Modular pathway engineering for the microbial production of branched-chain fatty alcohols

ATP biosensor reveals microbial energetic dynamics and facilitates bioproduction

Adenosine-5'-triphosphate (ATP), the primary energy currency in cellular processes, drives metabolic activities and biosynthesis. Despite its importance, understanding intracellular ATP dynamics' impact on bioproduction and exploiting it for enhanced bioproduction remains largely unexplored. Here, we harness an ATP biosensor to dissect ATP dynamics across different growth phases and carbon sources in multiple microbial strains. We find transient ATP accumulations during the transition from exponential to stationary growth phases in various conditions, coinciding with fatty acid (FA) and polyhydroxyalkanoate (PHA) production in Escherichia coli and Pseudomonas putida, respectively. We identify carbon sources (acetate for E. coli, oleate for P. putida) that elevate steady-state ATP levels and boost FA and PHA production. Moreover, we employ ATP dynamics as a diagnostic tool to assess metabolic burden, revealing bottlenecks that limit limonene bioproduction. Our results not only elucidate the relationship between ATP dynamics and bioproduction but also showcase its value in enhancing bioproduction in various microbial species.

Increased triacylglycerol production in Rhodococcus opacus by overexpressing transcriptional regulators

Lignocellulosic biomass is currently underutilized, but it offers promise as a resource for the generation of commercial end-products, such as biofuels, detergents, and other oleochemicals. Rhodococcus opacus PD630 is an oleaginous, Gram-positive bacterium with an exceptional ability to utilize recalcitrant aromatic lignin breakdown products to produce lipid molecules such as triacylglycerols (TAGs), which are an important biofuel precursor. Lipid carbon storage molecules accumulate only under growth-limiting low nitrogen conditions, representing a significant challenge toward using bacterial biorefineries for fuel precursor production. In this work, we screened overexpression of 27 native transcriptional regulators for their abilities to improve lipid accumulation under nitrogen-rich conditions, resulting in three strains that accumulate increased lipids, unconstrained by nitrogen availability when grown in phenol or glucose. Transcriptomic analyses revealed that the best strain (#13) enhanced FA production via activation of the β-ketoadipate pathway. Gene deletion experiments confirm that lipid accumulation in nitrogen-replete conditions requires reprogramming of phenylalanine metabolism. By generating mutants decoupling carbon storage from low nitrogen environments, we move closer toward optimizing R. opacus for efficient bioproduction on lignocellulosic biomass.

Co-Cultivated Enzyme Constraint Metabolic Network Model for Rational Guidance in Constructing Synthetic Consortia to Achieve Optimal Pathway Allocation Prediction

Synthetic consortia have emerged as a promising biosynthetic platform that offers new opportunities for biosynthesis. Genome-scale metabolic network models (GEMs) with complex constraints are extensively utilized to guide the synthesis in monocultures. However, few methods are currently available to guide the rational construction of synthetic consortia for predicting the optimal allocation strategy of synthetic pathways aimed at enhancing product synthesis. A standardized method to construct the co-cultivated Enzyme Constraint metabolic network model (CulECpy) is proposed, which integrates enzyme constraints and modular interaction scale constraints based on the research concept of "independent + global". This method is applied to construct several synthetic consortia models, which encompassed different target products, strains, synthetic pathways, and compositional structures. Analyzing the model, the optimal pathway allocation and initial inoculum ratio that enhance the synthesis of target products by synthetic consortia are predicted and verified. When comparing with the constructed co-culture synthesis system, the normalized root mean square error of all optimal theoretical yield simulations is found to be less than or equal to 0.25. The analyses and verifications demonstrate that the method CulECpy can guide the rational construction of synthetic consortia systems to facilitate biochemical synthesis.

