Heterologous Expression and Optimization of Fermentation Conditions for Recombinant Ikarugamycin Production

Harper C.P., Day A., Tsingos M., Ding E., Zeng E., Stumpf S.D., Qi Y., Robinson A., Greif J., Blodgett J.A.

ABSTRACT Polycyclic tetramate macrolactams (PTMs) are bioactive natural products commonly associated with certain actinobacterial and proteobacterial lineages. These molecules have been the subject of numerous structure-activity investigations since the 1970s. New members continue to be pursued in wild and engineered bacterial strains, and advances in PTM biosynthesis suggest their outwardly simplistic biosynthetic gene clusters (BGCs) belie unexpected product complexity. To address the origins of this complexity and understand its influence on PTM discovery, we engaged in a combination of bioinformatics to systematically classify PTM BGCs and PTM-targeted metabolomics to compare the products of select BGC types. By comparing groups of producers and BGC mutants, we exposed knowledge gaps that complicate bioinformatics-driven product predictions. In sum, we provide new insights into the evolution of PTM BGCs while systematically accounting for the PTMs discovered thus far. The combined computational and metabologenomic findings presented here should prove useful for guiding future discovery. IMPORTANCE Polycyclic tetramate macrolactam (PTM) pathways are frequently found within the genomes of biotechnologically important bacteria, including Streptomyces and Lysobacter spp . Their molecular products are typically bioactive, having substantial agricultural and therapeutic interest. Leveraging bacterial genomics for the discovery of new related molecules is thus desirable, but drawing accurate structural predictions from bioinformatics alone remains challenging. This difficulty stems from a combination of previously underappreciated biosynthetic complexity and remaining knowledge gaps, compounded by a stream of yet-uncharacterized PTM biosynthetic loci gleaned from recently sequenced bacterial genomes. We engaged in the following study to create a useful framework for cataloging historic PTM clusters, identifying new cluster variations, and tracing evolutionary paths for these molecules. Our data suggest new PTM chemistry remains discoverable in nature. However, our metabolomic and mutational analyses emphasize the practical limitations of genomics-based discovery by exposing hidden complexity.

D’Agostino P.M., Seel C.J., Ji X., Gulder T., Gulder T.A.

The γ-butyrolactone motif is found in many natural signaling molecules and other specialized metabolites. A prominent example is the potent aquatic phytotoxin cyanobacterin, which has a highly functionalized γ-butyrolactone core structure. The enzymatic machinery that assembles cyanobacterin and structurally related natural products (herein termed furanolides) has remained elusive for decades. Here, we elucidate the biosynthetic process of furanolide assembly. The cyanobacterin biosynthetic gene cluster was identified by targeted bioinformatic screening and validated by heterologous expression in Escherichia coli. Full functional evaluation of the recombinant key enzymes in vivo and in vitro, individually and in concert, provided in-depth mechanistic insights into a streamlined C–C bond-forming cascade that involves installation of compatible reactivity at seemingly unreactive Cα positions of amino acid precursors. Our work extends the biosynthetic and biocatalytic toolbox for γ-butyrolactone formation, provides a general paradigm for furanolide biosynthesis and sets the stage for their targeted discovery, biosynthetic engineering and enzymatic synthesis. Elucidation of the biosynthetic pathway of the γ-butyrolactone core structure of the furanolide natural product cyanobacterin reveals a carbon–carbon bond-forming cascade that features an enzyme catalyzing a Morita–Baylis–Hillman reaction.

Mo X., Gulder T.A.

Covering: up to the end of 2020Natural products bearing tetramic acid units as part of complex molecular architectures exhibit a broad range of potent biological activities. These compounds thus attract significant interest from both the biosynthetic and synthetic communities. Biosynthetically, most of the tetramic acids are derived from hybrid polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) machineries. To date, over 30 biosynthetic gene clusters (BGCs) involved in tetramate formation have been identified, from which different biosynthetic strategies evolved in Nature to assemble this intriguing structural unit were characterized. In this Highlight we focus on the biosynthetic concepts of tetramic acid formation and discuss the molecular mechanism towards selected representatives in detail, providing a systematic overview for the development of strategies for targeted tetramate genome mining and future applications of tetramate-forming biocatalysts for chemo-enzymatic synthesis.

