The structural analysis of a levan produced by Streptococcus salivarius SS2

New Levan-Based Chiral Stationary Phases: Synthesis and Comparative HPLC Enantioseparation of (±)-trans-β-Lactam Ureas in the Polar Organic Mode

In this paper, the preparation of three new polysaccharide-type chiral stationary phases (CSPs) based on levan carbamates (3,5-dimethylphenyl, 4-methylphenyl, and 1-naphthyl) is described. The enantioseparation of (±)-trans-β-lactam ureas 1a-h was investigated by high-performance liquid chromatography (HPLC) on six different chiral columns (Chiralpak AD-3, Chiralcel OD-3, Chirallica PST-7, Chirallica PST-8, Chirallica PST-9, and Chirallica PST-10) in the polar organic mode, using pure methanol (MeOH), ethanol (EtOH), and acetonitrile (ACN). Apart from the Chirallica PST-9 column (based on levan tris(1-naphthylcarbamate), the columns exhibited a satisfactory chiral recognition ability for the tested trans-β-lactam ureas 1a-h.

Isolation and characterization of exopolysaccharides from kombucha samples of different origins

Kombucha is prepared by fermenting sugared green or black tea with a symbiotic culture of bacteria and yeast (SCOBY). Some of the bacteria within the SCOBY are known to form exopolysaccharides (EPS) from sucrose. However, it is yet unknown whether water-soluble EPS are formed in kombucha, and if so, which specific EPS are present. Therefore, different kombucha samples were prepared by fermentation of green and black tea with SCOBYs from different manufacturers. Subsequently, the EPS were isolated and characterized by using various chromatographic methods, partial enzymatic hydrolyses and NMR spectroscopy. It was demonstrated that levans with a varying degree of branching at position O1 (4.3-7.9 %) are present, while only trace amounts of glucans were detected. Furthermore, levans isolated from kombucha had a comparably low molecular weight and the content of levan within the kombucha samples varied from 33 to 562 mg levan/L kombucha. Therefore, our study demonstrated that levans are the main EPS type in kombucha and that levan amounts and structures varied when different starter cultures and ingredients were used. Furthermore, we provide a comprehensive data set on the structural variability of levans from kombucha.

Comprehensive structural characterization of water-soluble and water-insoluble homoexopolysaccharides from seven lactic acid bacteria

Several lactic acid bacteria are able to produce water-soluble and water-insoluble homoexopolysaccharides (HoEPS) from sucrose. In this study, structures of all HoEPS which were fermentatively produced by Leuconostoc mesenteroides subsp. dextranicum NRRL B-1121 and B-1144, Leuconostoc mesenteroides subsp. mesenteroides NRRL B-1149, B-1438 and B-1118, Leuconostoc suionicum DSM 20241, and Liquorilactobacillus satsumensis DSM 16230 were systematically analyzed. Monosaccharide analysis, methylation analysis, NMR spectroscopy, size-exclusion chromatography, and different enzymatic fingerprinting methods were used to obtain detailed structural information. All strains produced water-soluble dextrans and/or levans as well as water-insoluble glucans. Levans showed different degrees of branching and high molecular weights, whereas dextrans had comparable structures and broader size distributions. Fine structures of water-soluble HoEPS were analyzed after endo-dextranase and endo-levanase hydrolysis. Water-insoluble glucans were composed of different portions of 1,3-linkages (5 to 40 %). Hydrolysis with endo-dextranase and endo-mutanase yielded further information on block sizes and varying fine structures. Overall, clear differences between HoEPS yields and structures were observed.

Elaboration of Nanostructured Levan-Based Colloid System as a Biological Alternative with Antimicrobial Activity for Applications in the Management of Pathogenic Microorganisms

