The Impact of Bacterial Genomics on Natural Product Research

Methods and options for the heterologous production of complex natural products

This review will detail the motivations, experimental approaches, and growing list of successful cases associated with the heterologous production of complex natural products.

Antibacterials from the sea

The ocean contains a host of macroscopic life in a great microbial soup. Unlike the terrestrial environment, an aqueous environment provides perpetual propinquity and blurs spatial distinctions. Marine organisms are under a persistent threat of infection by resident pathogenic microbes including bacteria, and in response they have engineered complex organic compounds with antibacterial activity from a diverse set of biological precursors. The diluting effect of the ocean drives the construction of potent molecules that are stable to harsh salty conditions. Members of each class of metabolite-ribosomal and non-ribosomal peptides, alkaloids, polyketides, and terpenes-have been shown to exhibit antibacterial activity. The sophistication and diversity of these metabolites points to the ingenuity and flexibility of biosynthetic processes in Nature. Compared with their terrestrial counterparts, antibacterial marine natural products have received much less attention. Thus, a concerted effort to discover new antibacterials from marine sources has the potential to contribute significantly to the treatment of the ever increasing drug-resistant infectious diseases.

Iso-migrastatin Titer Improvement in the Engineered Streptomyces lividans SB11002 Strain by Optimization of Fermentation Conditions

The heterologous production of iso-migrastatin (iso-MGS) was successfully demonstrated in an engineered S. lividans SB11002 strain, which was derived from S. lividans K4-114, following introduction of pBS11001, which harbored the entire mgs biosynthetic gene cluster. However, under similar fermentation conditions, the iso-MGS titer in the engineered strain was significantly lower than that in the native producer - Streptomyces platensis NRRL 18993. To circumvent the problem of low iso-MGS titers and to expand the utility of this heterologous system for iso-MGS biosynthesis and engineering, systematic optimization of the fermentation medium was carried out. The effects of major components in the cultivation medium, including carbon, organic and inorganic nitrogen sources, were investigated using a single factor optimization method. As a result, sucrose and yeast extract were determined to be the best carbon and organic nitrogen sources, resulting in optimized iso-MGS production. Conversely, all other inorganic nitrogen sources evaluated produced various levels of inhibition of iso-MGS production. The final optimized R2YE production medium produced iso-MGS with a titer of 86.5 mg/L, about 3.6-fold higher than that in the original R2YE medium, and 1.5 fold higher than that found within the native S. platensis NRRL 18993 producer.

Natural products as chemical probes

Natural products have evolved to encompass a broad spectrum of chemical and functional diversity. It is this diversity, along with their structural complexity, that enables nature's small molecules to target a nearly limitless number of biological macromolecules and to often do so in a highly selective fashion. Because of these characteristics, natural products have seen great success as therapeutic agents. However, this vast pool of compounds holds much promise beyond the development of future drugs. These features also make them ideal tools for the study of biological systems. Recent examples of the use of natural products and their derivatives as chemical probes to explore biological phenomena and assemble biochemical pathways are presented here.

The sequence of a 1.8-mb bacterial linear plasmid reveals a rich evolutionary reservoir of secondary metabolic pathways

Plasmids are mobile genetic elements that play a key role in the evolution of bacteria by mediating genome plasticity and lateral transfer of useful genetic information. Although originally considered to be exclusively circular, linear plasmids have also been identified in certain bacterial phyla, notably the actinomycetes. In some cases, linear plasmids engage with chromosomes in an intricate evolutionary interplay, facilitating the emergence of new genome configurations by transfer and recombination or plasmid integration. Genome sequencing of Streptomyces clavuligerus ATCC 27064, a Gram-positive soil bacterium known for its production of a diverse array of biotechnologically important secondary metabolites, revealed a giant linear plasmid of 1.8 Mb in length. This megaplasmid (pSCL4) is one of the largest plasmids ever identified and the largest linear plasmid to be sequenced. It contains more than 20% of the putative protein-coding genes of the species, but none of these is predicted to be essential for primary metabolism. Instead, the plasmid is densely packed with an exceptionally large number of gene clusters for the potential production of secondary metabolites, including a large number of putative antibiotics, such as staurosporine, moenomycin, β-lactams, and enediynes. Interestingly, cross-regulation occurs between chromosomal and plasmid-encoded genes. Several factors suggest that the megaplasmid came into existence through recombination of a smaller plasmid with the arms of the main chromosome. Phylogenetic analysis indicates that heavy traffic of genetic information between Streptomyces plasmids and chromosomes may facilitate the rapid evolution of secondary metabolite repertoires in these bacteria.

