Engineering a polyketide with a longer chain by insertion of an extra module into the erythromycin-producing polyketide synthase

Cross-Module Enoylreduction in the Azalomycin F Polyketide Synthase

The colinearity of canonical modular polyketide synthases, which creates a direct link between multienzyme structure and the chemical structure of the biosynthetic end-product, has become a cornerstone of knowledge-based genome mining. Herein, we report genetic and enzymatic evidence for the remarkable role of an enoylreductase in the polyketide synthase for azalomycin F biosynthesis. This internal enoylreductase domain, previously identified as acting only in the second of two chain extension cycles on an initial iterative module, is shown to also catalyze enoylreduction in trans within the next module. The mechanism for this rare deviation from colinearity appears to involve direct cross-modular interaction of the reductase with the longer acyl chain, rather than back transfer of the substrate into the iterative module, suggesting an additional and surprising plasticity in natural PKS assembly-line catalysis.

Repurposing Modular Polyketide Synthases and Non-ribosomal Peptide Synthetases for Novel Chemical Biosynthesis

In nature, various enzymes govern diverse biochemical reactions through their specific three-dimensional structures, which have been harnessed to produce many useful bioactive compounds including clinical agents and commodity chemicals. Polyketide synthases (PKSs) and non-ribosomal peptide synthetases (NRPSs) are particularly unique multifunctional enzymes that display modular organization. Individual modules incorporate their own specific substrates and collaborate to assemble complex polyketides or non-ribosomal polypeptides in a linear fashion. Due to the modular properties of PKSs and NRPSs, they have been attractive rational engineering targets for novel chemical production through the predictable modification of each moiety of the complex chemical through engineering of the cognate module. Thus, individual reactions of each module could be separated as a retro-biosynthetic biopart and repurposed to new biosynthetic pathways for the production of biofuels or commodity chemicals. Despite these potentials, repurposing attempts have often failed owing to impaired catalytic activity or the production of unintended products due to incompatible protein–protein interactions between the modules and structural perturbation of the enzyme. Recent advances in the structural, computational, and synthetic tools provide more opportunities for successful repurposing. In this review, we focused on the representative strategies and examples for the repurposing of modular PKSs and NRPSs, along with their advantages and current limitations. Thereafter, synthetic biology tools and perspectives were suggested for potential further advancement, including the rational and large-scale high-throughput approaches. Ultimately, the potential diverse reactions from modular PKSs and NRPSs would be leveraged to expand the reservoir of useful chemicals.

An in vitro platform for engineering and harnessing modular polyketide synthases

To harness the synthetic power of modular polyketide synthases (PKSs), many aspects of their biochemistry must be elucidated. A robust platform to study these megadalton assembly lines has not yet been described. Here, we in vitro reconstitute the venemycin PKS, a short assembly line that generates an aromatic product. Incubating its polypeptides, VemG and VemH, with 3,5-dihydroxybenzoic acid, ATP, malonate, coenzyme A, and the malonyl-CoA ligase MatB, venemycin production can be monitored by HPLC and NMR. Multi-milligram quantities of venemycin are isolable from dialysis-based reactors without chromatography, and the enzymes can be recycled. Assembly line engineering is performed using pikromycin modules, with synthases designed using the updated module boundaries outperforming those using the traditional module boundaries by over an order of magnitude. Using combinations of VemG, VemH, and their engineered derivatives, as well as the alternate starter unit 3-hydroxybenzoic acid, a combinatorial library of six polyketide products is readily accessed.

A robust platform to study modular polyketide synthases (PKSs) in vitro is still unavailable. Here, the authors report the reconstitution of the venemycin PKS, engineer hybrid venemycin/pikromycin PKSs, and obtain much improved yields through employing the updated module boundaries.

Emulating evolutionary processes to morph aureothin-type modular polyketide synthases and associated oxygenases

Polyketides produced by modular type I polyketide synthases (PKSs) play eminent roles in the development of medicines. Yet, the production of structural analogs by genetic engineering poses a major challenge. We report an evolution-guided morphing of modular PKSs inspired by recombination processes that lead to structural diversity in nature. By deletion and insertion of PKS modules we interconvert the assembly lines for related antibiotic and antifungal agents, aureothin (aur) and neoaureothin (nor) (aka spectinabilin), in both directions. Mutational and functional analyses of the polyketide-tailoring cytochrome P450 monooxygenases, and PKS phylogenies give contradictory clues on potential evolutionary scenarios (generalist-to-specialist enzyme evolution vs. most parsimonious ancestor). The KS-AT linker proves to be well suited as fusion site for both excision and insertion of modules, which supports a model for alternative module boundaries in some PKS systems. This study teaches important lessons on the evolution of PKSs, which may guide future engineering approaches.