Duell E.R., D’Agostino P.M., Shapiro N., Woyke T., Fuchs T.M., Gulder T.A.

Serratia plymuthica WS3236 was selected for whole genome sequencing based on preliminary genetic and chemical screening indicating the presence of multiple natural product pathways. This led to the identification of a putative sodorifen biosynthetic gene cluster (BGC). The natural product sodorifen is a volatile organic compound (VOC) with an unusual polymethylated hydrocarbon bicyclic structure (C16H26) produced by selected strains of S. plymuthica. The BGC encoding sodorifen consists of four genes, two of which (sodA, sodB) are homologs of genes encoding enzymes of the non-mevalonate pathway and are thought to enhance the amounts of available farnesyl pyrophosphate (FPP), the precursor of sodorifen. Proceeding from FPP, only two enzymes are necessary to produce sodorifen: an S-adenosyl methionine dependent methyltransferase (SodC) with additional cyclisation activity and a terpene-cyclase (SodD). Previous analysis of S. plymuthica found sodorifen production titers are generally low and vary significantly among different producer strains. This precludes studies on the still elusive biological function of this structurally and biosynthetically fascinating bacterial terpene. Sequencing and mining of the S. plymuthica WS3236 genome revealed the presence of 38 BGCs according to antiSMASH analysis, including a putative sodorifen BGC. Further genome mining for sodorifen and sodorifen-like BGCs throughout bacteria was performed using SodC and SodD as queries and identified a total of 28 sod-like gene clusters. Using direct pathway cloning (DiPaC) we intercepted the 4.6 kb candidate sodorifen BGC from S. plymuthica WS3236 (sodA–D) and transformed it into Escherichia coli BL21. Heterologous expression under the control of the tetracycline inducible PtetO promoter firmly linked this BGC to sodorifen production. By utilizing this newly established expression system, we increased the production yields by approximately 26-fold when compared to the native producer. In addition, sodorifen was easily isolated in high purity by simple head-space sampling. Genome mining of all available genomes within the NCBI and JGI IMG databases led to the identification of a wealth of sod-like pathways which may be responsible for producing a range of structurally unknown sodorifen analogs. Introduction of the S. plymuthica WS3236 sodorifen BGC into the fast-growing heterologous expression host E. coli with a very low VOC background led to a significant increase in both sodorifen product yield and purity compared to the native producer. By providing a reliable, high-level production system, this study sets the stage for future investigations of the biological role and function of sodorifen and for functionally unlocking the bioinformatically identified putative sod-like pathways.

Myronovskyi M., Rosenkränzer B., Nadmid S., Pujic P., Normand P., Luzhetskyy A.

Natural products are a rich source of potential drugs for many applications. Discovery of natural products through the activation of cryptic gene clusters encoding their biosynthetic pathways, engineering of those biosynthetic pathways and optimization of production yields often rely on the expression of these gene clusters in suitable heterologous host strains. Streptomyces albus J1074 provides high success rates of heterologous cluster expression with high levels of metabolite production, rapid growth and amenability to genetic manipulations. Here, we report the construction of S. albus chassis strains optimized for the discovery of natural products through heterologous expression of secondary metabolite clusters. 15 clusters encoding secondary metabolite biosynthetic pathways were deleted in the chromosome of S. albus Del14. This strain provides a substantially improved compound detection limit, owing to the lack of native secondary metabolites. Furthermore, the production yield of natural products heterologously expressed in S. albus Del14 was higher than in commonly used S. albus J1074 and S. coelicolor hosts. S. albus strains B2P1 and B4 were generated by introduction of additional phage phiC31 attB sites into the chromosome of S. albus Del14, allowing integration of up to four copies of a heterologous gene cluster. Amplification of gene clusters in the chromosome of the constructed strains further improved production yields of the encoded compounds. One cryptic cluster from Streptomyces spp. and two clusters from distantly related Frankia spp. strains were successfully activated in these new chassis strains, leading to the isolation of a new compound fralnimycin.

