Combinatorial polyketide biosynthesis by de novo design and rearrangement of modular polyketide synthase genes

Substrate Trapping in Polyketide Synthase Thioesterase Domains: Structural Basis for Macrolactone Formation

Emerging antibiotic resistance requires continual improvement in the arsenal of antimicrobial drugs, especially the critical macrolide antibiotics. Formation of the macrolactone scaffold of these polyketide natural products is catalyzed by a modular polyketide synthase (PKS) thioesterase (TE). The TE accepts a linear polyketide substrate from the termina PKS acyl carrier protein to generate an acyl-enzyme adduct that is resolved by attack of a substrate hydroxyl group to form the macrolactone. Our limited mechanistic understanding of TE selectivity for a substrate nucleophile and/or water has hampered development of TEs as biocatalysts that accommodate a variety of natural and non-natural substrates. To understand how TEs direct the substrate nucleophile for macrolactone formation, acyl-enzyme intermediates were trapped as stable amides by substituting the natural serine OH with an amino group. Incorporation of the unnatural amino acid, 1,3-diaminopropionic acid (DAP), was tested with five PKS TEs. DAP-modified TEs (TE DAP ) from the pikromycin and erythromycin pathways were purified and tested with six full-length polyketide intermediates from three pathways. The erythromycin TE had permissive substrate selectivity, whereas the pikromycin TE was selective for its native hexaketide and heptaketide substrates. In a crystal structure of a native substrate trapped in pikromycin TE DAP , the linear heptaketide was curled in the active site with the nucleophilic hydroxyl group positioned 4 Å from the amide-enzyme linkage. The curled heptaketide displayed remarkable shape complementarity with the TE acyl cavity. The strikingly different shapes of acyl cavities in TEs of known structure, including those reported here for juvenimicin, tylosin and fluvirucin biosynthesis, provide new insights to facilitate TE engineering and optimization.

What is the role of microbial biotechnology and genetic engineering in medicine?

Microbial products are essential for developing various therapeutic agents, including antibiotics, anticancer drugs, vaccines, and therapeutic enzymes. Genetic engineering techniques, functional genomics, and synthetic biology unlock previously uncharacterized natural products. This review highlights major advances in microbial biotechnology, focusing on gene-based technologies for medical applications.

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.

Current State-of-the-Art Toward Chemoenzymatic Synthesis of Polyketide Natural Products

Polyketide natural products have significant promise as pharmaceutical targets for human health and as molecular tools to probe disease and complex biological systems. While the biosynthetic logic of polyketide synthases (PKS) is well-understood, biosynthesis of designer polyketides remains challenging due to several bottlenecks, including substrate specificity constraints, disrupted protein-protein interactions, and protein solubility and folding issues. Focusing on substrate specificity, PKSs are typically interrogated using synthetic thioesters. PKS assembly lines and their products offer a wealth of information when studied in a chemoenzymatic fashion. This review provides an overview of the past two decades of polyketide chemoenzymatic synthesis and their contributions to the field of chemical biology. These synthetic strategies have successfully yielded natural product derivatives while providing critical insights into enzymatic promiscuity and mechanistic activity.

Biosensor Guided Polyketide Synthases Engineering for Optimization of Domain Exchange Boundaries

Type I modular polyketide synthases (PKSs) are multi-domain enzymes functioning like assembly lines. Many engineering attempts have been made for the last three decades to replace, delete and insert new functional domains into PKSs to produce novel molecules. However, inserting heterologous domains often destabilize PKSs, causing loss of activity and protein misfolding. To address this challenge, here we develop a fluorescence-based solubility biosensor that can quickly identify engineered PKSs variants with minimal structural disruptions. Using this biosensor, we screen a library of acyltransferase (AT)-exchanged PKS hybrids with randomly assigned domain boundaries, and we identify variants that maintain wild type production levels. We then probe each position in the AT linker region to determine how domain boundaries influence structural integrity and identify a set of optimized domain boundaries. Overall, we have successfully developed an experimentally validated, high-throughput method for making hybrid PKSs that produce novel molecules.

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.

Combinatorial biosynthesis: playing chess with the metabolism

Secondary metabolites are a group of natural products that produced by bacteria, fungi and plants. Many applications of these compounds from medicine to industry have been discovered. However, some changes in their structure and biosynthesis mechanism are necessary for their properties to be more suitable and also for their production to be profitable. The main and most useful method to achieve this goal is combinatorial biosynthesis. This technique uses the multi-unit essence of the secondary metabolites biosynthetic enzymes to make changes in their order, structure and also the organism that produces them.

Sequential Methods for Di- and Tetrahydro-Pyranone Synthesis Enable Concise Access to Tuscolid δ-Lactone

Two novel tandem sequences for stereoselective synthesis of elaborate hydropyranones are reported. The first one relies on an aldol-lactonization procedure of a chiral enolate with an unprotected aldehyde, while the second one is based on a challenging dienolate ketone addition with concomitant cyclization and substrate controlled reduction. Both approaches proceed with high efficiency and stereoselectivity and enable very short accesses to the authentic pyranone subunit of the complex polyketide tuscolid and will be important to develop a first total synthesis of this structurally unique macrolide and to evaluate the tuscolid-tuscoron rearrangement.

