Engineering a non-model yeast Rhodotorula mucilaginosa for terpenoids synthesis

Terpenoids have tremendous biological activities and are widely employed in food, healthcare and pharmaceutical industries. Using synthetic biology to product terpenoids from microbial cell factories presents a promising alternative route compared to conventional methods such as chemical synthesis or phytoextraction. The red yeast Rhodotorula mucilaginosa has been widely studied due to its natural production capacity of carotenoid and lipids, indicating a strong endogenous isoprene pathway with readily available metabolic intermediates. This study constructed several engineered strains of R. mucilaginosa with the aim of producing different terpenoids. Monoterpene α-terpineol was produced by expressing the α-terpineol synthase from Vitis vinifera. The titer of α-terpineol was further enhanced to 0.39 mg/L by overexpressing the endogenous rate-limiting gene of the MVA pathway. Overexpression of α-farnesene synthase from Malus domestica, in combination with MVA pathway rate-limiting gene resulted in significant increase in α-farnesene production, reaching a titer of 822 mg/L. The carotenoid degradation product β-ionone was produced at a titer of 0.87 mg/L by expressing the β-ionone synthase from Petunia hybrida. This study demonstrates the potential of R. mucilaginosa as a platform host for the direct biosynthesis of various terpenoids and provides insights for further development of such platforms.

Superficial scald development in 'Granny Smith' and 'Nicoter' apples: The role of key volatile compounds when fruit are stored under dynamic controlled atmosphere

A positive correlation of α-farnesene and its oxidation metabolites with superficial scald is commonly reported in apples stored in air or controlled atmosphere (CA) systems, where O2 levels are above the lower oxygen limit (LOL) tolerated by the fruit. Nevertheless, the LOL can be monitored by the dynamic controlled atmosphere (DCA) techniques and to provide different physiological responses. Therefore, this study aimed to evaluate key volatile metabolites from 'Granny Smith' and 'Nicoter' ('Kanzi®') apples stored under dynamic controlled atmosphere (DCA) monitored by respiratory quotient (RQ), i. e. at extremely low oxygen partial pressures (ELO) and correlate their emissions with the incidence of superficial scald (SS). The volatile compounds (VCs) were isolated by solid phase microextraction (HS-SPME) and analyzed by gas chromatography. For the first time, higher concentrations of α-farnesene and its oxidation metabolites (6-methyl-5-hepten-2-one, and 6-methyl-5-hepten-2-ol) were negatively correlated with SS in DCA-RQ. This is likely due to the higher levels of ethanol in fruit stored under this condition having an inhibitory effect on SS incidence even when α-farnesene, 6-methyl-5-hepten-2-one, and 6-methyl-5-hepten-2-ol accumulate. Additionally, SS is more correlated to internal ethylene concentration (IEC) than α-farnesene accumulation and their metabolites, even when fruit were stored under ELO, where ethylene action is reduced.

The hops (Humulus lupulus) genome contains a mid-sized terpene synthase family that shows wide functional and allelic diversity

BACKGROUND

Hops (Humulus lupulus L.) are a dioecious climbing perennial, with the dried mature "cones" (strobili) of the pistillate/female inflorescences being widely used as both a bittering agent and to enhance the flavour of beer. The glandular trichomes of the bract and bracteole flowering structures of the cones produce an abundance of secondary metabolites, such as terpenoids, bitter acids and prenylated phenolics depending on plant genetics, developmental stage and environment. More knowledge is required on the functional and allelic diversity of terpene synthase (TPS) genes responsible for the biosynthesis of volatile terpenes to assist in flavour-directed hop breeding.

