynthesis is addressed. Future directions for model heterologous host engineering and the optimisation of natural product biosynthetic gene cluster expression in heterologous hosts are also discussed.

Plasmid encoded antibiotics inhibit protozoan predation of Escherichia coli K12

Bacterial plasmids and phages encode the synthesis of toxic molecules that inhibit protozoan predation. One such toxic molecule is violacein, a purple pigmented, anti-tumour antibiotic produced by the Gram-negative soil bacterium Chromobacterium violaceum. In the current experiments a range of Escherichia coli K12 strains were genetically engineered to produce violacein and a number of its coloured, biosynthetic intermediates. A bactivorous predatory protozoan isolate, Colpoda sp.A4, was isolated from soil and tested for its ability to 'graze' on various violacein producing strains of E. coli K12. A grazing assay was developed based on protozoan "plaque" formation. Using this assay, E. coli K12 strains producing violacein were highly resistant to protozoan predation. However E. coli K12 strains producing violacein intermediates, showed low or no resistance to predation. In separate experiments, when either erythromycin or pentachlorophenol were added to the plaque assay medium, protozoan predation of E. coli K12 was markedly reduced. The inhibitory effects of these two molecules were removed if E. coli K12 strains were genetically engineered to inactivate the toxic molecules. In the case of erythromycin, the E. coli K12 assay strain was engineered to produce an erythromycin inactivating esterase, PlpA. For pentachlorophenol, the E. coli K12 assay strain was engineered to produce a PCP inactivating enzyme pentachlorophenol-4-monooxygenase (PcpB). This study indicates that in environments containing large numbers of protozoa, bacteria which use efflux pumps to remove toxins unchanged from the cell may have an evolutionary advantage over bacteria which enzymatically inactivate toxins.

Molecular insights on the biosynthesis of antitumour compounds by actinomycetes

Natural products are traditionally the main source of drug leads. In particular, many antitumour compounds are either natural products or derived from them. However, the search for novel antitumour drugs active against untreatable tumours, with fewer side-effects or with enhanced therapeutic efficiency, is a priority goal in cancer chemotherapy. Microorganisms, particularly actinomycetes, are prolific producers of bioactive compounds, including antitumour drugs, produced as secondary metabolites. Structural genes involved in the biosynthesis of such compounds are normally clustered together with resistance and regulatory genes, which facilitates the isolation of the gene cluster. The characterization of these clusters has represented, during the last 25 years, a great source of genes for the generation of novel derivatives by using combinatorial biosynthesis approaches: gene inactivation, gene expression, heterologous expression of the clusters or mutasynthesis. In addition, these techniques have been also applied to improve the production yields of natural and novel antitumour compounds. In this review we focus on some representative antitumour compounds produced by actinomycetes covering the genetic approaches used to isolate and validate their biosynthesis gene clusters, which finally led to generating novel derivatives and to improving the production yields.

Stable high level expression of the violacein indolocarbazole anti-tumour gene cluster and the Streptomyces lividans amyA gene in E. coli K12

Previous studies showed that when pPSX-vioABCDE was used to transform E. coli K12 DH5alpha the strain retained the plasmid even after 100 generations of unselected growth but produced a low level of the anti-tumour antibiotic violacein. Markedly higher levels of violacein synthesis were obtained from E. coli K12 DH5alpha pUC18-vioABCDE and Sphingomonas sp. JMP4092 pPSX-vioABCDE. Unfortunately, both strains were extremely unstable regardless of presence or absence of antibiotic selection to retain the plasmid. The current study was undertaken to determine if strains of E. coli K12 could be isolated which stably over produce violacein. When a range of E. coli K12 strains were transformed with pPSX-vioABCDE, most produced small amounts of violacein. However, a small number of related strains of E. coli K12 JM101, JM105 and JM109 not only over-produced violacein, but also maintained the high stability. In addition, E. coli K12 JM109 strongly expressed an alpha amylase gene (amyA) from Streptomyces lividans indicating that the S. lividans amyA promoter is highly active in E. coli K12 JM109. In another set of experiments, a violacein overproduction mutation (opv-1) of the plasmid pPSX-vioABCDE was isolated which enabled E. coli K12 DH5alpha to overproduce violacein while retaining high stability. The plasmid pPSX-vioABCDEopv-1 possesses a single base pair deletion in the promoter region of the violacein operon. By combining the over producing strain E. coli K12 JM109 and the over producing plasmid pPSX-vioABCDEopv-1, a stable hyper producing strain (E. coli K12 JM109 pPSX-vioABCDEopv-1) was constructed. Finally, two additional stable vectors, pPSX10 and pPSX20, were constructed to facilitate subcloning and functional analysis studies.