The wealth of complex polyketides is an essential source for drug discovery. Here, the authors report an evolution-guided rational morphing of modular polyketide synthases (PKSs) for aurothin and neoaurothin biosynthesis, and reveal engineering site suitable for diversifying PKS systems.

Computer-aided re-engineering of nonribosomal peptide and polyketide biosynthetic assembly lines

Covering: 2014 to 2019Nonribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs) have been the subject of engineering efforts for multiple decades. Their modular assembly line architecture potentially allows unlocking vast chemical space for biosynthesis. However, attempts thus far are often met with mixed success, due to limited molecular compatibility of the parts used for engineering. Now, new engineering strategies, increases in genomic data, and improved computational tools provide more opportunities for major progress. In this review we highlight some of the challenges and progressive strategies for the re-design of NRPSs & type I PKSs and survey useful computational tools and approaches to attain the ultimate goal of semi-automated and design-based engineering of novel peptide and polyketide products.

Biosynthesis of polyketides by trans-AT polyketide synthases in Burkholderiales

Widely used as drugs and agrochemicals, polyketides are a family of bioactive natural products, with diverse structures and functions. Polyketides are produced by megaenzymes termed as polyketide synthases (PKSs). PKS biosynthetic pathways are divided into the cis-AT PKSs and trans-AT PKSs; a division based mainly on the absence of an acyltransferase (AT) domain in the trans-AT PKS modules. In trans-AT biosynthesis, the AT activity is contributed via one or several independent proteins, and there are few other characteristics that distinguish trans-AT PKSs from cis-AT PKSs, especially in the formation of the β-branch. The trans-AT PKSs constitute a major PKS pathway, and many are found in Burkholderia species, which are prevalent in the environment and prolific sources of polyketides. This review summarizes studies from 1973 to 2017 on the biosynthesis of natural products by trans-AT PKSs from Burkholderia species.

Reprogramming of the antimycin NRPS-PKS assembly lines inspired by gene evolution

Reprogramming of the NRPS/PKS assembly line is an attractive method for the production of new bioactive molecules. However, it is usually hampered by the loss of intimate domain/module interactions required for the precise control of chain transfer and elongation reactions. In this study, we first establish heterologous expression systems of the unique antimycin-type cyclic depsipeptides: JBIR-06 (tri-lactone) and neoantimycin (tetra-lactone), and engineer their biosyntheses by taking advantage of bioinformatic analyses and evolutionary insights. As a result, we successfully accomplish three manipulations: (i) ring contraction of neoantimycin (from tetra-lactone to tri-lactone), (ii) ring expansion of JBIR-06 (from tri-lactone to tetra-lactone), and (iii) alkyl chain diversification of JBIR-06 by the incorporation of various alkylmalonyl-CoA extender units, to generate a set of unnatural derivatives in practical yields. This study presents a useful strategy for engineering NRPS-PKS module enzymes, based on nature’s diversification of the domain and module organizations.

Modifying the non-ribosomal peptide synthase (NRPS)/polyketide synthase (PKS) pathway to generate novel non-ribosomal peptides often results in a loss of productivity. Here the authors use evolutionary alignments of NRPS/PKS gene clusters to guide rational design of complexes that can produce novel lactones.

Engineering strategies for rational polyketide synthase design

In this review, we highlight strategies in engineering polyketide synthases (PKSs). We focus on important protein–protein interactions that constitute an intact PKS assembly line.