Genetically Engineered Protein-Based Bioadhesives with Programmable Material Properties

Silk-amyloid-mussel foot protein (SAM) hydrogels made from recombinant fusion proteins containing β-amyloid peptide, spider silk domain, and mussel foot protein (Mfp) are attractive bioadhesives as they display a unique combination of tunability, biocompatibility, bioabsorbability, strong cohesion, and underwater adhesion to a wide range of biological surfaces. To design tunable SAM hydrogels for tailored surgical repair applications, an understanding of the relationships between protein sequence and hydrogel properties is imperative. Here, we fabricated SAM hydrogels using fusion proteins of varying lengths of silk-amyloid repeats and Mfps to characterize their structure and properties. We found that increasing silk-amyloid repeats enhanced the hydrogel's β-sheet content (r = 0.74), leading to higher cohesive strength and toughness. Additionally, increasing the Mfp length beyond the half-length of the full Mfp sequence (1/2 Mfp) decreased the β-sheet content (r = -0.47), but increased hydrogel surface adhesion. Among different variants, the hydrogel made of 16xKLV-2Mfp displayed a high ultimate strength of 3.0 ± 0.3 MPa, an ultimate strain of 664 ± 119%, and an attractive underwater adhesivity of 416 ± 20 kPa to porcine skin. Collectively, the sequence-structure-property relationships learned from this study will be useful to guide the design of future protein adhesives with tunable characteristics for tailored surgical applications.

Protein-Based Hydrogels and Their Biomedical Applications

Hydrogels made from proteins are attractive materials for diverse medical applications, as they are biocompatible, biodegradable, and amenable to chemical and biological modifications. Recent advances in protein engineering, synthetic biology, and material science have enabled the fine-tuning of protein sequences, hydrogel structures, and hydrogel mechanical properties, allowing for a broad range of biomedical applications using protein hydrogels. This article reviews recent progresses on protein hydrogels with special focus on those made of microbially produced proteins. We discuss different hydrogel formation strategies and their associated hydrogel properties. We also review various biomedical applications, categorized by the origin of protein sequences. Lastly, current challenges and future opportunities in engineering protein-based hydrogels are discussed. We hope this review will inspire new ideas in material innovation, leading to advanced protein hydrogels with desirable properties for a wide range of biomedical applications.

Microbial Synthesis of High-Molecular-Weight, Highly Repetitive Protein Polymers

High molecular weight (MW), highly repetitive protein polymers are attractive candidates to replace petroleum-derived materials as these protein-based materials (PBMs) are renewable, biodegradable, and have outstanding mechanical properties. However, their high MW and highly repetitive sequence features make them difficult to synthesize in fast-growing microbial cells in sufficient amounts for real applications. To overcome this challenge, various methods were developed to synthesize repetitive PBMs. Here, we review recent strategies in the construction of repetitive genes, expression of repetitive proteins from circular mRNAs, and synthesis of repetitive proteins by ligation and protein polymerization. We discuss the advantages and limitations of each method and highlight future directions that will lead to scalable production of highly repetitive PBMs for a wide range of applications.

Applications and Tuning Strategies for Transcription Factor-Based Metabolite Biosensors

Transcription factor (TF)-based biosensors are widely used for the detection of metabolites and the regulation of cellular pathways in response to metabolites. Several challenges hinder the direct application of TF-based sensors to new hosts or metabolic pathways, which often requires extensive tuning to achieve the optimal performance. These tuning strategies can involve transcriptional or translational control depending on the parameter of interest. In this review, we highlight recent strategies for engineering TF-based biosensors to obtain the desired performance and discuss additional design considerations that may influence a biosensor's performance. We also examine applications of these sensors and suggest important areas for further work to continue the advancement of small-molecule biosensors.

Diverse mechanisms of bioproduction heterogeneity in fermentation and their control strategies

Microbial bioproduction often faces challenges related to populational heterogeneity, where cells exhibit varying biosynthesis capabilities. Bioproduction heterogeneity can stem from genetic and non-genetic factors, resulting in decreased titer, yield, stability, and reproducibility. Consequently, understanding and controlling bioproduction heterogeneity are crucial for enhancing the economic competitiveness of large-scale biomanufacturing. In this review, we provide a comprehensive overview of current understandings of the various mechanisms underlying bioproduction heterogeneity. Additionally, we examine common strategies for controlling bioproduction heterogeneity based on these mechanisms. By implementing more robust measures to mitigate heterogeneity, we anticipate substantial enhancements in the scalability and stability of bioproduction processes.