Divergent pathways in the biosynthesis of bisindole natural products

Two molecules of the amino acid L-tryptophan are the biosynthetic precursors to a class of natural products named the "bisindoles." Hundreds of these bisindole molecules have been isolated from natural sources, and many of these molecules have potent medicinal properties. Recent studies have clarified the biosynthetic construction of six bisindole molecules, revealing novel enzymatic mechanisms and leading to combinatorial synthesis of new bisindole compounds. Collectively, these results provide a vantage point for understanding how much of the diversity of the bisindole class is generated from a small number of diverging pathways from L-tryptophan, as well as enabling identification of bisindoles that are likely derived via completely distinct biosynthetic pathways.

Generation of potent and selective kinase inhibitors by combinatorial biosynthesis of glycosylated indolocarbazoles

We report the generation of novel glycosylated indolocarbazoles by combinatorial biosynthesis, and the identification of two novel potent and selective compounds inhibitors of JAK2 and Ikkb kinases.

Complete sequence and analysis of the stability functions of pPSX, a vector that allows stable cloning and expression of Streptomycete genes in Escherichia coli K12

The broad host range, cloning and expression vector pPSX has been completely sequenced and analysed. pPSX is 14.7kb in length and contains the fusion of two continuous segments of the parental 34kb, IncW plasmid pR388. pPSX appears to have retained at least three sets of gene/s which contribute in different ways to plasmid stability. The first of these parB, is a known participant in the partitioning of low-copy number plasmids. While the adjoining gene, orf35, has high homology with kfrA, a putative plasmid nucleoid organiser that is often associated with the ParAB family of proteins. The second set of genes; orfs18, 19, 20, whose exact functions are not clear, have homology to the stability operons of both IncW and IncN plasmids. The third is the resolvase, resP, which may resolve plasmid multimers that can lead to plasmid instability. pPSX is a small, stable cloning vector good for cloning and expression of a wide range of genes, including those from streptomycetes.

Antitumor compounds from actinomycetes: from gene clusters to new derivatives by combinatorial biosynthesis

Covering: up to October 2008. Antitumor compounds produced by actinomycetes and novel derivatives generated by combinatorial biosynthesis are reviewed (with 318 references cited.) The different structural groups for which the relevant gene clusters have been isolated and characterized are reviewed, with a description of the strategies used for the generation of the novel derivatives and the activities of these compounds against tumor cell lines.

Production of chromopyrrolic acid by coexpression of inkOD in a heterologous host Streptomyces albus

Two ink genes, inkO and inkD, responsible for the earliest steps of K252a biosynthetic pathway, from Nonomurea longicantena JCM 11136 were heterologously coexpressed in Streptomycesalbus J1074. The resultant strain accumulated compound that was purified by HPLC and studied by NMR. Coexpression of inkOD yielded chromopyrrolic acid, the key intermediate in an indolocarbazole biosynthesis.

Characterization of CalE10, the N-oxidase involved in calicheamicin hydroxyaminosugar formation

As the first in vitro characterization of a sugar N-oxidase, this study establishes CalE10 as the key oxidase involved in calicheamicin hydroxylamino glycoside formation. This study confirms that oxidation occurs at the sugar nucleotide stage prior to glycosyltransfer, and substrate specificity studies reveal CalE10-catalyzed oxidation to be regiospecific and to present trace amounts of the corresponding nitrosugar in vitro. This work also sets a precedent for the future study of other N-oxidases involved in hydroxylamino-, nitroso-, and/or nitrosugar formation.

