Improvement of polyunsaturated fatty acids synthesis by the coexpression of CYB5 with desaturase genes in Saccharomyces cerevisiae

Biotechnological advances in the production of unusual fatty acids in transgenic plants and recombinant microorganisms

Certain plants and microorganisms can produce high amounts of unusual fatty acids (UFAs) such as hydroxy, conjugated, cyclic, and very long-chain polyunsaturated fatty acids, which have distinct physicochemical properties and significant applications in the food, feed, and oleochemical industries. Since many natural sources of UFAs are not ideal for large-scale agricultural production or fermentation, it is attractive to produce them through synthetic biology. Although several UFAs have been commercially or pre-commercially produced in transgenic plants and microorganisms, their contents in transgenic hosts are generally much lower than in natural sources. Moreover, reproducing this success for a wider spectrum of UFAs has remained challenging. This review discusses recent advancements in our understanding of the biosynthesis, accumulation, and heterologous production of UFAs, and addresses the challenges and potential strategies for achieving high UFA content in engineered plants and microorganisms.

Establishment of Saccharomyces cerevisiae as a cell factory for efficient de novo production of monogalactosyldiacylglycerol

Monogalactosyldiacylglycerol (MGDG), a predominant photosynthetic membrane lipid derived from plants and microalgae, has important applications in feed additives, medicine, and other fields. The low content and various structural stereoselectivity differences of MGDG in plants limited the biological extraction or chemical synthesis of MGDG, resulting in a supply shortage of monogalactosyldiacylglycerol with a growing demand. Herein, we established Saccharomyces cerevisiae as a cell factory for efficient de novo production of monogalactosyldiacylglycerol for the first time. Heterologous production of monogalactosyldiacylglycerol was achieved by overexpression of codon-optimized monogalactosyldiacylglycerol synthase gene MGD1, the key Kennedy pathway genes (i.e. GAT1, ICT1, and PAH1), and multi-copy integration of the MGD1 expression cassette. The final engineered strain (MG-8) was capable of producing monogalactosyldiacylglycerol with titers as high as 16.58 nmol/mg DCW in a shake flask and 103.2 nmol/mg DCW in a 5 L fed-batch fermenter, respectively. This is the first report of heterologous biosynthesis of monogalactosyldiacylglycerol in microorganisms, which will provide a favorable reference for study on heterologous production of monogalactosyldiacylglycerol in yeasts.

Tuning Fatty Acid Profile and Yield in Pichia pastoris

Fatty acids have been supplied for diverse non-food, industrial applications from plant oils and animal fats for many decades. Due to the massively increasing world population demanding a nutritious diet and the thrive to provide feedstocks for industrial production lines in a sustainable way, i.e., independent from food supply chains, alternative fatty acid sources have massively gained in importance. Carbohydrate-rich side-streams of agricultural production, e.g., molasses, lignocellulosic waste, glycerol from biodiesel production, and even CO2, are considered and employed as carbon sources for the fermentative accumulation of fatty acids in selected microbial hosts. While certain fatty acid species are readily accumulated in native microbial metabolic routes, other fatty acid species are scarce, and host strains need to be metabolically engineered for their high-level production. We report the metabolic engineering of Pichia pastoris to produce palmitoleic acid from glucose and discuss the beneficial and detrimental engineering steps in detail. Fatty acid secretion was achieved through the deletion of fatty acyl-CoA synthetases and overexpression of the truncated E. coli thioesterase 'TesA. The best strains secreted >1 g/L free fatty acids into the culture medium. Additionally, the introduction of C16-specific ∆9-desaturases and fatty acid synthases, coupled with improved cultivation conditions, increased the palmitoleic acid content from 5.5% to 22%.