D’Agostino P.M., Gulder T.A.

The need for new pharmacological lead structures, especially against drug resistances, has led to a surge in natural product research and discovery. New biosynthetic gene cluster capturing methods to efficiently clone and heterologously express natural product pathways have thus been developed. Direct pathway cloning (DiPaC) is an emerging synthetic biology strategy that utilizes long-amplification PCR and HiFi DNA assembly for the capture and expression of natural product biosynthetic gene clusters. Here, we have further streamlined DiPaC by reducing cloning time and reagent costs by utilizing T4 DNA polymerase (sequence- and ligation-independent cloning, SLIC) for gene cluster capture. As a proof of principle, the majority of the cyanobacterial hapalosin gene cluster was cloned as a single piece (23 kb PCR product) using this approach, and predicted transcriptional terminators were removed by simultaneous pathway refactoring, leading to successful heterologous expression. The complementation of DiPaC with SLIC depicts a time and cost-efficient method for simple capture and expression of new natural product pathways.

Liu Y., Wang H., Song R., Chen J., Li T., Li Y., Du L., Shen Y.

Polycyclic tetramate macrolactams (PoTeMs) are a growing class of natural products with distinct structure and diverse biological activities. By promoter engineering and heterologous expression of the cryptic cbm gene cluster, four new PoTeMs, combamides A-E (1-4), were identified. Additionally, two new derivatives, combamides E (5) and F (6), were generated via combinatorial biosynthesis. Together, our findings provide a sound base for expanding the structure diversities of PoTeMs through genome mining and combinatorial biosynthesis.

Greunke C., Duell E.R., D’Agostino P.M., Glöckle A., Lamm K., Gulder T.A.

Specialized metabolites from bacteria are an important source of inspiration for drug development. The genes required for the biosynthesis of such metabolites in bacteria are usually organized in so-called biosynthetic gene clusters (BGCs). Using modern bioinformatic tools, the wealth of genomic data can be scanned for such BGCs and the expected products can often structurally be predicted in silico. This facilitates the directed discovery of putatively novel bacterial metabolites. However, the production of these molecules often requires genetic manipulation of the BGC for activation or the expression of the pathway in a heterologous host. The latter necessitates the transplantation of the BGC into a suitable expression system. To achieve this goal, powerful cloning strategies based on in vivo homologous recombination have recently been developed. This includes LCHR and LLHR in E. coli as well as TAR cloning in yeast. Here, we present Direct Pathway Cloning (DiPaC) as an efficient complementary BGC capturing strategy that relies on long-amplicon PCR and in vitro DNA assembly. This straightforward approach facilitates full pathway assembly, BGC refactoring and direct transfer into any vector backbone in vitro. The broad applicability and efficiency of DiPaC is demonstrated by the discovery of a new phenazine from Serratia fonticola, the first heterologous production of anabaenopeptins from Nostoc punctiforme and the transfer of the native erythromycin BGC from Saccharopolyspora erythraea into Streptomyces. Due to its simplicity, we envisage DiPaC to become an essential method for BGC cloning and metabolic pathways construction with significant applications in metabolic engineering, synthetic biology and biotechnology.

Greunke C., Glöckle A., Antosch J., Gulder T.A.

Nature provides an inexhaustible diversity of small organic molecules with beautiful molecular architectures that have strong and selective inhibitory activities. However, this tremendous biomedical potential often remains inaccessible, as the structural complexity of natural products can render their synthetic preparation extremely challenging. This problem is addressable by harnessing the biocatalytic procedures evolved by nature. In this work, we present an enzymatic total synthesis of ikarugamycin. The use of an iterative PKS/NRPS machinery and two reductases has allowed the construction of 15 carbon-carbon and 2 carbon-nitrogen bonds in a biocatalytic one-pot reaction. By scaling-up this method we demonstrate the applicability of biocatalytic approaches for the ex vivo synthesis of complex natural products.

Malcomson B., Wilson H., Veglia E., Thillaiyampalam G., Barsden R., Donegan S., El Banna A., Elborn J.S., Ennis M., Kelly C., Zhang S., Schock B.C.

