Molecular basis of ligand recognition specificity of flavone glucosyltransferases in Nemophila menziesii

Of the more than 100 families of glycosyltransferases, family 1 glycosyltransferases catalyze glycosylation using uridine diphosphate (UDP)-sugar as a sugar donor and are thus referred to as UDP-sugar:glycosyl transferases. The blue color of the Nemophila menziesii flower is derived from metalloanthocyanin, which consists of anthocyanin, flavone, and metal ions. Flavone 7-O-β-glucoside-4'-O-β-glucoside in the plant is sequentially biosynthesized from flavons by UDP-glucose:flavone 4'-O-glucosyltransferase (NmF4'GT) and UDP-glucose:flavone 4'-O-glucoside 7-O-glucosyltransferase (NmF4'G7GT). To identify the molecular mechanisms of glucosylation of flavone, the crystal structures of NmF4'G7GT in its apo form and in complex with UDP-glucose or luteolin were determined, and molecular structure prediction using AlphaFold2 was conducted for NmF4'GT. The crystal structures revealed that the size of the ligand-binding pocket and interaction environment for the glucose moiety at the pocket entrance plays a critical role in the substrate preference in NmF4'G7GT. The substrate specificity of NmF4'GT was examined by comparing its model structure with that of NmF4'G7GT. The structure of NmF4'GT may have a smaller acceptor pocket, leading to a substrate preference for non-glucosylated flavones (or flavone aglycones).

Structural Determination of Glucosyltransferase C by Cryo-Electron Microscopy

Biofilm formation is a critical factor in the development of cariogenic virulence of Streptococcus mutans. S. mutans has evolved a concerted mechanism to synthesize a biofilm matrix from dietary sugars by a family of glucosyltransferases (Gtfs). Three Gtfs, GtfB, C, and D utilize sucrose to form a sticky polymer consisting of insoluble and soluble glucans, a biofilm foundation. Each Gtf possesses two distinct domains, an N-terminal enzyme catalytic domain and a C-terminal glucan binding domain. X-ray crystallographic studies have determined a three-dimensional structure of the catalytic domain of GtfC, however, the structure of the C-terminal domain and the overall structure of Gtfs are unknown. Here, we provided a protocol that will guide us to solve Gtf structures using cryo-electron microscopy (cryo-EM). Cryo-EM and X-ray crystallography are two widely used techniques for determining protein structures, and both methods have their own advantages and limitations. Additionally, we also predicted the full-length GtfC structural using AlphaFold2. Overall, the combination of AlphaFold 2 with experimental approaches offers a powerful and synergistic strategy for protein structure determination, accelerating the pace of scientific discovery and enabling new insights into the molecular basis that govern the biosynthetic activities of Gtfs, which may inform the design of more effective treatments for cariogenic biofilms and associated diseases.

Molecular Identification of UDP-Sugar Dependent Glycosyltransferase and Acyltransferase Involved in the Phenylethanoid Glycoside Biosynthesis Induced by Methyl Jasmonate in Sesamum indicum L

Sesame (Sesamum indicum L.) plant contains large amounts of acteoside, a typical phenylethanoid glycoside (PhG) that exhibits various pharmacological activities. Although there is increasing interest in the biosynthesis of PhGs for improving their production, the pathway remains to be clarified. In this study, we established sesame-cultured cells and performed transcriptome analysis of methyl jasmonate (MeJA)-treated cultured cells to identify enzyme genes responsible for glucosylation and acylation in acteoside biosynthesis. Among the genes annotated as UDP-sugar dependent glycosyltransferase (UGT) and acyltransferase (AT), 34 genes and one gene, respectively, were up-regulated by MeJA in accordance with acteoside accumulation. Based on phylogenetic analysis, five UGT genes (SiUGT1-5) and one AT gene (SiAT1) were selected as candidate genes involved in acteoside biosynthesis. Additionally, two AT genes (SiAT2-3) were selected based on sequence identity. Enzyme assays using recombinant SiUGT proteins revealed that SiUGT1, named UGT85AF10, had the highest glucosyltransferase activity among the five candidates against hydroxytyrosol to produce hydroxytyrosol 1-O-glucoside. SiUGT1 also exhibited glucosyltransferase activity against tyrosol to produce salidroside (tyrosol 1-O-glucoside). SiUGT2, named UGT85AF11, had similar activity against hydroxytyrosol and tyrosol. Enzyme assay using the recombinant SiATs indicated that SiAT1 and SiAT2 had activity transferring the caffeoyl group to hydroxytyrosol 1-O-glucoside and salidroside (tyrosol 1-O-glucoside) but not to decaffeoylacteoside. The caffeoyl group was attached mainly at the 4-positon of glucose of hydroxytyrosol 1-O-glucoside, followed by attachment at the 6-position and the 3-position of glucose. Based on our results, we propose an acteoside biosynthetic pathway induced by MeJA treatment in sesame.

Functional identification of anthocyanin glucosyltransferase genes: a Ps3GT catalyzes pelargonidin to pelargonidin 3-O-glucoside painting the vivid red flower color of Paeonia

Glycosylation from an anthocyanidin 3-O-glucosyltransferase Ps3GT (PsUGT78A27) facilitates the accumulation of pelargonidin 3-O-glucoside, which defines the vivid red flower color and occurs only in specific peony tree cultivars. Although tree peony cultivars of Chinese and Japanese both originated from China, vivid red color is only found in flowers of Japanese cultivars but not of Chinese cultivar groups. In this study, a Japanese tree peony cultivar 'Taiyoh' with vivid red petals and a Chinese tree peony cultivar 'Hu Hong' with reddish pink petals were chosen as the experimental materials. Flavonoids profiling indicated that pelargonidin 3-O-glucoside (Pg3G) detected only in Japanese cultivar contributed to vivid red color of tree peony petals, while pelargonidin 3,5-di-O-glucoside (Pg3G5G) found in both of Japanese and Chinese cultivars was responsible for pink flower color. Through the integration of full-length transcriptome sequencing and in vitro enzymatic activity analysis, two anthocyanin glucosyltransferase genes PsUGT78A27 and PsUGT75L45 were isolated from the petals of tree peony, and their encoding products exhibited enzymatic activities of pelargonidin 3-O-glucosyltransferase and anthocyanin 5-O-glucosyltransferase, respectively. Further quantitative real-time PCR revealed that PsUGT78A27 displayed high expression in petals of both cultivars and PsUGT75L45 was expressed at high levels in cultivar 'Hu Hong' only. Using a gene gun technique, the GFP fusion proteins of PsUGT78A27 and PsUGT75L45 were visualized to be cytoplasmic and nuclear localization in the epidermal cells of tree peony petals, and the glucosylation function of PsUGT78A27 and PsUGT75L45 to alter petal color of tree peony and herbaceous peony had been directly validated in vivo. These results demonstrated that PsUGT78A27 and PsUGT75L45 are key players for the presence or absence of vivid red flower color in tree peony cultivars. Our findings further elucidated the chemical and molecular mechanism of petal pigmentation of Paeonia and could help breed the Paeonia cultivars possessing novel flower colors.

Synergistic inhibitory activity of Glycyrrhizae Radix and Rubi Fructus extracts on biofilm formation of Streptococcus mutans

BACKGROUND

Streptococcus mutans is a bacterium that causes oral diseases. Plaque, a biofilm produced by S. mutans and other bacteria, makes it difficult to remove cariogenic oral microorganisms, including biofilm producers. Glucan synthesis by glucosyltransferase is one of the mechanisms underlying plaque formation. This study demonstrates the effectiveness of inhibiting biofilm formation by interfering with the glucosyltransferase activity of S. mutans using edible herbal medicines.

METHODS

This study investigated the inhibitory activity of Glycyrrhizae Radix extract, Rubi Fructus extract, glycyrrhizin from Glycyrrhizae Radix, and ellagic acid from Rubi Fructus against glucosyltransferase activity of S. mutans. Enzyme kinetic analysis identified the mechanism by which glycyrrhizin and ellagic acid inhibit enzyme activity.

RESULTS

The conditions for synergistically inhibiting biofilm formation by combining Glycyrrhizae Radix and Rubi Fructus extracts were identified. Biofilm formation was also synergistically inhibited by mixing their respective active constituents, glycyrrhizin and ellagic acid. Glycyrrhizin and ellagic acid inhibited glucosyltransferase via noncompetitive and uncompetitive mechanisms, respectively, indicating that they inhibit it via distinct mechanisms.

CONCLUSIONS

This study presents an effective oral hygiene method using the synergistic activity of two natural plant extracts to inhibit biofilm formation through different inhibitory mechanisms against glucosyltransferase of S. mutans.

