Oxidative cleavage of premithramycin B is one of the last steps in the biosynthesis of the antitumor drug mithramycin

Metathramycin, a new bioactive aureolic acid discovered by heterologous expression of a metagenome derived biosynthetic pathway

Bacterial natural products have been a rich source of bioactive compounds for drug development, and advances in DNA sequencing, informatics and molecular biology have opened new avenues for their discovery. Here, we describe the isolation of an aureolic acid biosynthetic gene cluster from a metagenome library derived from a New Zealand soil sample. Heterologous expression of this pathway in Streptomyces albus resulted in the production and isolation of two new aureolic acid compounds, one of which (metathramycin, 6) possesses potent bioactivity against a human colon carcinoma cell line (HCT-116, IC₅₀ = 14.6 nM). As metathramycin was a minor constituent of the fermentation extract, its discovery relied on a combination of approaches including bioactivity guided fractionation, MS/MS characterisation and pathway engineering. This study not only demonstrates the presence of previously uncharacterised aureolic acids in the environment, but also the value of an integrated natural product discovery approach which may be generally applicable to low abundance bioactive metabolites.

Penipyranicins A-C: Antibacterial Methylpyran Polyketides from a Hydrothermal Spring Sediment Penicillium sp

Four new aromatic polyketides (1-4) were isolated from Penicillium sp. RO-11, obtained from the sediment of a hydrothermal spring in the southwestern region of Saudi Arabia. The new compounds are penipyranicins A-C (1-3), characterized by a 4-methyl-4H-pyran moiety, a structural motif unprecedented among fungal polyketides, and the naphthopyrone derivative isopyrenulin (4). The structures of the new compounds were elucidated on the basis of data from mass spectrometry, 1D and 2D NMR analysis, and comparison between experimental and time-dependent density functional theory-calculated electronic circular dichroism spectra. A plausible biosynthetic pathway connecting penipyranicins and isopyrenulin is proposed. The isolated compounds were active against Gram-positive and Gram-negative bacteria.

Biosynthesis of aromatic polyketides in microorganisms using type II polyketide synthases

Aromatic polyketides have attractive biological activities and pharmacological properties. Different from other polyketides, aromatic polyketides are characterized by their polycyclic aromatic structure. The biosynthesis of aromatic polyketides is usually accomplished by the type II polyketide synthases (PKSs), which produce highly diverse polyketide chains by sequential condensation of the starter units with extender units, followed by reduction, cyclization, aromatization and tailoring reactions. Recently, significant progress has been made in characterization and engineering of type II PKSs to produce novel products and improve product titers. In this review, we briefly summarize the architectural organizations and genetic contributions of PKS genes to provide insight into the biosynthetic process. We then review the most recent progress in engineered biosynthesis of aromatic polyketides, with emphasis on generating novel molecular structures. We also discuss the current challenges and future perspectives in the rational engineering of type II PKSs for large scale production of aromatic polyketides.

Discovery of a Cryptic Intermediate in Late Steps of Mithramycin Biosynthesis

MtmOIV and MtmW catalyze the final two reactions in the mithramycin (MTM) biosynthetic pathway, the Baeyer-Villiger opening of the fourth ring of premithramycin B (PMB), creating the C3 pentyl side chain, strictly followed by reduction of the distal keto group on the new side chain. Unexpectedly this results in a C2 stereoisomer of mithramycin, iso-mithramycin (iso-MTM). Iso-MTM undergoes a non-enzymatic isomerization to MTM catalyzed by Mg²⁺ ions. Crystal structures of MtmW and its complexes with co-substrate NADPH and PEG, suggest a catalytic mechanism of MtmW. The structures also show that a tetrameric assembly of this enzyme strikingly resembles the ring-shaped β subunit of a vertebrate ion channel. We show that MtmW and MtmOIV form a complex in the presence of PMB and NADPH, presumably to hand over the unstable MtmOIV product to MtmW, yielding iso-MTM, as a potential self-resistance mechanism against MTM toxicity.

How mithramycin stereochemistry dictates its structure and DNA binding function

An aureolic acid natural product mithramycin (MTM) has been known for its potent antineoplastic properties. MTM inhibits cell growth by binding in the minor groove of double-stranded DNA as a dimer, in which the two molecules of MTM are coordinated to each other through a divalent metal ion. A crystal structure of an MTM analogue, MTM SA-Phe, in the active metal ion-coordinated dimeric form demonstrates how the stereochemical features of MTM define the helicity of the dimeric scaffold for its binding to a right-handed DNA double helix. We also show crystallographically and biochemically that MTM, but not MTM SA-Phe, can be inactivated by boric acid through formation of a large macrocyclic species, in which two molecules of MTM are crosslinked to each other through 3-side chain-boron-sugar intermolecular bonds. We discuss these structural and biochemical properties in the context of MTM biosynthesis and the design of MTM analogues as anticancer therapeutics.

