Heterologous production of plant-derived isoprenoid products in microbes and the application of metabolic engineering and synthetic biology

QM/MM Study of the Reaction Mechanism of L-Tyrosine Hydroxylation Catalyzed by the Enzyme CYP76AD1

We have studied the hydroxylation mechanism of l-Tyr by the heme-dependent enzyme CYP76AD1 from the sugar beet (Beta vulgaris). This enzyme has a promising biotechnological application in modified yeast strains to produce medicinal alkaloids, an alternative to the traditional opium poppy harvest. A generative machine learning software based on AlphaFold was used to build the structure of CYP76AD1 since there are no structural data for this specific enzyme. After model validation, l-Tyr was docked in the active site of CYP76AD1 to assemble the reactive complex, whose catalytic distances remained stable throughout the 100 ns of MD simulation. Subsequent QM/MM calculations elucidated that l-Tyr hydroxylation occurs in two steps: hydrogen abstraction from l-Tyr by CpdI, forming an l-Tyr radical, and subsequent radical rebound, corresponding to a rate-limiting step of 16.0 kcal·mol-1. Our calculations suggest that the hydrogen abstraction step should occur in the doublet state, while the radical rebound should happen in the quartet state. The clarification of the reaction mechanism of CYP76AD1 provides insights into the rational optimization of the biosynthesis of alkaloids to eliminate the use of opium poppy.

Modern strategies for the diversification of the supply of natural compounds - the case of alkaloid painkillers

Plant-derived natural compounds are used for treating diseases since the beginning of humankind. The supply of many plant-derived natural compounds for medicinal purposes, such as thebaine, morphine, and codeine, is, nowadays, majorly dependent on opium poppy crop harvesting. This dependency puts an extra risk factor in ensuring the supply chain because crops are highly susceptible to environmental factors. Emerging technologies, such as biocatalysis, might help to solve this problem, by diversifying the sources of supply of these compounds. Here we review the first committed step in the production of alkaloid painkillers, the production of S-norcoclaurine, and the enzymes involved. The improvement of these enzymes can be carried out by experimental directed evolution and rational design strategies, supported by computational methods, to create variants that produce the S-norcoclaurine precursor for alkaloid painkillers in heterologous organisms, meeting the pharmaceutical industry standards and needs without depending on opium poppy crops.

Mining of the Catharanthus roseus Genome Leads to Identification of a Biosynthetic Gene Cluster for Fungicidal Sesquiterpenes

Characterization of cryptic biosynthetic gene clusters (BGCs) from microbial genomes has been proven to be a powerful approach to the discovery of new natural products. However, such a genome mining approach to the discovery of bioactive plant metabolites has been muted. The plant BGCs characterized to date encode pathways for antibiotics important in plant defense against microbial pathogens, providing a means to discover such phytoalexins by mining plant genomes. Here is reported the discovery and characterization of a minimal BGC from the medicinal plant Catharanthus roseus, consisting of an adjacent pair of genes encoding a terpene synthase (CrTPS18) and cytochrome P450 (CYP71D349). These two enzymes act sequentially, with CrTPS18 acting as a sesquiterpene synthase, producing 5-epi-jinkoheremol (1), which CYP71D349 further hydroxylates to debneyol (2). Infection studies with maize revealed that 1 and 2 exhibit more potent fungicidal activity than validamycin. Accordingly, this study demonstrates that characterization of such cryptic plant BGCs is a promising strategy for the discovery of potential agrochemical leads. Moreover, despite the observed absence of 1 and 2 in C. roseus, the observed transcriptional regulation is consistent with their differential fungicidal activity, suggesting that such conditional coexpression may be sufficient to drive BGC assembly in plants.