Evidence for an iterative module in chain elongation on the azalomycin polyketide synthase

The assembly-line synthases that produce bacterial polyketide natural products follow a modular paradigm in which each round of chain extension is catalysed by a different set or module of enzymes. Examples of deviation from this paradigm, in which a module catalyses either multiple extensions or none are of interest from both a mechanistic and an evolutionary viewpoint. We present evidence that in the biosynthesis of the 36-membered macrocyclic aminopolyol lactones (marginolactones) azalomycin and kanchanamycin, isolated respectively from Streptomyces malaysiensis DSM4137 and Streptomyces olivaceus Tü4018, the first extension module catalyses both the first and second cycles of polyketide chain extension. To confirm the integrity of the azl gene cluster, it was cloned intact on a bacterial artificial chromosome and transplanted into the heterologous host strain Streptomyces lividans, which does not possess the genes for marginolactone production. When furnished with 4-guanidinobutyramide, a specific precursor of the azalomycin starter unit, the recombinant S. lividans produced azalomycin, showing that the polyketide synthase genes in the sequenced cluster are sufficient to accomplish formation of the full-length polyketide chain. This provides strong support for module iteration in the azalomycin and kanchanamycin biosynthetic pathways. In contrast, re-sequencing of the gene cluster for biosynthesis of the polyketide β-lactone ebelactone in Streptomyces aburaviensis has shown that, contrary to a recently-published proposal, the ebelactone polyketide synthase faithfully follows the colinear modular paradigm.

Genetic engineering of modular PKSs: from combinatorial biosynthesis to synthetic biology

This reviews covers on-going efforts at engineering the gigantic modular polyketide synthases (PKSs), highlighting both notable successes and failures.

Broadening substrate specificity of a chain-extending ketosynthase through a single active-site mutation

An in vitro model system based on a ketosynthase domain of the erythromycin polyketide synthase was used to probe the apparent substrate tolerance of ketosynthase domains of the mycolactone polyketide synthase.

Identification of the Polyketide Biosynthetic Machinery for the Indolizidine Alkaloid Cyclizidine

The cyclizidine biosynthetic gene cluster was identified from Streptomyces NCIB 11649, which revealed the polyketide biosynthetic machinery for cyclizidine alkaloid biosynthesis. Both in vivo mutagenesis study and in vitro biochemical analysis provided insight into the timing and mechanism of the biosynthetic enzymes that produce cyclizidine-type indolizidine alkaloids.

Harnessing natural product assembly lines: structure, promiscuity, and engineering

Many therapeutically relevant natural products are biosynthesized by the action of giant mega-enzyme assembly lines. By leveraging the specificity, promiscuity, and modularity of assembly lines, a variety of strategies has been developed that enables the biosynthesis of modified natural products. This review briefly summarizes recent structural advances related to natural product assembly lines, discusses chemical approaches to probing assembly line structures in the absence of traditional biophysical data, and surveys efforts that harness the inherent or engineered promiscuity of assembly lines for the synthesis of non-natural polyketides and non-ribosomal peptide analogues.

Three new milbemycins from a genetically engineered strain S. avermitilis MHJ1011

Three new β-class milbemycins, 13α-hydroxy-4-ethy1 milbemycin β3 (1), 13α-hydroxy-25-ethy1 milbemycin β3 (2), 13α-hydroxy milbemycin β3 (3), were isolated from the broth of the genetically engineered strains Streptomyces avermitilis MHJ1011, whose aveA1 gene was replaced by milA1 gene seamlessly. Their structures were determined on the basis of extensive spectroscopic analysis and comparison with data from the literature. These three compounds, especially compound 1, exhibited potent acaricidal activity.

Chemical probes for the functionalization of polyketide intermediates

A library of functionalized chemical probes capable of reacting with ketosynthase-bound biosynthetic intermediates was prepared and utilized to explore in vivo polyketide diversification. Fermentation of ACP mutants of S. lasaliensis in the presence of the probes generated a range of unnatural polyketide derivatives, including novel putative lasalocid A derivatives characterized by variable aryl ketone moieties and linear polyketide chains (bearing alkyne/azide handles and fluorine) flanking the polyether scaffold. By providing direct information on microorganism tolerance and enzyme processing of unnatural malonyl-ACP analogues, as well as on the amenability of unnatural polyketides to further structural modifications, the chemical probes constitute invaluable tools for the development of novel mutasynthesis and synthetic biology.

