Cloning, Expression Analysis and Functional Characterization of Squalene Synthase (SQS) from Tripterygium wilfordii

Molecular characterization and transcriptional regulation analysis of the Torreya grandis squalene synthase gene involved in sitosterol biosynthesis and drought response

The kernel of Torreya grandis cv. 'Merrillii' (Cephalotaxaceae) is a rare nut with a variety of bioactive compounds and a high economic value. β-sitosterol is not only the most abundant plant sterol but also has various biological effects, such as antimicrobial, anticancer, anti-inflammatory, lipid-lowering, antioxidant, and antidiabetic activities. In this study, a squalene synthase gene from T. grandis, TgSQS, was identified and functionally characterized. TgSQS encodes a deduced protein of 410 amino acids. Prokaryotic expression of the TgSQS protein could catalyze farnesyl diphosphate to produce squalene. Transgenic Arabidopsis plants overexpressing TgSQS showed a significant increase in the content of both squalene and β-sitosterol; moreover, their drought tolerance was also stronger than that of the wild type. Transcriptome data from T. grandis seedlings showed that the expression levels of sterol biosynthesis pathway-related genes, such as HMGS, HMGR, MK, DXS, IPPI, FPPS, SQS, and DWF1, increased significantly after drought treatment. We also demonstrated that TgWRKY3 directly bound to the TgSQS promoter region and regulated its expression through a yeast one-hybrid experiment and a dual luciferase experiment. Together, these findings demonstrate that TgSQS has a positive role in β-sitosterol biosynthesis and in protecting against drought stress, emphasizing its importance as a metabolic engineering tool for the simultaneous improvement of β-sitosterol biosynthesis and drought tolerance.

Identification and functional characterization of the dirigent gene family in Phryma leptostachya and the contribution of PlDIR1 in lignan biosynthesis

BACKGROUND

Furofuran lignans, the main insecticidal ingredient in Phryma leptostachya, exhibit excellent controlling efficacy against a variety of pests. During the biosynthesis of furofuran lignans, Dirigent proteins (DIRs) are thought to be dominant in the stereoselective coupling of coniferyl alcohol to form ( ±)-pinoresinol. There are DIR family members in almost every vascular plant, but members of DIRs in P. leptostachya are unknown. To identify the PlDIR genes and elucidate their functions in lignan biosynthesis, this study performed transcriptome-wide analysis and characterized the catalytic activity of the PlDIR1 protein.

RESULTS

Fifteen full-length unique PlDIR genes were identified in P. leptostachya. A phylogenetic analysis of the PlDIRs classified them into four subfamilies (DIR-a, DIR-b/d, DIR-e, and DIR-g), and 12 conserved motifs were found among them. In tissue-specific expression analysis, except for PlDIR7, which displayed the highest transcript abundance in seeds, the other PlDIRs showed preferential expression in roots, leaves, and stems. Furthermore, the treatments with signaling molecules demonstrated that PlDIRs could be significantly induced by methyl jasmonate (MeJA), salicylic acid (SA), and ethylene (ETH), both in the roots and leaves of P. leptostachya. In examining the tertiary structure of the protein and the critical amino acids, it was found that PlDIR1, one of the DIR-a subfamily members, might be involved in the region- and stereo-selectivity of the phenoxy radical. Accordingly, LC-MS/MS analysis demonstrated the catalytic activity of recombinant PlDIR1 protein from Escherichia coli to direct coniferyl alcohol coupling into ( +)-pinoresinol. The active sites and hydrogen bonds of the interaction between PlDIR1 and bis-quinone methide (bisQM), the intermediate in ( +)-pinoresinol formation, were analyzed by molecular docking. As a result, 18 active sites and 4 hydrogen bonds (Asp-42, Ala-113, Leu-138, Arg-143) were discovered in the PlDIR1-bisQM complex. Moreover, correlation analysis indicated that the expression profile of PlDIR1 was closely connected with lignan accumulations after SA treatment.

CONCLUSIONS

The results of this study will provide useful clues for uncovering P. leptostachya's lignan biosynthesis pathway as well as facilitate further studies on the DIR family.