Amide N-glycosylation by Asm25, an N-glycosyltransferase of ansamitocins

Ansamitocins are potent antitumor maytansinoids produced by Actinosynnema pretiosum. Their biosynthesis involves the initial assembly of a macrolactam polyketide, followed by a series of postpolyketide synthase (PKS) modifications. Three ansamitocin glycosides were isolated from A. pretiosum and fully characterized structurally as novel ansamitocin derivatives, carrying a beta-D-glucosyl group attached to the macrolactam amide nitrogen in place of the N-methyl group. By gene inactivation and complementation, asm25 was identified as the N-glycosyltransferase gene responsible for the macrolactam amide N-glycosylation of ansamitocins. Soluble, enzymatically active Asm25 protein was obtained from asm25-expressing E. coli by solubilization from inclusion bodies. Its optimal reaction conditions, including temperature, pH, metal ion requirement, and Km/Kcat, were determined. Asm25 also showed broad substrate specificity toward other ansamycins and synthetic indolin-2-ones. To the best of our knowledge, this represents the first in vitro characterization of a purified antibiotic N-glycosyltransferase.

Cladoniamides A-G, tryptophan-derived alkaloids produced in culture by Streptomyces uncialis

Cladoniamides A-G (3- 9) have been isolated from cultures of Streptomyces uncialis, and their structures have been elucidated by a combination of spectroscopic analysis and an X-ray diffraction analysis of cladoniamide A (3). The cladoniamides have unprecedented rearranged and degraded alkaloid skeletons with putative biogenetic origins from indolocarbazole precursors. Cladoniamide G (9) is cytotoxic to MCF-7 cells in vitro at 10 microg/mL.

pPSY: a vector for the stable cloning and expression of streptomycete single gene phenotypes in Escherichia coli

pPSY is a 12kb cloning vector derived from the IncW plasmid R388, which provides a rapid and easy way to stably clone phenotypes encoded in DNA segments <10kb. In the present study three different genes were amplified by PCR, cloned into pGEM-T Easy and sub-cloned into the EcoRI site of pPSY. The first gene, vioA, is a FAD-dependent l-tryptophan amino acid oxygenase from the high G+C Gram-negative bacterium Chromobacterium violaceum. VioA is involved in the synthesis of the indolocarbazole antitumour antibiotic violacein. It was found that vioA was strongly expressed in Escherichia coli from its native promoter. Two other genes encoding recombinase A (recA) and an amylase (amyA), derived from the high G+C Gram-positive streptomycete, Streptomyces lividans, were also tested. Despite recA lacking its native promoter sequence, it was strongly expressed in E. coli using the lac promoter of pGEM-T Easy. Similar to vioA, S. lividansamyA was strongly expressed in E. coli from its native promoter. Unlike pGEM-T Easy, pPSY stably maintained all three genes without the requirement for antibiotic selection. These results demonstrate the applicability of pPSY as a stable amplicon cloning vector for the expression of heterologous genes in E. coli.

The biosynthetic genes encoding for the production of the dynemicin enediyne core in Micromonospora chersina ATCC53710

Dynemicin is a novel anthraquinone-fused member of the 10-membered enediyne antitumor antibiotic family. The development of a genetic system for the dynemicin producer Micromonospora chersina confirmed, for the first time, the requirement of the putative enediyne core biosynthetic genes (dynE8, U14 and U15) and a tailoring oxidase gene (orf23) for dynemicin production. Cloning and sequence analysis of a 76 kb of genomic sequence region containing dynE8 revealed a variety of genes conserved among known enediyne loci. Surprisingly, this fragment and flanking chromosomal DNA lacked any obvious genes encoding for the biosynthesis of the anthraquinone, suggesting that the location of genes encoding for the biosynthesis of the dynemicin enediyne core and the dynemicin anthraquinone are chromosomally distinct. The demonstrated trace production of a shunt product from mutant strain QGD23 (Deltaorf23) also sets the stage for subsequent studies to delineate the key steps in enediyne core biosynthesis and tailoring.