Diversity and divergence: evolution of secondary metabolism in the tropical tree genus Inga

Plants are widely recognized as chemical factories, with each species producing dozens to hundreds of unique secondary metabolites. These compounds shape the interactions between plants and their natural enemies. We explore the evolutionary patterns and processes by which plants generate chemical diversity, from evolving novel compounds to unique chemical profiles. We characterized the chemical profile of one-third of the species of tropical rainforest trees in the genus Inga (c. 100, Fabaceae) using ultraperformance liquid chromatography-mass spectrometry-based metabolomics and applied phylogenetic comparative methods to understand the mode of chemical evolution. We show: each Inga species contain structurally unrelated compounds and high levels of phytochemical diversity; closely related species have divergent chemical profiles, with individual compounds, compound classes, and chemical profiles showing little-to-no phylogenetic signal; at the evolutionary time scale, a species' chemical profile shows a signature of divergent adaptation. At the ecological time scale, sympatric species were the most divergent, implying it is also advantageous to maintain a unique chemical profile from community members; finally, we integrate these patterns with a model for how chemical diversity evolves. Taken together, these results show that phytochemical diversity and divergence are fundamental to the ecology and evolution of plants.

Fragment antigen binding domains (Fabs) as tools to study assembly-line polyketide synthases

The crystallization of proteins remains a bottleneck in our fundamental understanding of their functions. Therefore, discovering tools that aid crystallization is crucial. In this review, the versatility of fragment-antigen binding domains (Fabs) as protein crystallization chaperones is discussed. Fabs have aided the crystallization of membrane-bound and soluble proteins as well as RNA. The ability to bind three Fabs onto a single protein target has demonstrated their potential for crystallization of challenging proteins. We describe a high-throughput workflow for identifying Fabs to aid the crystallization of a protein of interest (POI) by leveraging phage display technologies and differential scanning fluorimetry (DSF). This workflow has proven to be especially effective in our structural studies of assembly-line polyketide synthases (PKSs), which harbor flexible domains and assume transient conformations. PKSs are of interest to us due to their ability to synthesize an unusually broad range of medicinally relevant compounds. Despite years of research studying these megasynthases, their overall topology has remained elusive. One Fab in particular, 1B2, has successfully enabled X-ray crystallographic and single particle cryo-electron microscopic (cryoEM) analyses of multiple modules from distinct assembly-line PKSs. Its use has not only facilitated multidomain protein crystallization but has also enhanced particle quality via cryoEM, thereby enabling the visualization of intact PKS modules at near-atomic (3-5 Å) resolution. The identification of PKS-binding Fabs can be expected to continue playing a key role in furthering our knowledge of polyketide biosynthesis on assembly-line PKSs.

Engineered Chimeras Unveil Swappable Modular Features of Fatty Acid and Polyketide Synthase Acyl Carrier Proteins

The strategic redesign of microbial biosynthetic pathways is a compelling route to access molecules of diverse structure and function in a potentially environmentally sustainable fashion. The promise of this approach hinges on an improved understanding of acyl carrier proteins (ACPs), which serve as central hubs in biosynthetic pathways. These small, flexible proteins mediate the transport of molecular building blocks and intermediates to enzymatic partners that extend and tailor the growing natural products. Past combinatorial biosynthesis efforts have failed due to incompatible ACP-enzyme pairings. Herein, we report the design of chimeric ACPs with features of the actinorhodin polyketide synthase ACP (ACT) and of the Escherichia coli fatty acid synthase (FAS) ACP (AcpP). We evaluate the ability of the chimeric ACPs to interact with the E. coli FAS ketosynthase FabF, which represents an interaction essential to building the carbon backbone of the synthase molecular output. Given that AcpP interacts with FabF but ACT does not, we sought to exchange modular features of ACT with AcpP to confer functionality with FabF. The interactions of chimeric ACPs with FabF were interrogated using sedimentation velocity experiments, surface plasmon resonance analyses, mechanism-based cross-linking assays, and molecular dynamics simulations. Results suggest that the residues guiding AcpP-FabF compatibility and ACT-FabF incompatibility may reside in the loop I, α-helix II region. These findings can inform the development of strategic secondary element swaps that expand the enzyme compatibility of ACPs across systems and therefore represent a critical step toward the strategic engineering of "un-natural" natural products.

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

The modular organization of the type I polyketide synthases (PKSs) would seem propitious for rational engineering of desirable analogous. However, despite decades of efforts, such experiments remain largely inefficient. Here, we combine multiple, state-of-the-art approaches to reprogram the stambomycin PKS by deleting seven internal modules. One system produces the target 37-membered mini-stambomycin metabolites − a reduction in chain length of 14 carbons relative to the 51-membered parental compounds − but also substantial quantities of shunt metabolites. Our data also support an unprecedented off-loading mechanism of such stalled intermediates involving the C-terminal thioesterase domain of the PKS. The mini-stambomycin yields are reduced relative to wild type, likely reflecting the poor tolerance of the modules downstream of the modified interfaces to the non-native substrates. Overall, we identify factors contributing to the productivity of engineered whole assembly lines, but our findings also highlight the need for further research to increase production titers.

Genetic engineering of the type I polyketide synthases (PKSs) to produce desirable analogous remains largely inefficient. Here, the authors leverage multiple approaches to delete seven internal modules from the stambomycin PKS and generate 37-membered mini-stambomycin macrolactones.