Expression of PnSS Promotes Squalene and Oleanolic Acid (OA) Accumulation in Aralia elata via Methyl Jasmonate (MeJA) Induction

Aralia elata is an important herb due to the abundance of pentacyclic triterpenoid saponins whose important precursors are squalene and OA. Here, we found that MeJA treatment promoted both precursors accumulation, especially the latter, in transgenic A. elata, overexpressing a squalene synthase gene from Panax notoginseng(PnSS). In this study, Rhizobium-mediated transformation was used to express the PnSS gene. Gene expression analysis and high-performance liquid chromatography (HPLC) were used to identify the effect of MeJA on squalene and OA accumulation. The PnSS gene was isolated and expressed in A. elata. Transgenic lines showed a very high expression of the PnSS gene and farnesyl diphosphate synthase gene (AeFPS) and a slightly higher squalene content than the wild-type, but endogenous squalene synthase (AeSS), squalene epoxidase (AeSE), and β-amyrin synthase (Aeβ-AS) gene were decreased as well as OA content. Following one day of MeJA treatment, the expression levels of PeSS, AeSS, and AeSE genes increased significantly. On day 3, the maximum content of both products reached 17.34 and 0.70 mg·g^-1, which increased 1.39- and 4.90-fold than in the same lines without treatment. Transgenic lines expressing PnSS gene had a limited capability to promote squalene and OA accumulation. MeJA strongly activated their biosynthesis pathways, leading to enhance yield.

Transcription Factors ZjMYB39 and ZjMYB4 Regulate Farnesyl Diphosphate Synthase- and Squalene Synthase-Mediated Triterpenoid Biosynthesis in Jujube

Jujube (Ziziphus jujuba Mill.) is rich in valuable bioactive triterpenoids. However, the regulatory mechanism underlying triterpenoid biosynthesis in jujube remains poorly studied. Here, we characterized the triterpenoid content in wild jujube and cultivated jujube. The triterpenoid content was higher in wild jujube than in cultivated jujube, triterpenoids were most abundant in young leaves, buds, and later stages of development. The transcriptome analysis and correlation analysis showed that differentially expressed genes (DEGs) were enriched in the terpenoid synthesis pathways, and triterpenoids content was strongly correlated with farnesyl diphosphate synthase (ZjFPS), squalene synthase (ZjSQS), and transcription factors ZjMYB39 and ZjMYB4 expression. Gene overexpression and silencing analysis indicated that ZjFPS and ZjSQS were key genes in triterpenoid biosynthesis and transcription factors ZjMYB39 and ZjMYB4 regulated triterpenoid biosynthesis. Subcellular localization experiments showed that ZjFPS and ZjSQS were localized to the nucleus and endoplasmic reticulum and ZjMYB39 and ZjMYB4 were localized to the nucleus. Yeast one-hybrid, glucuronidase activity, and dual-luciferase activity assays suggested that ZjMYB39 and ZjMYB4 regulate triterpenoid biosynthesis by directly binding and activating the promoters of ZjFPS and ZjSQS. These findings provide insights into the underlying regulatory network of triterpenoids metabolism in jujube and lay theoretical and practical foundation for molecular breeding.

Structural and Molecular Characterization of Squalene Synthase Belonging to the Marine Thraustochytrid Species Aurantiochytrium limacinum Using Bioinformatics Approach

The marine microorganisms thraustochytrids have been explored for their potential in the production of various bioactive compounds, such as DHA, carotenoids, and squalene. Squalene is a secondary metabolite of the triterpenoid class and is known for its importance in various industrial applications. The bioinformatic analysis for squalene synthase (SQS) gene (the first key enzyme in the tri-terpenoid synthesis pathway), that is prevailing among thraustochytrids, is poorly investigated. In-silico studies combining sequence alignments and bioinformatic tools helped in the preliminary characterization of squalene synthases found in Aurantiochytrium limacinum. The sequence contained highly conserved regions for SQS found among different species indicated the enzyme had all the regions for its functionality. The signal peptide sequence and transmembrane regions were absent, indicating an important aspect of the subcellular localization. Secondary and 3-D models generated using appropriate templates demonstrated the similarities with SQS of the other species. The 3-D model also provided important insights into possible active, binding, phosphorylation, and glycosylation sites.

Characterization of squalene synthase gene from Gymnema sylvestre R. Br

Background

Squalene synthase (SQS) is a rate-limiting enzyme necessary to produce pentacyclic triterpenes in plants. It is an important enzyme producing squalene molecules required to run steroidal and triterpenoid biosynthesis pathways working in competitive inhibition mode. Reports are available on information pertaining to SQS gene in several plants, but detailed information on SQS gene in Gymnema sylvestre R. Br. is not available. G. sylvestre is a priceless rare vine of central eco-region known for its medicinally important triterpenoids. Our work aims to characterize the GS-SQS gene in this high-value medicinal plant.