Significance This study reports that publicly available gene array expression data together with statistically significant connections’ map successfully predicts licensed drugs able to modify genes of interest. We used this method to predict drugs able to induce A20 [TNFα-induced protein 3 (TNFAIP3)], which is reduced in cystic fibrosis (CF) airway cells, and thus normalize the inflammatory response. Using CF and non-CF airway epithelial cells, ikarugamycin and quercetin had antiinflammatory effects mediated by induction of A20. We confirmed that this was mainly due to A20 induction, because no antiinflammatory effects were seen in bronchial epithelial cells with A20 knockdown. We have identified a process whereby already licensed drugs can be successfully repositioned for chronic inflammatory airway diseases.

Greunke C., Antosch J., Gulder T.A.

Application of PTM hydroxylases facilitates the selective late-stage functionalization of complex PTM core structuresin vivoandin vitro.

Lacret R., Oves-Costales D., Gómez C., Díaz C., de la Cruz M., Pérez-Victoria I., Vicente F., Genilloud O., Reyes F.

Zhang G., Zhang W., Zhang Q., Shi T., Ma L., Zhu Y., Li S., Zhang H., Zhao Y., Shi R., Zhang C.

Ikarugamycin is a member of the polycyclic tetramate macrolactams (PTMs) family of natural products with diverse biological activities. The biochemical mechanisms for the formation of polycyclic ring systems in PTMs remain elusive. The enzymatic mechanism of constructing an inner five-membered ring in ikarugamycin is reported. A three-gene-cassette ikaABC from the marine-derived Streptomyces sp. ZJ306 is sufficient for conferring ikarugamycin production in a heterologous host. IkaC catalyzes a reductive cyclization reaction to form the inner five-membered ring by a Michael addition-like reaction. This study provides the first biochemical evidence for polycycle formation in PTMs and suggests a reductive cyclization strategy which may be potentially applicable in general to the corresponding ring formation in other PTMs.

Olano C., García I., González A., Rodriguez M., Rozas D., Rubio J., Sánchez‐Hidalgo M., Braña A.F., Méndez C., Salas J.A.

Streptomyces albus J1074 is a streptomycete strain widely used as a host for expression of secondary metabolite gene clusters. Bioinformatic analysis of the genome of this organism predicts the presence of 27 gene clusters for secondary metabolites. We have used three different strategies for the activation of some of these silent/cryptic gene clusters in S. albus J1074: two hybrid polyketide-non-ribosomal peptides (PK-NRP) (antimycin and 6-epi-alteramides), a type I PK (candicidin), a non-ribosomal peptides (NRP) (indigoidine) and glycosylated compounds (paulomycins). By insertion of a strong and constitutive promoter in front of selected genes of two clusters, production of the blue pigment indigoidine and of two novel members of the polycyclic tetramate macrolactam family (6-epi-alteramides A and B) was activated. Overexpression of positive regulatory genes from the same organism also activated the biosynthesis of 6-epi-alteramides and heterologous expression of the regulatory gene pimM of the pimaricin cluster activated the simultaneous production of candicidins and antimycins, suggesting some kind of cross-regulation between both clusters. A cluster for glycosylated compounds (paulomycins) was also identified by comparison of the high-performance liquid chromatography profiles of the wild-type strain with that of a mutant in which two key enzymes of the cluster were simultaneously deleted.

Antosch J., Schaefers F., Gulder T.A.

Polycyclic tetramate macrolactams (PTMs) are a family of biomedically promising natural products with challenging molecular frameworks. Despite these interesting properties, so far only relatively little is known about the biosynthetic origin of PTMs, in particular concerning the mechanism by which their ring systems are formed. Herein we present the first insights into these processes by using the biosynthesis of ikarugamycin as an example. This has been facilitated by the first heterologous expression of a PTM biosynthetic gene cluster in Escherichia coli. With this approach it will not only become possible to mechanistically investigate already known PTM biosynthetic pathways in more detail in the future, but also to interrogate cryptic PTM biosynthetic pathways chemically and biochemically.