Engineering the stambomycin modular polyketide synthase yields 37-membered mini-stambomycins

Plug-and-play engineering of modular polyketide synthases

Modular polyketide synthases (PKSs) are multidomain, assembly line enzymes that biosynthesize complex antibiotics such as erythromycin and rapamycin. The modular characteristic of PKSs makes them an ideal platform for the custom production of designer polyketides by combinatorial biosynthesis. However, engineered hybrid PKS pathways often exhibit severe loss of enzyme activity, and a general principle for PKS reprogramming has not been established. Here we present a widely applicable strategy for designing hybrid PKSs. We reveal that two conserved motifs are robust cut sites to connect modules from different PKS pathways and demonstrate the custom production of polyketides with different starter units, extender units and variable reducing states. Furthermore, we expand the applicability of these cut sites to construct hybrid pathways involving cis-AT PKS, trans-AT PKS and even nonribosomal peptide synthetase. Collectively, our findings enable plug-and-play reprogramming of modular PKSs and facilitate the application of assembly line enzymes toward the bioproduction of designer molecules.

Exploiting the inherent promiscuity of the acyl transferase of the stambomycin polyketide synthase for the mutasynthesis of analogues

The polyketide specialized metabolites of bacteria are attractive targets for generating analogues, with the goal of improving their pharmaceutical properties. Here, we aimed to produce C-26 derivatives of the giant anti-cancer stambomycin macrolides using a mutasynthesis approach, as this position has been shown previously to directly impact bioactivity. For this, we leveraged the intrinsically broad specificity of the acyl transferase domain (AT12) of the modular polyketide synthase (PKS), which is responsible for the alkyl branching functionality at this position. Feeding of a panel of synthetic and commercially available dicarboxylic acid 'mutasynthons' to an engineered strain of Streptomyces ambofaciens (Sa) deficient in synthesis of the native α-carboxyacyl-CoA extender units, resulted in six new series of stambomycin derivatives as judged by LC-HRMS and NMR. Notably, the highest product yields were observed for substrates which were only poorly accepted when AT12 was transplanted into a different PKS module, suggesting a critical role for domain context in the overall functioning of PKS proteins. We also demonstrate the superiority of this mutasynthesis approach - both in terms of absolute titers and yields relative to the parental compounds - in comparison to the alternative precursor-directed strategy in which monoacid building blocks are supplied to the wild type strain. We further identify a malonyl-CoA synthetase, MatB_Sa, with specificity distinct from previously described promiscuous enzymes, making it a useful addition to a mutasynthesis toolbox for generating atypical, CoA activated extender units. Finally, we show that two of the obtained (deoxy)-butyl-stambomycins exhibit antibacterial and antiproliferative activities similar to the parental stambomycins, while an unexpected butyl-demethyl congener is less potent. Overall, this works confirms the interest of biosynthetic pathways which combine a dedicated route to extender unit synthesis and a broad specificity AT domain for producing bioactive derivatives of fully-elaborated complex polyketides.

Comparative genomics reveals genetic diversity and differential metabolic potentials of the species of Arachnia and suggests reclassification of Arachnia propionica E10012 (=NBRC_14587) as novel species

The genus Arachnia, including Arachnia propionica and Arachnia rubra, are part of the normal oral and respiratory microbiota but can act as opportunistic pathogens in humans. This study investigates the functional, phylogenomic and taxonomic characteristics of 10 completely sequenced Arachnia strains, to elucidate their evolutionary relationships and divergence patterns, focusing on genomic variability and functional diversity. Phylogenetic analyses revealed distinct patterns, with Arachnia propionica strains showing significant divergence compared to the conserved Arachnia rubra strains. Notably, E10012 (=NBRC 14587) emerged as a distinct lineage with unique adaptations, while NCTC11666 exhibited a unique phylogenetic position, suggesting subspecies-level classification. Functional analyses highlighted variability among Arachnia propionica strains, with E10012 (=NBRC 14587) showing genes linked to choline metabolism and metal resistance, and NCTC11666 enriched in carbohydrate-active enzymes like GH179. In contrast, Arachnia rubra demonstrated genomic conservation, indicative of evolutionary specialization. This study reveals that strains E10012 (=NBRC 14587) and NCTC11666 displayed unique genomic features and distinct phylogenetic positioning, suggesting their reclassification as potential novel species and subspecies respectively. This underscores the balance between genomic conservation and diversification in Arachnia, reflecting their ecological adaptability and functional roles in the oral microbiome.