Isolation, characterization, and comparative genomic analysis of vB_BviS-A10Y, a novel bacteriophage from mangrove sediments

Mangrove is among the most carbon-rich biomes on earth, and viruses are believed to play a significant role in modulating local and global carbon cycling. However, few viruses have been isolated from mangrove sediments to date. Here, we report the isolation of a novel Bacillus phage (named phage vB_BviS-A10Y) from mangrove sediments. Phage vB_BviS-A10Y has a hexameric head with a diameter of ~ 79.22 nm and a tail with a length of ~ 548.56 nm, which are typical features of siphophages. vB_BviS-A10Y initiated host lysis at 3.5 h postinfection with a burst size of 25 plaque-forming units (PFU)/cell. The genome of phage vB_BviS-A10Y is 162,435 bp long with 225 predicted genes, and the GC content is 34.03%. A comparison of the whole genome sequence of phage vB_BviS-A10Y with those of other phages from the NCBI viral genome database showed that phage vB_BviS-A10Y has the highest similarity (73.7% identity with 33% coverage) to Bacillus phage PBC2. Interestingly, abundant auxiliary metabolic genes (AMGs) were identified in the vB_BviS-A10Y genome. The presence of a β-1,3-glucosyltransferase gene in the phage genome supported our previous hypothesis that mangrove viruses may manipulate carbon cycling directly through their encoded carbohydrate-active enzyme (CAZyme) genes. Therefore, our study will contribute to a better understanding of the diversity and potential roles of viruses in mangrove ecosystems.

Inhibitory effects of Streptococcus salivarius K12 on formation of cariogenic biofilm

Bacground/purpose

Streptococcus salivarius (S. salivarius) K12 is known to be a probiotic bacterium. The purpose of this study was to investigate anti-cariogenic effects of S. salivarius K12 on cariogenic biofilm.

Materials and methods

S. salivarius K12 was cultured in M17 broth. The antimicrobial activity of spent culture medium (SCM) against Streptococcus mutans was investigated. S. salivarius K12 was co-cultivated with S. mutans using a membrane insert. When the biofilm was formed using salivary bacteria and S. mutans, the K12 was inoculated every day. The biomass of biofilm was investigated by a confocal laser scanning microscope. Also, bacterial DNA from the biofilm was extracted, and then bacteria proportion was analyzed by quantitative PCR using specific primers. The expression of gtf genes of S. mutans in the biofilm with or without S. salivarius K12 was analyzed by RT-PCR.

Results

The SCM of S. salivarius K12 inhibited the growth of S. mutans. Also, S. salivarius K12 reduced S. mutans growth in co-cultivation. The formation of cariogenic biofilm was reduced by adding S. salivarius K12, and the count of S. mutans in the biofilm was also decreased in the presence of S. salivarius K12. gtfB, gtfC, and gtfD expression of S. mutans in the biofilm was reduced in the presence of S. salivarius K12.

Conclusion

S. salivarius K12 may inhibit the formation of cariogenic biofilm by interrupting the growth and glucosyltransferase production of S. mutans.

Glucosyltransferase modulated Streptococcus mutans adhesion to different surfaces involved in biofilm formation by atomic force microscope

Biofilm on dental restorative materials is an important determinant in the aetiology of secondary caries development. Formation of biofilm involving adhesion of bacteria onto substrate, bacterial cell and biofilm surfaces. Glucosyltransferase B and C (GtfB and GtfC) are essential factors for regulation of Streptococcus mutans biofilm formation, but the mechanisms involving different kind of bacterial adhesion still lack detailed description. In this study, nanoscale adhesion force measurement was performed using atomic force microscopy (AFM). Bacteria-coated cantilevers were used to probe S. mutans adhesion to substrates, bacterial cells and early biofilms. Two representative dental materials, Glass ionomer cement (GIC) and composite resin, served as substrates. It was found that deletion of gtfB and gtfC genes both reduced adhesion forces of S. mutans towards substrate and bacterial cell surfaces (P<0.05). Notably, reduction of gtfB gene remarkably decreased bacterial adhesion to biofilm surfaces(P<0.05), while gtfC showed no obvious effect during this stage. Biofilms cultured on GIG further decreased cell-biofilm adhesion, compared with that on resin (P<0.05). Confocal fluorescence images and scanning electron microscopy images showed that deletion of gtfB lead to reduced microcolony formation and less production of exopolysaccharides (EPS) in the biofilm, and after bacteria culturing on GIC, the EPS content was further decreased. Our findings suggest that EPS mainly mediate bacterial adhesion to early biofilm surface. Deletion of gtfB and co-culture with GIC could significantly reduce the cell-biofilm adhesion, which is probably through decreasing of EPS production. gtfB exerts a critical role in the bacterial adhesion for the whole process of biofilm development, while gtfC possibly works only in the early stages. This article is protected by copyright. All rights reserved.

Identification and characterization of unique 5-hydroxyisoflavonoid biosynthetic key enzyme genes in Lupinus albus

5-Hydroxyisoflavonoids, no 5-deoxyisoflavonoids, in Lupinus species, are due to lack of CHRs and Type II CHIs, and the key enzymes of isoflavonoid biosynthetic pathway in white lupin were identified. White lupin (Lupinus albus) is used as food ingredients owing to rich protein, low starch, and rich bioactive compounds such as isoflavonoids. The isoflavonoids biosynthetic pathway in white lupin still remains unclear. In this study, only 5-hydroxyisoflavonoids, but no 5-deoxyisoflavonoids, were detected in white lupin and other Lupinus species. No 5-deoxyisoflavonoids in Lupinus species are due to lack of CHRs and Type II CHIs. We further found that the CHI gene cluster containing both Type I and Type II CHIs possibly arose after the divergence of Lupinus with other legume clade. LaCHI1 and LaCHI2 identified from white lupin metabolized naringenin chalcone to naringenin in yeast and tobacco (Nicotiana benthamiana), and were bona fide Type I CHIs. We further identified two isoflavone synthases (LaIFS1 and LaIFS2), catalyzing flavanone naringenin into isoflavone genistein and also catalyzing liquiritigenin into daidzein in yeast and tobacco. In addition, LaG6DT1 and LaG6DT2 prenylated genistein at the C-6 position into wighteone. Two glucosyltransferases LaUGT1 and LaUGT2 metabolized genistein and wighteone into its 7-O-glucosides. Taken together, our study not only revealed that exclusive 5-hydroxyisoflavonoids do exist in Lupinus species, but also identified key enzymes in the isoflavonoid biosynthetic pathway in white lupin.

A highly versatile fungal glucosyltransferase for specific production of quercetin-7-O-β-D-glucoside and quercetin-3-O-β-D-glucoside in different hosts

Glycosylation is an effective way to improve the water solubility of natural products. In this work, a novel glycosyltransferase gene (BbGT) was discovered from Beauveria bassiana ATCC 7159 and heterologously expressed in Escherichia coli. The purified enzyme was functionally characterized through in vitro enzymatic reactions as a UDP-glucosyltransferase, converting quercetin to five monoglucosylated and one diglucosylated products. The optimal pH and temperature for BbGT are 35 ℃ and 8.0, respectively. The activity of BbGT was stimulated by Ca²⁺, Mg²⁺, and Mn²⁺, but inhibited by Zn²⁺. BbGT enzyme is flexible and can glycosylate a variety of substrates such as curcumin, resveratrol, and zearalenone. The enzyme was also expressed in other microbial hosts including Saccharomyces cerevisiae, Pseudomonas putida, and Pichia pastoris. Interestingly, the major glycosylation product of quercetin in E. coli, P. putida, and P. pastoris was quercetin-7-O-β-D-glucoside, while the enzyme dominantly produced quercetin-3-O-β-D-glucoside in S. cerevisiae. The BbGT-harboring E. coli and S. cerevisiae strains were used as whole-cell biocatalysts to specifically produce the two valuable quercetin glucosides, respectively. The titer of quercetin-7-O-β-D-glucosides was 0.34 ± 0.02 mM from 0.83 mM quercetin in 24 h by BbGT-harboring E. coli. The yield of quercetin-3-O-β-D-glucoside was 0.22 ± 0.02 mM from 0.41 mM quercetin in 12 h by BbGT-harboring S. cerevisiae. This work thus provides an efficient way to produce two valuable quercetin glucosides through the expression of a versatile glucosyltransferase in different hosts. KEY POINTS: • A highly versatile glucosyltransferase was identified from B. bassiana ATCC 7159. • BbGT converts quercetin to five mono- and one di-glucosylated derivatives in vitro. • Different quercetin glucosides were produced by BbGT in E. coli and S. cerevisiae.

High-level de novo biosynthesis of glycosylated zeaxanthin and astaxanthin in Escherichia coli

Because of wide applications in food, feed, pharmaceutical and cosmetic industries, the carotenoid market is growing rapidly. Most carotenoids are hydrophobic, which limits their bioavailability. Glycosylation is a natural route that substantially increases the water solubility, as well as the bioavailability, photostability and biological activities of carotenoids. Here, we report metabolic engineering efforts (e.g., promoter and RBS engineering, optimization of carbon sources and supplementation of bottleneck genes) to produce glycosylated carotenoids in Escherichia coli. By fine-tuning the carotenoid-biosynthetic genes (crtX, crtZ and crtY), our strain produced up to 47.2 mg/L (~ 11,670 ppm) of zeaxanthin glucosides, ~ 78% of the total carotenoids produced. In another construct with mevalonate, astaxanthin pathway and crtX genes, the strain produced a mixture of carotenoid glucosides including astaxanthin and adonixanthin glucosides with a total yield of 8.1 mg/L (1774 ppm). Our work demonstrated a proof-of-concept study for the microbial biosynthesis of glycosylated carotenoids.