Discovery of 3-formyl-tyrosine metabolites from Pseudoalteromonas tunicata through heterologous expression

Genome mining and identification of natural product gene clusters typically relies on the presence of canonical nonribosomal polypeptide synthetase (NRPS) or polyketide synthase (PKS) domains. Recently, other condensation enzymes, such as the ATP-grasp ligases, have been recognized as important players in natural product biosynthesis. In this study, sequence based searching for homologues of DdaF, the ATP-grasp amide ligase from dapdiamide biosynthesis, led to the identification of a previously unannotated biosynthetic gene cluster in Pseudoalteromonas tunicata. Heterologous expression of the cluster in Escherichia coli allowed for the production and structure determination of two new 3-formyl tyrosine metabolites.

Quorum-sensing-regulated bactobolin production by Burkholderia thailandensis E264

Bacterial acyl-homoserine lactones upregulated an uncharacterized gene cluster (bta) in Burkholderia thailandensis E264 to produce an uncharacterized polar antibiotic. The antibiotic is identified as a mixture of four bactobolins. Annotation of the bta cluster allows us to propose a biosynthetic scheme for bactobolin and reveals unusual enzymatic reactions for further study.

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

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

Evolution of metabolic diversity: insights from microbial polyketide synthases

Polyketides are a family of complex natural products that are built from simple carboxylic acid building blocks. In microorganisms, the majority of these secondary metabolites are produced by exceptionally large, multifunctional proteins termed polyketide synthases (PKSs). Each unit of a type I PKS assembly line resembles a mammalian type fatty acid synthase (FAS), although certain domains are optionally missing. The evolutionary analysis of microbial PKS has revealed a long joint evolution process of PKSs and FASs. The phylogenomic analysis of modular type I PKSs as the most widespread PKS type in bacteria showed a large impact of gene duplications and gene losses on the evolution of type I PKS in different bacterial groups. The majority of type I PKSs in actinobacteria and cyanobacteria may have evolved from a common ancestor, whereas in proteobacteria most type I PKSs were acquired from other bacterial groups. The modularization of type I PKSs almost unexceptionally started with multiple duplications of a single ancestor module. The repeating modules represent ideal platforms for recombination events that can lead to corresponding changes in the actual chemistry of the products. The analysis of these "natural reprogramming" events of PKSs may assist in the development of concepts for the biocombinatorial design of bioactive compounds.

Escherichia coli allows efficient modular incorporation of newly isolated quinomycin biosynthetic enzyme into echinomycin biosynthetic pathway for rational design and synthesis of potent antibiotic unnatural natural product