Genetic organization of the biosynthetic gene cluster for the indolocarbazole K-252a in Nonomuraea longicatena JCM 11136

Indolocarbazole metabolite K-252a is a natural product that was previously reported as a potent protein kinase C inhibitor with in vitro and in vivo potency. From a biosynthetic viewpoint, this compound possesses structurally interesting features such as an unusual furanosyl sugar moiety, which are absent in the well-studied staurosporine and rebeccamycin. A cosmid library from genomic DNA of Nonomuraea longicatena JCM 11136 was constructed and screened for the presence of genes to be involved in the biosynthesis of indolocarbazole K-252a. Using as a probe an internal fragment of vioB, a Chromobacterium violaceum gene encoding a multifunctional enzyme that catalyzes tryptophan decarboxylation and condensation reaction in violacein biosynthesis, we isolated a DNA region that directed the biosynthesis of K-252a when introduced into the heterologous expression host Streptomyces albus. Sequence analysis of 45 kb revealed genes for indolocarbazole core formation, glycosylation, and sugar methylation, as well as a regulatory gene and two resistance/secretion genes. The cloned genes should help to elucidate the molecular basis for indolocarbazole biosynthesis and generate new indolocarbazole analogues by genetic engineering.

pPSX: a novel vector for the cloning and heterologous expression of antitumor antibiotic gene clusters

A cosmid cloning vector has been constructed that demonstrates high levels of segregational stability in Escherichia coli K12. pPSX is a 14-kilobase vector derived from the IncW plasmid pR388. pPSX is highly stable in E. coli in the absence of antibiotic selection, even while expressing the toxic indolocarbazole antitumor antibiotic violacein. The incorporation of the lambdacos sequence enables construction of cosmid libraries with inserts ranging from 24 to 36kb. The inclusion of a lacZalpha multiple cloning site (MCS) allows blue/white screening. pPSX cosmids can be extracted from the host cell with commercial plasmid extraction kits facilitating downstream analysis, sequencing and sub-cloning. pPSX can be transferred to a variety of heterologous hosts by either electroporation or mobilization from E. coli S17-1. While it is unstable in non-E. coli hosts without antibiotic selection, heterologous host strains such as Rhodobacter sphaeroides and Pseudomonas stutzeri will maintain the plasmid under antibiotic selection to allow screening of expressed inserts. pPSX provides the benefits of large insert sizes with high stability to allow cloning of chemotherapeutic gene clusters in E. coli and a range of other heterologous hosts.

Deciphering indolocarbazole and enediyne aminodideoxypentose biosynthesis through comparative genomics: insights from the AT2433 biosynthetic locus

AT2433, an indolocarbazole antitumor antibiotic, is structurally distinguished by its aminodideoxypentose-containing disaccharide and asymmetrically halogenated N-methylated aglycon. Cloning and sequence analysis of AT2433 gene cluster and comparison of this locus with that encoding for rebeccamycin and the gene cluster encoding calicheamicin present an opportunity to study the aminodideoxypentose biosynthesis via comparative genomics. The locus was confirmed via in vitro biochemical characterization of two methyltransferases--one common to AT2433 and rebeccamycin, the other unique to AT2433--as well as via heterologous expression and in vivo bioconversion experiments using the AT2433 N-glycosyltransferase. Preliminary studies of substrate tolerance for these three enzymes reveal the potential to expand upon the enzymatic diversification of indolocarbazoles. Moreover, this work sets the stage for future studies regarding the origins of the indolocarbazole maleimide nitrogen and indolocarbazole asymmetry.

Indolocarbazole natural products: occurrence, biosynthesis, and biological activity

The indolocarbazole family of natural products, including the biosynthetically related bisindolylmaleimides, is reviewed (with 316 references cited). The isolation of indolocarbazoles from natural sources and the biosynthesis of this class of compounds are thoroughly reviewed, including recent developments in molecular genetics, enzymology and metabolic engineering. The biological activities and underlying modes of action displayed by natural and synthetic indolocarbazoles is also presented, with an emphasis on the development of analogs that have entered clinical trials for its future use against cancer or other diseases.