Sulfated vizantin causes detachment of biofilms composed mainly of the genus Streptococcus without affecting bacterial growth and viability

BACKGROUND

Sulfated vizantin, a recently developed immunostimulant, has also been found to exert antibiofilm properties. It acts not as a bactericide, but as a detachment-promoting agent by reducing the biofilm structural stability. This study aimed to investigate the mechanism underlying this activity and its species specificity using two distinct ex vivo oral biofilm models derived from human saliva.

RESULTS

The biofilm, composed mainly of the genus Streptococcus and containing 50 μM of sulfated vizantin, detached significantly from its basal surface with rotation at 500 rpm for only 15 s, even when 0.2% sucrose was supplied. Expression analyses for genes associated with biofilm formation and bacterial adhesion following identification of the Streptococcus species, revealed that a variety of Streptococcus species in a cariogenic biofilm showed downregulation of genes encoding glucosyltransferases involved in the biosynthesis of water-soluble glucan. The expression of some genes encoding surface proteins was also downregulated. Of the two quorum sensing systems involved in the genus Streptococcus, the expression of luxS in three species, Streptococcus oralis, Streptococcus gordonii, and Streptococcus mutans, was significantly downregulated in the presence of 50 μM sulfated vizantin. Biofilm detachment may be facilitated by the reduced structural stability due to these modulations. As a non-specific reaction, 50 μM sulfated vizantin decreased cell surface hydrophobicity by binding to the cell surface, resulting in reduced bacterial adherence.

CONCLUSION

Sulfated vizantin may be a candidate for a new antibiofilm strategy targeting the biofilm matrix while preserving the resident microflora.

Bioconversion of Stevioside to Rebaudioside E Using Glycosyltransferase UGTSL2

Rebaudioside E, one of the minor components of steviol glycosides, was first isolated and identified from Stevia rebaudiana in 1977. It is a high-intensity sweetener that tastes about 150-200 times sweeter than sucrose and is also a precursor for biosynthesis of rebaudioside D and rebaudioside M, the next-generation Stevia sweeteners. In this work, new unknown steviol glycosides were enzymatically synthesized from stevioside by coupling UDP-glucosyltransferase UGTSL2 from Solanum lycopersicum and sucrose synthase StSUS1 from Solanum tuberosum. Rebaudioside E was speculated to be the main product of glucosylation of the Glc(β1→C-19) residue of stevioside along with the formation of a (β1→2) linkage based on the analysis of the regioselectivity and stereoselectivity of UGTSL2, and verified afterwards by LC-MS/MS with standard. In a 20-ml bioconversion reaction of 20 g/l stevioside by UGTSL2 and StSUS1, 15.92 g/l rebaudioside E was produced for 24 h.

Cationic hydrogels prepared from regioselectively azidated (1→3)-α-d-glucan via crosslinking and amination: Physical and adsorption properties

Cationic hydrogels with amino groups were successfully prepared using (1→3)-α-d-glucan synthesized by glucosyltransferase J (GtfJ) cloned from Streptococcus salivarius through a three-step reaction: (i) Azido groups were regioselectively introduced at the C6 position of (1→3)-α-d-glucan by a bromination-azidation process (degree of substitution 0.94), (ii) Azido groups were partially crosslinked with 1,8-nonadiyne via a copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction, (iii) Azido groups that were unused for crosslinking were reduced to amino groups by sodium borohydride (NaBH₄). The introduction of amino groups was confirmed quantitatively and qualitatively by elemental, Fourier transform infrared (FT-IR), and nuclear magnetic resonance (NMR) analyses. These cationic hydrogels showed a specific adsorption ability for bovine serum albumin (BSA) over a wide pH range of 4.5-8.0 due to their high pH values at the point of zero charge (pH_pzc 8.80-8.92).

Byproduct-free geraniol glycosylation by whole-cell biotransformation with recombinant Escherichia coli

Objective

Geraniol, a fragrance of great importance in the consumer goods industry, can be glucosylated by the UDP-glucose-dependent glucosyltransferase VvGT14a from Vitis vinifera, yielding more stable geranyl glucoside. Escherichia coli expressing VvGT14a is a convenient whole-cell biocatalyst for this biotransformation due to its intrinsic capability for UDP-glucose regeneration. The low water solubility and high cytotoxicity of geraniol can be overcome in a biphasic system where the non-aqueous phase functions as an in situ substrate reservoir. However, the effect of different process variables on the biphasic whole-cell biotransformation is unknown. Thus, the goal of this study was to identify potential bottlenecks during biotransformation with in situ geraniol supply via isopropyl myristate as second non-aqueous phase.

Results

First, insufficient UDP-glucose supply could be ruled out by measurement of intracellular UDP-glucose concentrations. Instead, oxygen supply was determined as a bottleneck. Moreover, the formation of the byproduct geranyl acetate by chloramphenicol acetyltransferase (CAT) was identified as a constraint for high product yields. The use of a CAT-deficient whole-cell biocatalyst prevented the formation of geranyl acetate, and geranyl glucoside could be obtained with 100% selectivity during a biotransformation on L-scale.

Conclusion

This study is the first to closely analyze the whole-cell biotransformation of geraniol with Escherichia coli expressing an UDP-glucose-dependent glucosyltransferase and can be used as an optimal starting point for the design of other glycosylation processes.

Electronic supplementary material

The online version of this article (10.1007/s10529-020-02993-z) contains supplementary material, which is available to authorized users.

Highly swellable hydrogel of regioselectively aminated (1→3)-α-d-glucan crosslinked with ethylene glycol diglycidyl ether

(1→3)-α-d-glucan synthesized by glucosyltransferase J (GtfJ) cloned from Streptococcus salivarius was regioselectively aminated as 6-amino-6-deoxy-(1→3)-α-d-glucan (aminoglucan) through three steps: bromination, azidation, and reduction. The degree of substitution of the amino group was determined by elemental analysis to be 0.97 and the molecular weight was 3.74×10⁴ as measured by size exclusion chromatography. The regioselective amination at the C6 position of every pyranose ring was confirmed by ¹H/¹³C NMR and solid state ¹⁵N cross polarization/magic angle spinning NMR spectroscopy. Aminoglucan was characterized by FT-IR, X-ray diffraction and thermogravimetric analysis. Solubility of aminoglucan in various solvents was investigated and confirmed in aqueous solution at pH ≤ 11. Therefore, aminoglucan was crosslinked with ethylene glycol diglycidyl ether (EGDE) by an epoxy-ring opening reaction under alkaline conditions. The obtained EGDE-crosslinked aminoglucan hydrogels were highly swellable in water owing to a strong water-holding ability and no water was released on compression and breaking of the gels.

Genome-wide analyses reveals a glucosyltransferase involved in rutin and emodin glucoside biosynthesis in tartary buckwheat

With people's increasing needs for health concern, rutin and emodin in tartary buckwheat have attracted much attention for their antioxidant, anti-diabetic and reducing weight function. However, the biosynthesis of rutin and emodin in tartary buckwheat is still unclear; especially their later glycosylation contributing to make them more stable and soluble is uncovered. Based on tartary buckwheat' genome, the gene structures of 106 UGTs were analyzed; 21 candidate FtUGTs were selected to enzymatic test by comparing their transcript patterns. Among them, FtUGT73BE5 and other 4 FtUGTs were identified to glucosylate flavonol or emodin in vitro; especially rFtUGT73BE5 could catalyze the glucosylation of all tested flavonoids and emodin. Furthermore, the identical in vivo functions of FtUGT73BE5 were demonstrated in tartary buckwheat hairy roots. The transcript profile of FtUGT73BE5 was consistent with the accumulation trend of rutin in plant; this gene may relate to anti-adversity for its transcripts were up-regulated by MeJA, and repressed by ABA.

Expression, purification, and characterization of a new Glucosyltransferase involved in the third step of O-antigen repeating-unit biosynthesis of Escherichia coli O152

The O antigen is indispensable for the full function and virulence of pathogenic bacteria. During O-repeating unit (RU) biosynthesis, committed glycosyltransferases (GTs) transfer various sugars from an activated sugar donor to the appropriate lipid carrier sequentially. While the nucleotide sequence specific for O antigen of pathogenic bacteria is already known, the exact substrate specificity of most hypothetical GTs have yet be characterized. In the present paper, we report the biochemical characterization of one alpha-glucosyltransferase, WfgE, a member of GT family 4. This enzyme is implicated in the pentasaccharide RU biosynthetic pathway of E. coli O152 O antigen. A chemoenzymatically synthesized acceptor (GlcGlcNAc α-PP-O(CH₂)₁₀CH₃) was used to characterize the WfgE activity. The enzyme product was determined to have a 1,2-linkage using strategy based on collision-induced dissociation electrospray ionization ion trap multiple tandem MS (CID-ESI-IT-MSⁿ). The lack of a DxD motif and its high activity without divalent metal ions suggests that WfgE belongs to the GT-B fold superfamily. The enzyme is specific for beta-glucose or galactose-terminating acceptor substrates, and in particular UDP-glucose but also UDP-galactose as donor substrates. Our results suggest that WfgE catalyses the addition of the third sugar residue of the E. coli O152 O-RU. The recombinant GST-WfgE was solubilized and further purified to homogeneity via GST affinity chromatography, paving the way for structure-function relationship studies.