Natural products display impressive activities against a wide range of targets, including viruses, microbes, and tumors. However, their clinical use is hampered frequently by their scarcity and undesirable toxicity. Not only can engineering Escherichia coli for plasmid-based pharmacophore biosynthesis offer alternative means of simple and easily scalable production of valuable yet hard-to-obtain compounds, but also carries a potential for providing a straightforward and efficient means of preparing natural product analogs. The quinomycin family of nonribosomal peptides, including echinomycin, triostin A, and SW-163s, are important secondary metabolites imparting antibiotic antitumor activity via DNA bisintercalation. Previously we have shown the production of echinomycin and triostin A in E. coli using our convenient and modular plasmid system to introduce these heterologous biosynthetic pathways into E. coli. However, we have yet to develop a novel biosynthetic pathway capable of producing bioactive unnatural natural products in E. coli. Here we report an identification of a new gene cluster responsible for the biosynthesis of SW-163s that involves previously unknown biosynthesis of (+)-(1S, 2S)-norcoronamic acid and generation of aliphatic side chains of various sizes via iterative methylation of an unactivated carbon center. Substituting an echinomycin biosynthetic gene with a gene from the newly identified SW-163 biosynthetic gene cluster, we were able to rationally re-engineer the plasmid-based echinomycin biosynthetic pathway for the production of a novel bioactive compound in E. coli.

Strategies for the discovery of new natural products by genome mining

Natural products have a very broad spectrum of applications. Many natural products are used clinically as antibacterial, antifungal, antiparasitic, anticancer and immunosuppressive agents and are therefore of utmost importance for our society. When in the 1940s the golden age of antibiotics was ushered in, a "gold rush fever" of natural product discovery in the pharmaceutical industry ensued for many decades. However, the traditional process of discovering new bioactive natural products is generally long and laborious, and known natural products are frequently rediscovered. A mass-withdrawal of pharmaceutical companies from new natural product discovery and natural products research has thus occurred in recent years. In this article, the concept of genome mining for novel natural product discovery, which promises to provide a myriad of new bioactive natural compounds, is summarized and discussed. Genome mining for new natural product discovery exploits the huge and constantly increasing quantity of DNA sequence data from a wide variety of organisms that is accumulating in publicly accessible databases. Genes encoding enzymes likely to be involved in natural product biosynthesis can be readily located in sequenced genomes by use of computational sequence comparison tools. This information can be exploited in a variety of ways in the search for new bioactive natural products.

Diversity and functional analysis of LuxR-type transcriptional regulators of cyclic lipopeptide biosynthesis in Pseudomonas fluorescens

Cyclic lipopeptides (CLPs) are produced by many Pseudomonas species and have several biological functions, including a role in surface motility, biofilm formation, virulence, and antimicrobial activity. This study focused on the diversity and role of LuxR-type transcriptional regulators in CLP biosynthesis in Pseudomonas species and, specifically, viscosin production by Pseudomonas fluorescens strain SBW25. Phylogenetic analyses showed that CLP biosynthesis genes in Pseudomonas strains are flanked by LuxR-type regulators that contain a DNA-binding helix-turn-helix domain but lack N-acylhomoserine lactone-binding or response regulator domains. For SBW25, site-directed mutagenesis of the genes coding for either of the two identified LuxR-type regulators, designated ViscAR and ViscBCR, strongly reduced transcript levels of the viscABC biosynthesis genes and resulted in a loss of viscosin production. Expression analyses further showed that a mutation in either viscAR or viscBCR did not substantially (change of <2.5-fold) affect transcription of the other regulator. Transformation of the DeltaviscAR mutant of SBW25 with a LuxR-type regulatory gene from P. fluorescens strain SS101 that produces massetolide, a CLP structurally related to viscosin, restored transcription of the viscABC genes and viscosin production. The results further showed that a functional viscAR gene was required for heterologous expression of the massetolide biosynthesis genes of strain SS101 in strain SBW25, leading to the production of both viscosin and massetolide. Collectively, these results indicate that the regulators flanking the CLP biosynthesis genes in Pseudomonas species represent a unique LuxR subfamily of proteins and that viscosin biosynthesis in P. fluorescens SBW25 is controlled by two LuxR-type transcriptional regulators.

Molecules derived from the extremes of life

In order to survive extremes of pH, temperature, salinity and pressure, organisms have been found to develop unique defences against their environment, leading to the biosynthesis of novel molecules ranging from simple osmolytes and lipids to complex secondary metabolites. This review highlights novel molecules isolated from microorganisms that either tolerate or favour extreme growth conditions.