CRISPR/Cas genome editing to optimize pharmacologically active plant natural products

Since time immemorial, human use medicinal plants as sources of food, therapy and industrial purpose. Classical biotechnology and recent next-generation sequencing (NGS) techniques have been successfully used to optimize plant-derived natural-products of biomedical significance. Earlier, protein based editing tools viz. zinc-finger nucleases (ZFNs) and transcription activator-like endonucleases (TALENs) have been popularized for transcriptional level genome manipulation. Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated9 (Cas9) endonuclease system is an efficient, robust and selective site-directed mutagenesis strategy for RNA-guided genome-editing. CRISPR/Cas9 genome-editing tool employs designed guide-RNAs that identifies a 3 base-pair protospacer adjacent motif (PAM) sequence occurring downstream of the target-DNA. The present review comprehensively complies the recent literature (2010-2020) retrieved from scientific-databases on the application of CRISPR/Cas9-editing-tools as potent genome-editing strategies in medicinal-plants discussing the recent developments, challenges and future-perspectives with notes on broader applicability of the technique in plants and lower-organisms. In plants, CRISPR/Cas-editing has been implemented successfully in relation to crop-yield and stress-tolerance. However, very few medicinal plants have been edited using CRISPR/Cas genome tool owing to the lack of whole-genome and mRNA-sequences and shortfall of suitable transformation and regeneration strategies. However, recently a number of plant secondary metabolic-pathways (viz. alkaloid, terpenoid, flavonoids, phenolic, saponin etc.) have been engineered employing CRISPR/Cas-editing via knock-out, knock-in, point-mutation, fine-tuning of gene-expression and targeted-mutagenesis. This genome-editing tool further extends its applicability incorporating the tools of synthetic- and systems-biology, functional-genomics and NGS to produce genetically-engineered medicinal-crops with advanced-traits facilitating the production of pharmaceuticals and nutraceuticals.

Enhance production of diterpenoids in yeast by overexpression of the fused enzyme of ERG20 and its mutant mERG20

Yeast has been widely used for large-scale production of terpenoids. In yeast, modifications of terpenoid biosynthetic pathways have been intensively studied. tHMG1 (encoding the catalytic domain of 3-hydroxy-3-methylglutaryl coenzyme A reductase of yeast) and UPC2-1 (the G888D mutant of UPC2 encoding a transcription factor) were integrated into yeast chromosome, and ERG9 (the squalene synthase gene of yeast) was knocked down to yield the chassis strain DH02. A F96C mutation in ERG20 (farnesyl diphosphate synthase of yeast) was conducted to obtain mERG20 which can function as a geranylgeranyl diphosphate synthase (GGPS). Then, three fused genes, including BTS1 (the yeast innate GGPS)-ERG20, ERG20-mERG20 and mERG20-ERG20, were constructed, and expressed either by the pESC-based plasmids in DH02, or by being integrated into DH02 chromosome. The highest geranylgeraniol (GGOH) content was observed in the extracts of DH12 integrated with ERG20-mERG20, corresponding to 3.2 and 2.3 folds of those of the strains integrated with BTS1 and mERG20, respectively. Finally, three genes encoding nor-copalyl diphosphate synthase (nor-CPS), ent-CPS and syn-CPS were integrated into the chromosome of DH12, respectively, to construct yeasts for producing corresponding copalyl diphosphates (CPPs). Thus, a yeast-based platform was built for characterizing all types of diterpene synthases using GGPP or various CPPs as their substrates.

Bioproduction of Isoprenoids and Other Secondary Metabolites Using Methanotrophic Bacteria as an Alternative Microbial Cell Factory Option: Current Stage and Future Aspects

Methane is a promising carbon feedstock for industrial biomanufacturing because of its low price and high abundance. Recent advances in metabolic engineering and systems biology in methanotrophs have made it possible to produce a variety of value-added compounds from methane, including secondary metabolites. Isoprenoids are one of the largest family of secondary metabolites and have many useful industrial applications. In this review, we highlight the current efforts invested to methanotrophs for the production of isoprenoids and other secondary metabolites, including riboflavin and ectoine. The future outlook for improving secondary metabolites production (especially of isoprenoids) using metabolic engineering of methanotrophs is also discussed.