RESULTS

Major volatile terpene compounds were identified using gas chromatography-mass spectrometry (GC-MS) in the ripe cones of twenty-one hop cultivars grown in New Zealand. All cultivars produced the monoterpene β-myrcene and the sesquiterpenes α-humulene and β-caryophyllene, but the quantities varied broadly. Other terpenes were found in large quantities in only a smaller subset of cultivars, e.g. β-farnesene (in seven cultivars) and α-pinene (in four). In four contrasting cultivars (Wakatu™, Wai-iti™, Nelson Sauvin™, and 'Nugget'), terpene production during cone development was investigated in detail, with concentrations of some of the major terpenes increasing up to 1000-fold during development and reaching maximal levels from 50-60 days after flowering. Utilising the published H. lupulus genome, 87 putative full-length and partial terpene synthase genes were identified. Alleles corresponding to seven TPS genes were amplified from ripe cone cDNA from multiple cultivars and subsequently functionally characterised by transient expression in planta. Alleles of the previously characterised HlSTS1 produced humulene/caryophyllene as the major terpenes. HlRLS alleles produced (R)-(-)-linalool, whilst alleles of two sesquiterpene synthase genes, HlAFS1 and HlAFS2 produced α-farnesene. Alleles of HlMTS1, HlMTS2 and HlTPS1 were inactive in all the hop cultivars studied.

CONCLUSIONS

Alleles of four TPS genes were identified and shown to produce key aroma volatiles in ripe hop cones. Multiple expressed but inactive TPS alleles were also identified, suggesting that extensive loss-of-function has occurred during domestication and breeding of hops. Our results can be used to develop hop cultivars with novel/improved terpene profiles using marker-assisted breeding strategies to select for, or against, specific TPS alleles.

Phase-Separated Multienzyme Compartmentalization for Terpene Biosynthesis in a Prokaryote

Liquid-liquid phase separation (LLPS) forms biomolecular condensates or coacervates in cells. Metabolic enzymes can form phase-separated subcellular compartments that enrich enzymes, cofactors, and substrates. Herein, we report the construction of synthetic multienzyme condensates that catalyze the biosynthesis of a terpene, α-farnesene, in the prokaryote E. coli. RGGRGG derived from LAF-1 was used as the scaffold protein to form the condensates by LLPS. Multienzyme condensates were then formed by assembling two enzymes Idi and IspA through an RIAD/RIDD interaction. Multienzyme condensates constructed inside E. coli cells compartmentalized the cytosolic space into regions of high and low enzyme density and led to a significant enhancement of α-farnesene production. This work demonstrates LLPS-driven compartmentalization of the cytosolic space of prokaryotic cells, condensation of a biosynthetic pathway, and enhancement of the biosynthesis of α-farnesene.

α-Farnesene production from lipid by engineered Yarrowia lipolytica

Producing high value-added products from waste lipid feedstock by microbial cell factory has great advantages to minimize the pollution as well as improve the economic value of wasted oils and fats. Yarrowia lipolytica is a non-conventional oleaginous yeast and can grow on a variety of hydrophobic substrates. In this study, we explored its ability to synthesize α-farnesene, an important sesquiterpene, using lipid feedstock. Based on the α-farnesene production strain, we constructed previously, we identified that Erg12 was the key limiting factor to further increase the α-farnesene production. The α-farnesene production was improved by 35.8% through increasing the copy number of ERG12 and FSERG20 on oleic acid substrate. Expression of heterologous VHb further improved α-farnesene production by 12.7%. Combining metabolic engineering with the optimization of fermentation conditions, the α-farnesene titer and yield reached 10.2 g/L and 0.1 g/g oleic acid, respectively, in fed-batch cultivation. The α-farnesene synthesis ability on waste cooking oil and other edible oils were also explored. Compared with using glucose as carbon source, using lipid substrates obtained higher α-farnesene yield and titer, but lower by-products accumulation, demonstrating the advantage of Y. lipolytica to synthesize high value-added products using lipid feedstock.

Aloe vera gel coating aggravates superficial scald incidence in 'Starking' apples during low-temperature storage

The effects of aloe vera (Aloe vera (L.) Burm. f.) gel treatment on the incidence of superficial scald in 'Starking' apples (Malus domestica Borkh. Var. Starking) during cold storage were studied. Apples were harvested at the pre-climacteric stage and treated with aloe vera gel. The treatment increased malondialdehyde content and membrane lipid damage. Furthermore, it inhibited the release of ethylene at the early stage but increased it in the later stage. The expression level of ACC synthase 1 (MdACS1) also increased, and the antioxidant capacity in apples, particularly, catalase, peroxidase, and superoxide dismutase activities, all decreased, while concomitantly, the content of α-farnesene and its oxidation product, conjugated triene increased, thereby aggravating superficial scald incidence during storage at low temperature.