Antibacterial discovery in actinomycetes strains with mutations in RNA polymerase or ribosomal protein S12

We show that selection of drug-resistant bacterial mutants allows the discovery of antibacterial compounds. Mutant strains of a soil-isolated Streptomyces species that does not produce antibacterials synthesize a previously unknown class of antibacterial, which we name piperidamycin. Overall, 6% of non-Streptomyces actinomycetes species and 43% of Streptomyces species that do not produce antibacterials are activated to produce them. The antibacterial-producing mutants all carried mutations in RNA polymerase and/or the ribosomal protein S12.

Myxobacteria--'microbial factories' for the production of bioactive secondary metabolites

In this article, we briefly review the potential of myxobacteria as 'natural product factories' by highlighting results from the recently sequenced myxobacterial model strain Myxococcus xanthus. We will focus on the production of polyketides, non-ribosomally-made peptides, and their hybrids, and discuss the evaluation of biosynthetic potential using genome-based methods, as well as biosynthetic process engineering.

Bioinformatic perspectives on NRPS/PKS megasynthases: advances and challenges

The increased understanding of both fundamental principles and mechanistic variations of NRPS/PKS megasynthases along with the unprecedented availability of microbial sequences has inspired a number of in silico studies of both enzyme families. The insights that can be extracted from these analyses go far beyond a rough classification of data and have turned bioinformatics into a frontier field of natural products research. As databases are flooded with NRPS/PKS gene sequence of microbial genomes and metagenomes, increasingly reliable structural prediction methods can help to uncover hidden treasures. Already, phylogenetic analyses have revealed that NRPS/PKS pathways should not simply be regarded as enzyme complexes, specifically evolved to product a selected natural product. Rather, they represent a collection of genetic opinions, allowing biosynthetic pathways to be shuffled in a process of perpetual chemical innovations and pathways diversification in nature can give impulses for specificities, protein interactions and genetic engineering of libraries of novel peptides and polyketides. The successful translation of the knowledge obtained from bioinformatic dissection of NRPS/PKS megasynthases into new techniques for drug discovery and design remain challenges for the future.

A novel polyketide biosynthesis gene cluster is involved in fruiting body morphogenesis in the filamentous fungi Sordaria macrospora and Neurospora crassa

During fungal fruiting body development, hyphae aggregate to form multicellular structures that protect and disperse the sexual spores. Analysis of microarray data revealed a gene cluster strongly upregulated during fruiting body development in the ascomycete Sordaria macrospora. Real time PCR analysis showed that the genes from the orthologous cluster in Neurospora crassa are also upregulated during development. The cluster encodes putative polyketide biosynthesis enzymes, including a reducing polyketide synthase. Analysis of knockout strains of a predicted dehydrogenase gene from the cluster showed that mutants in N. crassa and S. macrospora are delayed in fruiting body formation. In addition to the upregulated cluster, the N. crassa genome comprises another cluster containing a polyketide synthase gene, and five additional reducing polyketide synthase (rpks) genes that are not part of clusters. To study the role of these genes in sexual development, expression of the predicted rpks genes in S. macrospora (five genes) and N. crassa (six genes) was analyzed; all but one are upregulated during sexual development. Analysis of knockout strains for the N. crassa rpks genes showed that one of them is essential for fruiting body formation. These data indicate that polyketides produced by RPKSs are involved in sexual development in filamentous ascomycetes.

Cascade reactions during coronafacic acid biosynthesis: elongation, cyclization, and functionalization during Cfa7-catalyzed condensation

Herein, the biogenesis of the hydrindane ring system within coronafacic acid (CFA) has been investigated. These studies reveal that in addition to the canonical polyketide chain elongation and functionalization encoded by type I polyketide synthase (PKSs), cascade reactions can take place during assembly line-like biosynthesis. Indeed, upon Cfa7-catalyzed Claisen condensation between enzyme-bound malonate and an N-acetylcysteamine (SNAC) thioester, latent reactivity within the elongated enzyme-bound intermediate is unveiled. This reactivity translates into an intramolecular cyclization, which can proceed in a facile manner as observed by the enzyme-independent cyclization of a linear beta-ketothioester intermediate.