Biosynthetic Baeyer-Villiger Chemistry Enables Access to Two Anthracene Scaffolds from a Single Gene Cluster in Deep-Sea-Derived Streptomyces olivaceus SCSIO T05

Four known compounds, rishirilide B (1), rishirilide C (2), lupinacidin A (3), and galvaquinone B (4), representing two anthracene scaffolds typical of aromatic polyketides, were isolated from a culture of the deep-sea-derived Streptomyces olivaceus SCSIO T05. From the S. olivaceus producer was cloned and sequenced the rsd biosynthetic gene cluster (BGC) that drives rishirilide biosynthesis. The structural gene rsdK₂ inactivation and heterologous expression of the rsd BGC confirmed the single rsd BGC encodes construction of 1-4 and, thus, accounts for two anthracene scaffolds. Precursor incubation experiments with ¹³C-labeled acetate revealed that a Baeyer-Villiger-type rearrangement plays a central role in construction of 1-4. Two luciferase monooxygenase components, along with a reductase component, are presumably involved in the Baeyer-Villiger-type rearrangement reaction enabling access to the two anthracene scaffold variants. Engineering of the rsd BGC unveiled three SARP family transcriptional regulators, enhancing anthracene production. Inactivation of rsdR₄, a MarR family transcriptional regulator, failed to impact production of 1-4, although production of 3 was slightly improved; most importantly rsdR₄ inactivation led to the new adduct 6 in high titer. Notably, inactivation of rsdH, a putative amidohydrolase, substantially improved the overall titers of 1-4 by more than 4-fold.

Manipulation of two regulatory genes for efficient production of chromomycins in Streptomyces reseiscleroticus

Background

Regulatory genes play critical roles in natural product biosynthetic pathways. Chromomycins are promising anticancer natural products from actinomycetes. This study is aimed to create an efficient strain for production of these molecules by manipulating the regulatory genes.

Results

A putative but silent chromomycin biosynthetic gene cluster was discovered in Streptomyces reseiscleroticus. Heterologous expression of the ketosynthase, chain length factor, and acyl carrier protein in Streptomyces lividans confirmed that they are responsible for the assembly of a decaketide. Two regulatory genes are present in this gene cluster, including SARP-type activator SrcmRI and PadR-like repressor SrcmRII. Either overexpression of SrcmRI or disruption of SrcmRII turned on the biosynthetic pathway of chromomycins. The production titers of chromomycin A₃/A₂ in R5 agar in these two strains reached 8.9 ± 1.2/13.2 ± 1.6 and 49.3 ± 4.3/53.3 ± 3.6 mg/L, respectively. An engineered strain was then constructed with both SrcmRII disruption and SrcmRI overexpression, which produced chromomycins A₃ and A₂ in R5 agar at 69.4 ± 7.6 and 81.7 ± 7.2 mg/L, respectively. Optimization of the culture conditions further increased the titers of chromomycins A₃ and A₂ respectively to 145.1 ± 15.3 and 158.3 ± 15.4 mg/L in liquid fermentation.

Conclusions

This work revealed the synergistic effect of manipulation of pathway repressor and activator genes in the engineering of a natural product biosynthetic pathway. The resulting engineered strain showed the highest production titers of chromomycins by a strain of Streptomyces, providing an efficient way to produce these pharmaceutically valuable molecules.

Electronic supplementary material

The online version of this article (10.1186/s13036-018-0103-x) contains supplementary material, which is available to authorized users.

Dimerization and DNA recognition rules of mithramycin and its analogues

The antineoplastic and antibiotic natural product mithramycin (MTM) is used against cancer-related hypercalcemia and, experimentally, against Ewing sarcoma and lung cancers. MTM exerts its cytotoxic effect by binding DNA as a divalent metal ion (Me(2+))-coordinated dimer and disrupting the function of transcription factors. A precise molecular mechanism of action of MTM, needed to develop MTM analogues selective against desired transcription factors, is lacking. Although it is known that MTM binds G/C-rich DNA, the exact DNA recognition rules that would allow one to map MTM binding sites remain incompletely understood. Towards this goal, we quantitatively investigated dimerization of MTM and several of its analogues, MTM SDK (for Short side chain, DiKeto), MTM SA-Trp (for Short side chain and Acid), MTM SA-Ala, and a biosynthetic precursor premithramycin B (PreMTM B), and measured the binding affinities of these molecules to DNA oligomers of different sequences and structural forms at physiological salt concentrations. We show that MTM and its analogues form stable dimers even in the absence of DNA. All molecules, except for PreMTM B, can bind DNA with the following rank order of affinities (strong to weak): MTM=MTM SDK>MTM SA-Trp>MTM SA-Ala. An X(G/C)(G/C)X motif, where X is any base, is necessary and sufficient for MTM binding to DNA, without a strong dependence on DNA conformation. These recognition rules will aid in mapping MTM sites across different promoters towards development of MTM analogues as useful anticancer agents.