Deciphering the late steps in the biosynthesis of the anti-tumour indolocarbazole staurosporine: sugar donor substrate flexibility of the StaG glycosyltransferase

The indolocarbazole staurosporine is a potent inhibitor of a variety of protein kinases. It contains a sugar moiety attached through C-N linkages to both indole nitrogen atoms of the indolocarbazole core. Staurosporine biosynthesis was reconstituted in vivo in a heterologous host Streptomyces albus by using two different plasmids: the 'aglycone vector' expressing a set of genes involved in indolocarbazole biosynthesis together with staG (encoding a glycosyltransferase) and/or staN (coding for a P450 oxygenase), and the 'sugar vector' expressing a set of genes responsible for the biosynthesis of the sugar moiety. Attachment of the sugar to the two indole nitrogens of the indolocarbazole core was dependent on the combined action of StaG and StaN. When StaN was absent, the sugar was attached only to one of the nitrogen atoms, through an N-glycosidic linkage, as in the indolocarbazole rebeccamycin. The StaG glycosyltransferase showed flexibility with respect to the sugar donor. When the 'sugar vector' was substituted by constructs directing the biosynthesis of l-rhamnose, L-digitoxose, L-olivose and D-olivose, respectively, StaG and StaN were able to transfer and attach all of these sugars to the indolocarbazole aglycone.

RebG- and RebM-Catalyzed Indolocarbazole Diversification

Rebeccamycin and staurosporine represent two broad classes of indolocarbazole glycoside natural products with antitumor properties. Based upon previous sequence annotation and in vivo studies, rebG encodes for the rebeccamycin N-glucosyltransferase, and rebM for the requisite 4'-O-methyltransferase. In the current study, an efficient in vivo biotransformation system for RebG was established in both Streptomyces lividans and Escherichia coli. Bioconversion experiments revealed RebG to glucosylate a set of indolocarbazole surrogates, the products of which could be further modified by in vitro RebM-catalyzed 4'-O-methylation. Both RebG and RebM displayed substrate promiscuity, and evidence for a remarkable lack of RebG regioselectivity in the presence of asymmetric substrates is also provided. In the context of the created indolocarbazole analogues, cytotoxicity assays also highlight the importance of 4'-O-methylation for their biological activity.

Engineering biosynthetic pathways to generate antitumor indolocarbazole derivatives

The indolocarbazole family of natural products is a source of lead compounds with potential therapeutic applications in the treatment of cancer and neurodegenerative disorders. Rebeccamycin and staurosporine are two members of this family, which are produced by different actinomycete strains. Although both compounds display antitumor activity, their distinct structural features determine different modes of action: rebeccamycin targets DNA topoisomerase I, while staurosporine is a protein kinase inhibitor. Here we examine the biosyntheses of rebeccamycin and staurosporine while we summarize our recent work concerning (a) identification and characterization of genes involved in the biosynthesis of indolocarbazoles in actinomycetes, and (b) generation of novel indolocarbazole derivatives in microorganisms by combinatorial biosynthesis.

Biosynthesis of Indocarbazostatin B, Incorporation of D-[U-13C] Glucose and L-[2-13C] Tryptophan

High incorporation of D-[U-13C] glucose and L-[indole-2-13C] tryptophan into indocarbazostatin B (2) was observed in a biosynthetic study using a mutant strain, Streptomyces sp. MUV-7-8. The original strain, Streptomyces sp. TA-0403 produced a small amount of indocarbazostatin (1) and indocarbazostatin B (2), which displayed potent biological activities. To facilitate biosynthetic studies, we selected high indocarbazostatin producing mutant strains. The first mutants, Streptomyces sp. MUV-6-83 and MUV-6-17, produced indocarbazostatins C (3) and D (4) as well as 1 and 2. When the production medium was supplemented with D-tryptophan, the MUV-6-17 mutant produced K252c (5), whereas when L-tryptophan was added, it produced K252d (6). On further UV treatment of the mutant strain MUV-6-83, we finally obtained a new mutant producer, Streptomyces sp. MUV-7-8, that produced 2 as a major metabolite with higher productivity. This mutant producer enabled us to do a feeding experiment of the envisioned precursors, glucose and tryptophan.