Results

Coding DNA sequences (CDS) with 1245 bp length representing GS-SQS gene predicted from transcriptome data in G. sylvestre was used for further characterization. The SWISS protein structure modeled for the GS-SQS amino acid sequence data had MolProbity Score of 1.44 and the Clash Score 3.86. The quality estimates and statistical score of Ramachandran plots analysis indicated that the homology model was reliable. For full-length amplification of the gene, primers designed from flanking regions of CDS encoding GS-SQS were used to get amplification against genomic DNA as template which resulted in approximately 6.2-kb sized single-band product. The sequencing of this product through NGS was carried out generating 2.32 Gb data and 3347 number of scaffolds with N50 value of 457 bp. These scaffolds were compared to identify similarity with other SQS genes as well as the GS-SQSs of the transcriptome. Scaffold_3347 representing the GS-SQS gene harbored two introns of 101 and 164 bp size. Both these intronic regions were validated by primers designed from adjoining outside regions of the introns on the scaffold representing GS-SQS gene. The amplification took place when the template was genomic DNA and failed when the template was cDNA confirmed the presence of two introns in GS-SQS gene in Gymnema sylvestre R. Br.

Conclusion

This study shows GS-SQS gene was very closely related to Coffea arabica and Gardenia jasminoides and this gene harbored two introns of 101 and 164 bp size.

Molecular cloning and functional characterization of two squalene synthase genes in Atractylodes lancea

Two squalene synthase genes AlSQS1 and AlSQS2 were isolated from Atractylodes lancea and functionally characterized using in vitro enzymatic reactions. Atractylodes lancea is a traditional herb used for the treatment of rheumatic diseases, gastric disorders, and influenza. Its major active ingredients include sesquiterpenoids and triterpenes. Squalene synthase (SQS; EC 2.5.1.21) catalyzes the first enzymatic step in the central isoprenoid pathway towards sterol and triterpenoid biosynthesis. In this study, we aimed to investigate two SQSs from A. lancea using cloning and in vitro enzymatic characterization. Bioinformatics and phylogenetic analyses revealed that the AlSQSs exhibited high homology with other plant SQSs. Furthermore, AlSQS1 was observed to be localized in both the nucleus and cytoplasm, whereas AlSQS2 was localized in the cytoplasm and endoplasmic reticulum. To obtain soluble recombinant enzymes, AlSQS1 and AlSQS2 were successfully expressed as glutathione S-transferase (GST)-tagged fusion proteins in Escherichia coli Transetta (DE3). Approximately 68 kDa recombinant proteins were obtained using GST-tag affinity chromatography and Western blot analysis. Results of the in vitro enzymatic reactions established that both AlSQS1 and AlSQS2 were functional, which verifies their catalytic ability in converting two farnesyl pyrophosphates to squalene. The expression patterns of AlSQS and selected terpenoid genes were also investigated in two A. lancea chemotypes using available RNA sequencing data. AlSQS1 and AlSQS2, which showed relatively similar expression in the three tissues, were more highly expressed in the stems than in the leaves and rhizomes. Methyl jasmonate (MeJA) was used as an elicitor to analyze the expression profiles of AlSQSs. The results of qRT-PCR analysis revealed that the gene expression of AlSQS1 and AlSQS2 plummeted at lowest value at 12 h and reached its peak at 24 h. This study is the first report on the cloning, characterization, and expression of SQSs in A. lancea. Therefore, our findings contribute novel insights that may be useful for future studies regarding terpenoid biosynthesis in A. lancea.

Cloning and Functional Characterization of a Squalene Synthase from Paris polyphylla var. yunnanensis

Paris polyphylla Smith var. yunnanensis (Franch.) Hand. - Mazz. is a precious traditional Chinese medicine, and steroidal saponins are its major bioactive constituents possessing extensive biological activities. Squalene synthase (SQS) catalyzes the first dedicated step converting two molecular of farnesyl diphosphate (FDP) into squalene, a key intermediate in the biosynthetic pathway of steroidal saponins. In this study, a squalene synthase gene (PpSQS1) was cloned and functionally characterized from P. polyphylla var. yunnanensis, representing the first identified SQS from the genus Paris. The open reading frame of PpSQS1 is 1239 bp, which encodes a protein of 412 amino acids showing high similarity to those of other plant SQSs. Expression of PpSQS1 in Escherichia coli resulted in production of soluble recombinant proteins. Gas chromatography-mass spectrometry analysis showed that the purified recombinant PpSQS1 protein could produce squalene using FDP as a substrate in the in vitro enzymatic assay. qRT-PCR analysis indicated that PpSQS1 was highly expressed in rhizomes, consistent with the dominant accumulation of steroidal saponins there, suggesting that PpSQS1 is likely involved in the biosynthesis of steroidal saponins in the plant. The findings lay a foundation for further investigation on the biosynthesis and regulation of steroidal saponins, and also provide an alternative gene for manipulation of steroid production using synthetic biology.

