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Nematophilic bacteria as a source of novel macrocyclised antimicrobial non-ribosomal peptides
(2020)
A solution to ineffective clinical antimicrobials is the discovery of new ones from under-explored sources such as macrocyclic non-ribosomal peptides (NRP) from nematophilic bacteria. In this dissertation an antimicrobial discovery process –from soil sample to inhibitory peptide– is demonstrated through investigations on six nematophilic bacteria: Xenorhabdus griffiniae XN45, X. griffiniae VH1, Xenorhabdus sp. nov. BG5, Xenorhabdus sp. nov. BMMCB, X. ishibashii and Photorhabdus temperata. To demonstrate the first step of bacterium isolation and species delineation, endosymbionts were isolated from Steinernema sp. strains BG5 and VH1 that were isolated directly from soil samples in Western Kenya. After genome sequencing and assembly of novel Xenorhabdus isolates VH1 and BG5, species delineation was done via three overall genome relatedness indices. VH1 was identified as X. griffiniae VH1, BG5 as Xenorhabdus sp. nov. BG5 and X. griffiniae BMMCB was emended to Xenorhabdus sp. nov. BMMCB. The nematode host of X. griffiniae XN45, Steinernema sp. scarpo was highlighted as a putative novel species. To demonstrate the second step of genome mining and macrocyclic non-ribosomal peptide structure elucidation, chemosynthesis and biosynthesis, the non-ribosomal peptide whose production is encoded by the ishA-B genes in X. ishibashii was investigated. Through a combination of refactoring the ishA-B operon by a promoter exchange mechanism, isotope labelling experiments, high resolution tandem mass spectrometry analysis, bioinformatic protein domain analysis and chemoinformatic comparisons of actual to hypothetical mass spectrometry spectra, the structures of Ishipeptides were elucidated and confirmed by chemical synthesis. Ishipeptide A was a branch cyclic depsidodecapeptide macrocyclised via an ester bond between serine and the terminal glutamate. It chemosynthesis route was via a late stage macrolactamation and linearised Ishipeptide B was synthesised via solid phase iterative synthesis. Ishipeptides were not N-terminally acylated despite being biosynthesised from the IshA protein that had a C-starter domain. It was highlighted that more than restoration of the histidine active site of this domain is required to restore N-terminal acylation activity.
To demonstrate the final step of determination of antimicrobial activity, minimum inhibitory concentrations of Ishipeptides and Photoditritide from Photorhabdus temperata against fungi and bacteria were determined. None were antifungal while only the macrocyclic compounds were inhibitory, with Ishipeptide A inhibitory to Gram-positive bacteria at 37 µM. The cationic Photoditritide, a cyclic hexapeptide macrocyclised via a lactam bond between homoarginine and tryptophan, was 12 times more inhibitory (3.0 µM), even more effective than a current clinical compound, Ampicillin (4.2 µM). For both, macrocyclisation was hypothesised to contribute to antimicrobial activity. Ultimately, this dissertation demonstrated not only nematophilic bacteria as a source of novel macrocyclic antimicrobial non-ribosomal peptides but also a process of antimicrobial discovery–from soil sample to inhibitory peptide– from these useful bacteria genera. This is significant for the fight against antimicrobial resistance.