Understanding and Manipulating Assembly Line Biosynthesis by Heterologous Expression in Streptomyces

Assembly line enzymes, including polyketide synthases and nonribosomal peptide synthetases, play central roles in the construction of complex natural products. Due to the sequential biochemistry processed in each domain, the domain architecture of the assembly line enzymes strictly correlates with the product molecule. This colinearity makes assembly line enzymes an ideal target for rational reprogramming. Although many of the past engineering attempts suffered from decreased product yield, recent advancements in the bioinformatic analysis and engineering design now provide new opportunity to work on these modular megaenzymes. This chapter describes the methods for analyzing and engineering the assembly line enzymes, including module and domain analysis needed for designing the engineering of assembly line biosynthesis, and the expression vector construction with an example of two-vector heterologous expression system in Streptomyces.

How cis-Acyltransferase Assembly-Line Ketosynthases Gatekeep for Processed Polyketide Intermediates

With the redefinition of polyketide synthase (PKS) modules, a new appreciation of their most downstream domain, the ketosynthase (KS), is emerging. In addition to performing its well-established role of generating a carbon-carbon bond between an acyl-CoA building block and a growing polyketide, it may gatekeep against incompletely processed intermediates. Here, we investigate 739 KSs from 92 primarily actinomycete, cis-acyltransferase assembly lines. When KSs were separated into 16 families based on the chemistries at the α- and β-carbons of their polyketide substrates, a comparison of 32 substrate tunnel residues revealed unique sequence fingerprints. Surprisingly, additional fingerprints were detected when the chemistry at the γ-carbon was considered. Representative KSs were modeled bound to their natural polyketide substrates to better understand observed patterns, such as the substitution of a tryptophan by a smaller residue to accommodate an l-α-methyl group or the substitution of four smaller residues by larger ones to make better contact with a primer unit or diketide. Mutagenesis of a conserved glutamine in a KS within a model triketide synthase indicates that the substrate tunnel is sensitive to alteration and that engineering this KS to accept unnatural substrates may require several mutations.

Metagenomic sequencing-driven multidisciplinary approaches to shed light on the untapped microbial natural products

The advantage of metagenomics over the culture-based natural product (NP) discovery pipeline is the ability to access the biosynthetic potential of uncultivable microbes. Advances in DNA sequencing are revolutionizing conventional metagenomics approaches for microbial NP discovery. The genomes of (in)cultivable bugs can be resolved straightforwardly from environmental samples, enabling in situ prediction of biosynthetic gene clusters (BGCs). The predicted chemical diversities could be realized not only by heterologous expression of gene clusters originating from DNA synthesis or direct cloning, but also potentially by bioinformatic-directed organic synthesis or chemoenzymatic total synthesis. In this review, we suggest that metagenomic sequencing in tandem with multidisciplinary approaches will form a versatile platform to shed light on a plethora of microbial 'dark matter'.

Preparative production of an enantiomeric pair by engineered polyketide synthases

Using the updated module boundary of polyketide assembly lines, modules from the pikromycin synthase were recombined into engineered synthases that furnish an enantiomeric pair of 2-stereocenter triketide lactones at >99% ee with yields up to 0.39 g per liter of E. coli K207-3 in shake flasks.

Animal Fatty Acid Synthase: A Chemical Nanofactory

Fatty acids are crucial molecules for most living beings, very well spread and conserved across species. These molecules play a role in energy storage, cell membrane architecture, and cell signaling, the latter through their derivative metabolites. De novo synthesis of fatty acids is a complex chemical process that can be achieved either by a metabolic pathway built by a sequence of individual enzymes, such as in most bacteria, or by a single, large multi-enzyme, which incorporates all the chemical capabilities of the metabolic pathway, such as in animals and fungi, and in some bacteria. Here we focus on the multi-enzymes, specifically in the animal fatty acid synthase (FAS). We start by providing a historical overview of this vast field of research. We follow by describing the extraordinary architecture of animal FAS, a homodimeric multi-enzyme with seven different active sites per dimer, including a carrier protein that carries the intermediates from one active site to the next. We then delve into this multi-enzyme's detailed chemistry and critically discuss the current knowledge on the chemical mechanism of each of the steps necessary to synthesize a single fatty acid molecule with atomic detail. In line with this, we discuss the potential and achieved FAS applications in biotechnology, as biosynthetic machines, and compare them with their homologous polyketide synthases, which are also finding wide applications in the same field. Finally, we discuss some open questions on the architecture of FAS, such as their peculiar substrate-shuttling arm, and describe possible reasons for the emergence of large megasynthases during evolution, questions that have fascinated biochemists from long ago but are still far from answered and understood.