Considering the documented health benefits of bacterial exopolysaccharides (EPSs), specifically of bacterial levan (BL), including its intrinsic antimicrobial activity against certain pathogenic species, the current study concentrated on the development of active pharmaceutical ingredients (APIs) in the form of colloid systems (CoSs) containing silver nanoparticles (AgNPs) employing in-house biosynthesized BL as a reducing and capping agent. The established protocol of fermentation conditions implicating two species of lactic acid bacteria (LAB), i.e., Streptococcus salivarius K12 and Leuconostoc mesenteroides DSM 20343, ensured a yield of up to 25.7 and 13.7 g L-1 of BL within 72 h, respectively. An analytical approach accomplished by Fourier-transform infrared (FT-IR) spectroscopy allowed for the verification of structural features attributed to biosynthesized BL. Furthermore, scanning electron microscopy (SEM) revealed the crystalline morphology of biosynthesized BL with a smooth and glossy surface and highly porous structure. Molecular weight (Mw) estimated by multi-detector size-exclusion chromatography (SEC) indicated that BL biosynthesized using S. salivarius K12 has an impressively high Mw, corresponding to 15.435 × 104 kilodaltons (kDa). In turn, BL isolated from L. mesenteroides DSM 20343 was found to have an Mw of only 26.6 kDa. Polydispersity index estimation (PD = Mw/Mn) of produced BL displayed a monodispersed molecule isolated from S. salivarius K12, corresponding to 1.08, while this was 2.17 for L. mesenteroides DSM 20343 isolate. The presence of fructose as the main backbone and, to a lesser extent, glucose and galactose as side chain molecules in EPS hydrolysates was supported by HPLC-RID detection. In producing CoS-BL@AgNPs within green biosynthesis, the presence of nanostructured objects with a size distribution from 12.67 ± 5.56 nm to 46.97 ± 20.23 was confirmed by SEM and energy-dispersive X-ray spectroscopy (EDX). The prominent inhibitory potency of elaborated CoS-BL@AgNPs against both reference test cultures, i.e., Pseudomonas aeruginosa, Escherichia coli, Enterobacter aerogenes, and Staphylococcus aureus and those of clinical origin with multi-drug resistance (MDR), was confirmed by disc and well diffusion tests and supported by the values of the minimum inhibitory and bactericidal concentrations. CoS-BL@AgNPs can be treated as APIs suitable for designing new antimicrobial agents and modifying therapies in controlling MDR pathogens.

The fruB Gene of Streptococcus mutans Encodes an Endo-Levanase That Enhances Growth on Levan and Influences Global Gene Expression

Streptococcus mutans, the primary etiologic agent of human dental caries, and a variety of oral Streptococcus and Actinomyces spp. synthesize high molecular mass homopolymers of fructose (fructans) with predominantly β2,1- (inulins) or β2,6-linkages (levans). The ability of S. mutans to degrade fructans contributes to the severity of dental caries. The extracellular product of fruA of S. mutans is an exo- β-d-fructofuranosidase that releases fructose from levan and inulin. Located 70 bp downstream of fruA, fruB encodes a member of the glycoside hydrolase family 32, but the function of FruB has not been established. Growth assays performed using wild-type UA159 and fruB-deficient derivatives, with fructans as the sole carbohydrate source, showed a significant reduction in the growth rate of a fruB mutant on levan, but not on inulin. A purified, recombinant FruB protein degraded levan to release mainly fructooligosaccharides. Driven by the fruA promoter and a secondary promoter located in the 3′ region of the fruA sequence, the fruB gene is inducible by fructose and especially by levan, but a stable stem-loop structure in the intergenic region likely modulates transcriptional read-through from fruA. Transcriptomic analysis of UA159 and a fruB mutant grown on 0.2% levan revealed differential expression of genes encoding ABC transporters, transcriptional regulators and genes involved in growth and stress tolerance. The ability of FruB to enhance levan metabolism and the high degree of conservation of FruB across S. mutans isolates imply a significant contribution of FruB to the fitness and virulence of this pathogen in human dental biofilms.

IMPORTANCE Carbohydrate metabolism and acid production are essential for the development of dental caries. As a by-product of sucrose metabolism, formation, and degradation of fructans enhances the severity of caries by S. mutans in animal models. This study highlights a significant breakthrough in identifying FruB in S. mutans as an endolevanase that contributes to efficient utilization of levan, a specific type of fructan produced by certain commensals but not S. mutans. Transcriptomic analysis revealed that FruB-dependent levan metabolism impacted global gene regulation, including a large number of novel genes. Considering the preference for levan by both FruA and FruB, the conservation of fruAB in S. mutans might represent a competitive advantage in access to the energy storage produced by dental microbiome. This is the first report demonstrating the presence of an endolevanase in S. mutans, therefore should be of broad interest to the fields of dental caries and complex carbohydrate metabolism.