Refactoring the pikromycin synthase for the modular biosynthesis of macrolide antibiotics in E. coli

While engineering modular polyketide synthases (PKSs) using the recently updated module boundary has yielded libraries of triketide-pentaketides, this strategy has not yet been applied to the combinatorial biosynthesis of macrolactones or macrolide antibiotics. We developed a 2-plasmid system for the construction and expression of PKSs and employed it to obtain a refactored pikromycin synthase in E. coli that produces 85 mg of narbonolide per liter of culture. The replacement, insertion, deletion, and mutagenesis of modules enabled access to hexaketide, heptaketide, and octaketide derivatives. Supplying enzymes for desosamine biosynthesis and transfer enabled production of narbomycin, pikromycin, YC-17, methymycin, and 6 derivatives thereof. Knocking out pathways competing with desosamine biosynthesis and supplying the editing thioesterase PikAV boosted the titer of narbomycin 55-fold to 37 mgL-1. The replacement of the 3rd pikromycin module with its 5th yielded a new macrolide antibiotic and demonstrates how libraries of macrolide antibiotics can be readily accessed.

Synthetic Biology of Natural Products Engineering: Recent Advances Across the Discover-Design-Build-Test-Learn Cycle

Advances in genome engineering and associated technologies have reinvigorated natural products research. Here we highlight the latest developments in the field across the discover-design-build-test-learn cycle of bioengineering, from recent progress in computational tools for AI-supported genome mining, enzyme and pathway engineering, and compound identification to novel host systems and new techniques for improving production levels, and place these trends in the context of responsible research and innovation, emphasizing the importance of anticipatory analysis at the early stages of process development.

Modular polyketide synthase ketosynthases collaborate with upstream dehydratases to install double bonds

A VMYH motif was determined to help ketosynthases in polyketide assembly lines select α,β-unsaturated intermediates from an equilibrium mediated by an upstream dehydratase. Alterations of this motif decreased ketosynthase selectivity within a model tetraketide synthase, most significantly when replaced by the TNGQ motif of ketosynthases that accept D-β-hydroxy intermediates.

Assessing and harnessing updated polyketide synthase modules through combinatorial engineering

The modular nature of polyketide assembly lines and the significance of their products make them prime targets for combinatorial engineering. The recently updated module boundary has been successful for engineering short synthases, yet larger synthases constructed using the updated boundary have not been investigated. Here we describe our design and implementation of a BioBricks-like platform to rapidly construct 5 triketide, 25 tetraketide, and 125 pentaketide synthases to test every module combination of the pikromycin synthase. Anticipated products are detected from 60% of the triketide synthases, 32% of the tetraketide synthases, and 6.4% of the pentaketide synthases. We determine ketosynthase gatekeeping and module-skipping are the principal impediments to obtaining functional synthases. The platform is also employed to construct active hybrid synthases by incorporating modules from the erythromycin, spinosyn, and rapamycin assembly lines. The relaxed gatekeeping of a ketosynthase in the rapamycin synthase is especially encouraging in the quest to produce designer polyketides.

Insights into docking in megasynthases from the investigation of the toblerol trans-AT polyketide synthase: many α-helical means to an end

The fidelity of biosynthesis by modular polyketide synthases (PKSs) depends on specific moderate affinity interactions between successive polypeptide subunits mediated by docking domains (DDs). These sequence elements are notably portable, allowing their transplantation into alternative biosynthetic and metabolic contexts. Herein, we use integrative structural biology to characterize a pair of DDs from the toblerol trans-AT PKS. Both are intrinsically disordered regions (IDRs) that fold into a 3 α-helix docking complex of unprecedented topology. The C-terminal docking domain (CDD) resembles the 4 α-helix type (4HB) CDDs, which shows that the same type of DD can be redeployed to form complexes of distinct geometry. By carefully re-examining known DD structures, we further extend this observation to type 2 docking domains, establishing previously unsuspected structural relations between DD types. Taken together, these data illustrate the plasticity of α-helical DDs, which allow the formation of a diverse topological spectrum of docked complexes. The newly identified DDs should also find utility in modular PKS genetic engineering.

Discovery and Biosynthesis of Streptolateritic Acids A-D: Acyclic Pentacarboxylic Acids from Streptomyces sp. FXJ1.172 with Promising Activity against Potato Common Scab

Potato common scab (PCS) is a widespread plant disease that lacks effective control measures. Using a small molecule elicitor, we activate the production of a novel class of polyketide antibiotics, streptolateritic acids A-D, in Streptomyces sp. FXJ1.172. These compounds show a promising control efficacy against PCS and an unusual acyclic pentacarboxylic acid structure. A gene cluster encoding a type I modular polyketide synthase is identified to be responsible for the biosynthesis of these metabolites. A cytochrome P450 (CYP) and an aldehyde dehydrogenase (ADH) encoded by two genes in the cluster are proposed to catalyze iterative oxidation of the starter-unit-derived methyl group and three of six branching methyl groups to carboxylic acids during chain assembly. Our findings highlight how activation of silent biosynthetic gene clusters can be employed to discover completely new natural product classes able to combat PCS and new types of modular polyketide synthase-based biosynthetic machinery.

