Metabolic engineering of low-pH-tolerant non-model yeast, Issatchenkia orientalis, for production of citramalate

Issatchenkia orientalis as a platform organism for cost-effective production of organic acids

Driven by the urgent need to reduce the reliance on fossil fuels and mitigate environmental impacts, microbial cell factories capable of producing value-added products from renewable resources have gained significant attention over the past few decades. Notably, non-model yeasts with unique physiological characteristics have emerged as promising candidates for industrial applications, particularly for the production of organic acids. Among them, Issatchenkia orientalis stands out for its exceptional natural tolerance to low pH and high osmotic pressure, traits that are critical for overcoming the limitations of conventional microbial organisms. The acid tolerance of I. orientalis enables organic acid production under low pH conditions, bypassing the need for expensive neutral pH control typically required in conventional processes. Organic acids produced by I. orientalis, such as lactic acid, succinic acid, and itaconic acid, are widely used as building blocks for bioplastics, food additives, and pharmaceuticals. This review summarizes the key findings from systems biology studies on I. orientalis over the past two decades, providing insights into its unique metabolic and physiological traits. Advances in genetic tool development for this non-model yeast are also discussed, enabling targeted metabolic engineering to enhance its production capabilities. Additionally, case studies are highlighted to illustrate the potential of I. orientalis as a platform organism. Finally, the remaining challenges and future directions are addressed to further develop I. orientalis into a robust and versatile microbial cell factory for sustainable biomanufacturing.

Anti-Pdc1p Nanobody as a Genetically Encoded Inhibitor of Ethanol Production Enables Dual Transcriptional and Post-translational Controls of Yeast Fermentations

Microbial fermentation provides a sustainable method of producing valuable chemicals. Adding dynamic control to fermentations can significantly improve titers, but most systems rely on transcriptional controls of metabolic enzymes, leaving existing intracellular enzymes unregulated. This limits the ability of transcriptional controls to switch off metabolic pathways, especially when metabolic enzymes have long half-lives. We developed a two-layer transcriptional/post-translational control system for yeast fermentations. Specifically, the system uses blue light to transcriptionally activate the major pyruvate decarboxylase PDC1, required for cell growth and concomitant ethanol production. Switching to darkness transcriptionally inactivates PDC1 and instead activates the anti-Pdc1p nanobody, NbJRI, to act as a genetically encoded inhibitor of Pdc1p accumulated during the growth phase. This dual transcriptional/post-translational control improves the production of 2,3-BDO and citramalate by up to 100 and 92% compared to using transcriptional controls alone in dynamic two-phase fermentations. This study establishes the NbJRI nanobody as an effective genetically encoded inhibitor of Pdc1p that can enhance the production of pyruvate-derived chemicals.

Advantages of Mutant Generation by Genome Rearrangements of Non-Conventional Yeast via Direct Nuclease Transfection

We previously developed a genome engineering method (TAQing2.0) based on the direct delivery of DNA endonucleases into living cells, which induces genome rearrangements even in non-sporulating nonconventional yeasts without introducing foreign DNA. Using TAQing2.0 and conventional mutagenesis (by nitrosoguanidine), we obtained mutant asexual Candida utilis strains capable of growing under highly acidic conditions (pH 1.8). Whole genome resequencing revealed that the genomic sequences of mutants generated by both methods contain a negligible small population of unmappable sequences, suggesting that both types of mutants can be regarded as equivalent to naturally occurring mutants. TAQing2.0 mutants exhibit multiple genome rearrangements with few point mutations, whereas conventional mutagenesis produces numerous point mutations. This feature enabled us to easily identify candidate genes (e.g., LYP1 homolog) responsible for acid resistance. TAQing2.0 is a powerful and versatile tool for mutant production and gene hunting without invasion of foreign DNA.

Cell-free systems: A synthetic biology tool for rapid prototyping in metabolic engineering

Microbial cell factories provide sustainable alternatives to petroleum-based chemical production using cost-effective substrates. A deep understanding of their metabolism is essential to harness their potential along with continuous efforts to improve productivity and yield. However, the construction and evaluation of numerous genetic variants are time-consuming and labor-intensive. Cell-free systems (CFSs) serve as powerful platforms for rapid prototyping of genetic circuits, metabolic pathways, and enzyme functionality. They offer numerous advantages, including minimizing unwanted metabolic interference, precise control of reaction conditions, reduced labor, and shorter Design-Build-Test-Learn cycles. Additionally, the introduction of in vitro compartmentalization strategies in CFSs enables ultra-high-throughput screening in physically separated spaces, which significantly enhances prototyping efficiency. This review highlights the latest examples of using CFS to overcome prototyping limitations in living cells with a focus on rapid prototyping, particularly regarding gene regulation, enzymes, and multienzymatic reactions in bacteria. Finally, this review evaluates CFSs as a versatile prototyping platform and discusses its future applications, emphasizing its potential for producing high-value chemicals through microbial biosynthesis.