Single-Cell Immunoblotting based on a Photoclick Hydrogel Enables High-Throughput Screening and Accurate Profiling of Exogenous Gene Expression

Fast and accurate profiling of exogenous gene expression in host cells is crucial for studying gene function in cellular and molecular biology, but still faces the challenge of incomplete co-expression of reporter genes and target genes. Here, a single-cell transfection analysis chip (scTAC) is presented, which is based on the in situ microchip immunoblotting method, for rapid and accurate analysis of exogenous gene expression in thousands of individual host cells. scTAC not only can assign information of exogenous gene activity to specific transfected cells, but enables the acquisition of continuous protein expression even in low co-expression scenarios. It is demonstrated that scTAC can reveal the relationship of expression level between reporter genes and target genes, which is helpful for evaluating transient transfection strategy efficiency. The advantages of this method for the study of fusion protein expression and downstream protein expression in signaling pathway in rare cells are shown. Empirically, an EGFP-TSPAN8 fusion plasmid is transfected into MCF-7 breast cancer cells and the expressions of two cancer stemness biomarkers (ALDHA1 and SOX2) are analyzed. The scTAC method clearly reveals an interesting phenomenon that transfected adherent MCF-7 cells exhibit some stem cell characteristics, but they do not have stem cell appearance.

Engineering the acyltransferase domain of epothilone polyketide synthase to alter the substrate specificity

Background

Polyketide synthases (PKSs) include ketone synthase (KS), acyltransferase (AT) and acyl carrier protein (ACP) domains to catalyse the elongation of polyketide chains. Some PKSs also contain ketoreductase (KR), dehydratase (DH) and enoylreductase (ER) domains as modification domains. Insertion, deletion or substitution of the catalytic domains may lead to the production of novel polyketide derivatives or to the accumulation of desired products. Epothilones are 16-membered macrolides that have been used as anticancer drugs. The substrate promiscuity of the module 4 AT domain of the epothilone PKS (EPOAT4) results in production of epothilone mixtures; substitution of this domain may change the ratios of epothilones. In addition, there are two dormant domains in module 9 of the epothilone PKS. Removing these redundant domains to generate a simpler and more efficient assembly line is a desirable goal.

Results

The substitution of module 4 drastically diminished the activity of epothilone PKS. However, with careful design of the KS-AT linker and the post-AT linker, replacing EPOAT4 with EPOAT2, EPOAT6, EPOAT7 or EPOAT8 (specifically incorporating methylmalonyl-CoA (MMCoA)) significantly increased the ratio of epothilone D (4) to epothilone C (3) (the highest ratio of 4:3 = 4.6:1), whereas the ratio of 4:3 in the parental strain Schlegelella brevitalea 104-1 was 1.4:1. We also obtained three strains by swapping EPOAT4 with EPOAT3, EPOAT5, or EPOAT9, which specifically incorporate malonyl-CoA (MCoA). These strains produced only epothilone C, and the yield was increased by a factor of 1.8 compared to that of parental strain 104-1. Furthermore, mutations of five residues in the AT domain identified Ser310 as the critical factor for MMCoA recognition in EPOAT4. Then, the mutation of His308 to valine or tyrosine combined with the mutation of Phe310 to serine further altered the product ratios. At the same time, we successfully deleted the inactive module 9 DH and ER domains and fused the ΨKR domain with the KR domain through an ~ 25-residue linker to generate a productive and simplified epothilone PKS.

Conclusions

These results suggested that the substitution and deletion of catalytic domains effectively produces desirable compounds and that selection of the linkers between domains is crucial for maintaining intact PKS catalytic activity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01578-3.

Application of CRISPR/Cas System in the Metabolic Engineering of Small Molecules

Clustered regularly interspaced short palindromic repeats (CRISPR) and their associated Cas protein technology area is rapidly growing technique for genome editing and modulation of transcription of several microbes. Successful engineering in microbes requires an emphasis on the aspect of efficiency and targeted aiming, which can be employed using CRISPR/Cas system. Hence, this type of system is used to modify the genome of several microbes such as yeast and bacteria. In recent years, CRISPR/Cas systems have been chosen for metabolic engineering in microbes due to their specificity, orthogonality, and efficacy. Therefore, we need to review the scheme which was acquired for the execution of the CRISPR/Cas system for the modification and metabolic engineering in yeast and bacteria. In this review, we highlighted the application of the CRISPR/Cas system which has been used for the production of small molecules in the microbial system that is chemically and biologically important.

Enzymology and biosynthesis of the orsellinic acid derived medicinal meroterpenoids

The advent of synthetic biology has yielded fruitful studies on orsellinic acid-derived meroterpenoids, which reportedly possess important biological activities. Genomics and transcriptomics have significantly accelerated the discovery of the biosynthetic genes for orsellinic acid-derived fungal and plant meroterpenoids. Subsequently, a well-developed heterologous host provides a convenient platform to generate a supply of useful natural products. Furthermore, in vitro reconstitution and genome editing tools have been increasingly employed as efficient means to fully understand the enzyme reaction mechanisms. With the knowledge of the biosynthetic machinery, combinatorial and engineered biosyntheses have yielded novel molecules with improved bioactivities. These studies will lay the foundation for the production of meroterpenoids with novel medicinal properties.