Nematophilic bacteria as a source of novel macrocyclised antimicrobial non-ribosomal peptides
(2020)
A solution to ineffective clinical antimicrobials is the discovery of new ones from under-explored sources such as macrocyclic non-ribosomal peptides (NRP) from nematophilic bacteria. In this dissertation an antimicrobial discovery process –from soil sample to inhibitory peptide– is demonstrated through investigations on six nematophilic bacteria: Xenorhabdus griffiniae XN45, X. griffiniae VH1, Xenorhabdus sp. nov. BG5, Xenorhabdus sp. nov. BMMCB, X. ishibashii and Photorhabdus temperata. To demonstrate the first step of bacterium isolation and species delineation, endosymbionts were isolated from Steinernema sp. strains BG5 and VH1 that were isolated directly from soil samples in Western Kenya. After genome sequencing and assembly of novel Xenorhabdus isolates VH1 and BG5, species delineation was done via three overall genome relatedness indices. VH1 was identified as X. griffiniae VH1, BG5 as Xenorhabdus sp. nov. BG5 and X. griffiniae BMMCB was emended to Xenorhabdus sp. nov. BMMCB. The nematode host of X. griffiniae XN45, Steinernema sp. scarpo was highlighted as a putative novel species. To demonstrate the second step of genome mining and macrocyclic non-ribosomal peptide structure elucidation, chemosynthesis and biosynthesis, the non-ribosomal peptide whose production is encoded by the ishA-B genes in X. ishibashii was investigated. Through a combination of refactoring the ishA-B operon by a promoter exchange mechanism, isotope labelling experiments, high resolution tandem mass spectrometry analysis, bioinformatic protein domain analysis and chemoinformatic comparisons of actual to hypothetical mass spectrometry spectra, the structures of Ishipeptides were elucidated and confirmed by chemical synthesis. Ishipeptide A was a branch cyclic depsidodecapeptide macrocyclised via an ester bond between serine and the terminal glutamate. It chemosynthesis route was via a late stage macrolactamation and linearised Ishipeptide B was synthesised via solid phase iterative synthesis. Ishipeptides were not N-terminally acylated despite being biosynthesised from the IshA protein that had a C-starter domain. It was highlighted that more than restoration of the histidine active site of this domain is required to restore N-terminal acylation activity.
To demonstrate the final step of determination of antimicrobial activity, minimum inhibitory concentrations of Ishipeptides and Photoditritide from Photorhabdus temperata against fungi and bacteria were determined. None were antifungal while only the macrocyclic compounds were inhibitory, with Ishipeptide A inhibitory to Gram-positive bacteria at 37 µM. The cationic Photoditritide, a cyclic hexapeptide macrocyclised via a lactam bond between homoarginine and tryptophan, was 12 times more inhibitory (3.0 µM), even more effective than a current clinical compound, Ampicillin (4.2 µM). For both, macrocyclisation was hypothesised to contribute to antimicrobial activity. Ultimately, this dissertation demonstrated not only nematophilic bacteria as a source of novel macrocyclic antimicrobial non-ribosomal peptides but also a process of antimicrobial discovery–from soil sample to inhibitory peptide– from these useful bacteria genera. This is significant for the fight against antimicrobial resistance.
The early-diverging oomycetes contain a large number of holocarpic obligate parasites of diatoms, algae, aquatic phycomycetes, and invertebrate animals. These organisms are diverse and widespread. However, taxonomic placement most of the early-diverging oomycetes remains provisional and unresolved, since many have not been sequenced and studied for molecular phylogeny. Here, we report the taxonomy and phylogeny of several holocarpic oomycetes that we have rediscovered and newly classified, including several new species combinations. Phylogenetic reconstructions revealed that the type species of genus Ectrogella (E. bacillariacearum) is a member of the early-diverging Saprolegniales, while the type species of Olpidiopsis (O. saprolegniae) and Pontisma (P. lagenidioides) grouped within the early-diverging lineage of oomycetes forming distinct clades. Since the monophyletic red-algae parasitoids are unrelated to the Olpidiopsis, these were reclassified to the genus Pontisma, while genus Diatomophthora was introduced to accommodate all the diatom parasitoids that were previously assigned to Olpidiopsis. In addition, four new oomycete parasitoids, Miracula helgolandica, Miracula moenusica, Diatomophthora drebesii and Olpidiopsis parthenogenetica and a single rediscovered species, Diatomophthora gillii, are also classified here, including eight new species combinations of red-algae parasites (Pontisma bostrychiae, P. heterosiphoniae, P. muelleri, P. palmariae, P. porphyrae, P. pyropiae) and diatom parasitoids (Diatomophthora drebesii, D. gillii). The results obtained in this study have further improved the resolution and expanded the knowledge on the phylogeny of the earlydiverging oomycetes, leading to the establishment of three new orders (Miraculales, Diatomophthorales, Pontismatales) and one order (Anisolpidiales) being reintroduced.