Identification of additional players in the alternative biosynthesis pathway to isovaleryl-CoA in the myxobacterium Myxococcus xanthus

Isovaleryl-CoA (IV-CoA) is usually derived from the degradation of leucine by using the Bkd (branched-chain keto acid dehydrogenase) complex. We have previously identified an alternative pathway for IV-CoA formation in myxobacteria that branches from the well-known mevalonate-dependent isoprenoid biosynthesis pathway. We identified 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase (MvaS) to be involved in this pathway in Myxococcus xanthus, which is induced in mutants with impaired leucine degradation (e.g., bkd(-)) or during myxobacterial fruiting-body formation. Here, we show that the proteins required for leucine degradation are also involved in the alternative IV-CoA biosynthesis pathway through the efficient catalysis of the reverse reactions. Moreover, we conducted a global gene-expression experiment and compared vegetative wild-type cells with bkd mutants, and identified a five-gene operon that is highly up-regulated in bkd mutants and contains mvaS and other genes that are directly involved in the alternative pathway. Based on our experiments, we assigned roles to the genes required for the formation of IV-CoA from HMG-CoA. Additionally, several genes involved in outer-membrane biosynthesis and a plethora of genes encoding regulatory proteins were decreased in expression levels in the bkd(-) mutant; this explains the complex phenotype of bkd mutants including a lack of adhesion in developmental submerse culture.

Novel expression hosts for complex secondary metabolite megasynthetases: Production of myxochromide in the thermopilic isolate Corallococcus macrosporus GT-2

Although many secondary metabolites with diverse biological activities have been isolated from myxobacteria, most strains of these biotechnologically important gliding prokaryotes remain difficult to handle genetically. In this study we describe the new fast growing myxobacterial thermophilic isolate GT-2 as a heterologous host for the expression of natural product biosynthetic pathways isolated from other myxobacteria. According to the results of sequence analysis of the 16S rDNA, this moderately thermophilic isolate is closely related to Corallococcus macrosporus and was therefore named C. macrosporus GT-2. Fast growth of moderately thermophilic strains results in shorter fermentation and generation times, aspects which are of significant interest for molecular biological work as well as production of secondary metabolites. Development of a genetic manipulation system allowed the introduction of the complete myxochromide biosynthetic gene cluster, located on a transposable fragment, into the chromosome of GT-2. Genetic engineering of the biosynthetic gene cluster by promoter exchange leads to much higher production of myxochromides in the heterologous host C. macrosporus GT-2 in comparison to the original producer Stigmatella aurantiaca and to the previously described heterologous host Pseudomonas putida (600 mg/L versus 8 mg/L and 40 mg/L, respectively).

Heterologous production of polyketides in bacteria

Polyketide natural products are among the most important microbial metabolites in human medicine and are widely used to treat both acute and degenerative diseases. The need to develop new drugs has prompted the idea of using heterologous systems for the expression of polyketide biosynthetic pathways. The basic idea behind this approach is to use heterologous bacterial systems with better growth and genetic characteristics that could support better production of a certain compound than the original host or that could allow the generation of novel analogues through combinatorial biosynthesis. Moreover, these hosts could be used to express "cryptic" secondary metabolic pathways or serve as surrogate hosts in metagenomics experiments in order to find potential new bioactive compounds. In this chapter we discuss recent advances in the heterologous production of polyketides in bacteria and describe some methodological improvements of the systems.

Highly plastic genome of Microcystis aeruginosa PCC 7806, a ubiquitous toxic freshwater cyanobacterium

Background

The colonial cyanobacterium Microcystis proliferates in a wide range of freshwater ecosystems and is exposed to changing environmental factors during its life cycle. Microcystis blooms are often toxic, potentially fatal to animals and humans, and may cause environmental problems. There has been little investigation of the genomics of these cyanobacteria.