Structure of a modular polyketide synthase

Polyketide natural products constitute a broad class of compounds with diverse structural features and biological activities. Their biosynthetic machinery, represented by type I polyketide synthases (PKSs), has an architecture in which successive modules catalyse two-carbon linear extensions and keto-group processing reactions on intermediates covalently tethered to carrier domains. Here we used electron cryo-microscopy to determine sub-nanometre-resolution three-dimensional reconstructions of a full-length PKS module from the bacterium Streptomyces venezuelae that revealed an unexpectedly different architecture compared to the homologous dimeric mammalian fatty acid synthase. A single reaction chamber provides access to all catalytic sites for the intramodule carrier domain. In contrast, the carrier from the preceding module uses a separate entrance outside the reaction chamber to deliver the upstream polyketide intermediate for subsequent extension and modification. This study reveals for the first time, to our knowledge, the structural basis for both intramodule and intermodule substrate transfer in polyketide synthases, and establishes a new model for molecular dissection of these multifunctional enzyme systems.

Progress challenges and opportunities for the re-engineering of trans-AT polyketide synthases

Polyketides are a structurally and functionally diverse family of bioactive natural products that are used extensively as pharmaceuticals and agrochemicals. In bacteria these molecules are biosynthesized by giant, multi-functional enzymatic complexes, termed modular polyketide synthases (PKSs), that function in assembly-line like fashion to fuse and tailor simple carboxylic acid monomers into a vast array of elaborate chemical scaffolds. Modifying PKSs through targeted synthase re-engineering is a promising approach for accessing functionally-optimized polyketides. Due to their highly mosaic architectures the recently identified trans-AT family of modular synthases appear inherently more amenable to re-engineering than their well studied cis-AT counterparts. Here, we review recent progress in the re-engineering of trans-AT PKSs, summarize opportunities for harnessing the biosynthetic potential of these systems, and highlight challenges that such re-engineering approaches present.

Steps towards the synthetic biology of polyketide biosynthesis

Nature is providing a bountiful pool of valuable secondary metabolites, many of which possess therapeutic properties. However, the discovery of new bioactive secondary metabolites is slowing down, at a time when the rise of multidrug-resistant pathogens and the realization of acute and long-term side effects of widely used drugs lead to an urgent need for new therapeutic agents. Approaches such as synthetic biology are promising to deliver a much-needed boost to secondary metabolite drug development through plug-and-play optimized hosts and refactoring novel or cryptic bacterial gene clusters. Here, we discuss this prospect focusing on one comprehensively studied class of clinically relevant bioactive molecules, the polyketides. Extensive efforts towards optimization and derivatization of compounds via combinatorial biosynthesis and classical engineering have elucidated the modularity, flexibility and promiscuity of polyketide biosynthetic enzymes. Hence, a synthetic biology approach can build upon a solid basis of guidelines and principles, while providing a new perspective towards the discovery and generation of novel and new-to-nature compounds. We discuss the lessons learned from the classical engineering of polyketide synthases and indicate their importance when attempting to engineer biosynthetic pathways using synthetic biology approaches for the introduction of novelty and overexpression of products in a controllable manner.

Bioactive compounds synthesized by non-ribosomal peptide synthetases and type-I polyketide synthases discovered through genome-mining and metagenomics

Non-ribosomal peptide synthetases (NRPS) and type-I polyketide synthases (PKS-I) are multimodular enzymes involved in biosynthesis of oligopeptide and polyketide secondary metabolites produced by microorganisms such as bacteria and fungi. New findings regarding the mechanisms underlying NRPS and PKS-I evolution illustrate how microorganisms expand their metabolic potential. During the last decade rapid development of bioinformatics tools as well as improved sequencing and annotation of microbial genomes led to discovery of novel bioactive compounds synthesized by NRPS and PKS-I through genome-mining. Taking advantage of these technological developments metagenomics is a fast growing research field which directly studies microbial genomes or specific gene groups and their products. Discovery of novel bioactive compounds synthesized by NRPS and PKS-I will certainly be accelerated through metagenomics, allowing the exploitation of so far untapped microbial resources in biotechnology and medicine.