Characterization of the terminal activation step catalyzed by oxygenase CmmOIV of the chromomycin biosynthetic pathway from Streptomyces griseus

Inactivation and initial interrogation of key oxygenase CmmOIV of the biosynthetic pathway of chromomycin A(3) in Streptomyces griseus ssp. griseus revealed that a completely methylated and acetylated prechromomycin is the preferred substrate of this enzyme. This suggests that the three sugar decoration reactions, two O-acetylations and an O-methylation, which were previously believed to occur as the final steps of chromomycin A(3) biosynthesis, indeed take place prior to the CmmOIV reaction. Upon inactivation of CmmOIV, four new compounds accumulated; the fully decorated prechromomycin and its incompletely acetylated precursor along with a diketoprechromomycin-type compound were fully characterized and assayed with CmmOIV.

PKS and NRPS release mechanisms

This review covers the recent literature on the release mechanisms for polyketides and nonribosomal peptides produced by microorganisms. The emphasis is on the novel enzymology and mechanistic insights revealed by the biosynthetic studies of new natural products.

The role of antibiotics and antibiotic resistance in nature

Investigations of antibiotic resistance from an environmental prospective shed new light on a problem that was traditionally confined to a subset of clinically relevant antibiotic-resistant bacterial pathogens. It is clear that the environmental microbiota, even in apparently antibiotic-free environments, possess an enormous number and diversity of antibiotic resistance genes, some of which are very similar to the genes circulating in pathogenic microbiota. It is difficult to explain the role of antibiotics and antibiotic resistance in natural environments from an anthropocentric point of view, which is focused on clinical aspects such as the efficiency of antibiotics in clearing infections and pathogens that are resistant to antibiotic treatment. A broader overview of the role of antibiotics and antibiotic resistance in nature from the evolutionary and ecological prospective suggests that antibiotics have evolved as another way of intra- and inter-domain communication in various ecosystems. This signalling by non-clinical concentrations of antibiotics in the environment results in adaptive phenotypic and genotypic responses of microbiota and other members of the community. Understanding the complex picture of evolution and ecology of antibiotics and antibiotic resistance may help to understand the processes leading to the emergence and dissemination of antibiotic resistance and also help to control it, at least in relation to the newer antibiotics now entering clinical practice.

Crystal structure of Baeyer-Villiger monooxygenase MtmOIV, the key enzyme of the mithramycin biosynthetic pathway

Baeyer-Villiger monooxygenases (BVMOs), mostly flavoproteins, were shown to be powerful biocatalysts for synthetic organic chemistry applications and were also suggested to play key roles for the biosyntheses of various natural products. Here we present the three-dimensional structure of MtmOIV, a 56 kDa homodimeric FAD- and NADPH-dependent monooxygenase, which catalyzes the key frame-modifying step of the mithramycin biosynthetic pathway and currently the only BVMO proven to react with its natural substrate via a Baeyer-Villiger reaction. MtmOIV's structure was determined by X-ray crystallography using molecular replacement to a resolution of 2.9 A. MtmOIV cleaves a C-C bond, essential for the conversion of the biologically inactive precursor, premithramycin B, into the active drug mithramycin. The MtmOIV structure combined with substrate docking calculations and site-directed mutagenesis experiments identifies several residues that participate in cofactor and substrate binding. Future experimentation aimed at broadening the substrate specificity of the enzyme could facilitate the generation of chemically diverse mithramycin analogues through combinatorial biosynthesis.

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.

Generation of new derivatives of the antitumor antibiotic mithramycin by altering the glycosylation pattern through combinatorial biosynthesis

Mithramycin is an antitumor drug produced by Streptomyces argillaceus. It consists of a tricyclic aglycone and five deoxyhexoses that form a disaccharide and a trisaccharide chain, which are important for target interaction and therefore for the antitumor activity. Using a combinatorial biosynthesis approach, we have generated nine mithramycin derivatives, seven of which are new compounds, with alterations in the glycosylation pattern. The wild-type S. argillaceus strain and the mutant S. argillaceus M7U1, which has altered D-oliose biosynthesis, were used as hosts to express various "sugar plasmids", each one directing the biosynthesis of a different deoxyhexose. The newly formed compounds were purified and characterized by MS and NMR. Compared to mithramycin, they contained different sugar substitutions in the second (D-olivose, D-mycarose, or D-boivinose instead of D-oliose) and third (D-digitoxose instead of D-mycarose) sugar units of the trisaccharide as well as in the first (D-amicetose instead of D-olivose) sugar unit of the disaccharide. All compounds showed antitumor activity against different tumor cell lines. Structure-activity relationships are discussed on the basis of the number and type of deoxyhexoses present in these mithramycin derivatives.