Results

Deciphering the 5,172,804 bp sequence of Microcystis aeruginosa PCC 7806 has revealed the high plasticity of its genome: 11.7% DNA repeats containing more than 1,000 bases, 6.8% putative transposases and 21 putative restriction enzymes. Compared to the genomes of other cyanobacterial lineages, strain PCC 7806 contains a large number of atypical genes that may have been acquired by lateral transfers. Metabolic pathways, such as fermentation and a methionine salvage pathway, have been identified, as have genes for programmed cell death that may be related to the rapid disappearance of Microcystis blooms in nature. Analysis of the PCC 7806 genome also reveals striking novel biosynthetic features that might help to elucidate the ecological impact of secondary metabolites and lead to the discovery of novel metabolites for new biotechnological applications. M. aeruginosa and other large cyanobacterial genomes exhibit a rapid loss of synteny in contrast to other microbial genomes.

Conclusion

Microcystis aeruginosa PCC 7806 appears to have adopted an evolutionary strategy relying on unusual genome plasticity to adapt to eutrophic freshwater ecosystems, a property shared by another strain of M. aeruginosa (NIES-843). Comparisons of the genomes of PCC 7806 and other cyanobacterial strains indicate that a similar strategy may have also been used by the marine strain Crocosphaera watsonii WH8501 to adapt to other ecological niches, such as oligotrophic open oceans.

Discovering the hidden secondary metabolome of Myxococcus xanthus: a study of intraspecific diversity

As a monophyletic group, the myxobacteria are known to produce a broad spectrum of secondary metabolites. However, the degree of metabolic diversity that can be found within a single species remains unexplored. The model species Myxococcus xanthus produces several metabolites also present in other myxobacterial species, but only one compound unique to M. xanthus has been found to date. Here, we compare the metabolite profiles of 98 M. xanthus strains that originate from 78 locations worldwide and include 20 centimeter-scale isolates from one location. This screen reveals a strikingly high level of intraspecific diversity in the M. xanthus secondary metabolome. The identification of 37 nonubiquitous candidate compounds greatly exceeds the small number of secondary metabolites previously known to derive from this species. These results suggest that M. xanthus may be a promising source of future natural products and that thorough intraspecific screens of other species could reveal many new compounds of interest.

Visualizing the spatial distribution of secondary metabolites produced by marine cyanobacteria and sponges via MALDI-TOF imaging

Marine cyanobacteria and sponges are prolific sources of natural products with therapeutic applications. In this paper we introduce a mass spectrometry based approach to characterize the spatial distribution of these natural products from intact organisms of differing complexities. The natural product MALDI-TOF-imaging (npMALDI-I) approach readily identified a number of metabolites from the cyanobacteria Lyngbya majuscula 3L and JHB, Oscillatoria nigro-viridis, Lyngbya bouillonii, and a Phormidium species, even when they were present as mixtures. For example, jamaicamide B, a well established natural product from the cyanobacterium Lyngbya majuscula JHB, was readily detected as were the ions that correspond to the natural products curacin A and curazole from Lyngbya majuscula 3L. In addition to these known natural products, a large number of unknown ions co-localized with the different cyanobacteria, providing an indication that this method can be used for dereplication and drug discovery strategies. Finally, npMALDI-I was used to observe the secondary metabolites found within the sponge Dysidea herbacea. From these sponge data, more than 40 ions were shown to be co-localized, many of which were halogenated. The npMALDI-I data on the sponge indicates that, based on the differential distribution of secondary metabolites, sponges have differential chemical micro-environments within their tissues. Our data demonstrate that npMALDI-I can be used to provide spatial distribution of natural products, from single strands of cyanobacteria to the very complex marine assemblage of a sponge.