In the 'Golden Age of Antibiotics', between 1940 and 1970, the global pharmaceutical companies discovered many antibiotics, such as cephalosporins, tetracyclines, aminoglycosides, glycopeptides, etc., as well as antifungal and antiparisitic agents. Due to several reasons, e.g. the steady re-discovery of already known NPs and the associated high costs, many pharmaceutical companies have significantly scaled back or totally abandoned their NP discovery programs since the late 20th century. Instead those companies started to focus on drug discovery based on combinatorial synthesis and thereby on the creation of enormous synthetic libraries containing small molecules. Unfortunately, this synthetic approach dealing with the optimization of existing NP or antibiotic has its limitations. As a result, leading pharmaceutical companies are re-conducting NPs research to discover new antimicrobials for the upcoming antimicrobial resistance threat. The Natural Product Center of Excellence, a collaboration between Sanofi-Aventis and Fraunhofer IME, is advancing in this context the discovery and development of novel antimicrobial agents for the treatment of infectious diseases through the testing of Sanofi's microbial extract library and strain collection. The aim of the present PhD thesis was the discovery and isolation of novel antimicrobial compounds with improved activities and/or novel MOAs as potential lead compound for a further drug discovery.
Identification of new natural products from nematode-associated bacteria using mass spectrometry
(2023)
This work aims to find unknown natural products produced by bacteria, that live in close association with nematodes and to elucidate their structure by using mass spectrometry.
The first chapter of this work is dedicated to the detection of hitherto unknown natural products by using a metabolomics approach and subsequent structure elucidation of said compounds. This chapter includes metabolomics analysis of Xenorhabdus szentirmaii wild type and knockout mutants, overproduction of the target compound, identification of derivatives from other strains and MS based structure elucidation.
The second and third chapters are about natural products that protect C. elegans from B. thuringiensis infections.
The second chapter deals with natural products that protect the nematode host without killing the pathogen. I deployed molecular biology methods to generate deletion and overproduction strains of a target compound, identified it via LC-MS/MS analysis and used LC-MS/MS and lipidomics to analyse the chemical properties of the active compound.
The third chapter aims at finding natural products, which are produced by Pseudomonas strains MYb11 and MYb12, respectively. These natural products display the ability to protect C. elegans by killing B. thuringiensis. I identified said compounds via fractionation and subsequent bioactivity testing. After identification, I generated production strains of the target compounds and elucidated the structure of the bioactive derivative.
The last chapter deals with the structure elucidation of peptides produced by an unusual GameXPeptide synthetase in Xenorhabdus miraniensis. I analysed producer strains of GameXPeptides using LC-MS and elucidated the structural differences between the known GameXPeptides, produced by P. luminescens TT01, and the unusual ones produced by X. miraniensis.
Natural products (NPs) have been a rich source for pharmaceutically used anti-infectives and other drugs. However, the application of anti-infectives inevitably causes the development of resistant and multiresistant pathogens, which have to be treated with novel anti-infectives. The industrial research for novel anti-infectives has been concentrating on members of the bacterial Actinomycetales for a long time. Due to several reasons, e.g. the rediscovery of already known NPs, pharmaceutical companies abandoned their NP-research and focused on drug development based on combinatorial chemistry. However, the limited structural diversity of merely synthetic compound libraries has not been a fruitful source for bioactive compounds. Hence the discovery of novel bioactive NPs as a source for anti-infectives is still of economical and humanitarian interest and will remain to be an important branch of research in the future. One strategy to circumvent the rediscovery of bioactive NPs is the analysis of yet unexplored bacterial taxa. Based on this assumption, this work aimed at the discovery of novel NPs from the entomopathogenic bacterial genera Xenorhabdus and Photorhabdus and other promising taxa, as well as the investigation of their biosynthesis. ...