Effects of nano-hydroxyapatite on the formation of biofilms by Streptococcus mutans in two different media

OBJECTIVES

The aim of this study was to examine the effect of nano-hydroxyapatite (nHA) on biofilm formation by Streptococcus mutans, which is actively involved in the initiation of dental caries.

DESIGN

The effects of nHA on growth and biofilm formation by S. mutans were investigated in two media: a saliva analog medium, basal medium mucin (BMM); and a nutrient-rich medium, brain heart infusion (BHI); in the presence and absence of sucrose.

RESULTS

Sucrose enhanced the growth of S. mutans in both media. In the presence of sucrose, nHA enhanced bacterial growth and biofilm formation more in BMM medium than in BHI. nHA also affected the transcription of glucosyltransferase (gtf) genes and production of polysaccharide differently in the two media. In BHI medium, the transcription of all three gtf genes, coding for enzymes that synthesize soluble and insoluble glucans from sucrose, was increased more than 3-fold by nHA. However, in BMM medium, only the transcription of gtfB and gtfC, coding for insoluble glucans, was substantially enhanced by nHA.

CONCLUSIONS

nHA appeared to enhance biofilm formation by increasing glucosyltransferase transcription, which resulted in an increase in production of insoluble glucans. This effect was influenced by the growth conditions.

Design and study of anticaries effect of different medicinal plants against S.mutans glucosyltransferase

Background

The present study was aimed to evaluate the molecular level anticaries effect of different medicinal plants against Streptococcus mutans (S.mutans) glucosyltransferases (gtf).

Methods

A total of six natural sources named as Terminalia chebula (T.chebula), Psidium guajava (P.guajava), Azadirachta indica (A.indica) and Pongamia pinnata (P.pinnata); two essential oils, clove (Syzygium aromaticum) and peppermint oil (Mentha piperita) were selected as test samples. Hydroalcoholic plant extracts and essential oils were examined for their inhibitory potential on gtf isolated from S.mutans. Polyherbal mouth wash was prepared and its effect on gtf activity was compared with commercial chlorhexidine mouth wash (5%w/v). Enzyme kinetic study was carried out in order to explore the molecular mechanism of enzyme action.

Results

Out of six natural sources tested, A.indica has shown maximum inhibitory effect of 91.647% on gtf and T.chebula has shown IC₅₀ of 1.091 mg/ml which is significant when compared to standard chlorhexidine. From the final result of kinetic analysis it was found that T.chebula, P.guajava and P.pinnata have show uncompetitive inhibition where as A.indica has shown non-competitive inhibition. Surprisingly, both essential oils have shown allosteric inhibition (sigmoidal response). The polyherbal moutwash has shown significant inhibitory potential on gtf (95.936%) when compared to commercial chlorhexidine mouthwash (p < 0.05).

Conclusion

All the tested samples have shown considerable gtf inhibitory action. Moreover polyherbal mouth wash has shown promising noncompetitive inhibitory activity against gtf and it could be the future formulation to combat dental caries.

Oral and Systemic Effects of Xylitol Consumption

Recent results of randomized trials testing the efficacy of xylitol in caries prevention have been conflicting. This narrative review reveals the sources of discrepancy. The following databases were searched for the terms "xylitol" or "artificial sweeteners" restricted to the English language: PubMed, Web of Science, Evidenced-Based Medicine, Scopus, and the Cochrane database. In a separate search, the terms "dental caries" or "cariogenicity" or "glucosyltransferase" or "low glycemic" or "low insulinemic" or "dysbiosis" or "gut microbiome" were used and then combined. In section I, findings regarding the role of xylitol in dental caries prevention, the appropriateness of research methods, and the causes for potential biases are summarized. In section II, the systemic effects of xylitol on gut microbiota as well as low-glycemic/insulinogenic systemic effects are evaluated and summarized. The substitution of a carbonyl group with an alcohol radical in xylitol hinders its absorption and slowly releases sugar into the bloodstream. This quality of xylitol is beneficial for diabetic patients to maintain a constant glucose level. Although this quality of xylitol has been proven in in vitro and animal studies, it has yet to be proven in humans. Paradoxically, recent animal studies reported hyperglycemia and intestinal dysbiosis with artificial sweetener consumption. Upon careful inspection of evidence, it was revealed that these reports may be due to misinterpretation of original references or flaws in study methodology. Any systemic benefits of xylitol intake must be weighed in consideration with the well-established adverse gastrointestinal consequences. The contribution of xylitol to gut dysbiosis that may affect systemic immunity warrants further research.

Comparative study of different molecular weight pullulan productions by Aureobasidium pullulans CGMCC No.11602

Pullulan productions by Aureobasidium pullulans CGMCC No.11602 were conducted using the initial culture (IC) medium and the optimized culture (OC) medium, respectively, in which pullulan with significantly different molecular weights was obtained. Under the IC medium condition, the pullulan molecular weight (M w) reached 288,403 Da with a yield of 64.12 g/L after 96 h fermentation period. However, the pullulan molecular weight was much higher (M w, 3,715,352 Da), while the yield of pullulan was lower (40.12 g/L) using the OC medium. The FTIR spectra indicated that pullulan produced using the IC and OC medium were similar. Transcriptome analysis showed that a total of 871 differentially expressed genes (DEGs) were screened and "N-glycan biosynthesis" and "other types of O-glycan biosynthesis" were the most highly represented differential metabolic pathways (DMPs). Specifically, the genes involved in the two DMPs consistently pointed to glucosyltransferase genes (GTF), all of which were up-regulated in the OC medium when compared with those in the IC medium. Further studies showed that the activity and the relative quantity (RQ) of GTF transcription were remarkable higher, which were coincident with the slower decrease in the molecular weight of pullulan in the OC medium than those in the IC medium during the late stage of fermentation. The results indicated that GTF may be involved in the regulation of pullulan molecular weight.

Identification of a gene encoding glutathione S-transferase that is related to anthocyanin accumulation in buckwheat (Fagopyrum esculentum)

Anthocyanins are a group of flavonoids found in buckwheat (Fagopyrum esculentum) and many other plant species; however; little is known about their mechanisms of synthesis and regulation in buckwheat. We previously reported a spontaneous mutant buckwheat line that shows the green stem phenotype; this line does not accumulate anthocyanins but synthesizes flavonol and proanthocyanidin in the stem. Here, we used this line and lines developed by this line to search for genes related to anthocyanin accumulation in buckwheat. The lines with green stem showed flavonoid-3-O-glucosyltransferase activity against UDP-glucose, indicating that the flavonoid-3-O-glucosyltransferase gene was not controlling the green stem trait. We therefore searched the buckwheat genome database for a gene encoding glutathione S-transferase (GST), a flavonoid-binding protein that transports flavonoids to the vacuole, and identified a candidate gene, FeGST1. Expression analysis showed that FeGST1 was expressed in wild type buckwheat but not in the green stem lines. Linkage analysis with an F₂ segregating population produced by crossing between the green stem line and a self-compatible line showed that FeGST1 segregated with stem color without any recombination. This indicates that the green stem trait could be caused by homozygous non-functional alleles of the FeGST1 locus.

Inhibitory effects of β-caryophyllene on Streptococcus mutans biofilm

OBJECTIVE

The biofilm of Streptococcus mutans is associated with induction of dental caries. Also, they produce glucan as an extracellular polysaccharide through glucosyltransferases and help the formation of cariogenic biofilm. β-caryophyllene has been used for therapeutic agent in traditional medicine and has antimicrobial activity. The purpose of this study was to investigate the effect of β-caryophyllene on S. mutans biofilm and the expression of biofilm-related factor.

DESIGN

The susceptibility assay of S. mutans for β-caryophyllene was performed to investigate inhibitory concentration for S. mutans growth. To evaluated the effect of β-caryophyllene on S. mutans biofilm, β-caryophyllene was treated on S. mutans in the various concentrations before or after the biofilm formation. Live S. mutans in the biofilm was counted by inoculating on Mitis-salivarius agar plate, and S. mutans biofilm was analyzed by confocal laser scanning microscope after staining bacterial live/dead staining kit. Finally, the expression of glucosyltransferases of S. mutans was investigated by real-time RT-PCR after treating with β-caryophyllene at the non-killing concentration of S. mutans.

RESULTS

The growth of S. mutans was inhibited by β-caryophyllene in above concentration of 0.078%, S. mutans biofilm was inhibited by β-caryophyllene in above 0.32%. Also, 2.5% of β-caryophyllene showed anti-biofilm activity for S. mutans biofilm. β-caryophyllene reduced the expression of gtf genes at a non-killing concentration for S. mutans. On the basis on these results, β-caryophyllene may have anti-biofilm activity and the inhibitory effect on biofilm related factor.

CONCLUSIONS

β-caryophyllene may inhibit cariogenic biofilm and may be a candidate agent for prevention of dental caries.