A phosphopantetheinylating polyketide synthase producing a linear polyene to initiate enediyne antitumor antibiotic biosynthesis

The enediynes, unified by their unique molecular architecture and mode of action, represent some of the most potent anticancer drugs ever discovered. The biosynthesis of the enediyne core has been predicted to be initiated by a polyketide synthase (PKS) that is distinct from all known PKSs. Characterization of the enediyne PKS involved in C-1027 (SgcE) and neocarzinostatin (NcsE) biosynthesis has now revealed that (i) the PKSs contain a central acyl carrier protein domain and C-terminal phosphopantetheinyl transferase domain; (ii) the PKSs are functional in heterologous hosts, and coexpression with an enediyne thioesterase gene produces the first isolable compound, 1,3,5,7,9,11,13-pentadecaheptaene, in enediyne core biosynthesis; and (iii) the findings for SgcE and NcsE are likely shared among all nine-membered enediynes, thereby supporting a common mechanism to initiate enediyne biosynthesis.

Paradigm shifts in fungal secondary metabolite research

The 8th International Mycological Congress (IMC8; Cairns, Australia) hosted several plenary lectures, poster presentations, and even entire symposia dedicated to fungal secondary metabolites (extrolites). These advances, presented in this special issue, together demonstrated how the impact of molecular biology and genomics and the availability of sophisticated methods of analytical chemistry has resulted in paradigm shifts in our understanding of fungal secondary metabolism and its key role in fungal biology. Rather than focus on classical topics such as discovery of novel drug candidates and identification of toxins, here we address two major themes in this special issue: (1) the utility and importance of secondary metabolites and their genes in polyphasic taxonomy, phylogeny, and evolutionary history of kingdom Fungi (syn. Eumycota); and (2) the genetic processes regulating secondary metabolite biosynthesis. The history of fungal chemotaxonomy and some important classes of secondary metabolites are reviewed.

Activation of fungal silent gene clusters: a new avenue to drug discovery

The ongoing exponential growth of DNA sequence data will lead to the discovery of many natural-product biosynthesis pathways by genome mining for which no actual product has been characterised. In many cases, these clusters remain silent under laboratory conditions. New technologies based on genetic engineering are available to induce silent genes. Heterologous expression of a silent gene cluster under the control of defined promoters can be applied. Alternatively, promoters of biosynthesis genes within the genome can be exchanged by defined promoters. Most promising, however, is the activation of pathway-specific regulatory genes, which was recently demonstrated. Such regulatory genes are present in many secondary metabolite gene clusters. This approach is rendered feasible by the fact that all of the genes encoding the large number of enzymes required for the synthesis of a typical secondary metabolite are clustered and that in some cases, a single regulator controls the expression of all members of a gene cluster to a certain extent. The advantage of this technique is that only a small gene needs to be handled, and that an ectopic integration is sufficient, bypassing all limitations of homologous recombination. Most conveniently, this strategy can trigger the concerted expression of all pathway genes. The vast amount of DNA sequences in the public database represents only the beginning of this new genomics era. The activation of these gene clusters by genetic engineering will lead to the discovery of many so far unknown products and therefore represents a novel avenue to drug discovery.

Artificial reconstruction of two cryptic angucycline antibiotic biosynthetic pathways