This work deals with the characterization of three different type II polyketide synthase systems (PKS II) from the Gram-negative bacteria Xenorhabdus and Photorhabdus.
Particular attention was paid to a biochemically underexplored class of aryl polyene (APE) pigments. Bioinformatic analysis of enzymes involved in the biosynthesis and the in vitro reconstruction proved that the synthesis of APEs involves an unusual fatty acid-like elongation mechanism. Furthermore, the discovery of unexpected protein-protein interactions provided new insights into the multienzyme complex formation of this unusual PKS II system. Through collaboration with the groups from Prof. Michael Groll and junior Prof. Nina Morgner, two protein complexes were structurally solved and several native protein multimerization events were identified and allowed us to suggest a possible protein-interaction network. The results are summarized in publication ‘An Uncommon Type II PKS Catalyzes Biosynthesis of Aryl Polyene Pigments’ (first author; J. Am. Chem. Soc.).
In addition to in vitro-analysis, in vivo-studies were used to investigate the APE compound produced by X. doucetiae in more detail. The activation of the silent biosynthetic gene cluster (BGC) led to the detection of the APE compound in the homologous host. Further combination of homologous expression and targeted deletions of the APE BGC revealed an APE-lipid-like structure. MS-based analyses and purification of intermediates allowed us to deduce structural building blocks of the APE-lipid, which is composed of an APE structural core, a glucosamine residue and an unusual long-chain fatty acid with unusual conjugated double bonds and a phosphoethanolamine head group. In combination with the above stated in vitro-data, we assumed a plausible biosynthetic mechanism of the APE-lipid. The results are summarized in the section ‘Additional Results: Tracing the Full-length APE’.
The biosynthesis of isopropylstilbene (IPS) has already been well-studied by the Bode laboratory and the group of Prof. Ikuro Abe. Studies with Photorhabdus laumondii TT01 by the Bode group revealed the distributed locations and functions of the genes involved in biosynthesis, which originate from two pathways. Particularly, the Bode group first demonstrated that an unusual ketosynthase/cyclase (StlD) catalyzes the condensation of 5-phenyl-2,4-pentadienoyl-ACP and isovaleryl-beta-ketoacyl-ACP via a Michael addition. Such a pathway for stilbene formation is distinct from those widespread in plants. The Abe group solved the structure and biochemical mechanism of StlD and further investigated the aromatization reaction of the aromatase StlC. However, the generation of the required cinnamoyl-precursor 5-phenyl-2,4-pentadienoyl-ACP as a Michael acceptor for this cyclization reaction remained elusive. In this work, we were able to reconstitute the synthesis of the Michael acceptor in vitro, by the action of enzymes from the fatty acid biosynthesis. With the knowledge about the crucial cross-talk from primary and specialized metabolism, we further determined the minimal endowment for stilbene production in a heterologous host. Here, the discovered AasS enzyme StlB is responsible for the generation of cinnamoyl-ACP and among others, plFabH plays a key role as gatekeeper enzyme for further processing. With this information in hand, we were able to obtain IPS production in E. coli. These results are presented in the manuscript ‘Biosynthesis of the Multifunctional Isopropylstilbene in Photorhabdus laumondii Involves Cross-talk Between Specialized and Primary Metabolism’ (co-first author, manuscript).
The biosynthesis of the orange-to-red-pigmented anthraquinones (AQs) is the best-studied type II PKS system according to preliminary results. While several investigations by Brachmann et al. discovered the BGC and the overall product spectrum of the main AQ-256 and its methylated derivatives, data of Quiqin Zhou (Bode group) performed biochemical in vitro analysis paired with in vivo heterologous expression of the ant-genes antA-I. This led to the identification of shunt products that indicated an AQ-scaffold derived from an octaketide intermediate that gets shortened to a heptaketide by the hydrolase AntI, resulting in the main anthraquinone AQ-256. This PKS-shortening mechanism was further confirmed by the protein crystal structure of AntI by the Groll group (publication, minor contributions, co-author, Chem Sci. ‘Molecular Mechanism of Polyketide Shortening in Anthraquinone Biosynthesis of Photorhabdus luminescens’). Further substrate analysis of the P. luminescens AQ-producer and mutants revealed an inhibitory effect of cinnamic acid against the hydrolase AntI. Cinnamic acid might therefore be involved in regulation of AQ biosynthesis (‘Anthraquinone Production is Influenced by Cinnamic Acid’, first author, manuscript).