Artificial intelligence and synthetic biology approaches for human gut microbiome

The gut microbiome comprises a variety of microorganisms whose genes encode proteins to carry out crucial metabolic functions that are responsible for the majority of health-related issues in human beings. The advent of the technological revolution in artificial intelligence (AI) assisted synthetic biology (SB) approaches will play a vital role in the modulating the therapeutic and nutritive potential of probiotics. This can turn human gut as a reservoir of beneficial bacterial colonies having an immense role in immunity, digestion, brain function, and other health benefits. Hence, in the present review, we have discussed the role of several gene editing tools and approaches in synthetic biology that have equipped us with novel tools like Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR-Cas) systems to precisely engineer probiotics for diagnostic, therapeutic and nutritive value. A brief discussion over the AI techniques to understand the metagenomic data from the healthy and diseased gut microbiome is also presented. Further, the role of AI in potentially impacting the pace of developments in SB and its current challenges is also discussed. The review also describes the health benefits conferred by engineered microbes through the production of biochemicals, nutraceuticals, drugs or biotherapeutics molecules etc. Finally, the review concludes with the challenges and regulatory concerns in adopting synthetic biology engineered microbes for clinical applications. Thus, the review presents a synergistic approach of AI and SB toward human gut microbiome for better health which will provide interesting clues to researchers working in the area of rapidly evolving food and nutrition science.

Ketosynthase Domain Constrains the Design of Polyketide Synthases

Modular polyketide synthases (PKSs) produce complex, bioactive secondary metabolites in assembly line-like multistep reactions. Longstanding efforts to produce novel, biologically active compounds by recombining intact modules to new modular PKSs have mostly resulted in poorly active chimeras and decreased product yields. Recent findings demonstrate that the low efficiencies of modular chimeric PKSs also result from rate limitations in the transfer of the growing polyketide chain across the noncognate module:module interface and further processing of the non-native polyketide substrate by the ketosynthase (KS) domain. In this study, we aim at disclosing and understanding the low efficiency of chimeric modular PKSs and at establishing guidelines for modular PKSs engineering. To do so, we work with a bimodular PKS testbed and systematically vary substrate specificity, substrate identity, and domain:domain interfaces of the KS involved reactions. We observe that KS domains employed in our chimeric bimodular PKSs are bottlenecks with regards to both substrate specificity as well as interaction with the acyl carrier protein (ACP). Overall, our systematic study can explain in quantitative terms why early oversimplified engineering strategies based on the plain shuffling of modules mostly failed and why more recent approaches show improved success rates. We moreover identify two mutations of the KS domain that significantly increased turnover rates in chimeric systems and interpret this finding in mechanistic detail.

Structure-specific DNA recombination sites: Design, validation, and machine learning-based refinement

Recombination systems are widely used as bioengineering tools, but their sites have to be highly similar to a consensus sequence or to each other. To develop a recombination system free of these constraints, we turned toward attC sites from the bacterial integron system: single-stranded DNA hairpins specifically recombined by the integrase. Here, we present an algorithm that generates synthetic attC sites with conserved structural features and minimal sequence-level constraints. We demonstrate that all generated sites are functional, their recombination efficiency can reach 60%, and they can be embedded into protein coding sequences. To improve recombination of less efficient sites, we applied large-scale mutagenesis and library enrichment coupled to next-generation sequencing and machine learning. Our results validated the efficiency of this approach and allowed us to refine synthetic attC design principles. They can be embedded into virtually any sequence and constitute a unique example of a structure-specific DNA recombination system.

Site directed mutagenesis as a precision tool to enable synthetic biology with engineered modular polyketide synthases

Modular polyketide synthases (PKSs) are a multidomain megasynthase class of biosynthetic enzymes that have great promise for the development of new compounds, from new pharmaceuticals to high value commodity and specialty chemicals. Their colinear biosynthetic logic has been viewed as a promising platform for synthetic biology for decades. Due to this colinearity, domain swapping has long been used as a strategy to introduce molecular diversity. However, domain swapping often fails because it perturbs critical protein-protein interactions within the PKS. With our increased level of structural elucidation of PKSs, using judicious targeted mutations of individual residues is a more precise way to introduce molecular diversity with less potential for global disruption of the protein architecture. Here we review examples of targeted point mutagenesis to one or a few residues harbored within the PKS that alter domain specificity or selectivity, affect protein stability and interdomain communication, and promote more complex catalytic reactivity.

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.

Developing synthetic biology in Argentina: the Latin American TECNOx community as an alternative way for growth of the field

Synthetic biology emerged in the USA and Europe twenty years ago and quickly developed innovative research and technology as a result of continued funding. Synthetic biology is also growing in many developing countries of Africa, Asia and Latin America, where it could have a large economic impact by helping its use of genetic biodiversity in order to boost existing industries. Starting in 2011, Argentine synthetic biology developed along an idiosyncratic path. In 2011-2012, the main focus was not exclusively research but also on community building through teaching and participation in iGEM, following the template of the early "MIT school" of synthetic biology. In 2013-2015, activities diversified and included society-centered projects, social science studies on synthetic biology and bioart. Standard research outputs such as articles and industrial applications helped consolidate several academic working groups. Since 2016, the lack of a critical mass of researchers and a funding crisis were partially compensated by establishing links with Latin American synthetic biologists and with other socially oriented open technology collectives. The TECNOx community is a central node in this growing research and technology network. The first four annual TECNOx meetings brought together synthetic biologists with other open science and engineering platforms and explored the relationship of Latin American technologies with entrepreneurship, open hardware, ethics and human rights. In sum, the socioeconomic context encouraged Latin American synthetic biology to develop in a meandering and diversifying manner. This revealed alternative ways for growth of the field that may be relevant to other developing countries.

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.