ONE-SENTENCE SUMMARY

This review summarizes current understandings of different mechanisms of bioproduction heterogeneity and common control strategies based on these mechanisms.

Metabolic Engineering of Escherichia coli for Increased Bioproduction of N-Acetylneuraminic Acid

N-Acetylneuraminic acid (NeuAc) is widely used in the food and pharmaceutical industries. Therefore, it is important to develop an efficient and eco-friendly method for NeuAc production. Here, we achieved de novo biosynthesis of NeuAc in an engineered plasmid-free Escherichia coli strain, which efficiently synthesizes NeuAc using glycerol as the sole carbon source, via clustered regularly interspaced palindromic repeat (CRISPR)/CRISPR-associated protein 9-based genome editing. NeuAc key precursor, N-acetylmannosamine (ManNAc; 0.40 g/L), was produced by expressing UDP-N-acetylglucosamine-2-epimerase and glucosamine-6-phosphate synthase (GlmS) mutants and blocking the NeuAc catabolic pathway in E. coli BL21 (DE3). The expression levels of GlmM and GlmU-GlmS_A metabolic modules were optimized, significantly increasing the ManNAc titer to 8.95 g/L. Next, the expression levels of NeuAc synthase from different microorganisms were optimized, leading to the production of 6.27 g/L of NeuAc. Blocking the competing pathway of NeuAc biosynthesis increased the NeuAc titer to 9.65 g/L. In fed-batch culture in a 3 L fermenter, NeuAc titer reached 23.46 g/L with productivity of 0.69 g/L/h, which is the highest level achieved by microbial synthesis using glycerol as the sole carbon source in E. coli. The strategies used in our study can aid in the efficient bioproduction of NeuAc and its derivatives.

The De Novo Synthesis of 2-Phenylethanol from Glucose by the Synthetic Microbial Consortium Composed of Engineered Escherichia coli and Meyerozyma guilliermondii

Synthetic microbial consortia show promising applications for fine chemical production, especially with long metabolic pathways. In this study, a synthetic microbial consortium consisting of Escherichia coli YLC20 and Meyerozyma guilliermondii MG57 was successfully constructed, which could achieve efficient de novo 2-phenylethanol (2-PE) production from glucose. A tyrosine-deficient E. coli YLC20 overexpressing genes of aroF and pheA was first constructed, which could accumulate 29.5 g/L of l-phenylalanine (l-Phe) within 96 h from glucose accompanied by the coproduction of acetate and α-ketoglutarate (α-KG). Furthermore, the engineered M. guilliermondii MG57 was constructed through the stepwise metabolic engineering strategy, which could facilitate the 2-PE synthesis from l-Phe. Moreover, the cosubstrate and material intervention strategies were applied to improve the stability of the microbial consortium and 2-PE production. Finally, the synthetic microbial consortium could de novo synthesize 3.77 g/L of 2-PE from 80 g/L of glucose, providing a reference for the de novo synthesis of fine chemicals with long metabolic pathways.