Genome-sequencing projects have revealed that Streptomyces bacteria have the genetic potential to produce considerably larger numbers of natural products than can be observed under standard laboratory conditions. Cryptic angucycline-type aromatic polyketide gene clusters are particularly abundant. Sequencing of two such clusters from Streptomyces sp. PGA64 and H021 revealed the presence of several open reading frames that could be involved in processing the basic angucyclic carbon skeleton. The pga gene cluster contains one putative FAD-dependant monooxygenase (pgaE) and a putatively bifunctional monooxygenase/short chain alcohol reductase (pgaM), whereas the cab cluster contains two similar monooxygenases (cabE and cabM) and an independent reductase (cabV). In this study we have reconstructed the biosynthetic pathways for aglycone synthesis by cloning and sequentially expressing the angucycline tailoring genes with genes required for the synthesis of the unmodified angucycline metabolite-UWM6-in Streptomyces lividans TK24. The expression studies unequivocally showed that, after the production of UWM6, the pathways proceed through the action of the similar monooxygenases PgaE and CabE, followed by reactions catalysed by PgaM and CabMV. Analysis of the metabolites produced revealed that addition of pgaE and cabE genes directs both pathways to a known shunt product, rabelomycin, whereas expression of all genes from a given pathway results in the production of the novel angucycline metabolites gaudimycin A and B. However, one of the end products is most probably further modified by endogenous S. lividans TK24 enzymes. These experiments demonstrate that genes that are either inactive or cryptic in their native host can be used as biosynthetic tools to generate new compounds.

Total Synthesis of Myxovirescin A1

A convergent total synthesis of the antibiotic macrolide myxovirescin A1 (1) is described that is largely based on reagent- and catalyst-controlled transformations. This includes a highly regioselective Negishi reaction of dibromo-alkene 48 with an alkynylzinc reagent, and a palladium catalyzed alkyl-Suzuki coupling of the resulting enyne derivative 12 with the 9-BBN-adduct derived from alkene 61. The latter was obtained via an asymmetric hydrogenation of the chlorinated beta-ketoester 49 and an anti-selective oxyallylation of the functionalized aldehyde 53 as the key steps. The preparation of the bis-borylated allyl-donor 57 used in the oxyallylation step, however, required careful optimization and led to important insights into the nature of the classical hydroborating agent "di(isopinocampheyl)borane (Ipc2BH)". It was unambiguously shown by X-ray crystallography that in the solid state this compound is dimeric, but it is prone to undergo an essentially quantitative mono-deborylation when dissolved in CH2Cl2 or benzene; its composition in ethereal solvents is even more complex as evident from 11B NMR data. Product 71 derived from 12 and 61 was elaborated into the enyne-yne derivative 75, which served as the substrate for an exquisitely selective ring closing alkyne metathesis reaction (RCAM) catalyzed by the molybdenum tris-amido complex 20 activated in situ with CH2Cl2. The resulting cyclic enyne 76 was subjected to a ruthenium catalyzed trans-hydrosilylation/proto-desilylation tandem. Although [Cp*Ru(MeCN)3]PF6 had previously been recommended as catalyst of choice for trans-hydrosilylation reactions of internal alkynes, this complex failed to afford the desired product, whereas its sterically less hindered congener [CpRu(MeCN)3]PF6 permitted the reaction to be performed in appreciable yield, but at the expense of a lower stereoselectivity. AgF-mediated proto-desilylation of the isomeric silanes 79 and 80 followed by cleavage of the remaining acetal protecting groups afforded myxovirescin A1 and its hitherto unknown 14Z-isomer 81, respectively.

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.

Metabolic engineering of Pseudomonas putida for methylmalonyl-CoA biosynthesis to enable complex heterologous secondary metabolite formation

An operon consisting of three open reading frames, annotated in silico as methylmalonyl-CoA (mm-CoA) epimerase, mm-CoA mutase (MCM), and meaB, was identified in the sequencing project of the myxobacterium Sorangium cellulosum So ce56. This putative MCM pathway operon was subcloned from a bacterial artificial chromosome by Red/ET recombineering onto a minimal replicon derived from p15A. This plasmid was modified for integration and heterologous expression in Pseudomonas putida to enable the production of complex secondary metabolites requiring mm-CoA as precursor. Methylmalonate was identified in the recombinant P. putida strain by an analysis method based on gas chromatography/mass spectrometry. The engineered strain is able to synthesize polyketides requiring mm-CoA as an extender unit, which was demonstrated by the production of myxothiazol after integration of the biosynthetic gene cluster into the chromosome, followed by induction of expression.