High-throughput mapping of CoA metabolites by SAMDI-MS to optimize the cell-free biosynthesis of HMG-CoA

Metabolic engineering uses enzymes to produce small molecules with industrial, pharmaceutical, and energy applications. However, efforts to optimize enzymatic pathways for commercial production are limited by the throughput of assays for quantifying metabolic intermediates and end products. We developed a multiplexed method for profiling CoA-dependent pathways that uses a cysteine-terminated peptide to covalently capture CoA-bound metabolites. Captured metabolites are then rapidly separated from the complex mixture by immobilization onto arrays of self-assembled monolayers and directly quantified by SAMDI mass spectrometry. We demonstrate the throughput of the assay by characterizing the cell-free synthesis of HMG-CoA, a key intermediate in the biosynthesis of isoprenoids, collecting over 10,000 individual spectra to map more than 800 unique reaction conditions. We anticipate that our rapid and robust analytical method will accelerate efforts to engineer metabolic pathways.

Efficient production of glycyrrhetinic acid in metabolically engineered Saccharomyces cerevisiae via an integrated strategy

Background

Glycyrrhetinic acid (GA) is the most important ingredient in licorice due to its outstanding anti-inflammatory activity and wide application in the medicine and cosmetics industries. Contemporary industrial production of GA by acid hydrolysis of glycyrrhizin which was extracted from Glycyrrhiza plants, is not environment-friendly and devastates farmland since the Glycyrrhiza rhizomes grow up to 10 m underground.

Results

In this study, GA was produced through metabolically engineering Saccharomyces cerevisiae by introducing the entire heterogeneous biosynthetic pathway of GA. Codon optimized CYP88D6 and CYP72A154, combined with β-AS (β-amyrin synthase encoding gene) and the NADPH-cytochrome P450 reductase gene of Arabidopsis thaliana were introduced into S. cerevisiae. The resulting strain (Y1) produced 2.5 mg/L of β-amyrin and 14 μg/L of GA. The cytochrome b5 from G. uralensis (GuCYB5) was identified and the introduction of this novel GuCYB5 increased the efficiency of GA production by eightfold. The joint utilization of the GuCYB5 gene along with 10 known MVA pathway genes from S. cerevisiae were overexpressed in a stable chromosome integration to achieve higher GA production. Using the combined strategy, GA concentration improved by 40-fold during batch fermentation. The production was further improved to 8.78 mg/L in fed-batch fermentation, which was increased by a factor of nearly 630.

Conclusions

This study first investigated the influence of carbon flux in the upstream module and the introduction of a newly identified GuCYB5 on GA production. The newly identified GuCYB5 was highly effective in improving GA production. An integrated strategy including enzyme discovery, pathway optimization, and fusion protein construction was provided in improving GA production, achieving a 630 fold increase in GA production. The metabolically engineered yeast cell factories provide an alternative approach to glycyrrhetinic acid production, replacing the traditional method of plant extraction.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1138-5) contains supplementary material, which is available to authorized users.

Cell-free biosynthesis of limonene using enzyme-enriched Escherichia coli lysates

Isoprenoids are an attractive class of metabolites for enzymatic synthesis from renewable substrates. However, metabolic engineering of microorganisms for monoterpenoid production is limited by the need for time-consuming, and often non-intuitive, combinatorial tuning of biosynthetic pathway variations to meet design criteria. Towards alleviating this limitation, the goal of this work was to build a modular, cell-free platform for construction and testing of monoterpenoid pathways, using the fragrance and flavoring molecule limonene as a model. In this platform, multiple Escherichia coli lysates, each enriched with a single overexpressed pathway enzyme, are mixed to construct the full biosynthetic pathway. First, we show the ability to synthesize limonene from six enriched lysates with mevalonate substrate, an adenosine triphosphate (ATP) source, and cofactors. Next, we extend the pathway to use glucose as a substrate, which relies on native metabolism in the extract to convert glucose to acetyl-CoA along with three additional enzymes to convert acetyl-CoA to mevalonate. We find that the native E. coli farnesyl diphosphate synthase (IspA) is active in the lysate and diverts flux from the pathway intermediate geranyl pyrophospahte to farnesyl pyrophsophate and the byproduct farnesol. By adjusting the relative levels of cofactors NAD+, ATP and CoA, the system can synthesize 0.66 mM (90.2 mg l-1) limonene over 24 h, a productivity of 3.8 mg l-1 h-1. Our results highlight the flexibility of crude lysates to sustain complex metabolism and, by activating a glucose-to-limonene pathway with 9 heterologous enzymes encompassing 20 biosynthetic steps, expands an approach of using enzyme-enriched lysates for constructing, characterizing and prototyping enzymatic pathways.