Engineering diverse fatty acid compositions of phospholipids in Escherichia coli

Bacterial fatty acids (FAs) are an essential component of the cellular membrane and are an important source of renewable chemicals as they can be converted to fatty alcohols, esters, ketones, and alkanes, and used as biofuels, detergents, lubricants, and commodity chemicals. Most prior FA bioconversions have been performed on the carboxylic acid group. Modification of the FA hydrocarbon chain could substantially expand the structural and functional diversity of FA-derived products. Additionally, the effects of such modified FAs on the growth and metabolic state of their producing cells are not well understood. Here we engineer novel Escherichia coli phospholipid biosynthetic pathways, creating strains with distinct FA profiles enriched in ω7-unsaturated FAs (ω7-UFAs, 75%), Δ5-unsaturated FAs (Δ5-UFAs, 60%), cyclopropane FAs (CFAs, 55%), internally-branched FAs (IBFAs, 40%), and Δ5,ω7-double unsaturated FAs (DUFAs, 46%). Although bearing drastically different FA profiles in phospholipids, UFA, CFA, and IBFA enriched strains display wild-type-like phenotypic profiling and growth. Transcriptomic analysis reveals DUFA production drives increased differential expression and the induction of the fur iron starvation transcriptional cascade, but higher TCA cycle activation compared to the UFA producing strain. This likely reflects a slight cost imparted for DUFA production, which resulted in lower maximum growth in some, but not all, environmental conditions. The IBFA-enriched strain was further engineered to produce free IBFAs, releasing 96 mg/L free IBFAs from 154 mg/L of the total cellular IBFA pool. This work has resulted in significantly altered FA profiles of membrane lipids in E. coli, greatly increasing our understanding of the effects of FA structure diversity on the transcriptome, growth, and ability to react to stress.

The Growth Dependent Design Constraints of Transcription-Factor-Based Metabolite Biosensors

Metabolite biosensors based on metabolite-responsive transcription factors are key synthetic biology components for sensing and precisely controlling cellular metabolism. Biosensors are often designed under laboratory conditions but are deployed in applications where cellular growth rate differs drastically from its initial characterization. Here we asked how growth rate impacts the minimum and maximum biosensor outputs and the dynamic range, which are key metrics of biosensor performance. Using LacI, TetR, and FadR-based biosensors in Escherichia coli as models, we find that the dynamic range of different biosensors have different growth rate dependencies. We developed a kinetic model to explore how tuning biosensor parameters impact the dynamic range growth rate dependence. Our modeling and experimental results revealed that the effects to dynamic range and its growth rate dependence are often coupled, and the metabolite transport mechanisms shape the dynamic range-growth rate response. This work provides a systematic understanding of biosensor performance under different growth rates, which will be useful for predicting biosensor behavior in broad synthetic biology and metabolic engineering applications.

Transient Antibiotic Tolerance Triggered by Nutrient Shifts From Gluconeogenic Carbon Sources to Fatty Acid

Nutrient shifts from glycolytic-to-gluconeogenic carbon sources can create large sub-populations of extremely antibiotic tolerant bacteria, called persisters. Positive feedback in Escherichia coli central metabolism was believed to play a key role in the formation of persister cells. To examine whether positive feedback in nutrient transport can also support high persistence to β-lactams, we performed nutrient shifts for E. coli from gluconeogenic carbon sources to fatty acid (FA). We observed tri-phasic antibiotic killing kinetics characterized by a transient period of high antibiotic tolerance, followed by rapid killing then a slower persister-killing phase. The duration of transient tolerance (3-44 h) varies with pre-shift carbon source and correlates strongly with the time needed to accumulate the FA degradation enzyme FadD after the shift. Additionally, FadD accumulation time and thus transient tolerance time can be reduced by induction of the glyoxylate bypass prior to switching, highlighting that two interacting feedback loops simultaneously control the length of transient tolerance. Our results demonstrate that nutrient switches along with positive feedback are not sufficient to trigger persistence in a majority of the population but instead triggers only a temporary tolerance. Additionally, our results demonstrate that the pre-shift metabolic state determines the duration of transient tolerance and that supplying glyoxylate can facilitate antibiotic killing of bacteria.