Biosynthesis of rapamycin and its regulation: past achievements and recent progress

Rapamycin and its analogs are clinically important macrolide compounds produced by Streptomyces hygroscopicus. They exhibit antifungal, immunosuppressive, antitumor, neuroprotective and antiaging activities. The core macrolactone ring of rapamycin is biosynthesized by hybrid type I modular polyketide synthase (PKS)/nonribosomal peptide synthetase systems primed with 4,5-dihydrocyclohex-1-ene-carboxylic acid. The linear polyketide chain is condensed with pipecolate by peptide synthetase, followed by cyclization to form the macrolide ring and modified by a series of post-PKS tailoring steps. The aim of this review was to outline past and recent advances in the biosynthesis and regulation of rapamycin, with an emphasis on the distinguished contributions of Professor Demain to the study of rapamycin. In addition, this article describes the biological activities as well as mechanism of action of rapamycin and its derivatives. Recent attempts to improve the productivity of rapamycin and generate diverse rapamycin analogs through mutasynthesis and mutagenesis are also introduced, along with some future perspectives.

Genetic engineering of macrolide biosynthesis: past advances, current state, and future prospects

Polyketides comprise one of the major families of natural products. They are found in a wide variety of bacteria, fungi, and plants and include a large number of medically important compounds. Polyketides are biosynthesized by polyketide synthases (PKSs). One of the major groups of polyketides are the macrolides, the activities of which are derived from the presence of a macrolactone ring to which one or more 6-deoxysugars are attached. The core macrocyclic ring is biosynthesized from acyl-CoA precursors by PKS. Genetic manipulation of PKS-encoding genes can result in predictable changes in the structure of the macrolactone component, many of which are not easily achieved through standard chemical derivatization or total synthesis. Furthermore, many of the changes, including post-PKS modifications such as glycosylation and oxidation, can be combined for further structural diversification. This review highlights the current state of novel macrolide production with a focus on the genetic engineering of PKS and post-PKS tailoring genes. Such engineering of the metabolic pathways for macrolide biosynthesis provides attractive alternatives for the production of diverse non-natural compounds. Other issues of importance, including the engineering of precursor pathways and heterologous expression of macrolide biosynthetic genes, are also considered.

The changing patterns of covalent active site occupancy during catalysis on a modular polyketide synthase multienzyme revealed by ion-trap mass spectrometry

A catalytically competent, homodimeric diketide synthase comprising the first extension module of the erythromycin polyketide synthase was analysed using MS, after limited proteolysis to release functional domains, to determine the pattern of covalent attachment of substrates and intermediates to active sites during catalysis. Using the natural substrates, the acyltransferase and acylcarrier protein of the loading module were found to be heavily loaded with propionyl starter groups, while the ketosynthase was fully propionylated. The acylcarrier protein of the extension module was partly occupied by the product diketide, and the adjacent chain-releasing thioesterase domain was vacant, implying that the rate-limiting step is transfer of the diketide from the acylcarrier protein to the thioesterase domain. The data suggest an attractive model for preventing iterative chain extension by efficient repriming of the ketosynthase domain after condensation. Use of the alternative starter unit valeryl-CoA produced an altered pattern, in which a significant proportion of the extension acylcarrier protein was loaded with methylmalonate, not diketide, consistent with the condensation step having become an additional slow step. Strikingly, when NADPH was omitted, the extension acylcarrier protein contained methylmalonate and none of the expected keto diketide, in contrast to results obtained previously by mixing individual recombinant domains, showing the importance of also studying intact modules. The detailed patterns of loading of the extension acylcarrier protein (of which there are two in the homodimer) also provided the first evidence for simultaneous loading of both acylcarrier proteins and for the coordination of timing between the two active centres for chain extension.

Functional dissection of a multimodular polypeptide of the pikromycin polyketide synthase into monomodules by using a matched pair of heterologous docking domains

The pikromyin polyketide synthase (PKS) in Streptomyces venezulae is comprised of a loading module and six extension modules, which generate the corresponding 14-membered macrolactone product. PikAI is a multimodular component of this PKS and houses both the loading domain and the first two extension modules, joined by short intraprotein linkers. We have shown that PikAI can be separated into two proteins at either of these linkers, only when matched pairs of docking domains (DDs) from a heterologous modular phoslactomycin PKS are used in place of the intraprotein linker. In both cases the yields of pikromycin produced by the S. venezuelae mutant were 50% of that of a S. venezuelae strain expressing the native trimodular PikAI. This observation provides the first demonstration that such separations do not dramatically impact the efficiency of the entire in vivo biosynthetic process. Expression of module 2 as a monomodular protein fused to a heterologous N-terminal docking domain was also observed to give almost a tenfold improvement in the in vivo generation of pikromycin from a synthetic diketide intermediate. These results demonstrate the utility of DDs to manipulate biosynthetic processes catalyzed by modular PKSs and the quest to generate novel polyketide products.