Mithramycin analogues generated by combinatorial biosynthesis show improved bioactivity

Plasmid pLNBIV was used to overexpress the biosynthetic pathway of nucleoside-diphosphate (NDP)-activated l-digitoxose in the mithramycin producer Streptomyces argillaceus. This led to a "flooding" of the biosynthetic pathway of the antitumor drug mithramycin (MTM) with NDP-activated deoxysugars, which do not normally occur in the pathway, and consequently to the production of the four new mithramycin derivatives 1- 4 with altered saccharide patterns. Their structures reflect that NDP sugars produced by pLNBIV, namely, l-digitoxose and its biosynthetic intermediates, influenced the glycosyl transfer to positions B, D, and E, while positions A and C remained unaffected. All four new structures have unique, previously not found sugar decoration patterns, which arise from either overcoming the substrate specificity or inhibition of certain glycosyltransferases (GTs) of the MTM pathway with the foreign NDP sugars expressed by pLNBIV. An apoptosis TUNEL (=terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) assay revealed that compounds 1 (demycarosyl-3D-beta- d-digitoxosyl-MTM) and 3 (deoliosyl-3C-beta- d-mycarosyl-MTM) show improved activity (64.8 +/- 2% and 50.3 +/- 2.5% induction of apoptosis, respectively) against the estrogen receptor (ER)-positive human breast cancer cell line MCF-7 compared with the parent drug MTM (37.8 +/- 2.5% induction of apoptosis). In addition, compounds 1 and 4 (3A-deolivosyl-MTM) show significant effects on the ER-negative human breast cancer cell line MDA-231 (63.6 +/- 2% and 12.6 +/- 2.5% induction of apoptosis, respectively), which is not inhibited by the parent drug MTM itself (2.6 +/- 1.5% induction of apoptosis), but for which chemotherapeutic agents are urgently needed.

From genetic diversity to metabolic unity: studies on the biosynthesis of aurafurones and aurafuron-like structures in myxobacteria and streptomycetes

The myxobacterial polyketide secondary metabolites aurafuron A and B were identified by genome mining in the myxobacterial strain Stigmatella aurantiaca DW4/3-1. The compounds contain an unusual furanone moiety and resemble metabolites isolated from soil-dwelling and marine actinobacteria, a fungus and mollusks. We describe here the cloning and functional analysis of the aurafuron biosynthetic gene cluster, including site-directed mutagenesis and feeding studies using labeled precursors. The polyketide core of the aurafurones is assembled by a modular polyketide synthase (PKS). As with many such systems described from myxobacteria, the aurafuron PKS exhibits a number of unusual features, including the apparent iterative use of a module, redundant modules and domains, a trans acting dehydratase and the absence of a terminal thioesterase domain. Four oxidoreductases are encoded within the gene locus, some of which likely participate in formation of the furanone moiety via a Baeyer-Villiger type oxidation. Indeed, inactivation of a gene encoding a cytochrome P(450) monooxygenase completely abolished production of both compounds. We also compare the complete gene locus to biosynthetic gene clusters from two Streptomyces sp., which produce close structural analogues of the aurafurones. A portion of the post-PKS biosynthetic machinery is strikingly similar in all three cases, in contrast to the PKS genes, which are highly divergent. Phylogenetic analysis of the ketosynthase domains further indicates that the PKSs have developed independently (polyphyletically) during evolution. These findings point to a currently unknown but important biological function of aurafuron-like compounds for the producing organisms.

Engineering the glycosylation of natural products in actinomycetes

Bioactive natural products are frequently glycosylated with saccharide chains of different length, in which the sugars contribute to specific interactions with the biological target. Combinatorial biosynthesis approaches are being used in antibiotic-producing actinomycetes to generate derivatives with novel sugars in their architecture. Recent advances in this area indicate that glycosyltransferases involved in the biosynthesis of natural products have substrate flexibility regarding the sugar donor but also, less frequently, with respect to the aglycon acceptor. Therefore, the possibility exists of altering the glycosylation pattern of natural products, thus enabling an increase in the structural diversity of natural products.

Manipulating nature's sugar biosynthetic machineries for glycodiversification of macrolides: Recent advances and future prospects

Changing the sugar structures and glycosylation patterns of natural products is an effective means of altering the biological activity of clinically useful drugs. Several recent strategies have provided researchers with the opportunity to manipulate sugar structures and to change the sugar moieties attached to these natural products via a biosynthetic approach. In this review, we explore the utility of contemporary in vivo and in vitro methods to achieve natural product glycodiversification. This study will focus on recent progress from our laboratory in elucidating the biosynthesis of D-desosamine, a deoxysugar component of many macrolide antibiotics, and will highlight how we have engineered the D-desosamine biosynthetic pathway in Streptomyces venezuelae through targeted disruption and heterologous expression of the sugar biosynthetic genes to generate a variety of new glycoforms. The in vitro exploitation of the substrate flexibility of the endogenous D-desosamine glycosyltransferase (GT) to generate many non-natural glycoforms will also be discussed. These experiments are compared with recent work from other research groups on the same topics. Finally, the significance of these studies for the future prospects of natural product glycodiversification is discussed.

Type II polyketide synthases: gaining a deeper insight into enzymatic teamwork

This review covers advances in understanding of the biosynthesis of polyketides produced by type II PKS systems at the genetic, biochemical and structural levels.