Biosynthesis of novel desferrioxamine derivatives requires unprecedented crosstalk between separate NRPS-independent siderophore pathways

Iron is essential to many biological processes but its poor solubility in aerobic environments restricts its bioavailability. To overcome this limitation, bacteria have evolved a variety of strategies, including the production and secretion of iron-chelating siderophores. Here, we describe the discovery of four series of siderophores from Streptomyces ambofaciens ATCC23877, three of which are unprecedented. MS/MS-based molecular networking revealed that one of these series corresponds to acylated desferrioxamines (acyl-DFOs) recently identified from S. coelicolor. The remaining sets include tetra- and penta-hydroxamate acyl-DFO derivatives, all of which incorporate a previously undescribed building block. Stable isotope labeling and gene deletion experiments provide evidence that biosynthesis of the acyl-DFO congeners requires unprecedented crosstalk between two separate non-ribosomal peptide synthetase (NRPS)-independent siderophore (NIS) pathways in the producing organism. Although the biological role(s) of these new derivatives remain to be elucidated, they may confer advantages in terms of metal chelation in the competitive soil environment due to the additional bidentate hydroxamic functional groups. The metabolites may also find application in various fields including biotechnology, bioremediation, and immuno-PET imaging.IMPORTANCEIron-chelating siderophores play important roles for their bacterial producers in the environment, but they have also found application in human medicine both in iron chelation therapy to prevent iron overload and in diagnostic imaging, as well as in biotechnology, including as agents for biocontrol of pathogens and bioremediation. In this study, we report the discovery of three novel series of related siderophores, whose biosynthesis depends on the interplay between two NRPS-independent (NIS) pathways in the producing organism S. ambofaciens-the first example to our knowledge of such functional cross-talk. We further reveal that two of these series correspond to acyl-desferrioxamines which incorporate four or five hydroxamate units. Although the biological importance of these novel derivatives is unknown, the increased chelating capacity of these metabolites may find utility in diagnostic imaging (for instance, 89Zr-based immuno-PET imaging) and other applications of metal chelators.

Module-Based Polyketide Synthase Engineering for de Novo Polyketide Biosynthesis

Polyketide retrobiosynthesis, where the biosynthetic pathway of a given polyketide can be reversibly engineered due to the colinearity of the polyketide synthase (PKS) structure and function, has the potential to produce millions of organic molecules. Mixing and matching modules from natural PKSs is one of the routes to produce many of these molecules. Evolutionary analysis of PKSs suggests that traditionally used module boundaries may not lead to the most productive hybrid PKSs and that new boundaries around and within the ketosynthase domain may be more active when constructing hybrid PKSs. As this is still a nascent area of research, the generality of these design principles based on existing engineering efforts remains inconclusive. Recent advances in structural modeling and synthetic biology present an opportunity to accelerate PKS engineering by re-evaluating insights gained from previous engineering efforts with cutting edge tools.

Assessing and harnessing updated polyketide synthase modules through combinatorial engineering

The modular nature of polyketide assembly lines and the significance of their products make them prime targets for combinatorial engineering. While short synthases constructed using the recently updated module boundary have been shown to outperform those using the traditional boundary, larger synthases constructed using the updated boundary have not been investigated. Here we describe our design and implementation of a BioBricks-like platform to rapidly construct 5 triketide, 25 tetraketide, and 125 pentaketide synthases from the updated modules of the Pikromycin synthase. Every combinatorial possibility of modules 2-6 inserted between the first and last modules of the native synthase was constructed and assayed. Anticipated products were observed from 60% of the triketide synthases, 32% of the tetraketide synthases, and 6.4% of the pentaketide synthases. Ketosynthase gatekeeping and module-skipping were determined to be the principal impediments to obtaining functional synthases. The platform was also used to create functional hybrid synthases through the incorporation of modules from the Erythromycin, Spinosyn, and Rapamycin assembly lines. The relaxed gatekeeping observed from a ketosynthase in the Rapamycin synthase is especially encouraging in the quest to produce designer polyketides.