3-Methyl-3-buten-1-ol (isoprenol) confers longevity and stress tolerance in Caenorhabditis elegans

The present investigation demonstrates the longevity-promoting effects of 3-methyl-3-buten-1-ol (isoprenol) in the animal model Caenorhabditis elegans that might be served as a lead nutraceutical in geriatric research. Our results showed that 0.5 mM isoprenol extended the mean lifespan of worms by 25% in comparison to control worms. Isoprenol also significantly enhanced survival of the worms under various stress conditions. It was found that the longevity-promoting effects of isoprenol were associated with improved age-associated physiological behaviour and reduced intracellular reactive oxygen species (ROS) accumulation. Finally, studies with gene-specific mutants revealed the involvement of pro-longevity transcription factors (TFs) DAF-16 and SKN-1 with simultaneous over-expression of GST-4 and SOD-3 in isoprenol treated worms. In silico analysis revealed the binding affinity of isoprenol with DAF-16 and SKN-1 TFs. Together, the findings suggest that isoprenol is able to enhance the lifespan of C. elegans and embarks its potential in the developments of formulations for age-related ailments.

Functional characterization of squalene epoxidase and NADPH-cytochrome P450 reductase in Dioscorea zingiberensis

Dioscorea zingiberensis is a perennial medicinal herb rich in a variety of pharmaceutical steroidal saponins. Squalene epoxidase (SE) is the key enzyme in the biosynthesis pathways of triterpenoids and sterols, and catalyzes the epoxidation of squalene in coordination with NADPH-cytochrome P450 reductase (CPR). In this study, we cloned DzSE and DzCPR gene sequences from D. zingiberensis leaves, encoding proteins with 514 and 692 amino acids, respectively. Recombinant proteins were successfully expressed in vitro, and enzymatic analysis indicated that, when SE and CPR were incubated with the substrates squalene and NADPH, 2,3-oxidosqualene was formed as the product. Subcellular localization revealed that both the DzSE and DzCPR proteins are localized to the endoplasmic reticulum. The changes in transcription of DzSE and DzCPR were similar in several tissues. DzSE expression was enhanced in a time-dependent manner after methyl jasmonate (MeJA) treatments, while DzCPR expression was not inducible.

P450s controlling metabolic bifurcations in plant terpene specialized metabolism

ABSTRACT

Catalyzing stereo- and regio-specific oxidation of inert hydrocarbon backbones, and a range of more exotic reactions inherently difficult in formal chemical synthesis, cytochromes P450 (P450s) offer outstanding potential for biotechnological engineering. Plants and their dazzling diversity of specialized metabolites have emerged as rich repository for functional P450s with the advances of deep transcriptomics and genome wide discovery. P450s are of outstanding interest for understanding chemical diversification throughout evolution, for gaining mechanistic insights through the study of their structure-function relationship, and for exploitation in Synthetic Biology. In this review, we highlight recent developments and examples in the discovery of plant P450s involved in the biosynthesis of industrially relevant monoterpenoids, sesquiterpenoids, diterpenoids and triterpenoids, throughout 2016 and early 2017. Examples were selected to illustrate the spectrum of value from commodity chemicals, flavor and fragrance compounds to pharmacologically active terpenoids. We focus on a recently emerging theme, where P450s control metabolic bifurcations and chemical diversity of the final product profile, either within a pathway, or through neo-functionalization in related species. The implications may inform approaches for rational assembly of recombinant pathways, biotechnological production of high value terpenoids and generation of novel chemical entities.