Studies on solvent precipitation of levan synthesized using Bacillus subtilis MTCC 441

Levan is a water soluble biopolymer widely used in food, pharma, personal care and aquaculture industries. In this work, levan was synthesized by Bacillus subtilis MTCC 441 using sucrose as a sole carbon source. Effects of pH, sucrose concentration, nitrogen source, nitrogen concentration, inoculum size and agitation speed on levan production were studied. Yeast extract (YE) was found to be the best nitrogen source. Sucrose concentration - 100 g/L, pH - 7, YE concentration - 2 g/L, inoculum size 10% (v/v) and RPM - 150 were found to be optimal values for levan production. Effects of precipitation pH (3-12), choice of solvent (ethanol, isopropanol, acetone, and methanol) and supernatant to solvent ratio (1:1 to 1:6) on levan yield were also studied. Isopropanol resulted in maximum levan recovery among the four solvents considered. Optimal pH and supernatant to solvent ratio for levan precipitation were found to be 11 and 1:5, respectively. Corresponding levan yield was 0.395 g/g of sucrose supplied. The product obtained was characterized using FTIR, 1H-NMR, 13C-NMR, and GPC. The cytotoxicity of the precipitated levan was studied on EA.hy926 cell line using MTT assay and the compound was proven to be non-toxic to the cells.

Structural characterization of the exopolysaccharides from water kefir

Water kefir is a beverage which is produced by initiating fermentation of a fruit extract/sucrose solution with insoluble kefir grains. Exopolysaccharides that are formed from sucrose play a major role in the kefir grain formation, but the exopolysaccharides in the kefir beverage and the detailed structural composition of the whole kefir grains have not been studied yet. Therefore, kefir grains and the corresponding kefir beverage were analyzed for exopolysaccharides by multiple chromatographic approaches and two-dimensional NMR spectroscopy. Furthermore, different fractionation techniques were applied to obtain further information about the exopolysaccharides. The exopolysaccharide-fraction of the investigated kefir beverage was predominantly composed of O3- and O2-branched dextrans as well as lower amounts of levans. The insoluble dextrans from the kefir grains were mostly O3-branched and contained an elevated portion of 1,3-linked glucose units compared to the soluble dextrans. The structurally different exopolysaccharides in water kefir suggest the involvement of multiple bacteria.

Brachybacterium sp. CH-KOV3 isolated from an oil-polluted environment-a new producer of levan

Various microorganisms isolated from polluted environments, such as Pseudomonas sp. and Micrococcus sp. can synthesize exopolysaccharides (EPSs) which are natural, non-toxic and biodegradable polymers. EPSs play a key role in protection of microbial cells under various external influences. For humans, these substances have potential use in many industries. EPSs can be applied as a flavor or a fragrance carrier, an emulsifier, a stabilizer, a prebiotic, an antioxidant or an antitumor agent. In this study, we characterized an environmental microorganism that produces EPS, optimized EPS production by this strain and characterized the EPS produced. Isolate CH-KOV3 was identified as Brachybacterium paraconglomeratum. The sucrose level in the growth medium greatly influenced EPS production, and the highest yield was when the microorganism was incubated in media with 500g/L of sucrose. The optimal temperature and pH were 28°C and 7.0, respectively. The nuclear magnetic resonance (NMR) results and GC-MS analysis confirmed that the residues were d-fructofuranosyl residues with β-configuration, where fructose units are linked by β-2,6-glycosidic bonds, with β-2,1-linked branches. All these data indicate that the investigated EPS is a levan-type polysaccharide. Thus, it was concluded that Brachybacterium sp. CH-KOV3 could constitute a new source for production of the bioactive polysaccharide, levan.

Fermentation pH Modulates the Size Distributions and Functional Properties of Gluconobacter albidus TMW 2.1191 Levan

Bacterial levan has gained an increasing interest over the last decades due to its unique characteristics and multiple possible applications. Levan and other exopolysaccharides (EPSs) production are usually optimized to obtain the highest concentration or yield while a possible change of the molecular size and mass during the production process is mostly neglected. In this study, the molar mass and radius of levan samples were monitored during fermentations with the food-grade, levan-producing acetic acid bacterium Gluconobacter (G.) albidus TMW 2.1191 in shake flasks (without pH control) and bioreactors (with pH control at 4.5, 5.5 and 6.5, respectively). In uncontrolled fermentations, the levan size/molar mass continuously decreased concomitantly with the continuous acidification of the nutrient medium. On the contrary, the amount, molar mass and size of levan could be directly influenced by controlling the pH during fermentation. Using equal initial substrate amounts, the largest weight average molar mass and geometric radius of levan were observed at constant pH 6.5, while the highest levan concentration was obtained at constant pH 4.5. Since there is a special demand to find suitable hydrocolloids from food-grade bacteria to develop novel gluten-free (GF) products, these differently produced levans were used for baking of GF breads, and the best quality improvement was obtained by addition of levan with the highest mass and radius. This work, therefore, demonstrates for the first time that one bacterial strain can produce specific high molecular weight fractions of one EPS type, which differ in properties and sizes among each other in dependence of the controllable production conditions.