Biochemical analysis from Quiqin Zhou with the minimal PKS of the AQ-synthase further revealed the exclusive activation of the AQ-ACP by the PPTase AntB. The PPTase is insoluble alone but gets stabilized by the CoA-ligase, most likely inactive, working as a chaperone. Thus, the minimal PKS endowment to produce the octaketide scaffold compromises, besides the ACP, the KS:CLF heterodimer and the MCAT, the co-occurrence of the PPTase AntB and the CoA-ligase AntG. For the first time, X-ray crystallography depicted a minimal PKS in action, by obtaining the structural data of native complexes from an ACP:KS:CLF, the KS:CLF alone and an ACP:MCAT in their non-active and active forms. It was possible to confirm a KS-bound hexaketide, which was built upon heterologous expression of the KS:CLF. Mutagenesis with amino-acids proposed to be involved in protein-protein interactions in the ACP:KS:CLF complex revealed some interesting protein-interaction sites. Additionally, an induced-fit mechanism of the MCAT with the ACP during the malonylation reaction confirmed a monodirectional transfer reaction (‘Structural Snapshots of the Minimal PKS System Responsible for Octaketide Biosynthesis’ co-author, manuscript under review).
As fossil resources are diminishing, environmental concerns arise and chemical synthesis often involves expensive catalysts or extensive extraction procedures, the demand for production of industrially relevant compounds from renewable resources increases. In this context, engineering microorganisms for production of specialty chemicals, such as 3-alkylphenols, presents an attractive, environmental-friendly approach. 3-alkylphenols have various applications: due to their antiseptic and stabilizing properties many 3-alkylphenols, including 3-methylphenol (3-MP), are utilized as additives in disinfectant reagents and biological products, while they can be also implemented as platform chemicals for production of lubricating oil additives or flavors. Some 3-akylphenols have potential for transmission control of the disease sleeping sickness that is transmitted by tsetse flies in sub-saharan Africa, since 3-ethylphenol (3-EP) and 3-propylphenol (3-PP) and to a lesser degree 3-MP were found to attract tsetse flies and improved catch rates in impregnated tsetse fly traps. Microbial fermentation of 3-alkylphenols would provide a simple and inexpensive way for local communities in Africa to produce these compounds and prepare their own tsetse fly traps.
Some molds synthesize 3-MP as an intermediate during biosynthesis of the mycotoxin patulin. However, the heterologous host Saccharomyces cerevisiae has advantageous traits for industrial application, since it is well characterized, robust, simple to handle and easily genetically accessible. In this thesis, genetical engineering approaches were utilized to establish the yeast S. cerevisiae for biotechnological production of 3-alkylphenols. As a proof of concept, the iterative polyketide synthase from Penicillium patulum, 6-methylsalicylic acid synthase (MSAS), and 6-methylsalicylic acid (6-MSA) decarboxylase PatG from Aspergillus clavatus were heterologously expressed in S. cerevisiae resulting in the first reported de novo biosynthesis of 3-MP via 6-MSA in yeast from sugars (Hitschler & Boles, 2019). It was shown that codon-optimization and genomic integration of heterologous genes, high initial cell densities and a balanced expression of PatG were beneficial for heterologous production of up to 589 mg/L 3-MP in S. cerevisiae. However, toxicity of 3-MP limited higher product accumulation.