Opportunities and challenges for the sustainable production of structurally complex diterpenoids in recombinant microbial systems

With over 50.000 identified compounds terpenes are the largest and most structurally diverse group of natural products. They are ubiquitous in bacteria, plants, animals and fungi, conducting several biological functions such as cell wall components or defense mechanisms. Industrial applications entail among others pharmaceuticals, food additives, vitamins, fragrances, fuels and fuel additives. Central building blocks of all terpenes are the isoprenoid compounds isopentenyl diphosphate and dimethylallyl diphosphate. Bacteria like Escherichia coli harbor a native metabolic pathway for these isoprenoids that is quite amenable for genetic engineering. Together with recombinant terpene biosynthesis modules, they are very suitable hosts for heterologous production of high value terpenes. Yet, in contrast to the number of extracted and characterized terpenes, little is known about the specific biosynthetic enzymes that are involved especially in the formation of highly functionalized compounds. Novel approaches discussed in this review include metabolic engineering as well as site-directed mutagenesis to expand the natural terpene landscape. Focusing mainly on the validation of successful integration of engineered biosynthetic pathways into optimized terpene producing Escherichia coli, this review shall give an insight in recent progresses regarding manipulation of mostly diterpene synthases.

Engineering a novel biosynthetic pathway in Escherichia coli for the production of caffeine

This work demonstrated a novel biosynthetic pathway to produce caffeine in Escherichia coli.

Cell-Free Mixing of Escherichia coli Crude Extracts to Prototype and Rationally Engineer High-Titer Mevalonate Synthesis

Cell-free metabolic engineering (CFME) is advancing a powerful paradigm for accelerating the design and synthesis of biosynthetic pathways. However, as most cell-free biomolecule synthesis systems to date use purified enzymes, energy and cofactor balance can be limiting. To address this challenge, we report a new CFME framework for building biosynthetic pathways by mixing multiple crude lysates, or extracts. In our modular approach, cell-free lysates, each selectively enriched with an overexpressed enzyme, are generated in parallel and then combinatorically mixed to construct a full biosynthetic pathway. Endogenous enzymes in the cell-free extract fuel high-level energy and cofactor regeneration. As a model, we apply our framework to synthesize mevalonate, an intermediate in isoprenoid synthesis. We use our approach to rapidly screen enzyme variants, optimize enzyme ratios, and explore cofactor landscapes for improving pathway performance. Further, we show that genomic deletions in the source strain redirect metabolic flux in resultant lysates. In an optimized system, mevalonate was synthesized at 17.6 g·L^-1 (119 mM) over 20 h, resulting in a volumetric productivity of 0.88 g·L^-1·hr^-1. We also demonstrate that this system can be lyophilized and retain biosynthesis capability. Our system catalyzes ∼1250 turnover events for the cofactor NAD⁺ and demonstrates the ability to rapidly prototype and debug enzymatic pathways in vitro for compelling metabolic engineering and synthetic biology applications.

Biosynthesis of therapeutic natural products using synthetic biology

Natural products are a group of bioactive structurally diverse chemicals produced by microorganisms and plants. These molecules and their derivatives have contributed to over a third of the therapeutic drugs produced in the last century. However, over the last few decades traditional drug discovery pipelines from natural products have become far less productive and far more expensive. One recent development with promise to combat this trend is the application of synthetic biology to therapeutic natural product biosynthesis. Synthetic biology is a young discipline with roots in systems biology, genetic engineering, and metabolic engineering. In this review, we discuss the use of synthetic biology to engineer improved yields of existing therapeutic natural products. We further describe the use of synthetic biology to combine and express natural product biosynthetic genes in unprecedented ways, and how this holds promise for opening up completely new avenues for drug discovery and production.