Evolution and Diversity of Assembly-Line Polyketide Synthases

Assembly-line polyketide synthases (PKSs) are among the most complex protein machineries known in nature, responsible for the biosynthesis of numerous compounds used in the clinic. Their present-day diversity is the result of an evolutionary path that has involved the emergence of a multimodular architecture and further diversification of assembly-line PKSs. In this review, we provide an overview of previous studies that investigated PKS evolution and propose a model that challenges the currently prevailing view that gene duplication has played a major role in the emergence of multimodularity. We also analyze the ensemble of orphan PKS clusters sequenced so far to evaluate how large the entire diversity of assembly-line PKS clusters and their chemical products could be. Finally, we examine the existing techniques to access the natural PKS diversity in natural and heterologous hosts and describe approaches to further expand this diversity through engineering.

Towards scaling elementary flux mode computation

While elementary flux mode (EFM) analysis is now recognized as a cornerstone computational technique for cellular pathway analysis and engineering, EFM application to genome-scale models remains computationally prohibitive. This article provides a review of aspects of EFM computation that elucidates bottlenecks in scaling EFM computation. First, algorithms for computing EFMs are reviewed. Next, the impact of redundant constraints, sensitivity to constraint ordering and network compression are evaluated. Then, the advantages and limitations of recent parallelization and GPU-based efforts are highlighted. The article then reviews alternative pathway analysis approaches that aim to reduce the EFM solution space. Despite advances in EFM computation, our review concludes that continued scaling of EFM computation is necessary to apply EFM to genome-scale models. Further, our review concludes that pathway analysis methods that target specific pathway properties can provide powerful alternatives to EFM analysis.

Self-Assembled Multienzyme Nanostructures on Synthetic Protein Scaffolds

Sequential enzymes in a biosynthetic pathway often self-assemble to form nanomachineries known as multienzyme complexes inside cells. Enzyme self-assembly insulates toxic intermediates, increases the efficiency of intermediate transfer, minimizes metabolic crosstalk, streamlines flux, and improves the product yield. Artful structures and superior catalytic functions of these natural nanomachines inspired the development of synthetic multienzyme complexes to expedite biosynthesis. Here we present a versatile self-assembly strategy to construct multienzyme nanostructures based on synthetic protein scaffolds. The protein scaffolds were formed using the spontaneous protein reaction of SpyCatcher and SpyTag. Two types of protein scaffolds were generated: two skeleton proteins cross-linked and hierarchically assembled into heterogeneous nanostructures (the cross-linked scaffold), and head-to-tail cyclization of a dual-reactive skeleton protein gave a homogeneous cyclic scaffold. Sequential enzymes from the menaquinone biosynthetic pathway were assembled on both scaffolds through the docking domain interactions derived from polyketide synthases. Both scaffolded assemblies effectively increased the yield of the final product of the cascade catalytic reaction in menaquinone biosynthesis. Surprisingly, the rate enhancements were driven by different mechanisms: the cross-linked scaffold assembly streamlined the overall flow of the reactants, whereas the cyclic scaffold assembly accelerated the catalytic efficiency of the rate-limiting enzyme. Altogether, self-assembly of sequential enzymes by combining the SpyCatcher/SpyTag reaction and the docking domain interactions yielded protein-based nanostructures with special architecture, exceptional catalytic activity, and unexpected catalytic mechanisms. This work demonstrates a versatile strategy of gaining more powerful biocatalysts by protein self-assembly for efficient bioconversion of valuable chemicals.

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.

Diversification of polyketide structures via synthase engineering

Polyketide natural products possess diverse biological activities including antibiotic, anticancer, and immunosuppressive. Their equally varied and complex structures arise from head-to-tail condensation of simple carboxyacyl monomers. Since the seminal discovery that biosynthesis of polyketides such as the macrolide erythromycin is catalyzed by uncharacteristically large, multifunctional enzymes, termed modular type I polyketide synthases, chemists and biologists alike have been inspired to harness the apparent modularity of the synthases to further diversify polyketide structures. Yet, initial attempts to perform "combinatorial biosynthesis" failed due to challenges associated with maintaining the structural and catalytic integrity of large, chimeric synthases. Fast forward nearly 30 years, and advancements in our understanding of polyketide synthase structure and function have allowed the field to make significant progress toward effecting desired modifications to polyketide scaffolds in addition to engineering small, chiral fragments. This review highlights selected examples of polyketide diversification via control of monomer selection, oxidation state, stereochemistry, and cyclization. We conclude with a perspective on the present and future of polyketide structure diversification and hope that the examples presented here will encourage medicinal chemists to embrace polyketide synthetic biology as a means to revitalize polyketide drug discovery.

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.

Commodity Chemicals From Engineered Modular Type I Polyketide Synthases

Reduced polyketides are a subclass of natural products that have a variety of medical, veterinary, and agricultural applications and are well known for their structural diversity. Although these compounds do not resemble each other, they are all made by a class of enzymes known as modular polyketide synthases (PKSs). The commonality of PKS domains/modules that compose PKSs and the understanding of the relationship between the sequence of the PKS and the structure of the compound it produces render modular PKSs as excellent targets for engineering to produce novel compounds with predicted structures. Here, we describe experimental protocols and considerations for modular PKS engineering and two case studies to produce commodity chemicals by engineered PKSs.