Opportunities and Challenges for Microbial Synthesis of Fatty Acid-Derived Chemicals (FACs)

Global warming and uneven distribution of fossil fuels worldwide concerns have spurred the development of alternative, renewable, sustainable, and environmentally friendly resources. From an engineering perspective, biosynthesis of fatty acid-derived chemicals (FACs) is an attractive and promising solution to produce chemicals from abundant renewable feedstocks and carbon dioxide in microbial chassis. However, several factors limit the viability of this process. This review first summarizes the types of FACs and their widely applications. Next, we take a deep look into the microbial platform to produce FACs, give an outlook for the platform development. Then we discuss the bottlenecks in metabolic pathways and supply possible solutions correspondingly. Finally, we highlight the most recent advances in the fast-growing model-based strain design for FACs biosynthesis.

Dynamic control in metabolic engineering: Theories, tools, and applications

Metabolic engineering has allowed the production of a diverse number of valuable chemicals using microbial organisms. Many biological challenges for improving bio-production exist which limit performance and slow the commercialization of metabolically engineered systems. Dynamic metabolic engineering is a rapidly developing field that seeks to address these challenges through the design of genetically encoded metabolic control systems which allow cells to autonomously adjust their flux in response to their external and internal metabolic state. This review first discusses theoretical works which provide mechanistic insights and design choices for dynamic control systems including two-stage, continuous, and population behavior control strategies. Next, we summarize molecular mechanisms for various sensors and actuators which enable dynamic metabolic control in microbial systems. Finally, important applications of dynamic control to the production of several metabolite products are highlighted, including fatty acids, aromatics, and terpene compounds. Altogether, this review provides a comprehensive overview of the progress, advances, and prospects in the design of dynamic control systems for improved titer, rate, and yield metrics in metabolic engineering.

Biosynthesis of Fatty Alcohols in Engineered Microbial Cell Factories: Advances and Limitations

Concerns about climate change and environmental destruction have led to interest in technologies that can replace fossil fuels and petrochemicals with compounds derived from sustainable sources that have lower environmental impact. Fatty alcohols produced by chemical synthesis from ethylene or by chemical conversion of plant oils have a large range of industrial applications. These chemicals can be synthesized through biological routes but their free forms are produced in trace amounts naturally. This review focuses on how genetic engineering of endogenous fatty acid metabolism and heterologous expression of fatty alcohol producing enzymes have come together resulting in the current state of the field for production of fatty alcohols by microbial cell factories. We provide an overview of endogenous fatty acid synthesis, enzymatic methods of conversion to fatty alcohols and review the research to date on microbial fatty alcohol production. The primary focus is on work performed in the model microorganisms, Escherichia coli and Saccharomyces cerevisiae but advances made with cyanobacteria and oleaginous yeasts are also considered. The limitations to production of fatty alcohols by microbial cell factories are detailed along with consideration to potential research directions that may aid in achieving viable commercial scale production of fatty alcohols from renewable feedstock.

Functional replacement of isoprenoid pathways in Rhodobacter sphaeroides

Advances in synthetic biology and metabolic engineering have proven the potential of introducing metabolic by-passes within cell factories. These pathways can provide a more efficient alternative to endogenous counterparts due to their insensitivity to host's regulatory mechanisms. In this work, we replaced the endogenous essential 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway for isoprenoid biosynthesis in the industrially relevant bacterium Rhodobacter sphaeroides by an orthogonal metabolic route. The native 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway was successfully replaced by a heterologous mevalonate (MVA) pathway from a related bacterium. The functional replacement was confirmed by analysis of the reporter molecule amorpha-4,11-diene after cultivation with [4-13 C]glucose. The engineered R. sphaeroides strain relying exclusively on the MVA pathway was completely functional in conditions for sesquiterpene production and, upon increased expression of the MVA enzymes, it reached even higher sesquiterpene yields than the control strain coexpressing both MEP and MVA modules. This work represents an example where substitution of an essential biochemical pathway by an alternative, heterologous pathway leads to enhanced biosynthetic performance.