Engineering the oleaginous yeast Yarrowia lipolytica for production of α-farnesene

BACKGROUND

Yarrowia lipolytica, a non-traditional oil yeast, has been widely used as a platform for lipid production. However, the production of other chemicals such as terpenoids in engineered Y. lipolytica is still low. α-Farnesene, a sesquiterpene, can be used in medicine, bioenergy and other fields, and has very high economic value. Here, we used α-farnesene as an example to explore the potential of Y. lipolytica for terpenoid production.

RESULTS

We constructed libraries of strains overexpressing mevalonate pathway and α-farnesene synthase genes by non-homologous end-joining (NHEJ) mediated integration into the Y. lipolytica chromosome. First, a mevalonate overproduction strain was selected by overexpressing relevant genes and changing the cofactor specificity. Based on this strain, the downstream α-farnesene synthesis pathway was overexpressed by iterative integration. Culture conditions were also optimized. A strain that produced 25.55 g/L α-farnesene was obtained. This is the highest terpenoid titer reported in Y. lipolytica.

CONCLUSIONS

Yarrowia lipolytica is a potentially valuable species for terpenoid production, and NHEJ-mediated modular integration is effective for expression library construction and screening of high-producer strains.

High toxicity of camphene and γ-elemene from Wedelia prostrata essential oil against larvae of Spodoptera litura (Lepidoptera: Noctuidae)

The development of eco-friendly biopesticides in the fight against agricultural pests is an important challenge nowadays. The essential oil of Wedelia prostrata Hemsl. is known for its multipurpose biological activities in Chinese folk medicine. However, limited efforts attempted to understand the potential insecticidal activity of its main individual constituents and related mechanism of action. In this research, we investigated the insecticidal activity of W. prostrata against the crop pest Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae). Toxicity on 4th instar larvae was assessed after 24 h of exposure. The LC₅₀ and LC₉₀ of the W. prostrata essential oil were 167.46 and 322.12 μg/ml, respectively. GC-MS analyses were carried out to shed light on the oil chemical composition. Main constituents were the monoterpene camphene (9.6%) and the sesquiterpenes γ-elemene (7.6%), α-humulene (6.9%), and (E,E)-α-farnesene (7.3%). The pure constituents were evaluated for their insecticidal activity on S. litura 4th instar larvae. The most toxic molecule was camphene (LC₅₀ = 6.28 μg/ml), followed by γ-elemene, (LC₅₀ = 10.64 μg/ml), α-humulene (LC₅₀ = 12.89 μg/ml), and (E,E)-α-farnesene (LC₅₀ = 16.77 μg/ml). Overall, our experiments highlighted the promising potential of camphene and γ-elemene from W. prostrata essential oil against larvae of S. litura, allowing us to propose these two compounds for the development of newer pesticides in the fight against crop pests. Further studies aimed at evaluating the potential synergy between these two molecules, as well as their stability in field conditions are ongoing.

Ozonolysis of α/β-farnesene mixture: analysis of gas-phase and particulate reaction products

Atmospheric oxidation of sesquiterpenes has been of considerable interest recently because of their likely contribution to ambient organic aerosol, but farnesene oxidation has been reported in only a few studies and with limited data. In the present study, a detailed chemical analysis of the organic fraction of gas and particle phases originating from the ozonolysis of a mixture of α-farnesene and β-farnesene was carried out in a 14.5 m³ smog chamber. More than 80 organic compounds bearing OH functionality were detected for the first time in this system in the gas and particle phases. The major secondary organic aerosol (SOA) components included conjugated α-farnesene trienols, hydroxyl carboxylic acid and its corresponding lactones, C3-C7 linear dicarboxylic acids, and hydroxy/carbonyl/carboxylic compounds. Of particular importance was 5,6-dihydroxy-6-methylheptan-2-one (DHMHO), which was detected at high concentration. In the gas phase, the main species identified were trienols and their corresponding epoxides and diepoxides. Proposed reaction schemes are provided for selected compounds. A similar analysis was performed for ambient PM_2.5 samples collected during summer 2013 as part of the SOAS to determine farnesene contributions to PM_2.5. Gas chromatography-mass spectrometry analysis were consistent with the occurrence of several farnesene SOA compounds, indicating the potential impact of farnesene on the regional aerosol burden. The high abundance of DHMHO in chamber SOA and its presence in ambient PM_2.5 is particularly important because to our knowledge it is specific to farnesene and therefore could serve as an indicator for farnesene emitted into ambient aerosol. In the absence of authentic standards, however, it is difficult to accurately quantify the contribution of SOA originating from farnesene to ambient PM_2.5.