Comparison of stress tolerance mechanisms between Saccharomyces cerevisiae and the multistress-tolerant Pichia kudriavzevii

Yeasts play a vital role in both research and industrial biomanufacturing. Saccharomyces cerevisiae has been extensively utilized as a model system. However, its application is often constrained by limited tolerance to the diverse stress conditions encountered in bioprocesses. These challenges have driven increasing interest in nonconventional, multistress-tolerant yeasts as alternative biomanufacturing hosts. This review highlights Pichia kudriavzevii as a promising nonconventional yeast for industrial applications. Unlike S. cerevisiae, P. kudriavzevii exhibits exceptional tolerance to high temperatures, elevated concentrations of furanic and phenolic inhibitors, osmotic stress, salinity, and extreme pH. These traits make it an attractive candidate for industrial processes without requiring extensive genetic modifications to enhance stress resistance. As a result, P. kudriavzevii has emerged as a flagship species for advancing bioeconomy. Despite its industrial potential, the molecular mechanisms underlying P. kudriavzevii's superior stress tolerance remain poorly understood. This review compiles current knowledge on P. kudriavzevii and compares its stress tolerance mechanisms with those of S. cerevisiae, providing insights into its innate resilience. By expanding our understanding of nonconventional yeasts, this review aims to facilitate their broader adoption as robust microbial platforms for industrial biomanufacturing.

Production of (R)-citramalate by engineered Saccharomyces cerevisiae

The budding yeast, Saccharomyces cerevisiae, has a high tolerance to organic acids and alcohols, and thus grows well under toxic concentrations of various compounds in the culture medium, potentially allowing for highly efficient compound production. (R)-citramalate is a raw material for methyl methacrylate and can be used as a metabolic intermediate in the biosynthesis of higher alcohols. (R)-citramalate is synthesized from pyruvate and acetyl-CoA. Unlike Escherichia coli, S. cerevisiae has organelles, and its intracellular metabolites are compartmentalized, preventing full use of intracellular acetyl-CoA. Therefore, in this study, to increase the amount of cytosolic acetyl-CoA for highly efficient production of (R)-citramalate, we inhibited the transport of cytosolic acetyl-CoA and pyruvate to the mitochondria. We also constructed a heterologous pathway to supply cytosolic acetyl-CoA. Additionally, we attempted to export (R)-citramalate from cells by expressing a heterologous dicarboxylate transporter gene. We evaluated the effects of these approaches on (R)-citramalate production and constructed a final strain by combining these positive approaches. The resulting strain produced 16.5 mM (R)-citramalate in batch culture flasks. This is the first report of (R)-citramalate production by recombinant S. cerevisiae, and the (R)-citramalate production by recombinant yeast achieved in this study was the highest reported to date.

Biocontrol agents transform the stability and functional characteristics of the grape phyllosphere microenvironment

The spread of grape leaf diseases has a negative impact on the sustainable development of agriculture. Diseases induced by Uncinula necator significantly affect the quality of grapes. Bacillus biocontrol agents have been proven effective in disease management. However, limited research has been conducted on the impact of biocontrol agents on the assembly and potential functions of plant phyllosphere microbial communities. This study used high-throughput sequencing combined with bioinformatics analysis and culture omics technology for analysis. The results showed that biocontrol bacteria B. subtilis utilized in this study can significantly reduce the disease index of powdery mildew (p<0.05); concurrently, it exhibits a lower disease index compared to traditional fungicides. A comprehensive analysis has revealed that biocontrol bacteria have no significant impact on the diversity of phyllosphere fungi and bacteria, while fungicides can significantly reduce bacterial diversity. Additionally, biocontrol agents can increase the complexity of fungal networks and enhance the degree of modularity and stability of the bacterial network. The results also showed that the biocontrol agents, which contained a high amount of B. subtilis, were able to effectively colonize the grapevine phyllosphere, creating a microenvironment that significantly inhibits pathogenic bacteria on grape leaves while enhancing leaf photosynthetic capacity. In conclusion, biocontrol agents significantly reduce the grape powdery mildew disease index, promote a microenvironment conducive to symbiotic microorganisms and beneficial bacteria, and enhance plant photosynthetic capacity. These findings provide a basis for promoting biocontrol agents and offer valuable insights into sustainable agriculture development.