Structure and function of the macrolide biosensor protein, MphR(A), with and without erythromycin

The regulatory protein MphR(A) has recently seen extensive use in synthetic biological applications, such as metabolite sensing and exogenous control of gene expression. This protein negatively regulates the expression of a macrolide 2'-phosphotransferase I resistance gene (mphA) via binding to a 35-bp DNA operator upstream of the start codon and is de-repressed by the presence of erythromycin. Here, we present the refined crystal structure of the MphR(A) protein free of erythromycin and that of the MphR(A) protein with bound erythromycin at 2.00- and 1.76-A resolutions, respectively. We also studied the DNA binding properties of the protein and identified mutants of MphR(A) that are defective in gene repression and ligand binding in a cell-based reporter assay. The combination of these two structures illustrates the molecular basis of erythromycin-induced gene expression and provides a framework for additional applied uses of this protein in the isolation and engineered biosynthesis of polyketide natural products.

Interrogating the molecular basis for multiple macrolactone ring formation by the pikromycin polyketide synthase

The pikromycin polyketide synthase (PKS) is unique in its ability to generate both 12 and 14 membered ring macrolactones. As such, dissection of the molecular basis for controlling metabolic diversity in this system remains an important objective for understanding modular PKS function and expanding chemical diversity. Here, we describe a series of experiments designed to probe the importance of the protein-protein interaction that occurs between the final two monomodules, PikAIII (module 5) and PikAIV (module 6), for the production of the 12 membered ring macrolactone 10-deoxymethynolide. The results obtained from these in vitro studies demonstrate that PikAIII and PikAIV generate the 12 membered ring macrocycle most efficiently when engaged in their native protein-protein interaction. Accordingly, the data are consistent with PikAIV adopting an alternative conformation that enables the terminal thioesterase domain to directly off-load the PikAIII-bound hexaketide intermediate for macrocyclization.

Insights into polyether biosynthesis from analysis of the nigericin biosynthetic gene cluster in Streptomyces sp. DSM4137

Nigericin was among the first polyether ionophores to be discovered, but its biosynthesis remains obscure. The biosynthetic gene cluster for nigericin has been serendipitously cloned from Streptomyces sp. DSM4137, and deletion of this gene cluster abolished the production of both nigericin and the closely related metabolite abierixin. Detailed comparison of the nigericin biosynthetic genes with their counterparts in the biosynthetic clusters for other polyketides has prompted a significant revision of the proposed common pathway for polyether biosynthesis. In particular, we present evidence that in nigericin, nanchangmycin, and monensin, an unusual ketosynthase-like protein, KSX, transfers the initially formed linear polyketide chain to a discrete acyl carrier protein, ACPX, for oxidative cyclization. Consistent with this, deletion of either monACPX or monKSX from the monensin gene cluster effectively abolished monensin A biosynthesis.

Structure and Mechanism of the 6-Deoxyerythronolide B Synthase

6-Deoxyerythronolide B, the macrocyclic aglycone of the antibiotic erythromycin, is synthesized by a polyketide synthase (PKS) that has emerged as the prototypical modular megasynthase. A variety of molecular biological, protein chemical, and biosynthetic experiments over the past two decades have yielded insights into its mechanistic features. More recently, high-resolution structural images of portions of the 6-deoxyerythronolide B synthase have provided a platform for interpreting this wealth of biochemical data, while at the same time presenting a fundamentally new basis for the design of more detailed investigations into this remarkable enzyme. For example, the critical roles of domain-domain interactions and nonconserved linkers, as well as large interdomain movements in the structure and function of modular PKSs, have been highlighted. In turn, these insights point the way forward for more sophisticated and efficient biosynthetic engineering of complex polyketide natural products.