Leveraging synthetic biology for producing bioactive polyketides and non-ribosomal peptides in bacterial heterologous hosts

Bacteria have historically been a rich source of natural products (e.g. polyketides and non-ribosomal peptides) that possess medically-relevant activities. Despite extensive discovery programs in both industry and academia, a plethora of biosynthetic pathways remain uncharacterized and the corresponding molecular products untested for potential bioactivities. This knowledge gap comes in part from the fact that many putative natural product producers have not been cultured in conventional laboratory settings in which the corresponding products are produced at detectable levels. Next-generation sequencing technologies are further increasing the knowledge gap by obtaining metagenomic sequence information from complex communities where production of the desired compound cannot be isolated in the laboratory. For these reasons, many groups are turning to synthetic biology to produce putative natural products in heterologous hosts. This strategy depends on the ability to heterologously express putative biosynthetic gene clusters and produce relevant quantities of the corresponding products. Actinobacteria remain the most abundant source of natural products and the most promising heterologous hosts for natural product discovery and production. However, researchers are discovering more natural products from other groups of bacteria, such as myxobacteria and cyanobacteria. Therefore, phylogenetically similar heterologous hosts have become promising candidates for synthesizing these novel molecules. The downside of working with these microbes is the lack of well-characterized genetic tools for optimizing expression of gene clusters and product titers. This review examines heterologous expression of natural product gene clusters in terms of the motivations for this research, the traits desired in an ideal host, tools available to the field, and a survey of recent progress.

Analysis of modular bioengineered antimicrobial lanthipeptides at nanoliter scale

The rise of antibiotic resistance demands the acceleration of molecular diversification strategies to inspire new chemical entities for antibiotic medicines. We report here on the large-scale engineering of ribosomally synthesized and post-translationally modified antimicrobial peptides carrying the ring-forming amino acid lanthionine. New-to-nature variants featuring distinct properties were obtained by combinatorial shuffling of peptide modules derived from 12 natural antimicrobial lanthipeptides and processing by a promiscuous post-translational modification machinery. For experimental characterization, we developed the nanoFleming, a miniaturized and parallelized high-throughput inhibition assay. On the basis of a hit set of >100 molecules, we identified variants with improved activity against pathogenic bacteria and shifted activity profiles, and extrapolated design guidelines that will simplify the identification of peptide-based anti-infectives in the future.

Engineering of Chimeric Polyketide Synthases Using SYNZIP Docking Domains

Engineering of assembly line polyketide synthases (PKSs) to produce novel bioactive compounds has been a goal for over 20 years. The apparent modularity of PKSs has inspired many engineering attempts in which entire modules or single domains were exchanged. In recent years, it has become evident that certain domain-domain interactions are evolutionarily optimized and, if disrupted, cause a decrease of the overall turnover rate of the chimeric PKS. In this study, we compared different types of chimeric PKSs in order to define the least invasive interface and to expand the toolbox for PKS engineering. We generated bimodular chimeric PKSs in which entire modules were exchanged, while either retaining a covalent linker between heterologous modules or introducing a noncovalent docking domain, or SYNZIP domain, mediated interface. These chimeric systems exhibited non-native domain-domain interactions during intermodular polyketide chain translocation. They were compared to otherwise equivalent bimodular PKSs in which a noncovalent interface was introduced between the condensing and processing parts of a module, resulting in non-native domain interactions during the extender unit acylation and polyketide chain elongation steps of their catalytic cycles. We show that the natural PKS docking domains can be efficiently substituted with SYNZIP domains and that the newly introduced noncovalent interface between the condensing and processing parts of a module can be harnessed for PKS engineering. Additionally, we established SYNZIP domains as a new tool for engineering PKSs by efficiently bridging non-native interfaces without perturbing PKS activity.

Exploiting enzyme promiscuity to shape plant specialized metabolism

The amazing variability of plant metabolism and its rapid divergence during evolution pose fundamental questions as to the driving forces, mechanisms, and players in metabolic differentiation. This review examines concepts that help us understand adaptive pathway evolution, with a particular emphasis on plant specialized metabolism, previously often termed secondary metabolism. Following a general introduction to pathway and metabolite evolution, the focus is directed to enzyme promiscuity and its classification. Promiscuous enzymes (or substrates), 'silent' elements of the metabolome, and the 'underground metabolism' may be used and combined to evolve 'new' metabolic pathways. It appears that new pathways rarely appear from scratch, but instead emerge from 'floppy' enzymes and elements of a 'messy' metabolism, and in this way a range of metabolites is generated, some of which may provide benefits to the plant.

Engineering the Substrate Specificity of a Modular Polyketide Synthase for Installation of Consecutive Non-Natural Extender Units

There is significant interest in diversifying the structures of polyketides to create new analogues of these bioactive molecules. This has traditionally been done by focusing on engineering the acyltransferase (AT) domains of polyketide synthases (PKSs) responsible for the incorporation of malonyl-CoA extender units. Non-natural extender units have been utilized by engineered PKSs previously; however, most of the work to date has been accomplished with ATs that are either naturally promiscuous and/or located in terminal modules lacking downstream bottlenecks. These limitations have prevented the engineering of ATs with low native promiscuity and the study of any potential gatekeeping effects by domains downstream of an engineered AT. In an effort to address this gap in PKS engineering knowledge, the substrate preferences of the final two modules of the pikromycin PKS were compared for several non-natural extender units and through active site mutagenesis. This led to engineering of the methylmalonyl-CoA specificity of both modules and inversion of their selectivity to prefer consecutive non-natural derivatives. Analysis of the product distributions of these bimodular reactions revealed unexpected metabolites resulting from gatekeeping by the downstream ketoreductase and ketosynthase domains. Despite these new bottlenecks, AT engineering provided the first full-length polyketide products incorporating two non-natural extender units. Together, this combination of tandem AT engineering and the identification of previously poorly characterized bottlenecks provides a platform for future advancements in the field.