Robust in vitro activity of RebF and RebH, a two-component reductase/halogenase, generating 7-chlorotryptophan during rebeccamycin biosynthesis

The indolocarbazole antitumor agent rebeccamycin is modified by chlorine atoms on each of two indole moieties of the aglycone scaffold. These halogens are incorporated during the initial step of its biosynthesis from conversion of L-Trp to 7-chlorotryptophan. Two genes in the biosynthetic cluster, rebF and rebH, are predicted to encode the flavin reductase and halogenase components of an FADH2-dependent halogenase, a class of enzymes involved in the biosynthesis of numerous halogenated natural products. Here, we report that, in the presence of O2, chloride ion, and L-Trp as cosubstrates, purified RebH displays robust regiospecific halogenating activity to generate 7-chlorotryptophan over at least 50 catalytic cycles. Halogenation by RebH required the addition of RebF, which catalyzes the NADH-dependent reduction of FAD to provide FADH2 for the halogenase. Maximal rates were achieved at a RebF/RebH ratio of 3:1. In air-saturated solutions, a k(cat) of 1.4 min(-1) was observed for the RebF/RebH system but increased at least 10-fold in low-pO2 conditions. RebH was also able to use bromide ions to generate monobrominated Trp. The demonstration of robust chlorinating activity by RebF/RebH sets up this system for the probing of mechanistic questions regarding this intriguing class of enzymes.

Molecular analysis of the rebeccamycin L-amino acid oxidase from Lechevalieria aerocolonigenes ATCC 39243

Rebeccamycin, a member of the tryptophan-derived indolocarbazole family, is produced by Lechevalieria aerocolonigenes ATCC 39243. The biosynthetic pathway that specifies biosynthesis of this important metabolite is comprised of 11 genes spanning 18 kb of DNA. A presumed early enzyme involved in elaboration of the rebeccamycin aglycone is encoded by rebO, located at the left-hand region of the reb gene cluster. The deduced protein product, RebO (51.9 kDa), is an L-amino acid oxidase (L-AAO) that has 27% identity to an L-AAO from Scomber japonicus (animal, mackerel) and is a member of the family of FAD-dependent oxidase enzymes. In order to study the biochemical properties of this key enzyme, the rebO gene was overexpressed and purified from Escherichia coli. Biochemical characterization showed that RebO is dimeric, with a molecular mass of approximately 101 kDa. Further analysis revealed that the enzyme contains a noncovalently bound FAD cofactor and is reoxidized at the expense of molecular oxygen by producing one molecule of hydrogen peroxide. Based on kinetic studies, RebO shows significant preference for 7-chloro-L-tryptophan, suggesting its likely role as the natural early pathway substrate. Furthermore, the native RebO enzyme has evident, albeit limited, flexibility as shown by bioconversion studies with unnatural substrates. This work provides the first analysis of a structural enzyme involved in construction of this important class of indolocarbazole natural products.

Combinatorial biosynthesis of antitumor indolocarbazole compounds

Rebeccamycin and staurosporine are natural products with antitumor properties, which belong to the family of indolocarbazole alkaloids. An intense effort currently exists for the generation of indolocarbazole derivatives for the treatment of several diseases, including cancer and neurodegenerative disorders. Here, we report a biological process based on combinatorial biosynthesis for the production of indolocarbazole compounds (or their precursors) in engineered microorganisms as a complementary approach to chemical synthesis. We have dissected and reconstituted the entire biosynthetic pathway for rebeccamycin in a convenient actinomycete host, Streptomyces albus. This task was achieved by coexpressing different combinations of genes isolated from the rebeccamycin-producing microorganism. Also, a gene (staC) was identified in staurosporine-producing microbes and was shown to have a key role to differentiate the biosynthetic pathways for the two indolocarbazoles. Last, incorporation of the pyrH and thal genes, encoding halogenases from different microorganisms, resulted in production of derivatives with chlorine atoms at novel positions. We produced >30 different compounds by using the recombinant strains generated in this work.

Natural product glycorandomization

glycorandomization is a chemoenzymatic strategy that overcomes the limitations in natural product derivatization associated with both solely chemistry-based approaches or in vivo engineering. In this article we present the basic strategies for glycorandomization development as a next-generation tool in drug discovery.

Accessing Natural Products by Combinatorial Biosynthesis

Enhancement and selective production of the protein phosphatase IIa inhibitor phoslactomycin (PLM) B by rational engineering of the PLM biosynthetic pathway highlights the effectiveness of combinatorial biosynthesis as a promising way to prepare complex natural products and their analogs.