The aureolic acid family of antitumor compounds: structure, mode of action, biosynthesis, and novel derivatives

Members of the aureolic acid family are tricyclic polyketides with antitumor activity which are produced by different streptomycete species. These members are glycosylated compounds with two oligosaccharide chains of variable sugar length. They interact with the DNA minor groove in high-GC-content regions in a nonintercalative way and with a requirement for magnesium ions. Mithramycin and chromomycins are the most representative members of the family, mithramycin being used as a chemotherapeutic agent for the treatment of several cancer diseases. For chromomycin and durhamycin A, antiviral activity has also been reported. The biosynthesis gene clusters for mithramycin and chromomycin A(3) have been studied in detail by gene sequencing, insertional inactivation, and gene expression. Most of the biosynthetic intermediates in these pathways have been isolated and characterized. Some of these compounds showed an increase in antitumor activity in comparison with the parent compounds. A common step in the biosynthesis of all members of the family is the formation of the tetracyclic intermediate premithramycinone. Further biosynthetic steps (glycosylation, methylations, acylations) proceed through tetracyclic intermediates which are finally converted into tricyclic compounds by the action of a monooxygenase, a key event for the biological activity. Heterologous expression of biosynthetic genes from other aromatic polyketide pathways in the mithramycin producer (or some mutants) led to the isolation of novel hybrid compounds.

Flavoprotein monooxygenases, a diverse class of oxidative biocatalysts

During the last decades a large number of flavin-dependent monooxygenases have been isolated and studied. This has revealed that flavoprotein monooxygenases are able to catalyze a remarkable wide variety of oxidative reactions such as regioselective hydroxylations and enantioselective sulfoxidations. These oxidation reactions are often difficult, if not impossible, to be achieved using chemical approaches. Analysis of the available genome sequences has indicated that many more flavoprotein monooxygenases exist and await biocatalytic exploration. Based on the known biochemical properties of a number of flavoprotein monooxygenases and sequence and structural analyses, flavoprotein monooxygenases can be classified into six distinct flavoprotein monooxygenase subclasses. This review provides an inventory of known flavoprotein monooxygenases belonging to these different enzyme subclasses. Furthermore, the biocatalytic potential of a selected number of flavoprotein monooxygenases is highlighted.

Isolation and identification of three new 5-alkenyl-3,3(2H)-furanones from two streptomyces species using a genomic screening approach

Analyses of biosynthetic gene clusters derived from Streptomyces aculeolatus NRRL 18422 and Streptomyces sp. Eco86 indicated that both microorganisms have similar type I polyketide synthase (PKS) gene clusters with relatively few genes encoding post-PKS elaborative enzymes. However both gene clusters included a sequence coding for a relatively uncommon oxidative enzyme related to Baeyer-Villiger, flavin-type monooxygenases. Screening of culture extracts for compounds with the predicted physicochemical properties of the end products from these loci, led to the isolation of three 5-alkenyl-3,3(2H)-furanones, one (E-837, 1) from the former and two (E-492, 2, E-975, 3) from the latter strain. The structures, confirmed by spectral analyses including MS, and ID and 2D NMR experiments, were in accord with those predicted by genomic analyses. Baeyer-Villiger type oxidation is postulated to be involved in the formation of the furanone moieties in these molecules. All three new compounds were tested for their electron transport inhibitory activities. They had IC50 values of 1-4 microg/ml against Ascaris suum NADH-fumarate reductase and 1-12 microg/ml against bovine heart NADH oxidase.

Novel GC-rich DNA-binding compound produced by a genetically engineered mutant of the mithramycin producer Streptomyces argillaceus exhibits improved transcriptional repressor activity: implications for cancer therapy

The aureolic acid antibiotic mithramycin (MTM) binds selectively to GC-rich DNA sequences and blocks preferentially binding of proteins, like Sp1 transcription factors, to GC-rich elements in gene promoters. Genetic approaches can be applied to alter the MTM biosynthetic pathway in the producing microorganism and obtain new products with improved pharmacological properties. Here, we report on a new analog, MTM SDK, obtained by targeted gene inactivation of the ketoreductase MtmW catalyzing the last step in MTM biosynthesis. SDK exhibited greater activity as transcriptional inhibitor compared to MTM. SDK was a potent inhibitor of Sp1-dependent reporter activity and interfered minimally with reporters of other transcription factors, indicating that it retained a high degree of selectivity toward GC-rich DNA-binding transcription factors. RT-PCR and microarray analysis showed that SDK repressed transcription of multiple genes implicated in critical aspects of cancer development and progression, including cell cycle, apoptosis, migration, invasion and angiogenesis, consistent with the pleiotropic role of Sp1 family transcription factors. SDK inhibited proliferation and was a potent inducer of apoptosis in ovarian cancer cells while it had minimal effects on viability of normal cells. The new MTM derivative SDK could be an effective agent for treatment of cancer and other diseases with abnormal expression or activity of GC-rich DNA-binding transcription factors.