Identification of UDP glucosyltransferases from the aluminum-resistant tree Eucalyptus camaldulensis forming β-glucogallin, the precursor of hydrolyzable tannins

In the highly aluminum-resistant tree Eucalyptus camaldulensis, hydrolyzable tannins are proposed to play a role in internal detoxification of aluminum, which is a major factor inhibiting plant growth on acid soils. To understand and modulate the molecular mechanisms of aluminum detoxification by hydrolyzable tannins, the biosynthetic genes need to be identified. In this study, we identified and characterized genes encoding UDP-glucose:gallate glucosyltransferase, which catalyzes the formation of 1-O-galloyl-β-d-glucose (β-glucogallin), the precursor of hydrolyzable tannins. By homology-based cloning, seven full-length candidate cDNAs were isolated from E. camaldulensis and expressed in Escherichia coli as recombinant N-terminal His-tagged proteins. Phylogenetic analysis classified four of these as UDP glycosyltransferase (UGT) 84A subfamily proteins (UGT84A25a, -b, UGT84A26a, -b) and the other three as UGT84J subfamily proteins (UGT84J3, -4, -5). In vitro enzyme assays showed that the UGT84A proteins catalyzed esterification of UDP-glucose and gallic acid to form 1-O-galloyl-β-d-glucose, whereas the UGT84J proteins were inactive. Further analyses with UGT84A25a and -26a indicated that they also formed 1-O-glucose esters of other structurally related hydroxybenzoic and hydroxycinnamic acids with a preference for hydroxybenzoic acids. The UGT84A genes were expressed in leaves, stems, and roots of E. camaldulensis, regardless of aluminum stress. Taken together, our results suggest that the UGT84A subfamily enzymes of E. camaldulensis are responsible for constitutive production of 1-O-galloyl-β-d-glucose, which is the first step of hydrolyzable tannin biosynthesis.

Structural Aspects of ER Glycoprotein Quality-Control System Mediated by Glucose Tagging

N-linked oligosaccharides attached to proteins act as tags for glycoprotein quality control, ensuring their appropriate folding and trafficking in cells. Interactions with a variety of intracellular lectins determine glycoprotein fates. Monoglucosylated glycoforms are the hallmarks of incompletely folded glycoproteins in the protein quality-control system, in which glucosidase II and UDP-glucose/glycoprotein glucosyltransferase are, respectively, responsible for glucose trimming and attachment. In this review, we summarize a recently emerging view of the structural basis of the functional mechanisms of these key enzymes as well as substrate N-linked oligosaccharides exhibiting flexible structures, as revealed by applying a series of biophysical techniques including small-angle X-ray scattering, X-ray crystallography, high-speed atomic force microscopy , electron microscopy , and computational simulation in conjunction with NMR spectroscopy.

A fusion protein strategy for soluble expression of Stevia glycosyltransferase UGT76G1 in Escherichia coli

The UDP-glucosyltransferase UGT76G1 from Stevia rebaudiana converts stevioside to rebaudioside A via a one-step glycosylation reaction, which increases the amount of sweet-tasting rebaudioside A and decreases the amount of stevioside that has a bitter aftertaste. This enzyme could, therefore, conceivably be used to improve the organoleptic properties of steviol glycosides and offer a cost-effective preparation of high-purity rebaudioside A. Producing soluble enzymes by overexpression is a prerequisite for large-scale biocatalysis. However, most of the UGT76G1 overexpressed in Escherichia coli is in inclusion bodies. In this study, three N-terminal fusion partners, 3'-phosphoadenosine-5'-phosphatase (CysQ), 2-keto-3-deoxy-6-phosphogluconate aldolase (EDA) and N-utilisation substance A (NusA), were tested to improve UGT76G1 expression and solubility in E. coli. Compared with the fusion-free protein, the solubility of UGT76G1 was increased 40% by fusion with CysQ, and the glucosyltransferase activity of the crude extract was increased 82%. This successful CysQ fusion strategy could be applied to enhance the expression and solubility of other plant-derived glucosyltransferases and presumably other unrelated proteins in the popular, convenient and cost-effective E. coli host.

Characterization of crystalline linear (1→3)-α-d-glucan synthesized in vitro

We investigated the crystal structure and molecular arrangement of the linear (1→3)-α-d-glucan synthesized by glucosyltransferase GtfJ cloned from Streptococcus salivarius using sucrose as a substrate. The synthetic products had two morphologies: wavy fibril-like crystals as major and thin lamellae as minor products. Their structures were analyzed using electron microdiffraction, synchrotron X-ray powder diffraction, and solid-state ¹³C NMR spectroscopy. The fibrils and lamellae had the same allomorphic form but different molecular arrangements. The wet crystals were in a hydrated form, which converted into an anhydrous form with a significant decrease in crystallinity on drying. The hydrated and anhydrous forms had an extended-chain conformation with 2/1 helix, and the hydrated form was estimated to contain one water molecule per glucose residue. The long glucan chains were folded in the fibril crystals, while the short, extended chains were arranged perpendicular to the base plane of the lamellae.

Molecular cloning of an ester-forming triterpenoid: UDP-glucose 28-O-glucosyltransferase involved in saponin biosynthesis from the medicinal plant Centella asiatica

Triterpene saponins include bioactive compounds with structures consisting of triterpene aglycones (sapogenins) and one or more sugar moieties linked through acetal or ester glycosidic linkages at one or more sites. Centella asiatica (L.) Urban is a medicinal plant that contains bioactive ursane-type saponins, such as madecassoside and asiaticoside. In this work, glucosylation of triterpenoids in C. asiatica was investigated starting with plant extracts. An enzyme capable of glucosylating asiatic and madecassic acids was partially purified. Proteomics methods and cDNA sequence data were employed as tools to obtain a full-length cDNA clone encoding a glucosyltransferase. The recombinant gene product, UGT73AD1, was functionally expressed in Escherichia coli and purified by immobilized metal-affinity chromatography. Purified recombinant UGT73AD1 was found to have a narrow specificity, glucosylating asiatic and madecassic acids at the C28 carboxyl. mRNA accumulated in all tissues tested (leaves, stems, roots and flowers), with highest expression in leaves. Thus, UGT73AD1 was identified as a triterpenoid carboxylic acid: UDP-glucose 28-O-glucosyltransferase that appears to be involved in saponin biosynthesis in C. asiatica.

Genome-Wide Identification and Functional Characterization of UDP-Glucosyltransferase Genes Involved in Flavonoid Biosynthesis in Glycine max

Flavonoids, natural products abundant in the model legume Glycine max, confer benefits to plants and to animal health. Flavonoids are present in soybean mainly as glycoconjugates. However, the mechanisms of biosynthesis of flavonoid glycosides are largely unknown in G. max. In the present study, 212 putative UDP-glycosyltransferase (UGT) genes were identified in G. max by genome-wide searching. The GmUGT genes were distributed differentially among the 20 chromosomes, and they were expressed in various tissues with distinct expression profiles. We further analyzed the enzymatic activities of 11 GmUGTs that are potentially involved in flavonoid glycosylation, and found that six of them (UGT72X4, UGT72Z3, UGT73C20, UGT88A13, UGT88E19 and UGT92G4) exhibited activity toward flavonol, isoflavone, flavone and flavanol aglycones with different kinetic properties. Among them, UGT72X4, UGT72Z3 and UGT92G4 are flavonol-specific UGTs, and UGT73C20 and UGT88E19 exhibited activity toward both flavonol and isoflavone aglycones. In particular, UGT88A13 exhibited activity toward epicatechin, but not for the flavonol aglycones kaempferol and quercetin. Overexpression of these six GmUGT genes significantly increased the contents of isoflavone and flavonol glucosides in soybean hairy roots. In addition, overexpression of these six GmUGT genes also affected flavonol glycoside contents differently in seedlings and seeds of transgenic Arabidopsis thaliana. We provide valuable information on the identification of all UGT genes in soybean, and candidate GmUGT genes for potential metabolic engineering of flavonoid compounds in both Escherichia coli and plants.

Clostridium difficile toxin glucosyltransferase domains in complex with a non-hydrolyzable UDP-glucose analogue

Clostridium difficile is the leading cause of hospital-acquired diarrhea and pseudomembranous colitis worldwide. The organism produces two homologous toxins, TcdA and TcdB, which enter and disrupt host cell function by glucosylating and thereby inactivating key signalling molecules within the host. As a toxin-mediated disease, there has been a significant interest in identifying small molecule inhibitors of the toxins' glucosyltransferase activities. This study was initiated as part of an effort to identify the mode of inhibition for a small molecule inhibitor of glucosyltransferase activity called apigenin. In the course of trying to get co-crystals with this inhibitor, we determined five different structures of the TcdA and TcdB glucosyltransferase domains and made use of a non-hydrolyzable UDP-glucose substrate. While we were able to visualize apigenin bound in one of our structures, the site was a crystal packing interface and not likely to explain the mode of inhibition. Nevertheless, the structure allowed us to capture an apo-state (one without the sugar nucleotide substrate) of the TcdB glycosyltransferase domain that had not been previously observed. Comparison of this structure with structures obtained in the presence of a non-hydrolyzable UDP-glucose analogue have allowed us to document multiple conformations of a C-terminal loop important for catalysis. We present our analysis of these five new structures with the hope that it will advance inhibitor design efforts for this important class of biological toxins.