De Novo Biosynthesis of Free Vaccenic Acid with a Low Content of Oleic Acid in Saccharomyces cerevisiae

Omega-7 (ω-7) fatty acids have potential application in the fields of nutraceutical, agricultural, and food industry. The natural ω-7 fatty acids are currently from plants or vegetable oils, which are unsustainable and limited by the availability of plant sources. Here, we developed an innovative biosynthetic route to produce vaccenic acid (C18:1 ω-7) while minimizing oleic acid (C18:1 ω-9) content in Saccharomyces cerevisiae. We have engineered S. cerevisiaeto produce C18:1 ω-7 by expressing a fatty acid elongase from Rattus norvegicus. To reduce the content of C18:1 ω-9, the endogenous desaturase Ole1 was replaced by the desaturase, which has specific activity on palmitoyl-coenzyme A (C16:0-CoA). Finally, the production of free C18:1 ω-7 was improved by optimizing the source of cytochrome b5 and overexpressing endoplasmic reticulum chaperones. After combining these strategies, the yield of C18:1 ω-7 was increased from 0 to 9.3 mg/g DCW and C18:1 ω-9 was decreased from 25.2 mg/g DCW to 1.6 mg/g DCW. This work shows a de novo synthetic pathway to produce the highest amount of free C18:1 ω-7 with a low content of C18:1 ω-9 in S. cerevisiae.

Harnessing bioengineered microbes as a versatile platform for space nutrition

Human enterprises through the solar system will entail long-duration voyages and habitation creating challenges in maintaining healthy diets. We discuss consolidating multiple sensory and nutritional attributes into microorganisms to develop customizable food production systems with minimal inputs, physical footprint, and waste. We envisage that a yeast collection bioengineered for one-carbon metabolism, optimal nutrition, and diverse textures, tastes, aromas, and colors could serve as a flexible food-production platform. Beyond its potential for supporting humans in space, bioengineered microbial-based food could lead to a new paradigm for Earth's food manufacturing that provides greater self-sufficiency and removes pressure from natural ecosystems.

Catalytic mechanisms underlying fungal fatty acid desaturases activities

Polyunsaturated fatty acids (PUFAs) have beneficial roles in a variety of human pathologies and disorders. Owing to the limited source of PUFAs in animals and plants, microorganisms, especially fungi, have become a new source of PUFAs. In fungi, fatty acid desaturases (F-FADS) are the main enzymes that convert saturated fatty acids (SFAs) into PUFAs. Their catalytic activities and substrate specificities, which are directly dependent on the structure of the FADS proteins, determine their efficiency to convert SFAs to PUFAs. Catalytic mechanisms underlying F-FADS activities can be determined from the findings of the relationship between their structure and function. In this review, the advances made in the past decade in terms of catalytic activities and substrate specificities of the fungal FADS cluster are summarized. The relationship between the key domain(s) and site(s) in F-FADS proteins and their catalytic activity is highlighted, and the FADS cluster is analyzed phylogenetically. In addition, subcellular localization of F-FADS is discussed. Finally, we provide prospective crystal structures of F-FADSs. The findings may provide a reference for the resolution of the crystal structures of F-FADS proteins and facilitate the increase in fungal PUFA production for human health.

ER stress induced by the OCH1 mutation triggers changes in lipid homeostasis in Kluyveromyces lactis

In Kluyveromyces lactis yeast, OCH1 encodes for the α-1,6-mannosyltrasferase that adds the initial α-1,6-mannose to the outer-chains of N-glycoproteins. Kloch1-1 mutant cells showed altered calcium homeostasis and endoplasmic reticulum (ER) stress. Since ER plays a major role in lipid biosynthesis and lipid droplet (LD) formation, herein the impact of Och1p depletion on lipid homeostasis was investigated. Transcriptional profiles of genes involved in biosynthesis of fatty acids, their amount and composition changed in mutant cells. An increased amount of ergosterol was determined in these cells. Enhanced transcription of genes involved in both synthesis and mobilization of LDs was also found in Kloch1-1 cells, accompanied by a reduced amount of LDs. We provide evidence that ER alterations, determined by protein misfolding as a result of reduced N-glycosylation, induced altered lipid homeostasis in Kloch1-1 cells. Chemical chaperone 4-phenyl butyrate (4-PBA) slightly alleviated the LD phenotype in cells depleted of Och1p. Remarkably, complete suppression of ER stress, via increased expression of plasma membrane calcium channel subunit Mid1, fully restored lipid homeostasis in mutant cells. To further reinforce this finding, low numbers of LDs were observed in wild type cells when ER stress was triggered by DTT treatment.