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

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

Engineering a non-oxidative glycolysis pathway in escherichia coli for high-level citramalate production

BACKGROUND

Methyl methacrylate (MMA) is a key precursor of polymethyl methacrylate, extensively used as a transparent thermoplastic in various industries. Conventional MMA production poses health and environmental risks; hence, citramalate serves as an alternative bacterial compound precursor for MMA production. The highest citramalate titer was previously achieved by Escherichia coli BW25113. However, studies on further improving citramalate production through metabolic engineering are limited, and phage contamination is a persistent problem in E. coli fermentation.

RESULTS

This study aimed to construct a phage-resistant E. coli BW25113 strain capable of producing high citramalate titers from glucose. First, promoters and heterologous cimA genes were screened, and an effective biosynthetic pathway for citramalate was established by overexpressing MjcimA3.7, a mutated cimA gene from Methanococcus jannaschii, regulated by the BBa_J23100 promoter in E. coli. Subsequently, a phage-resistant E. coli strain was engineered by integrating the Ssp defense system into the genome and mutating key components of the phage infection cycle. Then, the strain was engineered to include the non-oxidative glycolysis pathway while removing the acetate synthesis pathway to enhance the supply of acetyl-CoA. Furthermore, glucose utilization by the strain improved, thereby increasing citramalate production. Ultimately, 110.2 g/L of citramalate was obtained after 80 h fed-batch fermentation. The citramalate yield from glucose and productivity were 0.4 g/g glucose and 1.4 g/(L·h), respectively.

CONCLUSION

This is the highest reported citramalate titer and productivity in E. coli without the addition of expensive yeast extract and additional induction in fed-bath fermentation, emphasizing its potential for practical applications in producing citramalate and its derivatives.

Strategies in engineering sustainable biochemical synthesis through microbial systems

Growing environmental concerns and the urgency to address climate change have increased demand for the development of sustainable alternatives to fossil-derived fuels and chemicals. Microbial systems, possessing inherent biosynthetic capabilities, present a promising approach for achieving this goal. This review discusses the coupling of systems and synthetic biology to enable the elucidation and manipulation of microbial phenotypes for the production of chemicals that can substitute for petroleum-derived counterparts and contribute to advancing green biotechnology. The integration of artificial intelligence with metabolic engineering to facilitate precise and data-driven design of biosynthetic pathways is also discussed, along with the identification of current limitations and proposition of strategies for optimizing biosystems, thereby propelling the field of chemical biology towards sustainable chemical production.

Enabling pathway design by multiplex experimentation and machine learning

The remarkable metabolic diversity observed in nature has provided a foundation for sustainable production of a wide array of valuable molecules. However, transferring the biosynthetic pathway to the desired host often runs into inherent failures that arise from intermediate accumulation and reduced flux resulting from competing pathways within the host cell. Moreover, the conventional trial and error methods utilized in pathway optimization struggle to fully grasp the intricacies of installed pathways, leading to time-consuming and labor-intensive experiments, ultimately resulting in suboptimal yields. Considering these obstacles, there is a pressing need to explore the enzyme expression landscape and identify the optimal pathway configuration for enhanced production of molecules. This review delves into recent advancements in pathway engineering, with a focus on multiplex experimentation and machine learning techniques. These approaches play a pivotal role in overcoming the limitations of traditional methods, enabling exploration of a broader design space and increasing the likelihood of discovering optimal pathway configurations for enhanced production of molecules. We discuss several tools and strategies for pathway design, construction, and optimization for sustainable and cost-effective microbial production of molecules ranging from bulk to fine chemicals. We also highlight major successes in academia and industry through compelling case studies.

Sustainable metabolic engineering requires a perfect trifecta

The versatility of cellular metabolism in converting various substrates to products inspires sustainable alternatives to conventional chemical processes. Metabolism can be engineered to maximize the yield, rate, and titer of product generation. However, the numerous combinations of substrate, product, and organism make metabolic engineering projects difficult to navigate. A perfect trifecta of substrate, product, and organism is prerequisite for an environmentally and economically sustainable metabolic engineering endeavor. As a step toward this endeavor, we propose a reverse engineering strategy that starts with product selection, followed by substrate and organism pairing. While a large bioproduct space has been explored, the top-ten compounds have been synthesized mainly using glucose and model organisms. Unconventional feedstocks (e.g. hemicellulosic sugars and CO2) and non-model organisms are increasingly gaining traction for advanced bioproduct synthesis due to their specialized metabolic modes. Judicious selection of the substrate-organism-product combination will illuminate the untapped territory of sustainable metabolic engineering.