Isoprenoid-Based Biofuels: Homologous Expression and Heterologous Expression in Prokaryotes

Enthusiasm for mining advanced biofuels from microbial hosts has increased remarkably in recent years. Isoprenoids are one of the highly diverse groups of secondary metabolites and are foreseen as an alternative to petroleum-based fuels. Most of the prokaryotes synthesize their isoprenoid backbone via the deoxyxylulose-5-phosphate pathway from glyceraldehyde-3-phosphate and pyruvate, whereas eukaryotes synthesize isoprenoids via the mevalonate pathway from acetyl coenzyme A (acetyl-CoA). Microorganisms do not accumulate isoprenoids in large quantities naturally, which restricts their application for fuel purposes. Various metabolic engineering efforts have been utilized to overcome the limitations associated with their natural and nonnatural production. The introduction of heterologous pathways/genes and overexpression of endogenous/homologous genes have shown a remarkable increase in isoprenoid yield and substrate utilization in microbial hosts. Such modifications in the hosts' genomes have enabled researchers to develop commercially competent microbial strains for isoprenoid-based biofuel production utilizing a vast array of substrates. The present minireview briefly discusses the recent advancement in metabolic engineering efforts in prokaryotic hosts for the production of isoprenoid-based biofuels, with an emphasis on endogenous, homologous, and heterologous expression strategies.

New developments in engineering plant metabolic pathways

Plants contain countless metabolic pathways that are responsible for the biosynthesis of complex metabolites. Armed with new tools in sequencing and bioinformatics, the genes that encode these plant biosynthetic pathways have become easier to discover, putting us in an excellent position to fully harness the wealth of compounds and biocatalysts (enzymes) that plants provide. For overproduction and isolation of high-value plant-derived chemicals, plant pathways can be reconstituted in heterologous hosts. Alternatively, plant pathways can be modified in the native producer to confer new properties to the plant, such as better biofuel production or enhanced nutritional value. This perspective highlights a range of examples that demonstrate how the metabolic pathways of plants can be successfully harnessed with a variety of metabolic engineering approaches.

The potential of the mevalonate pathway for enhanced isoprenoid production

The cytosol-localised mevalonic acid (MVA) pathway delivers the basic isoprene unit isopentenyl diphosphate (IPP). In higher plants, this central metabolic intermediate is also synthesised by the plastid-localised methylerythritol phosphate (MEP) pathway. Both MVA and MEP pathways conspire through exchange of intermediates and regulatory interactions. Products downstream of IPP such as phytosterols, carotenoids, vitamin E, artemisinin, tanshinone and paclitaxel demonstrate antioxidant, cholesterol-reducing, anti-ageing, anticancer, antimalarial, anti-inflammatory and antibacterial activities. Other isoprenoid precursors including isoprene, isoprenol, geraniol, farnesene and farnesol are economically valuable. An update on the MVA pathway and its interaction with the MEP pathway is presented, including the improvement in the production of phytosterols and other isoprenoid derivatives. Such attempts are for instance based on the bioengineering of microbes such as Escherichia coli and Saccharomyces cerevisiae, as well as plants. The function of relevant genes in the MVA pathway that can be utilised in metabolic engineering is reviewed and future perspectives are presented.

Discovery and resupply of pharmacologically active plant-derived natural products: A review

Medicinal plants have historically proven their value as a source of molecules with therapeutic potential, and nowadays still represent an important pool for the identification of novel drug leads. In the past decades, pharmaceutical industry focused mainly on libraries of synthetic compounds as drug discovery source. They are comparably easy to produce and resupply, and demonstrate good compatibility with established high throughput screening (HTS) platforms. However, at the same time there has been a declining trend in the number of new drugs reaching the market, raising renewed scientific interest in drug discovery from natural sources, despite of its known challenges. In this survey, a brief outline of historical development is provided together with a comprehensive overview of used approaches and recent developments relevant to plant-derived natural product drug discovery. Associated challenges and major strengths of natural product-based drug discovery are critically discussed. A snapshot of the advanced plant-derived natural products that are currently in actively recruiting clinical trials is also presented. Importantly, the transition of a natural compound from a "screening hit" through a "drug lead" to a "marketed drug" is associated with increasingly challenging demands for compound amount, which often cannot be met by re-isolation from the respective plant sources. In this regard, existing alternatives for resupply are also discussed, including different biotechnology approaches and total organic synthesis. While the intrinsic complexity of natural product-based drug discovery necessitates highly integrated interdisciplinary approaches, the reviewed scientific developments, recent technological advances, and research trends clearly indicate that natural products will be among the most important sources of new drugs also in the future.