Exploiting nonionic surfactants to enhance fatty alcohol production in Rhodosporidium toruloides

Fatty alcohols (FOHs) are important feedstocks in the chemical industry to produce detergents, cosmetics, and lubricants. Microbial production of FOHs has become an attractive alternative to production in plants and animals due to growing energy demands and environmental concerns. However, inhibition of cell growth caused by intracellular FOH accumulation is one major issue that limits FOH titers in microbial hosts. In addition, identification of FOH‐specific exporters remains a challenge and previous studies towards this end are limited. To alleviate the toxicity issue, we exploited nonionic surfactants to promote the export of FOHs in Rhodosporidium toruloides, an oleaginous yeast that is considered an attractive next‐generation host for the production of fatty acid‐derived chemicals. Our results showed FOH export efficiency was dramatically improved and the growth inhibition was alleviated in the presence of small amounts of tergitol and other surfactants. As a result, FOH titers increase by 4.3‐fold at bench scale to 352.6 mg/L. With further process optimization in a 2‐L bioreactor, the titer was further increased to 1.6 g/L. The method we show here can potentially be applied to other microbial hosts and may facilitate the commercialization of microbial FOH production.

Heterogeneity coordinates bacterial multi-gene expression in single cells

For a genetically identical microbial population, multi-gene expression in various environments requires effective allocation of limited resources and precise control of heterogeneity among individual cells. However, it is unclear how resource allocation and cell-to-cell variation jointly shape the overall performance. Here we demonstrate a Simpson’s paradox during overexpression of multiple genes: two competing proteins in single cells correlated positively for every induction condition, but the overall correlation was negative. Yet this phenomenon was not observed between two competing mRNAs in single cells. Our analytical framework shows that the phenomenon arises from competition for translational resource, with the correlation modulated by both mRNA and ribosome variability. Thus, heterogeneity plays a key role in single-cell multi-gene expression and provides the population with an evolutionary advantage, as demonstrated in this study.

Microbes perform multitasking for a wide range of purposes, including survival, adaptation, colonization, and evolution. Both modelling and experimental results at the ensemble level reveal trade-offs between different tasks due to resource competition, but it is unclear how single cells allocate limited intracellular resources to perform multitasking, and how does a population coordinate single cell performances during multitasking to maximize population efficiencies. In this study, we address this question by using bacterial multi-gene overexpression as the basic form of multitasking. We discovered and analyzed a statistical phenomenon called Simpson’s paradox, where competing proteins in single cells correlate positively at each constant condition, although the proteins correlate negatively when all conditions are combined. We demonstrate that the phenomenon arises from competition for translational resources, with the correlation modulated by heterogeneity of both mRNA and ribosomes. We further show that heterogeneity coordinates multiple functional modules, conferring an evolutionary advantage on the population. Our work discloses that heterogeneity in the form of Simpson’s paradox is an important phenomenon in coordinating multi-gene expression.

Biosynthesis, regulation, and engineering of microbially produced branched biofuels

The steadily increasing demand on transportation fuels calls for renewable fuel replacements. This has attracted a growing amount of research to develop advanced biofuels that have similar physical, chemical, and combustion properties with petroleum-derived fossil fuels. Early generations of biofuels, such as ethanol, butanol, and straight-chain fatty acid-derived esters or hydrocarbons suffer from various undesirable properties and can only be blended in limited amounts. Recent research has shifted to the production of branched-chain biofuels that, compared to straight-chain fuels, have higher octane values, better cold flow, and lower cloud points, making them more suitable for existing engines, particularly for diesel and jet engines. This review focuses on several types of branched-chain biofuels and their immediate precursors, including branched short-chain (C4-C8) and long-chain (C15-C19)-alcohols, alkanes, and esters. We discuss their biosynthesis, regulation, and recent efforts in their overproduction by engineered microbes.