Characterization of kinetics and products of the Baeyer-Villiger oxygenase MtmOIV, the key enzyme of the biosynthetic pathway toward the natural product anticancer drug mithramycin from Streptomyces argillaceus

MtmOIV, the key oxygenase of the mithramycin biosynthetic pathway in Streptomyces argillaceus, was proven to act initially as Baeyer-Villiger monooxygenase, but may also catalyze various follow-up reaction steps. The reaction of the overexpressed pure His6-tagged enzyme with its substrate premithramycin B was studied. Various intermediates and products were isolated and physicochemically characterized, several of them being previously unknown compounds. This is the first example in which a bacterial enzyme was unequivocally proven to act as Baeyer-Villigerase with its natural substrate, that is, in its natural context.

Deoxysugar transfer during chromomycin A3 biosynthesis in Streptomyces griseus subsp. griseus: new derivatives with antitumor activity

Chromomycin A3 is an antitumor drug produced by Streptomyces griseus subsp. griseus. It consists of a tricyclic aglycone with two aliphatic side chains and two O-glycosidically linked saccharide chains, a disaccharide of 4-O-acetyl-D-oliose (sugar A) and 4-O-methyl-D-oliose (sugar B), and a trisaccharide of D-olivose (sugar C), D-olivose (sugar D), and 4-O-acetyl-L-chromose B (sugar E). The chromomycin gene cluster contains four glycosyltransferase genes (cmmGI, cmmGII, cmmGIII, and cmmGIV), which were independently inactivated through gene replacement, generating mutants C60GI, C10GII, C10GIII, and C10GIV. Mutants C10GIV and C10GIII produced the known compounds premithramycinone and premithramycin A1, respectively, indicating the involvement of CmmGIV and CmmGIII in the sequential transfer of sugars C and D and possibly also of sugar E of the trisaccharide chain, to the 12a position of the tetracyclic intermediate premithramycinone. Mutant C10GII produced two new tetracyclic compounds lacking the disaccharide chain at the 8 position, named prechromomycin A3 and prechromomycin A2. All three compounds accumulated by mutant C60GI were tricyclic and lacked sugar B of the disaccharide chain, and they were named prechromomycin A4, 4A-O-deacetyl-3A-O-acetyl-prechromomycin A4, and 3A-O-acetyl-prechromomycin A4. CmmGII and CmmGI are therefore responsible for the formation of the disaccharide chain by incorporating, in a sequential manner, two D-oliosyl residues to the 8 position of the biosynthetic intermediate prechromomycin A3. A biosynthetic pathway is proposed for the glycosylation events in chromomycin A3 biosynthesis.

The biosynthesis, molecular genetics and enzymology of the polyketide-derived metabolites

This review covers the biosynthesis of aliphatic and aromatic polyketides as well as mixed polyketide/NRPS metabolites, and discusses the molecular genetics and enzymology of the proteins responsible for their formation.

Functional analyses of oxygenases in jadomycin biosynthesis and identification of JadH as a bifunctional oxygenase/dehydrase

A novel angucycline metabolite, 2,3-dehydro-UWM6, was identified in a jadH mutant of Streptomyces venezuelae ISP5230. Both UWM6 and 2,3-dehydro-UWM6 could be converted to jadomycin A or B by a ketosynthase alpha (jadA) mutant of S. venezuelae. These angucycline intermediates were also converted to jadomycin A by transformant of the heterologous host Streptomyces lividans expressing the jadFGH oxygenases in vivo and by its cell-free extracts in vitro; thus the three gene products JadFGH are implicated in catalysis of the post-polyketide synthase biosynthetic reactions converting UWM6 to jadomycin aglycone. Genetic and biochemical analyses indicate that JadH possesses dehydrase activity, not previously associated with polyketide-modifying oxygenase. Since the formation of aromatic polyketides often requires multiple dehydration steps, bifunctionality of oxygenases modifying aromatic polyketides may be a general phenomenon.

Biosynthesis of the antitumor chromomycin A3 in Streptomyces griseus: analysis of the gene cluster and rational design of novel chromomycin analogs

The biosynthetic gene cluster of the aureolic acid type antitumor drug chromomycin A3 from S. griseus subsp. griseus has been identified and characterized. It spans 43 kb and contains 36 genes involved in polyketide biosynthesis and modification, deoxysugar biosynthesis and sugar transfer, pathway regulation and resistance. The organization of the cluster clearly differs from that of the closely related mithramycin. Involvement of the cluster in chromomycin A3 biosynthesis was demonstrated by disrupting the cmmWI gene encoding a polyketide reductase involved in side chain reduction. Three novel chromomycin derivatives were obtained, named chromomycin SK, chromomycin SA, and chromomycin SDK, which show antitumor activity and differ with respect to their 3-side chains. A pathway for the biosynthesis of chromomycin A3 and its deoxysugars is proposed.