Isomelezitose formation by glucansucrases

Several glucansucrases were surveyed for their ability to produce isomelezitose, a trisaccharide with the structure α-D-glucopyranosyl (1 → 6) β-D-fructofuranosyl (2 ↔ 1) α-D-glucopyranoside. Nearly all strains tested, with one exception, produced at least trace levels of isomelezitose. Yields were low but significant, ranging from less than 1% to approximately 5% based on sucrose. This trisaccharide may arise in either of two ways: glucopyranosyl transfer to the 6^Fru-OH position of sucrose, or to the anomeric OH position of isomaltulose. This study indicates that isomelezitose formation may be a general phenomenon of many glucansucrase reactions.

Inhibition of streptococcal biofilm by hydrogen water

OBJECTIVES

The accumulation of oral bacterial biofilm is the main etiological factor of oral diseases. Recently, electrolyzed hydrogen-rich water (H-water) has been shown to act as an effective antioxidant by reducing oxidative stress. In addition to this general health benefit, H-water has antibacterial activity for disease-associated oral bacteria. However, little is known about the effect of H-water on oral bacterial biofilm. The objective of this study was to confirm the effect of H-water on streptococcal biofilm formation.

METHODS

In vitro streptococcal biofilm was quantified using crystal violet staining after culture on a polystyrene plate. The effect of H-water on the expression of genes involved in insoluble glucan synthesis and glucan binding, which are critical steps for oral biofilm formation, was evaluated in MS. In addition, we compared the number of salivary streptococci after oral rinse with H-water and that with control tap water. Salivary streptococci were quantified by counting viable colonies on Mitis Salivarius agar-bacitracin.

RESULTS

Our data showed that H-water caused a significant decrease in in vitro streptococcal biofilm formation. The expression level of the mRNA of glucosyltransferases (gtfB, gtfc, and gtfI) and glucan-binding proteins (gbpC, dblB) were decreased remarkably in MS after H-water exposure for 60s. Furthermore, oral rinse with H-water for 1 week led to significantly fewer salivary streptococci than did that with control tap water.

CONCLUSIONS

Our data suggest that oral rinse with H-water would be helpful in treating dental biofilm-dependent diseases with ease and efficiency.

From Plant Extract to a cDNA Encoding a Glucosyltransferase Candidate: Proteomics and Transcriptomics as Tools to Help Elucidate Saponin Biosynthesis in Centella asiatica

Centella asiatica (L.) Urban (Apiaceae), a small annual plant that grows in India, Sri Lanka, Malaysia, and other parts of Asia, is well-known as a medicinal herb with a long history of therapeutic uses. The bioactive compounds present in C. asiatica leaves include ursane-type triterpene sapogenins and saponins-asiatic acid, madecassic acid, asiaticoside, and madecassoside. Various bioactivities have been shown for these compounds, although most of the steps in the biosynthesis of triterpene saponins, including glycosylation, remain uncharacterized at the molecular level. This chapter describes an approach that integrates partial enzyme purification, proteomics methods, and transcriptomics, with the aim of reducing the number of cDNA candidates encoding for a glucosyltransferase involved in saponin biosynthesis and facilitating the elucidation of the pathway in this medicinal plant.

Cloning and characterization of soybean gene Fg1 encoding flavonol 3-O-glucoside/galactoside (1→6) glucosyltransferase

KEY MESSAGE

Flavonoids are important secondary metabolites in plants. Sugar-sugar glycosyltransferases are involved in the final step of flavonoid biosynthesis and contribute to the structural diversity of flavonoids. This manuscript describes the first cloning of a sugar-sugar glucosyltransferase gene in the UGT family that attaches glucose to the 6″-position of sugar bound to a flavonol. The results provide a glimpse on the possible evolution of sugar-sugar glycosyltransferase genes and identify putative amino acids responsible for the recognition of the hydroxyl group of the sugar moiety and specification of sugar. A scheme for the genetic control of flavonol glycoside biosynthesis is proposed. Flavonol glycosides (FGs) are predominant in soybean leaves and they show substantial differences among genotypes. In previous studies, we identified two flavonoid glycoside glycosyltransferase genes that segregated in recombinant inbred lines developed from a cross between cultivars Nezumisaya and Harosoy; one was responsible for the attachment of glucose to the 2″-position of glucose or galactose that is bound to the 3-position of kaempferol and the other was involved in the attachment of glucose to the 6″-position. This study was conducted to clone and characterize the 6″-glucosyltransferase gene. Linkage mapping indicated that the gene was located in the molecular linkage group I (chromosome 20). Based on the genome sequence, we cloned a candidate cDNA, GmF3G6"Gt from Harosoy but the corresponding cDNA could not be amplified by PCR from Nezumisaya. The coding region of GmF3G6″Gt in Harosoy is 1386 bp long encoding 462 amino acids. This gene was not expressed in leaves of Nezumisaya. The GmF3G6″Gt recombinant protein converted UDP-glucose and kaempferol 3-O-glucoside or kaempferol 3-O-galactoside to kaempferol 3-O-glucosyl-(1→6)-glucoside or kaempferol 3-O-glucosyl-(1→6)-galactoside, respectively. These results indicate that GmF3G6″Gt encodes a flavonol 3-O-glucoside/galactoside (1→6) glucosyltransferase and corresponds to the Fg1 gene. GmF3G6″Gt had an amino acid similarity of 82 % with GmF3G6″Rt encoding flavonol 3-O-glucoside/galactoside (1→6) rhamnosyltransferase, suggesting a recent evolutionary divergence of the two genes. This may be the first cloning of a sugar-sugar glucosyltransferase gene in the UGT family that attaches glucose to the 6″-position of sugar bound to a flavonol. A scheme for the control of FG biosynthesis is proposed.

Engineering Saccharomyces cerevisiae with the deletion of endogenous glucosidases for the production of flavonoid glucosides

BACKGROUND

Glycosylation of flavonoids is a promising approach to improve the pharmacokinetic properties and biological activities of flavonoids. Recently, many efforts such as enzymatic biocatalysis and the engineered Escherichia coli biotransformation have increased the production of flavonoid glucosides. However, the low yield of flavonoid glucosides can not meet the increasing demand for human medical and dietary needs. Saccharomyces cerevisiae is a generally regarded as safe (GRAS) organism that has several attractive characteristics as a metabolic engineering platform for the production of flavonoid glucosides. However, endogenous glucosidases of S. cerevisiae as a whole-cell biocatalyst reversibly hydrolyse the glucosidic bond and hinder the biosynthesis of the desired products. In this study, a model flavonoid, scutellarein, was used to exploit how to enhance the production of flavonoid glucosides in the engineered S. cerevisiae.

RESULTS

To produce flavonoid glucosides, three flavonoid glucosyltransferases (SbGTs) from Scutellaria baicalensis Georgi were successfully expressed in E. coli, and their biochemical characterizations were identified. In addition, to synthesize the flavonoid glucosides in whole-cell S. cerevisiae, SbGT34 was selected for constructing the engineering yeast. Three glucosidase genes (EXG1, SPR1, YIR007W) were knocked out using homologous integration, and the EXG1 gene was determined to be the decisive gene of S. cerevisiae in the process of hydrolysing flavonoid glucosides. To further enhance the potential glycosylation activity of S. cerevisiae, two genes encoding phosphoglucomutase and UTP-glucose-1-phosphate uridylyltransferase involved in the synthetic system of uridine diphosphate glucose were over-expressed in S. cerevisiae. Consequently, approximately 4.8 g (1.2 g/L) of scutellarein 7-O-glucoside (S7G) was produced in 4 L of medium after 54 h of incubation in a 10-L fermenter while being supplied with ~3.5 g of scutellarein.

CONCLUSIONS

The engineered yeast harbouring SbGT with a deletion of glucosidases produced more flavonoid glucosides than strains without a deletion of glucosidases. This platform without glucosidase activity could be used to modify a wide range of valued plant secondary metabolites and to explore of their biological functions using whole-cell S. cerevisiae as a biocatalyst.

Shigella flexneri serotype 1c derived from serotype 1a by acquisition of gtrIC gene cluster via a bacteriophage

Background

Shigella spp. are the primary causative agents of bacillary dysentery. Since its emergence in the late 1980s, the S. flexneri serotype 1c remains poorly understood, particularly with regard to its origin and genetic evolution. This article provides a molecular insight into this novel serotype and the gtrIC gene cluster that determines its unique immune recognition.

Results

A PCR of the gtrIC cluster showed that serotype 1c isolates from different geographical origins were genetically conserved. An analysis of sequences flanking the gtrIC cluster revealed remnants of a prophage genome, in particular integrase and tRNA^Pro genes. Meanwhile, Southern blot analyses on serotype 1c, 1a and 1b strains indicated that all the tested serotype 1c strains may have had a common origin that has since remained distinct from the closely related 1a and 1b serotypes. The identification of prophage genes upstream of the gtrIC cluster is consistent with the notion of bacteriophage-mediated integration of the gtrIC cluster into a pre-existing serotype.