Depletion of casein kinase I leads to a NAD(P)(+)/NAD(P)H balance-dependent metabolic adaptation as determined by NMR spectroscopy-metabolomic profile in Kluyveromyces lactis

BACKGROUND

In the Crabtree-negative Kluyveromyces lactis yeast the rag8 mutant is one of nineteen complementation groups constituting the fermentative-deficient model equivalent to the Saccharomyces cerevisiae respiratory petite mutants. These mutants display pleiotropic defects in membrane fatty acids and/or cell walls, osmo-sensitivity and the inability to grow under strictly anaerobic conditions (Rag(-) phenotype). RAG8 is an essential gene coding for the casein kinase I, an evolutionary conserved activity involved in a wide range of cellular processes coordinating morphogenesis and glycolytic flux with glucose/oxygen sensing.

METHODS

A metabolomic approach was performed by NMR spectroscopy to investigate how the broad physiological roles of Rag8, taken as a model for all rag mutants, coordinate cellular responses.

RESULTS

Statistical analysis of metabolomic data showed a significant increase in the level of metabolites in reactions directly involved in the reoxidation of the NAD(P)H in rag8 mutant samples with respect to the wild type ones. We also observed an increased de novo synthesis of nicotinamide adenine dinucleotide. On the contrary, the production of metabolites in pathways leading to the reduction of the cofactors was reduced.

CONCLUSIONS

The changes in metabolite levels in rag8 showed a metabolic adaptation that is determined by the intracellular NAD(P)(+)/NAD(P)H redox balance state.

GENERAL SIGNIFICANCE

The inadequate glycolytic flux of the mutant leads to a reduced/asymmetric distribution of acetyl-CoA to the different cellular compartments with loss of the fatty acid dynamic respiratory/fermentative adaptive balance response.

Transcriptional regulation of desaturase genes in Pichia pastoris GS115

Here we investigated the regulation of Pichia pastoris desaturase genes by low temperature and exogenous fatty acids in the late-exponential phase at the transcriptional level. Time-course studies of gene expression showed that mRNA levels of four desaturase genes were rapidly and transiently enhanced by low temperature and suppressed by exogenous oleic acid. Stearic acid showed no obvious repression of mRNA levels of Fad12 and Fad15 and a slight increase in Fad9A and Fad9B mRNA levels. Using a promoter-reporter gene construct, we demonstrated that the pFAD15 promoter activity was induced by low temperature in a time-dependent manner and reduced in a dose- and time-dependent manner by unsaturated fatty acids. Also, there was no absolute correlation between mRNA abundance and production of corresponding fatty acids. Disruption of Spt23 resulted in a decrease in transcript levels of Fad9A and Fad9B, but had little effect on the other desaturase genes. Consistent with these observations, a decrease in the relative amount of oleic acid (OLA) and an increase in the relative content of linoleic acid and ALA with different degrees were clearly observed in the stationary phase cells of ΔSpt23 mutant. Further analysis showed that the effect of low-temperature activation and OLA inhibition on expression of Fad9A and Fad9B seemed to disappear after disruption of the Spt23 gene, which indicated that Spt23p is essential for the expression of two Δ9-desaturase genes internally and probably involved in the regulation of Δ9-desaturase genes transcription in response to external stimuli, and thereby plays a role in the synthesis of OLA.

Engineered high content of ricinoleic acid in fission yeast Schizosaccharomyces pombe

In an effort to produce ricinoleic acid (12-hydroxy-octadeca-cis-9-enoic acid: C18:1-OH) as a petrochemical replacement in a variety of industrial processes, we introduced Claviceps purpurea oleate ∆12-hydroxylase gene (CpFAH12) to Schizosaccharomyces pombe, putting it under the control of inducible nmt1 promoter. Since Fah12p is able to convert oleic acid to ricinoleic acid, we thought that S. pombe, in which around 75% of total fatty acid (FA) is oleic acid, would accordingly be an ideal microorganism for high production of ricinoleic acid. Unfortunately, at the normal growth temperature of 30 °C, S. pombe cells harboring CpFAH12 grew poorly when the CpFAH12 gene expression was induced, perhaps implicating ricinoleic acid as toxic in S. pombe. However, in line with a likely thermoinstability of Fah12p, there was almost no growth inhibition at 37 °C or, by contrast with 30 °C and lower temperatures, ricinoleic acid accumulation. Accordingly, various optimization steps led to a regime with preliminary growth at 37 °C followed by a 5-day incubation at 20 °C, and the level of ricinoleic acid reached 137.4 μg/ml of culture that corresponded to 52.6% of total FA.