Biosynthesis, total synthesis, structural modifications, bioactivity, and mechanism of action of the quinone-methide triterpenoid celastrol

Celastrol, a quinone-methide triterpenoid, was extracted from Tripterygium wilfordii Hook. F. in 1936 for the first time. Almost 70 years later, it is considered one of the molecules most likely to be developed into modern drugs, as it exhibits notable bioactivity, including anticancer and anti-inflammatory activity, and exerts antiobesity effects. In addition, the molecular mechanisms underlying its bioactivity are being widely studied, which offers new avenues for its development as a pharmaceutical reagent. Owing to its potential therapeutic effects and unique chemical structure, celastrol has attracted considerable interest in the fields of organic, biosynthesis, and medicinal chemistry. As several steps in the biosynthesis of celastrol have been revealed, the mechanisms of key enzymes catalyzing the formation and postmodifications of the celastrol scaffold have been gradually elucidated, which lays a good foundation for the future heterogeneous biosynthesis of celastrol. Chemical synthesis is also an effective approach to obtain celastrol. The total synthesis of celastrol was realized for the first time in 2015, which established a new strategy to obtain celastroid natural products. However, owing to the toxic effects and suboptimal pharmacological properties of celastrol, its clinical applications remain limited. To search for drug-like derivatives, several structurally modified compounds were synthesized and tested. This review focuses primarily on the latest research progress in the biosynthesis, total synthesis, structural modifications, bioactivity, and mechanism of action of celastrol. We anticipate that this paper will facilitate a more comprehensive understanding of this promising compound and provide constructive references for future research in this field.

Molecular Cloning and Functional Identification of a Squalene Synthase Encoding Gene from Alfalfa (Medicago sativa L.)

The quality of alfalfa, a main forage legume worldwide, is of great importance for the dairy industry and is affected by the content of triterpene saponins. These natural terpenoid products of triterpene aglycones are catalyzed by squalene synthase (SQS), a highly conserved enzyme present in eukaryotes. However, there is scare information on alfalfa SQS. Here, an open reading frame (ORF) of SQS was cloned from alfalfa. Sequence analysis showed MsSQS had the same exon/intron composition and shared high homology with its orthologs. Bioinformatic analysis revealed the deduced MsSQS had two transmembrane domains. When transiently expressed, GFP-MsSQS fusion protein was localized on the plasma membrane of onion epidermal cells. Removal of the C-terminal transmembrane domain of MsSQS improved solubility in Escherichia coli. MsSQS was preferably expressed in roots, followed by leaves and stems. MeJA treatment induced MsSQS expression and increased the content of total saponins. Overexpression of MsSQS in alfalfa led to the accumulation of total saponins, suggesting a correlation between MsSQS expression level with saponins content. Therefore, MsSQS is a canonical squalene synthase and contributes to saponin synthesis in alfalfa. This study provides a key candidate gene for genetic manipulation of the synthesis of triterpene saponins, which impact both plant and animal health.

Validation of suitable reference genes for quantitative gene expression analysis in Tripterygium wilfordii

Validation of suitable reference genes is critical in quantitative real-time polymerase chain reaction (qRT-PCR) analysis. Suitable and reliable reference genes for the normalization of gene expression data are characterized by high gene expression stability across tissues and different experimental conditions. This study evaluated the gene expression stability of ten reference genes commonly used in Arabidopsis thaliana for their suitability in qRT-PCR analysis in Tripterygium wilfordii Hook.f. The orthologous sequences of these ten candidate genes were identified from T. wilfordii transcriptomic data (Project No. SRX472292). Five algorithms including GeNorm, NormFinder, BestKeeper, ΔCt, and RefFinder were used to assess the gene expression stability of these putative reference genes in different plant tissues and different stress conditions. The results identified ACTINT7 and TBP as the most suitable reference genes across all samples. The gene expressions of TwHMGR (3-hydroxy-3-methylglutaryl coenzyme A reductase, KU246037.1) and of TwDXR (1-deoxy-D-xylulose-5-phosphate reductoisomerase, KJ174341.1) were investigated to validate the suitability of the reference genes. The validation analysis confirmed the suitability of ACTINT7 and TBP as the best reference genes for elucidating secondary metabolite biosynthesis pathway in T. wilfordii. In summary, this study identified the most suitable and reliable reference genes for future qRT-PCR- based studies in T. wilfordii.