Different in vivo detoxification strategies were implemented to face this bottleneck. Growth tests revealed that 3-methylanisole (3-MA) is less toxic to the yeast cells than 3-MP. Expression of an orcinol-O-methyltransferase from chinese rose hybrids (OOMT2) was combined with in situ extraction converting the toxic 3-MP product into the volatile 3-MA and accumulating up to 211 mg/L 3-MA in the dodecane phase. Alternatively, up to 533 mg/L 3-MP glucoside were synthesized by expression of a UDP-glycosyltransferase (UGT72B27) from Vitis vinifera in the 3-MP producing strain, revealing saccharose as beneficial carbon source and ethanol growth phase as essential for high 3-MP production, although 3-MP conversions were not yet complete. Both detoxification strategies allowed circumvention of the toxicity imposed limited product accumulation. This was demonstrated when both detoxification strategies were combined with redirection of the carbon flux through deletion of phosphoglucose isomerase gene PGI1 and feeding a mixture of fructose and glucose leading to majorly improved product formation, with up to 899 mg/L 3-MA/3-MP and 873 mg/L 3-MP/3-MP glucoside, compared to less than 313 mg/L product titers in the wild type controls (Hitschler & Boles, 2020).
For provision of the tsetse fly attractants 3-EP from propionyl-CoA and 3-PP from butyryl-CoA, the substrate promiscuities of MSAS and PatG were exploited. However, slower formation rates with the alternative substrates propionyl-CoA and butyryl-CoA suggested that competing formation of 6-MSA from the preferred priming unit acetyl-CoA was dominating in vivo. Indeed, 3-EP or 3-PP formation was not observed in 3-MP producing yeast strains. Assuming that intracellular levels of propionyl-CoA and butyryl-CoA were limiting 3-EP and 3-PP formation, different strategies were implemented to raise the supply of these alternative priming units and successfully compete with acetyl-CoA for MSAS priming.
Supplementation of propionate increased propionyl-CoA levels by endogenous pathways sufficiently to enable 3-EP formation in yeast mediated by MSAS and PatG. Deletion of the 2-methylcitrate synthases CIT2 and CIT3 revealed that degradation of propionyl-CoA was not limiting 3-EP formation at this stage. In order to raise propionyl-CoA levels further, a heterologous propionyl-CoA synthase (PrpE) was expressed in the 3-MP producing yeast strain leading to up to 12.5 mg/L 3-EP with propionate feeding and blockage of degradation. Moreover, PrpE enabled also 3-EP formation without propionate supplementation suggesting that an endogenous supply of propionate existed that was reactivated by PrpE. As threonine or 2-ketobutyrate feeding increased 3-EP titers in combination with PrpE, this indicated that threonine degradation via 2-ketobutyrate was responsible for the endogenous propionate supply. Moreover, expression of branched-chain ketoacid dehydrogenase complex from Pseudomonas putida combined with PrpE provided propionyl-CoA from endogenous 2-ketobutyrate and raised 3-EP titers up to 5.9 mg/L compared to 2.8 mg/L with only PrpE indicating a potential route for optimization of 3-EP titers independent of propionate or threonine feeding.
For 3-PP production from butyryl-CoA, a heterologous ‘reverse ß-oxidation’ pathway was introduced in the 3-MP producing yeast strain providing sufficient butyryl-CoA for biosynthesis of up to 2 mg/L 3-PP. Degradation of the precursor via ß-oxidation was slightly limiting, since deletion of fatty acyl-CoA oxidase POX1 increased 3-PP titers slightly to 2.6 mg/L.
As the concentrations of 3-alkylphenols are close to the concentrations implemented in tsetse fly traps, the engineered yeast strains have the potential for simple and inexpensive on-site production of 3-alkylphenols as tsetse fly attractants by local rural communities in Africa. In spite of this success, 3-MP remained the main product in the developed yeast strains. Since 3-EP and 3-PP are more efficient tsetse fly attractants, a shift in substrate specificities of MSAS and PatG is desirable for a more favorable 3-EP/3-MP and 3-PP/3-MP product ratio regarding tsetse fly attraction. During rational engineering of MSAS, the MSASQ625A/I752V mutant showed a beneficial shift of product ratios with up to 11 mg/L 3-EP/63 mg/L 3-MP and 4.5 mg/L 3-PP/116 mg/L 3-MP, compared to a higher proportion of 3-MP with up to 343 mg/L, 11 mg/L 3-EP and 1.5 mg/L 3-PP in the wild type controls. Further engineering of MSAS and PatG might majorly improve production of 3-EP and 3-PP.