A genetic screen for increasing metabolic flux in the isoprenoid pathway of Saccharomyces cerevisiae: Isolation of SPT15 mutants using the screen

A genetic screen to identify mutants that can increase flux in the isoprenoid pathway of yeast has been lacking. We describe a carotenoid-based visual screen built with the core carotenogenic enzymes from the red yeast Rhodosporidium toruloides. Enzymes from this yeast displayed the required, higher capacity in the carotenoid pathway. The development also included the identification of the metabolic bottlenecks, primarily phytoene dehydrogenase, that was subjected to a directed evolution strategy to yield more active mutants. To further limit phytoene pools, a less efficient version of GGPP synthase was employed. The screen was validated with a known flux increasing gene, tHMG1. New mutants in the TATA binding protein SPT15 were isolated using this screen that increased the yield of carotenoids, and an alternate isoprenoid, α-Farnesene confirming increase in overall flux. The findings indicate the presence of previously unknown links to the isoprenoid pathway that can be uncovered using this screen.

Effect of sunlight-exposure on antioxidants and antioxidant enzyme activities in 'd'Anjou' pear in relation to superficial scald development

Influence of preharvest sunlight exposure on superficial scald development in 'd'Anjou' pears during cold storage was investigated. The biochemical changes related to scald including α-farnesene, conjugated trienols (CTols), antioxidants, antioxidant enzyme activities were monitored among separated blushed and shaded peels of unbagged fruit as well as the whole peel of bagged fruit. In unbagged fruit, scald symptom was restricted to shaded peel; while there was no difference in α-farnesene between blushed and shaded peels, CTols increased significantly in shaded peel along with scald development after 3months storage. Bagging treatment increased both α-farnesene and CTols significantly and enhanced scald. Preharvest sunlight exposure significantly increased certain antioxidant contents and antioxidant enzyme activities in blushed peel at harvest and during storage. These results reveal a direct role of CTols during development of scald, however, antioxidant systems may play an important role in α-farnesene oxidation to CTols and scald susceptibility in 'd'Anjou pears.

Anticariogenic and phytochemical evaluation of Eucalyptus globules Labill

In the present study, in vitro anticariogenic potential of ethyl acetate, hexane and methanol and aqueous extracts of plant leaves of Eucalyptus globules Labill. were evaluated by using four cariogenic bacteria, Lactobacillus acidophilus, Lactobacillus casei, Staphylococcus aureus and Streptococcus mutans. Agar well diffusion method and minimum inhibitory concentration (MIC) were used for this purpose. The ethyl acetate extracted fraction of plant leaves showed good inhibitory effects against all selected bacteria. In Eucalyptus globules, hexane and ethyl acetate extracts found highly effective against, Lactobacillus acidophilus with MIC value of 0.031 and 0.062 mg/mL, respectively. Qualitative phytochemical investigation of above extracts showed the presence of alkaloids, phenolic compounds, steroids, cardiac glycosides and terpenes. Based on the MIC value and bioautography, ethyl acetate of plant leaf was selected for further study. Further investigation on the structure elucidation of the bioactive compound using IR, GC-MS and NMR techniques revealed the presence of alpha-farnesene, a sesquiterpene. Eucalyptus globules plant leaf extracts have great potential as anticariogenic agents that may be useful in the treatment of oral disease.