Evidence that a novel thioesterase is responsible for polyketide chain release during biosynthesis of the polyether ionophore monensin

Polyether ionophores, such as monensin A, are known to be biosynthesised, like many other antibiotic polyketides, on giant modular polyketide synthases (PKSs), but the intermediates and enzymes involved in the subsequent steps of oxidative cyclisation remain undefined. In particular there has been no agreement on the mechanism and timing of the final polyketide chain release. We now report evidence that MonCII from the monensin biosynthetic gene cluster in Streptomyces cinnamonensis, which was previously thought to be an epoxide hydrolase, is a novel thioesterase that belongs to the alpha/beta-hydrolase structural family and might catalyse this step. Purified recombinant MonCII was found to hydrolyse several thioester substrates, including an N-acetylcysteamine thioester derivative of monensin A. Further, incubation with a hallmark inhibitor of such enzymes, phenylmethanesulfonyl fluoride, led to inhibition of the thioesterase activity and to the accumulation of an acylated form of MonCII. These findings require a reassessment of the role of other enzymes implicated in the late stages of polyether ionophore biosynthesis.

Activating hybrid modular interfaces in synthetic polyketide synthases by cassette replacement of ketosynthase domains

Unnatural combinations of polyketide synthase modules often fail to make a polyketide product. The causes of these failures are likely complex and are not yet amenable to rational correction. One possible explanation is the inability of the ketosynthase (KS) domain to extend the ketide donated to it by the upstream module. We therefore addressed the problem by exchanging KS domains of the acceptor module in a combinatorial fashion and coexpressing these chimeric modules with ketide-donor modules that naturally interact with the transplanted KS. This approach was remarkably successful in activating previously unproductive bimodular combinations, and the results augur well for the ongoing development of molecular tools to design and produce novel polyketides.

Biogenetic proposals and biosynthetic studies on secondary metabolites of opisthobranch molluscs

Marine chemical diversity is generated by a large number of transformations often not noted in terrestrial counterparts. Life in the oceans differs in most respects from life on land and our knowledge of the genetics and biochemistry of marine organisms is still very limited to a small number of species. Biosynthetic studies and biogenetic speculations can therefore be crucial in predicting relevant enzymes and their encoding genes, with a view to setting the stage for rational engineering of marine natural products. A further useful outcome to the identification of biosynthetic pathways is the resulting classification of natural products, which can serve to correlate chemical diversity and biodiversity. This review summarizes the present knowledge on secondary metabolites biogenesis in marine opisthobranchs, a class of organisms that has been emerging as a prolific source of structurally diverse metabolites possessing a broad variety of biological activities.

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.

Combinatorial biosynthesis of reduced polyketides

The bacterial multienzyme polyketide synthases (PKSs) produce a diverse array of products that have been developed into medicines, including antibiotics and anticancer agents. The modular genetic architecture of these PKSs suggests that it might be possible to engineer the enzymes to produce novel drug candidates, a strategy known as 'combinatorial biosynthesis'. So far, directed engineering of modular PKSs has resulted in the production of more than 200 new polyketides, but key challenges remain before the potential of combinatorial biosynthesis can be fully realized.

The Impact of Bacterial Genomics on Natural Product Research

"There's life in the old dog yet!" This adage also holds true for natural product research. After the era of natural products was declared to be over, because of the introduction of combinatorial synthesis techniques, natural product research has taken a surprising turn back towards a major field of pharmaceutical research. Current challenges, such as emerging multidrug-resistant bacteria, might be overcome by developments which combine genomic knowledge with applied biology and chemistry to identify, produce, and alter the structure of new lead compounds. Significant biological activity is reported much less frequently for synthetic compounds, a fact reflected in the large proportion of natural products and their derivatives in clinical use. This Review describes the impact of microbial genomics on natural products research, in particularly the search for new lead structures and their optimization. The limitations of this research are also discussed, thus allowing a look into future developments.

A crevice adjoining the ribosome tunnel: hints for cotranslational folding

RNA protection experiments and the crystal structure of a complex of the large ribosomal subunit from the eubacterium Deinococcus radiodurans with rapamycin, a polyketide compound resembling macrolides and ketolides, showed that rapamycin binds to a crevice located at the boundaries of the nascent protein exit tunnel, near its entrance. At this location rapamycin cannot occlude the ribosome exit tunnel, consistent with its failure to act as a ribosomal antibiotic drug. In accord with recent biochemical data, this crevice may play a role in facilitating local cotranslational folding of nascent chains, in particular for transmembrane proteins.