In summary, this thesis successfully established the yeast S. cerevisiae as cell factory for production of different 3-alkylphenols optimizing expression of the heterologous production pathway, elucidating means to detoxify products and establishing different approaches to increase intracellular levels of acyl-CoA precursors. The engineered yeast strains can be potentially implemented for simple and inexpensive fermentation of tsetse fly attractants in Africa.
In this thesis, we characterized megasynthases such as fatty acid synthases (FASs) and polyketide synthases. The obtained insights into structure and function were used to engineer such systems to produce new-to-nature compounds.
The in vitro characterization of megasynthases requires reproducible access to these enzymes in high quality. Therefore, we established purification strategies for the yeast FAS and the methylsalicylic acid synthase (MSAS) from Saccharopolyspora erythraea (SerMSAS) and applied the latter one on MSAS from Penicillium patulum (PenPaMSAS) and on 6-deoxyerythronolide B synthase (DEBS) module 6. With the purified samples, we were able to obtain initial structural data for SerMSAS and solve the complete structure of the yeast FAS (PDB: 6TA1). On the example of the yeast FAS, we could show that the sample can suffer from adsorption to the water-air interface during the grid preparation for electron microscopy and presented how the use of graphene-based grids can overcome this problem. The combined structural and functional analysis of the yeast FAS showed that the structural domains trimerization module and dimerization module 2 are not essential for the assembly of the whole system. Therefore, they can potentially be used for domain exchange approaches. The in-depth functional analysis of SerMSAS revealed that not SerMSAS itself releases the product, but a 3-oxoacyl-(acyl-carrier protein) synthase like enzyme within the gene cluster transfers 6-methyl salicylic acid from SerMSAS to another carrier protein for subsequent modifications. In contrast, we showed that PenPaMSAS can release its product by hydrolysis and that non-native substrates can be incorporated although at significantly slower turnover rates compared to the native starter substrate. Our further investigation demonstrated that the substrate specificity of the acyltransferase (AT) is a critical factor for the incorporation of non-native substrates.
With the insight from the functional and structural characterization, we engineered megasynthases for the biosynthesis of natural product derivatives. We targeted the AT of PenPaMSAS for active site mutagenesis and discovered a mutant which can transfer non-native substrates significantly faster (~200-300%). Additionally, the malonyl/acetyl transferase (MAT) of the mammalian FAS was used as a promising target for protein engineering because of its previously reported properties including polyspecificity, fast transfer kinetics, robustness, and plasticity. We showed that the MAT can transfer fluorinated substrates and accept the acyl carrier protein of DEBS module 6. By exchanging the substrate specific AT of DEBS with the polyspecific MAT of the mammalian FAS, we demonstrated an efficient DEBS/FAS hybrid and an optimal truncation site for the applied ATs. In contrast to the wild type system, the DEBS/FAS enzyme was able to synthesize demethylated and fluorinated derivatives. The production and purification of a fluoro-methyl-disubstituted polyketide was of particular interest, as it has a high potential for the generation of new drugs and shows the potential of protein engineering. Furthermore, the incorporation of the disubstituted substrate had important implication in the mechanistic details of the ketosynthase-mediated C-C bond formation.
This work addresses the investigation of the biosynthesis mechanisms of type II polyketide synthase (PKS) and fatty acid synthase (FAS) derived specialized metabolites (SMs) from Photorhabdus laumondii.