Aureolic acids: similar antibiotics with different biosynthetic gene clusters

In this issue of Chemistry & Biology, Méndez and colleagues describe the sequence and organization of the chromomycin gene cluster. Unexpectedly, the arrangement is starkly different from the mithramycin biosynthetic cluster, despite similarity in the individual genes and the near identical structures of the two antibiotic aureolic acids.

MtmMII-mediated C-Methylation during Biosynthesis of the Antitumor Drug Mithramycin Is Essential for Biological Activity and DNA-Drug Interaction

The antitumor drug mithramycin consists of a polyketide chromophore glycosylated with a trisaccharide and a disaccharide. Two post-polyketide methylations take place during mithramycin biosynthesis. One of these methylations has been shown to be very relevant for biological activity, that is the introduction of a methyl group at aromatic C-7. We have purified to 282- fold the MtmMII methyltransferase involved in this reaction. The protein is a monomer, and results from kinetic studies were consistent with a model for the enzyme acting via a compulsory order mechanism. The enzyme showed high substrate specificity and was unable to operate on structurally closely related molecules. Structural predictions suggest that the molecule is integrated by two domains, an essentially all alpha-amino domain and an alpha/beta-carboxyl domain displaying a variation of a Rossmann-fold containing the cofactor binding site. Although 7-demethyl-mithramycin did not show any biological activity, it was able to reach the nucleus of eukaryotic cells, with subsequent binding to DNA. Mithramycin and 7-demethylmithramycin were able to form similar complexes with Mg(2+), although their respective DNA binding isotherms were very different. The dinucleotide binding model fit well the isotherms recorded for both compounds, predicting that the C-7 methyl group was essential for high affinity binding to specific GC and CG sequences. Considering previous structural studies, we propose that this effect is performed by positioning the group in the floor of the minor groove, allowing the interaction with the third sugar moiety of the trisaccharide, d-mycarose, which is involved in sequence selectivity.

Inhibition of c-src transcription by mithramycin: structure-activity relationships of biosynthetically produced mithramycin analogues using the c-src promoter as target

The aureolic acid antitumor antibiotic mithramycin (MTM) inhibits both cancer growth and bone resorption by cross-linking GC-rich DNA, thus blocking binding of Sp-family transcription factors to gene regulatory elements. Transcription of c-src, a gene implicated in many human cancers and required for osteoclast-dependent bone resorption, is regulated by the binding of Sp factors to specific elements in its promoter. Therefore, this gene represents an important anticancer target and a potential lead target through which MTM displays its so far uncharacterized action against osteoclastic bone resorption. Here we demonstrate, using DNA binding studies, promoter reporter assays, and RT-PCR, that MTM inhibits Sp binding to the c-src promoter region, thereby decreasing its expression in human cancer cells. Furthermore, selected mithramycin analogues, namely, premithramycin B, mithramycin SK, 7-demethylmithramycin, 4E-ketomithramycin, and 4C-ketodemycarosylmithramycin, generated through combinatorial biosynthesis, were compared with MTM for their ability to block Sp binding to the c-src promoter. Although most of the tested compounds lost their ability to bind to the DNA, alteration of the MTM 3-pentyl side chain led to a compound (mithramycin SK) with the same DNA binding specificity but with lower binding affinity than MTM. While this compound was comparable to MTM in promoter reporter, gene expression, and anticancer assays, given its weaker interaction with the DNA, it may be much less toxic than MTM. The results presented here supplement recent findings and, moreover, allow new conclusions to be made regarding both the structure-activity relationships, particularly with respect to the alkyl side chains, and the mechanism of action of aureolic acid drugs.

Purification and characterization of a monooxygenase involved in the biosynthetic pathway of the antitumor drug mithramycin

A monooxygenase encoded by the mtmOIV gene from the mithramycin gene cluster of Streptomyces argillaceus was purified 21-fold by a three-step purification procedure. This monooxygenase catalyzes the oxidative cleavage of the fourth ring of premithramycin B. The enzyme was dependent on NADPH and flavin adenine dinucleotide for activity with optimal pH at 9.5, and the K(m) values for NADPH and premithramycin B were 269.22 and 23.35 micro M, respectively. The reaction catalyzed by MtmOIV yields two possible isomers of the same basic shortened aliphatic chain molecule. One of the reaction products showed important biological activity, thus highlighting the importance of the cleavage of the fourth ring of the aglycon for biological activity.