A Genetic Circuit Compiler: Generating Combinatorial Genetic Circuits with Web Semantics and Inference

A central strategy of synthetic biology is to understand the basic processes of living creatures through engineering organisms using the same building blocks. Biological machines described in terms of parts can be studied by computer simulation in any of several languages or robotically assembled in vitro. In this paper we present a language, the Genetic Circuit Description Language (GCDL) and a compiler, the Genetic Circuit Compiler (GCC). This language describes genetic circuits at a level of granularity appropriate both for automated assembly in the laboratory and deriving simulation code. The GCDL follows Semantic Web practice, and the compiler makes novel use of the logical inference facilities that are therefore available. We present the GCDL and compiler structure as a study of a tool for generating κ-language simulations from semantic descriptions of genetic circuits.

Probing the modularity of megasynthases by rational engineering of a fatty acid synthase Type I

Modularity is a fundamental property of megasynthases such as polyketide synthases (PKSs). In this study, we exploit the close resemblance between PKSs and animal fatty acid synthase (FAS) to re-engineer animal FAS to probe the modularity of the FAS/PKS family. Guided by sequence and structural information, we truncate and dissect animal FAS into its components, and reassemble them to generate new PKS-like modules as well as bimodular constructs. The novel re-engineered modules resemble all four common types of PKSs and demonstrate that this approach can be a powerful tool to deliver products with higher catalytic efficiency. Our data exemplify the inherent plasticity and robustness of the overall FAS/PKS fold, and open new avenues to explore FAS-based biosynthetic pathways for custom compound design.

Biochemical characterization of the minimal domains of an iterative eukaryotic polyketide synthase

Iterative type I polyketide synthases (PKS) are megaenzymes essential to the biosynthesis of an enormously diverse array of bioactive natural products. Each PKS contains minimally three functional domains, β-ketosynthase (KS), acyltransferase (AT), and acyl carrier protein (ACP), and a subset of reducing domains such as ketoreductase (KR), dehydratase (DH), and enoylreductase (ER). The substrate selection, condensation reactions, and β-keto processing of the polyketide growing chain are highly controlled in a programmed manner. However, the structural features and mechanistic rules that orchestrate the iterative cycles, processing domains functionality, and chain termination in this kind of megaenzymes are often poorly understood. Here, we present a biochemical and functional characterization of the KS and the AT domains of a PKS from the mallard duck Anas platyrhynchos (ApPKS). ApPKS belongs to an animal PKS family phylogenetically more related to bacterial PKS than to metazoan fatty acid synthases. Through the dissection of the ApPKS enzyme into mono- to didomain fragments and its reconstitution in vitro, we determined its substrate specificity toward different starters and extender units. ApPKS AT domain can effectively transfer acetyl-CoA and malonyl-CoA to the ApPKS ACP stand-alone domain. Furthermore, the KS and KR domains, in the presence of Escherichia coli ACP, acetyl-CoA, and malonyl-CoA, showed the ability to catalyze the chain elongation and the β-keto reduction steps necessary to yield a 3-hydroxybutyryl-ACP derivate. These results provide new insights into the catalytic efficiency and specificity of this uncharacterized family of PKSs.

Protein-protein interactions in "cis-AT" polyketide synthases

Covering: up to the end of 2018 Polyketides are a valuable source of bioactive and clinically important molecules. The biosynthesis of these chemically complex molecules has led to the discovery of equally complex polyketide synthase (PKS) pathways. Crystallography has yielded snapshots of individual catalytic domains, di-domains, and multi-domains from a variety of PKS megasynthases, and cryo-EM studies have provided initial views of a PKS module in a series of defined biochemical states. Here, we review the structural and biochemical results that shed light on the protein-protein interactions critical to catalysis by PKS systems with an embedded acyltransferase. Interactions include those that occur both within and between PKS modules, as well as with accessory enzymes.

Synthetic biology of polyketide synthases

Complex reduced polyketides represent the largest class of natural products that have applications in medicine, agriculture, and animal health. This structurally diverse class of compounds shares a common methodology of biosynthesis employing modular enzyme systems called polyketide synthases (PKSs). The modules are composed of enzymatic domains that share sequence and functional similarity across all known PKSs. We have used the nomenclature of synthetic biology to classify the enzymatic domains and modules as parts and devices, respectively, and have generated detailed lists of both. In addition, we describe the chassis (hosts) that are used to assemble, express, and engineer the parts and devices to produce polyketides. We describe a recently developed software tool to design PKS system and provide an example of its use. Finally, we provide perspectives of what needs to be accomplished to fully realize the potential that synthetic biology approaches bring to this class of molecules.