Metabolic engineering for isoprenoid-based biofuel production

Sustainable economic and industrial growth is the need of the hour and it requires renewable energy resources having better performance and compatibility with existing fuel infrastructure from biological routes. Isoprenoids (C ≥ 5) can be a potential alternative due to their diverse nature and physiochemical properties similar to that of petroleum based fuels. In the past decade, extensive research has been done to utilize metabolic engineering strategies in micro-organisms primarily, (i) to overcome the limitations associated with their natural and non-natural production and (ii) to develop commercially competent microbial strain for isoprenoid-based biofuel production. This review briefly describes the engineered isoprenoid biosynthetic pathways in well-characterized microbial systems for the production of several isoprenoid-based biofuels and fuel precursors.

Pseudomonas putida-a versatile host for the production of natural products

The biosynthesis of natural products by heterologous expression of biosynthetic pathways in amenable production strains enables biotechnological access to a variety of valuable compounds by conversion of renewable resources. Pseudomonas putida has emerged as a microbial laboratory work horse, with elaborated techniques for cultivation and genetic manipulation available. Beyond that, this bacterium offers several particular advantages with regard to natural product biosynthesis, notably a versatile intrinsic metabolism with diverse enzymatic capacities as well as an outstanding tolerance to xenobiotics. Therefore, it has been applied for recombinant biosynthesis of several valuable natural products. This review provides an overview of applications of P. putida as a host organism for the recombinant biosynthesis of such natural products, including rhamnolipids, terpenoids, polyketides and non-ribosomal peptides, and other amino acid-derived compounds. The focus is on de novo natural product synthesis from intrinsic building blocks by means of heterologous gene expression and strain engineering. Finally, the future potential of the bacterium as a chassis organism for synthetic microbiology is pointed out.

Development of bio-based fine chemical production through synthetic bioengineering

Fine chemicals that are physiologically active, such as pharmaceuticals, cosmetics, nutritional supplements, flavoring agents as well as additives for foods, feed, and fertilizer are produced by enzymatically or through microbial fermentation. The identification of enzymes that catalyze the target reaction makes possible the enzymatic synthesis of the desired fine chemical. The genes encoding these enzymes are then introduced into suitable microbial hosts that are cultured with inexpensive, naturally abundant carbon sources, and other nutrients. Metabolic engineering create efficient microbial cell factories for producing chemicals at higher yields. Molecular genetic techniques are then used to optimize metabolic pathways of genetically and metabolically well-characterized hosts. Synthetic bioengineering represents a novel approach to employ a combination of computer simulation and metabolic analysis to design artificial metabolic pathways suitable for mass production of target chemicals in host strains. In the present review, we summarize recent studies on bio-based fine chemical production and assess the potential of synthetic bioengineering for further improving their productivity.

Current and emerging options for taxol production

Paclitaxel (trademark "Taxol") is a plant-derived isoprenoid natural product that exhibits potent anticancer activity. Taxol was originally isolated from the Pacific yew tree in 1967 and triggered an intense scientific and engineering venture to provide the compound reliably to cancer patients. The choices available for production include synthetic and biosynthetic routes (and combinations thereof). This chapter focuses on the currently utilized and emerging biosynthetic options for Taxol production. A particular emphasis is placed on the biosynthetic production hosts including macroscopic and unicellular plant species and more recent attempts to elucidate, transfer, and reconstitute the Taxol pathway within technically advanced microbial hosts. In so doing, we provide the reader with relevant background related to Taxol and more general information related to producing valuable, but structurally complex, natural products through biosynthetic strategies.