Conclusions

This is the first study to show that serotype 1c isolates from different geographical origins share an identical pattern of genetic arrangement, suggesting that serotype 1c strains may have originated from a single parental strain. Analysis of the sequence around the gtrIC cluster revealed a new site for the integration of the serotype converting phages of S. flexneri. Understanding the origin of new pathogenic serotypes and the molecular basis of serotype conversion in S. flexneri would provide information for developing cross-reactive Shigella vaccines.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0746-z) contains supplementary material, which is available to authorized users.

Effect of a single point mutation on the interaction of glucans with a glucansucrase from Leuconostoc mesenteroides NRRL B-1118

Our previous work showed that substitution of an amino acid that is coupled with the +2 subsite adjacent to the transition stabilizer of a glucansucrase, which produces a water-insoluble glucan, resulted in significant changes in the structures and yields of the water-insoluble glucans produced. We now describe how these changes affect the ability of the glucansucrase to bind to exogenous glucans, and how these glucans can influence the yield, product structures, and kinetics of the mutant glucansucrases. The activity of the wild-type enzyme, with threonine at position 654, is not significantly activated by added dextran, and the yield of water-insoluble glucan from sucrose is only slightly increased by dextran. Mutant T654Y is not affected at all by the addition of dextran. However, several mutant enzymes exhibit markedly lower yields of glucan relative to the wild type; these lower yields can be partially or completely overcome by the addition of water-soluble dextran. Although evidence indicates that the soluble dextran is incorporated into water-insoluble glucan, the increased yields cannot be accounted for solely by incorporation of the dextran into insoluble product. Furthermore, these DsrI mutants are significantly activated by exogenous glucans. The addition of dextran does not markedly change the KM for sucrose in the mutant enzymes, but does increase the Vmax of the reaction. These effects apparently depend on the presence of unbranched sequences of α1→6-linked D-glucose units in the glucan.

Dynamic metabolic changes in seeds and seedlings of Brassica napus (oilseed rape) suppressing UGT84A9 reveal plasticity and molecular regulation of the phenylpropanoid pathway

In Brassica napus, suppression of the key biosynthetic enzyme UDP-glucose:sinapic acid glucosyltransferase (UGT84A9) inhibits the biosynthesis of sinapine (sinapoylcholine), the major phenolic component of seeds. Based on the accumulation kinetics of a total of 158 compounds (110 secondary and 48 primary metabolites), we investigated how suppression of the major sink pathway of sinapic acid impacts the metabolome of developing seeds and seedlings. In UGT84A9-suppressing (UGT84A9i) lines massive alterations became evident in late stages of seed development affecting the accumulation levels of 58 secondary and 7 primary metabolites. UGT84A9i seeds were characterized by decreased amounts of various hydroxycinnamic acid (HCA) esters, and increased formation of sinapic and syringic acid glycosides. This indicates glycosylation and β-oxidation as metabolic detoxification strategies to bypass intracellular accumulation of sinapic acid. In addition, a net loss of sinapic acid upon UGT84A9 suppression may point to a feedback regulation of HCA biosynthesis. Surprisingly, suppression of UGT84A9 under control of the seed-specific NAPINC promoter was maintained in cotyledons during the first two weeks of seedling development and associated with a reduced and delayed transformation of sinapine into sinapoylmalate. The lack of sinapoylmalate did not interfere with plant fitness under UV-B stress. Increased UV-B radiation triggered the accumulation of quercetin conjugates whereas the sinapoylmalate level was not affected.

Base J glucosyltransferase does not regulate the sequence specificity of J synthesis in trypanosomatid telomeric DNA

Telomeric DNA of trypanosomatids possesses a modified thymine base, called base J, that is synthesized in a two-step process; the base is hydroxylated by a thymidine hydroxylase forming hydroxymethyluracil (hmU) and a glucose moiety is then attached by the J-associated glucosyltransferase (JGT). To examine the importance of JGT in modifiying specific thymine in DNA, we used a Leishmania episome system to demonstrate that the telomeric repeat (GGGTTA) stimulates J synthesis in vivo while mutant telomeric sequences (GGGTTT, GGGATT, and GGGAAA) do not. Utilizing an in vitro GT assay we find that JGT can glycosylate hmU within any sequence with no significant change in Km or kcat, even mutant telomeric sequences that are unable to be J-modified in vivo. The data suggests that JGT possesses no DNA sequence specificity in vitro, lending support to the hypothesis that the specificity of base J synthesis is not at the level of the JGT reaction.

Diversity of sugar acceptor of glycosyltransferase 1 from Bacillus cereus and its application for glucoside synthesis

Glycosyltransferase 1 from Bacillus cereus (BcGT1) catalyzes the transfer of a glucosyl moiety from uridine diphosphate glucose (UDP-glucose) to various acceptors; it was expressed and characterized. The specificity of acceptors was found to be broad: more than 20 compounds classified into O-, S-, and N-linkage glucosides can be prepared with BcGT1 catalysis. Based on this work, we conclude that the corresponding acceptors of these compounds must possess the following features: (1) the acceptors must contain at least one aromatic or fused-aromatic or heteroaromatic ring; (2) the reactive hydroxyl or sulfhydryl or amino group can attach either on the aromatic ring or on its aliphatic side chain; and (3) the acceptors can be a primary, secondary, or even a tertiary amine. Four representative acceptors-fluorescein methyl ester, 17-β-estradiol, 7-mercapto-4-methylcoumarin, and 6-benzylaminopurine-were chosen as a candidate acceptor for O-, S-, and N-glucosidation, respectively. These enzymatic products were purified and the structures were confirmed with mass and NMR spectra. As all isolated glucosides are β-anomers, BcGT1 is confirmed to be an inverting enzyme. This study not only demonstrates the substrate promiscuity of BcGT1 but also showed the great application prospect of this enzyme in bioconversion of valuable bioactive molecules.

Purification of a Recombinant Polyhistidine-Tagged Glucosyltransferase Using Immobilized Metal-Affinity Chromatography (IMAC)

Short peptide tags genetically fused to recombinant proteins have been widely used to facilitate detection or purification without the need to develop specific procedures. In general, an ideal affinity tag would allow the efficient purification of tagged proteins in high yield, without affecting its function. Here, we describe the purification steps to purify a recombinant polyhistidine-tagged glucosyltransferase from Centella asiatica using immobilized metal affinity chromatography.

Inhibition of Streptococcus mutans biofilm formation, extracellular polysaccharide production, and virulence by an oxazole derivative

Dental caries, a biofilm-related oral disease, is a result of disruption of the microbial ecological balance in the oral environment. Streptococcus mutans, which is one of the primary cariogenic bacteria, produces glucosyltransferases (Gtfs) that synthesize extracellular polysaccharides (EPSs). The EPSs, especially water-insoluble glucans, contribute to the formation of dental plaque, biofilm stability, and structural integrity, by allowing bacteria to adhere to tooth surfaces and supplying the bacteria with protection against noxious stimuli and other environmental attacks. The identification of novel alternatives that selectively inhibit cariogenic organisms without suppressing oral microbial residents is required. The goal of the current study is to investigate the influence of an oxazole derivative on S. mutans biofilm formation and the development of dental caries in rats, given that oxazole and its derivatives often exhibit extensive and pharmacologically important biological activities. Our data shows that one particular oxazole derivative, named 5H6, inhibited the formation of S. mutans biofilms and prevented synthesis of extracellular polysaccharides by antagonizing Gtfs in vitro, without affecting the growth of the bacteria. In addition, topical applications with the inhibitor resulted in diminished incidence and severity of both smooth and sulcal surface caries in vivo with a lower percentage of S. mutans in the animals' dental plaque compared to the control group (P < 0.05). Our results showed that this oxazole derivative has the capacity to inhibit biofilm formation and cariogenicity of S. mutans.

Indole-3-acetic acid UDP-glucosyltransferase from immature seeds of pea is involved in modification of glycoproteins

The glycosylation of auxin is one of mechanisms contributing to hormonal homeostasis. The enzyme UDPG: indole-3-ylacetyl-β-D-glucosyltransferase (IAA glucosyltransferase, IAGlc synthase) catalyzes the reversible reaction: IAA+UDPG↔1-O-IA-glucose+UDP, which is the first step in the biosynthesis of IAA-ester conjugates in monocotyledonous plants. In this study, we report IAA-glucosyltransferase isolated using a biochemical approach from immature seed of pea (Pisum sativum). The enzyme was purified by PEG fractionation, DEAE-Sephacel anion-exchange chromatography and preparative PAGE. LC-MS/MS analysis of tryptic peptides of the enzyme revealed the high identity with maize IAGlc synthase, but lack of homology with other IAA-glucosyltransferases from dicots. Biochemical characterization showed that of several acyl acceptors tested, the enzyme had the highest activity on IAA as the glucosyl acceptor (Km=0.52 mM, Vmax=161 nmol min(-1), kcat/Km=4.36 mM s(-1)) and lower activity on indole-3-propionic acid and 1-naphthalene acetic acid. Whereas indole-3-butyric acid and indole-3-propionic acid were competitive inhibitors of IAGlc synthase, D-gluconic acid lactone, an inhibitor of β-glucosidase activity, potentiated the enzyme activity at the optimal concentration of 0.3mM. Moreover, we demonstrated that the 1-O-IA-glucose synthesized by IAGlc synthase is the substrate for IAA labeling of glycoproteins from pea seeds indicating a possible role of this enzyme in the covalent modification of a class of proteins by a plant hormone.