Mithramycin SK, a novel antitumor drug with improved therapeutic index, mithramycin SA, and demycarosyl-mithramycin SK: three new products generated in the mithramycin producer Streptomyces argillaceus through combinatorial biosynthesis

To gain initial structure-activity relationships regarding the highly functionalized pentyl side chain attached at C-3 of mithramycin (MTM), we focused on a post-polyketide synthase (post-PKS) tailoring step of the MTM biosynthesis by Streptomyces argillaceus ATCC 12956, which was proposed to be catalyzed by ketoreductase (KR) MtmW. In this last step of the MTM biosynthesis, a keto group of the pentyl side chain is reduced to a secondary alcohol, and we anticipated the generation of an MTM derivative with an additional keto group in the 3-side chain. Insertional inactivation of mtmW, a gene located ca. 8 kb downstream of the mithramycin-PKS genes, yielded an S. argillaceus mutant, which accumulated three new mithramycin analogues, namely mithramycin SA, demycarosyl-mithramycin SK, and mithramycin SK (MTM-SK). The structures of these three compounds confirmed indirectly the proposed role of MtmW in MTM biosynthesis. However, the new mithramycin derivatives bear unexpectedly shorter 3-side chains (ethyl or butyl) than MTM, presumably caused by nonenzymatic rearrangement or cleavage reactions of the initially formed pentyl side chain with a reactive beta-dicarbonyl functional group. The major product, MTM-SK, was tested in vitro against a variety of human cancer cell lines, as well as in an in vitro toxicity assay, and showed an improved therapeutic index, in comparison to the parent drug, MTM.

Essential role of an activator protein-2 (AP-2)/specificity protein 1 (Sp1) cluster in the UVB-mediated induction of the human vascular endothelial growth factor in HaCaT keratinocytes

Chronic sun exposure of the skin has long been postulated to enhance cutaneous angiogenesis, resulting in highly vascularized skin cancers. As the UVB component of sunlight is a major contributor to photocarcinogenesis, we aimed to explore the effects of UVB radiation on vascular endothelial growth factor (VEGF) gene expression, using the immortalized keratinocyte cell line HaCaT as a model for transformed premalignant epithelial cells. In the present paper, we studied the molecular mechanism of UVB-induced VEGF providing a major angiogenic activity in tumour progression and invasion. After 12-24 h of UVB irradiation, a 2.4- to 2.7-fold increase in endogenous VEGF protein level was measured, correlating with an up to 2.5-fold induction of promoter-based reporter gene constructs of VEGF. Furthermore, we identified a GC-rich UVB-responsive region between -87 and -65 bp of the VEGF promoter. In electrophoretic mobility-shift assays, this region binds Sp1-dependent protein complexes constitutively and an additional UVB-inducible protein complex distinct from Sp1 protein. The transcription factor AP-2 (activator protein-2) was detected as a component of the UVB-inducible protein complex. The critical role of the AP-2/Sp1 (specificity protein 1) cluster was supported by demonstration of a significant reduction of UVB-mediated promoter activity upon deletion of this recognition site. The specificity of this region for UVB irradiation was demonstrated using PMA, which increased VEGF activity in HaCaT cells after transient transfection of the deleted promoter construct. In conclusion, our data clarified regulatory mechanisms of UVB-dependent VEGF stimulation which may be critical for angiogenic processes in the skin.

A polyketide synthase-peptide synthetase gene cluster from an uncultured bacterial symbiont of Paederus beetles

Many drug candidates from marine and terrestrial invertebrates are suspected metabolites of uncultured bacterial symbionts. The antitumor polyketides of the pederin family, isolated from beetles and sponges, are an example. Drug development from such sources is commonly hampered by low yields and the difficulty of sustaining invertebrate cultures. To obtain insight into the true producer and find alternative supplies of these rare drug candidates, the putative pederin biosynthesis genes were cloned from total DNA of Paederus fuscipes beetles, which use this compound for chemical defense. Sequence analysis of the gene cluster and adjacent regions revealed the presence of ORFs with typical bacterial architecture and homologies. The ped cluster, which is present only in beetle specimens with high pederin content, is located on a 54-kb region bordered by transposase pseudogenes and encodes a mixed modular polyketide synthase/nonribosomal peptide synthetase. Notably, none of the modules contains regions with homology to acyltransferase domains, but two copies of isolated monodomain acyltransferase genes were found at the upstream end of the cluster. In line with an involvement in pederin biosynthesis, the upstream cluster region perfectly mirrors pederin structure. The unexpected presence of additional polyketide synthase/nonribosomal peptide synthetase modules reveals surprising insights into the evolutionary relationship between pederin-type pathways in beetles and sponges.

A gene cluster from a marine Streptomyces encoding the biosynthesis of the aromatic spiroketal polyketide griseorhodin A

The telomerase inhibitor griseorhodin A is probably the most heavily oxidized bacterial polyketide known and features a unique epoxyspiroketal moiety crucial for its activity. To gain insight into which tailoring enzymes generate this pharmacophore, we have cloned and fully sequenced the griseorhodin biosynthesis gene cluster. Among other unusual features, this aromatic polyketide synthase (PKS) system encodes an unprecedented number of functionally diverse oxidoreductases, which are involved in the oxidative modification of a polyaromatic tridecaketide precursor by cleavage of three carbon-carbon bonds. The cluster was highly unstable on a variety of shuttle plasmids but could finally be functionally expressed in its entirety in Streptomyces lividans using a novel integrative cosmid vector. The availability of the tailoring system now opens up the possibility of engineering nonnatural biosynthetic pathways yielding novel pharmacologically active analogs with a similar pharmacophore.