Intrinsic Levanase Activity of Bacillus subtilis 168 Levansucrase (SacB)

Levansucrase catalyzes the synthesis of fructose polymers through the transfer of fructosyl units from sucrose to a growing fructan chain. Levanase activity of Bacillus subtilis levansucrase has been described since the very first publications dealing with the mechanism of levan synthesis. However, there is a lack of qualitative and quantitative evidence regarding the importance of the intrinsic levan hydrolysis of B. subtilis levansucrase and its role in the levan synthesis process. Particularly, little attention has been paid to the long-term hydrolysis products, including its participation in the final levan molecules distribution. Here, we explored the hydrolytic and transferase activity of the B. subtilis levansucrase (SacB) when levans produced by the same enzyme are used as substrate. We found that levan is hydrolyzed through a first order exo-type mechanism, which is limited to a conversion extent of around 30% when all polymer molecules reach a structure no longer suitable to SacB hydrolysis. To characterize the reaction, Isothermal Titration Calorimetry (ITC) was employed and the evolution of the hydrolysis products profile followed by HPLC, GPC and HPAEC-PAD. The ITC measurements revealed a second step, taking place at the end of the reaction, most probably resulting from disproportionation of accumulated fructo-oligosaccharides. As levanase, levansucrase may use levan as substrate and, through a fructosyl-enzyme complex, behave as a hydrolytic enzyme or as a transferase, as demonstrated when glucose and fructose are added as acceptors. These reactions result in a wide variety of oligosaccharides that are also suitable acceptors for fructo-oligosaccharide synthesis. Moreover, we demonstrate that SacB in the presence of levan and glucose, through blastose and sucrose synthesis, results in the same fructooligosaccharides profile as that observed in sucrose reactions. We conclude that SacB has an intrinsic levanase activity that contributes to the final levan profile in reactions with sucrose as substrate.

A fructan, from Radix ophiopogonis, stimulates the proliferation of cultured lymphocytes: structural and functional analyses

A fructan, Opaw-2, with molecular mass of about 14 kDa, was isolated from the tuberous roots of Ophiopogon japonicus. Opaw-2 comprises fructose and glucose with a molar ratio of 30:1. Linkage and NMR analyses indicated that Opaw-2 has a backbone structure of beta-(1-->2)-Fruf and beta-(2-->6)-Fruf residues that branches at O-6 of beta-(1-->2)-Fruf residues with alpha-1-linkage to the Glcp residues and terminates with Fruf residues. In cultured lymphocytes, the application of Opaw-2 significantly stimulated the proliferation of lymphocytes in a dose-dependent manner. By using atomic force microscopy, Opaw-2 showed a morphological change from globular to helical fibrous shape at increasing concentrations.

Structure-function relationships of glucansucrase and fructansucrase enzymes from lactic acid bacteria

Lactic acid bacteria (LAB) employ sucrase-type enzymes to convert sucrose into homopolysaccharides consisting of either glucosyl units (glucans) or fructosyl units (fructans). The enzymes involved are labeled glucansucrases (GS) and fructansucrases (FS), respectively. The available molecular, biochemical, and structural information on sucrase genes and enzymes from various LAB and their fructan and alpha-glucan products is reviewed. The GS and FS enzymes are both glycoside hydrolase enzymes that act on the same substrate (sucrose) and catalyze (retaining) transglycosylation reactions that result in polysaccharide formation, but they possess completely different protein structures. GS enzymes (family GH70) are large multidomain proteins that occur exclusively in LAB. Their catalytic domain displays clear secondary-structure similarity with alpha-amylase enzymes (family GH13), with a predicted permuted (beta/alpha)(8) barrel structure for which detailed structural and mechanistic information is available. Emphasis now is on identification of residues and regions important for GS enzyme activity and product specificity (synthesis of alpha-glucans differing in glycosidic linkage type, degree and type of branching, glucan molecular mass, and solubility). FS enzymes (family GH68) occur in both gram-negative and gram-positive bacteria and synthesize beta-fructan polymers with either beta-(2-->6) (inulin) or beta-(2-->1) (levan) glycosidic bonds. Recently, the first high-resolution three-dimensional structures have become available for FS (levansucrase) proteins, revealing a rare five-bladed beta-propeller structure with a deep, negatively charged central pocket. Although these structures have provided detailed mechanistic insights, the structural features in FS enzymes dictating the synthesis of either beta-(2-->6) or beta-(2-->1) linkages, degree and type of branching, and fructan molecular mass remain to be identified.