Boosting titers of engineered triketide and tetraketide synthases to record levels through T7 promoter tuning

Modular polyketide synthases (PKS's) are promising platforms for the rational engineering of designer polyketides and commodity chemicals, yet their low productivities are a barrier to the practical biosynthesis of these compounds. Previously, we engineered triketide lactone synthases such as Pik167 using the recently updated module definition and showed they generate hundreds of milligrams of product per liter of Escherichia coli K207-3 shake flask culture. As the molar ratio between the 2 polypeptides of Pik167 is highly skewed, we sought to attenuate the strength of the T7 promoter controlling the production of the smaller, better-expressing polypeptide and thereby increase production of the first polypeptide under the control of an unoptimized T7 promoter. Through this strategy, a 1.8-fold boost in titer was obtained. After a further 1.5-fold boost obtained by increasing the propionate concentration in the media from 20 to 80 mM, a record titer of 791 mg L-1 (627 mg L-1 isolated) was achieved, a 2.6-fold increase overall. Spurred on by this result, the tetraketide synthase Pik1567 was engineered and the T7 promoter attenuation strategy was applied to its second and third genes. A 5-fold boost, from 20 mg L-1 to 100 mg L-1, in the titer of its tetraketide product was achieved.

Enzymology of assembly line synthesis by modular polyketide synthases

Modular polyketide synthases (PKSs) run catalytic reactions over dozens of steps in a highly orchestrated manner. To accomplish this synthetic feat, they form megadalton multienzyme complexes that are among the most intricate proteins on earth. Polyketide products are of elaborate chemistry with molecular weights of usually several hundred daltons and include clinically important drugs such as erythromycin (antibiotic), rapamycin (immunosuppressant) and epothilone (anticancer drug). The term 'modular' refers to a hierarchical structuring of modules and domains within an overall assembly line arrangement, in which PKS organization is colinearly translated into the polyketide structure. New structural information obtained during the past few years provides substantial direct insight into the orchestration of catalytic events within a PKS module and leads to plausible models for synthetic progress along assembly lines. In light of these structural insights, the PKS engineering field is poised to enter a new era of engineering.

Insights into azalomycin F assembly-line contribute to evolution-guided polyketide synthase engineering and identification of intermodular recognition

Modular polyketide synthase (PKS) is an ingenious core machine that catalyzes abundant polyketides in nature. Exploring interactions among modules in PKS is very important for understanding the overall biosynthetic process and for engineering PKS assembly-lines. Here, we show that intermodular recognition between the enoylreductase domain ER_1/2 inside module 1/2 and the ketosynthase domain KS₃ inside module 3 is required for the cross-module enoylreduction in azalomycin F (AZL) biosynthesis. We also show that KS₄ of module 4 acts as a gatekeeper facilitating cross-module enoylreduction. Additionally, evidence is provided that module 3 and module 6 in the AZL PKS are evolutionarily homologous, which makes evolution-oriented PKS engineering possible. These results reveal intermodular recognition, furthering understanding of the mechanism of the PKS assembly-line, thus providing different insights into PKS engineering. This also reveals that gene duplication/conversion and subsequent combinations may be a neofunctionalization process in modular PKS assembly-lines, hence providing a different case for supporting the investigation of modular PKS evolution.

Metabolic pathway assembly using docking domains from type I cis-AT polyketide synthases

Engineered metabolic pathways in microbial cell factories often have no natural organization and have challenging flux imbalances, leading to low biocatalytic efficiency. Modular polyketide synthases (PKSs) are multienzyme complexes that synthesize polyketide products via an assembly line thiotemplate mechanism. Here, we develop a strategy named mimic PKS enzyme assembly line (mPKSeal) that assembles key cascade enzymes to enhance biocatalytic efficiency and increase target production by recruiting cascade enzymes tagged with docking domains from type I cis-AT PKS. We apply this strategy to the astaxanthin biosynthetic pathway in engineered Escherichia coli for multienzyme assembly to increase astaxanthin production by 2.4-fold. The docking pairs, from the same PKSs or those from different cis-AT PKSs evidently belonging to distinct classes, are effective enzyme assembly tools for increasing astaxanthin production. This study addresses the challenge of cascade catalytic efficiency and highlights the potential for engineering enzyme assembly.

Assembly artificial pathway in design connecting media can increase biosynthetic efficiency, but the choice of connecting media is limited. Here, the authors develop a new protein assembly strategy using a pool of docking peptides from polyketide synthase and show its application in astaxanthin biosynthesis in E. coli.

Engineering Modular Polyketide Biosynthesis in Streptomyces Using CRISPR/Cas: A Practical Guide

The CRISPR/Cas system, which has been widely applied to organisms ranging from microbes to animals, is currently being adapted for use in Streptomyces bacteria. In this case, it is notably applied to rationally modify the biosynthetic pathways giving rise to the polyketide natural products, which are heavily exploited in the medical and agricultural arenas. Our aim here is to provide the potential user with a practical guide to exploit this approach for manipulating polyketide biosynthesis, by treating key experimental aspects including vector choice, design of the basic engineering components, and trouble-shooting.