The elucidation of the biosynthetic pathway of the bacterial 3,5-dihydroxy-4-isopropyl-trans-stilbene (IPS) was one of the major topics of this thesis. IPS exhibits several bioactive characteristics as it inhibits the phenoloxidase of insects, acts antibacterial, but also influences the soluble epoxide hydrolase which is involved in inflammatory reactions. It was recently approved as a treatment against psoriasis by the FDA and is the first Photorhabdus derived drug.
The stilbene generation in Photorhabdus requires the formation of the two acyl-carrier-protein (ACP) bound 5-phenyl-2,4-pentadienoyl- and isovaleryl-β-ketoacyl-moieties. The ketosynthase (KS)/cyclase StlD catalyzes a ring formation via a Michael-addition between the two intermediates which is then further processed by an aromatase. The formation of 5-phenyl-2,4-pentadienoyl-ACP was shown via in vitro assays with purified proteins by proving the influence of the KS FabH, ketoreductase FabG and dehydratase FabA or FabZ of the fatty acid metabolism. While E. coli was able to complement most of these enzymes in attempts to produce IPS in the heterologous host, the Photorhabdus derived FabH was not replaceable despite 73 % sequence identity with the E. coli based isoenzyme, acting as a gatekeeper enzyme for cinnamic acid (CA) moieties. Furthermore, the ability to incorporate meta-substituted halogenated CA-derivatives was shown in order to produce 3-chloro- and 3-bromo-IPS. While studying the stilbene biosynthesis, the ability of Photorhabdus and Xenorhabdus to produce hydrazines was also discovered.
The second investigated biosynthesis was the formation of benzylideneacetone (BZA). BZA is produced by Photorhabdus and Xenorhabdus strains acting as a suppressor for the immune cascade of insects and has also antibiotic activities towards Gram-negative bacteria. Due to its structural similarity towards CA and the intermediates during the stilbene formation, a shared mechanism for Photorhabdus and Xenorhabdus budapestensis was proposed due to their ability to produce CA. The production of BZA was also dependent on the stilbene related CoA-ligase, the ACP and FabH. It was verified in vitro and in vivo in E. coli yielding a 150-fold increase of the BZA production compared to the Photorhabdus and Xenorhabdus wildtype (WT) strains.
The second part of this work deals with the optimization of P. laumondii strains regarding the production titers of IPS. Therefore, several deletions of other SM related genes as well as promoter exchanges in front of stilbene related genes were carried out. These approaches were combined with the upregulation of the phenylalanine by heterologous plasmid expression, since it is the precursor of CA. Another approach applied in parallel was the optimization of the cultivation conditions with different media and supplementation with XAD-resins. It was proved that media rich on fatty acids or peptides led to higher optical densities of the cultures and thus to higher titers of stilbenes. Since IPS is inhibiting the phenoloxidase, an enzyme important for the insect immunity, it was hypothesized that cultivation in media containing insects might enhance the output of this SM. Starting from 23 mg/l of IPS in the P. laumondii WT strain, it was possible to increase the production levels to more than 860 mg/l by utilizing the mentioned approaches.
The last topic of this thesis focuses on the production of epoxidated IPS (EPS) and its derivatives. Under laboratory conditions, only a low titer of EPS was observed for the wildtype strain. However, the optimized IPS strains and IPS-production conditions could also be applied for EPS which led to higher productions and also to the detection of many new derivatives. Most of the EPS derivatives were amino acid or peptide derived acting as nucleophiles to open the epoxide ring and yielding β-amino-alcohols. However, purification and chemical synthesis attempts to obtain EPS failed due to its poor stability. Epoxides were utilized in in vitro assays with amino acids, peptides and proteins to get insights whether epoxidations might act as posttranslational modification in Photorhabdus. The reactions were performed with styrene oxide and stilbene oxide replacing EPS based on their structural similarity. The modifications were executed successfully although proteomics approaches with in vivo data are required to confirm these findings. During the purification attempts of EPS, further derivatives were detected. The structures of dimerized stilbenes, a cis-isomer of IPS and another derivative that might incorporate an amino-group in the resveratrol ring were proposed on the basis of the HPLC-MS data.