Engineering a Coenzyme A Detour To Expand the Product Scope and Enhance the Selectivity of the Ehrlich Pathway

The Ehrlich pathway is a major route for the renewable production of higher alcohols. However, the product scope of the Ehrlich pathway is restricted, and the product selectivity is suboptimal. Here, we demonstrate that a Coenzyme A (CoA) detour, which involves conversion of the 2-keto acids into acyl-CoAs, expands the biological toolkit of reaction chemistries available in the Ehrlich pathway to include the gamut of CoA-dependent enzymes. As a proof-of-concept, we demonstrated the first biosynthesis of a tertiary branched-alcohol, pivalcohol, at a level of ∼10 mg/L from glucose in Escherichia coli, using a pivalyl-CoA mutase from Xanthobacter autotrophicus. Furthermore, engineering an enzyme in the CoA detour, the Lactobacillus brevis CoA-acylating aldehyde dehydrogenase, allowed stringent product selectivity. Targeted production of 3-methyl-1-butanol (3-MB) in E. coli mediated by the CoA detour showed a 3-MB:side-product (isobutanol) ratio of >20, an increase over the ratios previously achieved using the conventional Ehrlich pathway.

Systematic Analysis of Bottlenecks in a Multibranched and Multilevel Regulated Pathway: The Molecular Fundamentals of l-Methionine Biosynthesis in Escherichia coli

To produce chemicals and fuels from renewable resources, various strategies and genetic tools have been developed to redesign pathways and optimize the metabolic flux in microorganisms. However, in most successful cases, the target chemicals are synthesized through a linear pathway, and regular methodologies for the identification of bottlenecks and metabolic flux optimization in multibranched and multilevel regulated pathways, such as the l-methionine biosynthetic pathway, have rarely been reported. In the present study, a systematic analysis strategy was employed to gradually reveal and remove the potential bottlenecks limiting the l-methionine biosynthesis in E. coli. 80 genes in central metabolism and selected amino acids biosynthetic pathways were first repressed or upregulated to probe their effects on l-methionine accumulation. The l-methionine biosynthetic pathway was then modularized and iteratively genetic modifications were performed to uncover the multiple layers of limitations and stepwise improve the l-methionine titer. The metabolomics data further revealed a more evenly distributed metabolic flux in l-methionine biosynthesis pathway of the optimal strain and provided valuable suggestions for further optimization. The optimal strain produced 16.86 g/L of l-methionine in 48 h by fed-batch fermentation. This work is the first to our knowledge to systematically elucidate the molecular fundamentals of multilevel regulation of l-methionine biosynthesis. It also demonstrated that the systematic analysis strategy can boost our ability to identify the potential bottlenecks and optimize the metabolic flux in multibranched and multilevel regulated pathways for the production of corresponding chemicals.

Engineering the oleaginous yeast Yarrowia lipolytica to produce the aroma compound β-ionone

Background

β-Ionone is a fragrant terpenoid that generates a pleasant floral scent and is used in diverse applications as a cosmetic and flavoring ingredient. A growing consumer desire for natural products has increased the market demand for natural β-ionone. To date, chemical extraction from plants remains the main approach for commercial natural β-ionone production. Unfortunately, changing climate and geopolitical issues can cause instability in the β-ionone supply chain. Microbial fermentation using generally recognized as safe (GRAS) yeast offers an alternative method for producing natural β-ionone. Yarrowia lipolytica is an attractive host due to its oleaginous nature, established genetic tools, and large intercellular pool size of acetyl-CoA (the terpenoid backbone precursor).

Results

A push–pull strategy via genome engineering was applied to a Y. lipolytica PO1f derived strain. Heterologous and native genes in the mevalonate pathway were overexpressed to push production to the terpenoid backbone geranylgeranyl pyrophosphate, while the carB and biofunction carRP genes from Mucor circinelloides were introduced to pull flux towards β-carotene (i.e., ionone precursor). Medium tests combined with machine learning based data analysis and ¹³C metabolite labeling investigated influential nutrients for the β-carotene strain that achieved > 2.5 g/L β-carotene in a rich medium. Further introduction of the carotenoid cleavage dioxygenase 1 (CCD1) from Osmanthus fragrans resulted in the β-ionone production. Utilization of in situ dodecane trapping avoided ionone loss from vaporization (with recovery efficiencies of ~ 76%) during fermentation operations, which resulted in titers of 68 mg/L β-ionone in shaking flasks and 380 mg/L in a 2 L fermenter. Both β-carotene medium tests and β-ionone fermentation outcomes indicated the last enzymatic step CCD1 (rather than acetyl-CoA supply) as the key bottleneck.