Identification of a Highly Specific Isoflavone 7-O-glucosyltransferase in the soybean (Glycine max (L.) Merr.)

Isoflavone conjugates [7-O-β-D-glucosides and 7-O-(6″-malonyl-β-D-glucosides) of daidzein and genistein] accumulate in soybean roots and serve as the stored precursors of isoflavones (aglycons), which play very important roles in the rhizobia-mediated nodulation of this plant. Thus far, the isoflavone 7-O-glucosyltransferase (GmIF7GT or GmUGT1) has been biochemically characterized and is believed to be involved in isoflavone conjugate biosynthesis. The soybean genome encodes many other glycosyltransferase homologs (GmUGTs) that are related to GmUGT1; however, their catalytic properties, substrate specificities, and role(s) in isoflavone conjugation are unknown. In this study, nine different GmUGT1-related GmUGT cDNAs were isolated; six of these cDNAs belonged to two distinct phylogenetic subgroups (A and B), and these six were functionally characterized. The results showed that GmUGT4, a representative of subgroup A, encoded a UGT that was highly specific for isoflavones showing kcat and kcat/Km values for daidzein of 5.89 ± 0.65 s(-1) and 2.91 × 10(5) s(-1)M(-1), respectively. Moreover, GmUGT4 was expressed in the roots (mainly in lateral roots) of the 7-day-old seedlings and seeds, both of which contained abundant amounts of isoflavone conjugates. By contrast, GmUGT1 and GmUGT7, which were subgroup B members, encoded enzymes with broad glucosyl-acceptor specificities and were mainly expressed in the aerial portions (cotyledons and hypocotyls) of the seedlings. In the present study, we proposed that the role of isoflavone glucosylation in a soybean plant is assigned to different GmUGT members in an organ/tissue-dependent manner. We also established the functional importance of GmUGT4 in the biosynthesis of isoflavone conjugates in lateral roots that make a major contribution to overall N2 fixation.

A sweet code for glycoprotein folding

Glycoprotein synthesis is initiated in the endoplasmic reticulum (ER) lumen upon transfer of a glycan (Glc3Man9GlcNAc2) from a lipid derivative to Asn residues (N-glycosylation). N-Glycan-dependent quality control of glycoprotein folding in the ER prevents exit to Golgi of folding intermediates, irreparably misfolded glycoproteins and incompletely assembled multimeric complexes. It also enhances folding efficiency by preventing aggregation and facilitating formation of proper disulfide bonds. The control mechanism essentially involves four components, resident lectin-chaperones (calnexin and calreticulin) that recognize monoglucosylated polymannose protein-linked glycans, lectin-associated oxidoreductase acting on monoglucosylated glycoproteins (ERp57), a glucosyltransferase that creates monoglucosylated epitopes in protein-linked glycans (UGGT) and a glucosidase (GII) that removes the glucose units added by UGGT. This last enzyme is the only mechanism component sensing glycoprotein conformations as it creates monoglucosylated glycans exclusively in not properly folded glycoproteins or in not completely assembled multimeric glycoprotein complexes. Glycoproteins that fail to properly fold are eventually driven to proteasomal degradation in the cytosol following the ER-associated degradation pathway, in which the extent of N-glycan demannosylation by ER mannosidases play a relevant role in the identification of irreparably misfolded glycoproteins.

Nicotine promotes Streptococcus mutans extracellular polysaccharide synthesis, cell aggregation and overall lactate dehydrogenase activity

Several epidemiology studies have reported a positive relationship between smoking and dental caries. Nicotine, an alkaloid component of tobacco, has been demonstrated to stimulate biofilm formation and metabolic activity of Streptococcus mutans, one of the most important pathogens of dental caries. The first aim of the present study was to explore the possible mechanisms leading to increased biofilm by nicotine treatment from three aspects, extracellular polysaccharides (EPS) synthesis, glucosyltransferase (Gtf) synthesis and glucan-binding protein (Gbp) synthesis at the mRNA and protein levels. The second aim was to investigate how nicotine affects S. mutans virulence, particular in lactate dehydrogenase (LDH) activity. Confocal laser scanning microscopy results demonstrated that both biofilm bacterial cell numbers and EPS were increased by nicotine. Gtf and GbpA protein expression of S. mutans planktonic cells were upregulated while GbpB protein expression of biofilm cells were downregulated by nicotine. The mRNA expression trends of those genes were mostly consistent with results on protein level but not statistically significant, and gtfD and gbpD of biofilm cells were inhibited. Nicotine was not directly involved in S. mutans LDH activity. However, since it increases the total number of bacterial cells in biofilm, the overall LDH activity of S. mutans biofilm is increased. In conclusion, nicotine stimulates S. mutans planktonic cell Gtf and Gbp expression. This leads to more planktonic cells attaching to the dental biofilm. Increased cell numbers within biofilm results in higher overall LDH activity. This contributes to caries development in smokers.

Molecular cloning and characterization of the ABA-specific glucosyltransferase gene from bean (Phaseolus vulgaris L.)

Levels of the plant hormone abscisic acid (ABA) are maintained in homeostasis by a balance of its biosynthesis, catabolism and conjugation. The detailed molecular and signaling events leading to strict homeostasis are not completely understood in crop plants. In this study, we obtained cDNA of an ABA-inducible, ABA-specific UDP-glucosyltransferase (ABAGT) from the bean plant (Phaseolus vulgaris L.) involved in conjugation of a glucose residue to ABA to form inactive ABA-glucose ester (ABA-GE) to examine its role during development and abiotic stress in bean. The bacterially expressed PvABAGTase enzyme showed ABA-specific glucosylation activity in vitro. A higher level of the PvABAGT transcript was observed in mature leaves, mature flowers, roots, seed coats and embryos as well as upon rehydration following a period of dehydration. Overexpression of 35S::PvABAGT in Arabidopsis showed reduced sensitivity to ABA compared with WT. The transgenic plants showed a high level of ABA-GE without significant decrease in the level of ABA compared with the wild type (WT) during dehydration stress. Upon rehydration, the levels of ABA and phaseic acid (PA) decreased in the WT and the PvABAGT-overexpressing lines with high levels of ABA-GE only in the transgenic plants. Our findings suggest that the PvABAGT gene could play a role in ABA homeostasis during development and stress responses in bean and its overexpression in Arabidopsis did not alter ABA homeostasis during dehydration stress.

Expression and purification of functional human glycogen synthase-1:glycogenin-1 complex in insect cells

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GYS1:GN1 complex expressed using bicistronic pFastBac-Dual vector in insect cells.

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A large quantity of highly-pure stoichiometric GYS1:GN1 complex obtained.

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Purified GYS1 is functional and heavily phosphorylated at several Ser/Thr residues.

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GYS1:GN1 complex will be useful to reveal its structural and biochemical properties.

We report the successful expression and purification of functional human muscle glycogen synthase (GYS1) in complex with human glycogenin-1 (GN1). Stoichiometric GYS1:GN1 complex was produced by co-expression of GYS1 and GN1 using a bicistronic pFastBac™-Dual expression vector, followed by affinity purification and subsequent size-exclusion chromatography. Mass spectrometry analysis identified that GYS1 is phosphorylated at several well-characterised and uncharacterised Ser/Thr residues. Biochemical analysis, including activity ratio (in the absence relative to that in the presence of glucose-6-phosphate) measurement, covalently attached phosphate estimation as well as phosphatase treatment, revealed that recombinant GYS1 is substantially more heavily phosphorylated than would be observed in intact human or rodent muscle tissues. A large quantity of highly-pure stoichiometric GYS1:GN1 complex will be useful to study its structural and biochemical properties in the future, which would reveal mechanistic insights into its functional role in glycogen biosynthesis.

A phylogenetic examination of the primary anthocyanin production pathway of the Plantae

BACKGROUND

Anthocyanin pigments aid in reproduction and provide ultraviolet protection to land plants. We have examined the phylogenetic relationships among the five primary enzymes responsible for producing anthocyanin pigment in its three major forms. Dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), Flavonoid 3'glucosyltransferase (F3GT), flavonoid 3'hydroxylase (F3'H), and flavonoid 3'5' hydroxylase (F3'5'H) are responsible for the final steps in anthocyanin pigment production.

RESULTS

We were interested in how conserved the anthocyanin pathway genes may be among land plants, and evolutionarily how far back into the plant lineage anthocyanin production may be traced. The DFR, ANS, F3GT, and F3'H genes date back 450 million years to the first land plants. Mosses, spike mosses, and ferns express these four products, although there is no evidence of sequence orthologues for these genes in algae. Additionally, F3'5'H is not evident in organisms that predated gymnosperms.

CONCLUSION

Our findings support the hypothesis that "blue" anthocyanin pigments did not evolve until 300-350 mya along with the gymnosperms, although the "red" anthocyanin pigments may be as ancient as the mosses (~450 mya).