Conclusions

We engineered a GRAS Y. lipolytica platform for sustainable and economical production of the natural aroma β-ionone. Although β-carotene could be produced at high titers by Y. lipolytica, the synthesis of β-ionone was relatively poor, possibly due to low CCD1 activity and non-specific CCD1 cleavage of β-carotene. In addition, both β-carotene and β-ionone strains showed decreased performances after successive sub-cultures. For industrial application, β-ionone fermentation efforts should focus on both CCD enzyme engineering and strain stability improvement.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0984-x) contains supplementary material, which is available to authorized users.

Modular Pathway Rewiring of Yeast for Amino Acid Production

Amino acids find various applications in biotechnology in view of their importance in the food, feed, pharmaceutical, and personal care industries as nutrients, additives, and drugs, respectively. For the large-scale production of amino acids, microbial cell factories are widely used and the development of amino acid-producing strains has mainly focused on prokaryotes Corynebacterium glutamicum and Escherichia coli. However, the eukaryote Saccharomyces cerevisiae is becoming an even more appealing microbial host for production of amino acids and derivatives because of its superior molecular and physiological features, such as amenable to genetic engineering and high tolerance to harsh conditions. To transform S. cerevisiae into an industrial amino acid production platform, the highly coordinated and multiple layers regulation in its amino acid metabolism should be relieved and reconstituted to optimize the metabolic flux toward synthesis of target products. This chapter describes principles, strategies, and applications of modular pathway rewiring in yeast using the engineering of l-ornithine metabolism as a paradigm. Additionally, detailed protocols for in vitro module construction and CRISPR/Cas-mediated pathway assembly are provided.

Rewiring FadR regulon for the selective production of ω-hydroxy palmitic acid from glucose in Escherichia coli

ω-Hydroxy palmitic acid (ω-HPA) is a valuable compound for an ingredient of artificially synthesized ceramides and an additive for lubricants and adhesives. Production of such a fatty acid derivative is limited by chemical catalysis, but plausible by biocatalysis. However, its low productivity issue, including formations of unsaturated fatty acid (UFA) byproducts in host cells, remains as a hurdle toward industrial biological processes. In this study, to achieve selective and high-level production of ω-HPA from glucose in Escherichia coli, FadR, a native transcriptional regulator of fatty acid metabolism, and its regulon were engineered. First, FadR was co-expressed with a thioesterase with a specificity toward palmitic acid production to enhance palmitic acid production yield, but a considerable quantity of UFAs was also produced. In order to avoid the UFA production caused by fadR overexpression, FadR regulon was rewired by i) mutating FadR consensus binding sites of fabA or fabB, ii) integrating fabZ into fabI operon, and iii) enhancing the strength of fabI promoter. This approach led to dramatic increases in both proportion (48.3-83.0%) and titer (377.8 mg/L to 675.8 mg/L) of palmitic acid, mainly due to the decrease in UFA synthesis. Introducing a fatty acid ω-hydroxylase, CYP153A35, into the engineered strain resulted in a highly selective production of ω-HPA (83.5 mg/L) accounting for 87.5% of total ω-hydroxy fatty acids. Furthermore, strategies, such as i) enhancement in CYP153A35 activity, ii) expression of a fatty acid transporter, iii) supplementation of triton X-100, and iv) separation of the ω-HPA synthetic pathway into two strains for a co-culture system, were applied and resulted in 401.0 mg/L of ω-HPA production. For such selective productions of palmitic acid and ω-HPA, the rewiring of FadR regulation in E. coli is a promising strategy to develop an industrial process with economical downstream processing.