Antitumor activity of levan polysaccharides from selected microorganisms

Levans were isolated from the cultures of Gluconoacetobacter xylinus (G-levan; Mw = 40,000), Microbacterium laevaniformans (M; Mw = 710,000), Rahnella aquatilis (R; Mw = 380,000), and Zymomonas mobilis (Z; Mw = 570,000). The levans were composed mainly of fructose residues when analyzed by TLC and HPLC, and their main backbones were beta-(2,6)-linkages with beta-(2,1)-branches by GC-MS and NMR. In the in vitro antitumor activity test of the levans against eight different tumor cell lines, relatively stronger activity was observed from the SNU-1 and HepG2. The M- (52.54-62.05%) and R-levan (52.15-58.58%) showed the significantly high activity against SNU-1, while M-levan showed the highest (49.93-61.82%) activity against HepG2. During the in vivo analysis of inhibitory activity of the levans against Sarcoma-180 growth, M-, R- and Z-levans showed strong antitumor activity (average 66%) but G-levan (42%) had significantly lower activity.

Biochemical and molecular characterization of a levansucrase from Lactobacillus reuteri

Lactobacillus reuteri strain 121 employs a fructosyltransferase (FTF) to synthesize a fructose polymer [a fructan of the levan type, with beta(2-->6) linkages] from sucrose or raffinose. Purification of this FTF (a levansucrase), and identification of peptide amino acid sequences, allowed isolation of the first Lactobacillus levansucrase gene (lev), encoding a protein (Lev) consisting of 804 amino acids. Lev showed highest similarity with an inulosucrase of L. reuteri 121 [Inu; producing an inulin polymer with beta(2-->1)-linked fructosyl units] and with FTFs from streptococci. Expression of lev in Escherichia coli resulted in an active FTF (Lev Delta 773His) that produced the same levan polymer [with only 2-3 % beta(2-->1-->6) branching points] as L. reuteri 121 cells grown on raffinose. The low degree of branching of the L. reuteri levan is very different from bacterial levans known up to now, such as that of Streptococcus salivarius, having up to 30 % branches. Although Lev is unusual in showing a higher hydrolysis than transferase activity, significant amounts of levan polymer are produced both in vivo and in vitro. Lev is strongly dependent on Ca(2+) ions for activity. Unique properties of L. reuteri Lev together with Inu are: (i) the presence of a C-terminal cell-wall-anchoring motif causing similar expression problems in Escherichia coli, (ii) a relatively high optimum temperature for activity for FTF enzymes, and (iii) at 50 degrees C, kinetics that are best described by the Hill equation.

Structural elucidation and antitumor activity of a fructan from Cyathula officinalis Kuan

A fructan named CoPS3 was isolated from Cyathula officinalis Kuan. The structure of CoPS3 was determined by methylation, by the reductive-cleavage method combined with GC-MS analysis, and both 1D and 2D 1H and 13C NMR spectroscopy. These results show that CoPS3 is a graminans-type fructan that is comprised of a beta-D-fructofuranosyl backbone having residues linked (2-->1)- and (2-->6) with branches and an alpha-D-glucopyranose residue on the nonreducing end of the fructan chain. Each branch is terminated by a beta-D-Fruf residue. Bioassay showed that it could inhibit growth of Lewis pulmonary carcinoma implanted in mice.

Production of long-chain levan by a sacC insertional mutant from Bacillus subtilis 327UH

A hyper extracellular protein producer, Bacillus subtilis 327UH, produced large amounts of levan in a medium containing 20% sucrose, and the yield of levan after 10 hours was more than 60%, when based on the fructose amount of sucrose. After transformation of 327UH with a levanase-deficient 168SC (sacC::Cm(r)) chromosomal DNA, a Cm(r) transformant 327UHSC (sacC::Cm(r) degSU(Hy)) produced 3 times longer levan than that of the wild type.

The production of a new water-soluble polysaccharide by Acetobacter xylinum NCI 1005 and its structural analysis by NMR spectroscopy

A new water-soluble polysaccharide (WSP) was isolated from a culture of Acetobacter xylinum NCI 1005 grown on sucrose. The structure of the WSP was analysed by nuclear magnetic resonance spectroscopy and determined to be a beta-(2-->6)-linked polyfructan, which is structurally different from the polymer synthesized from glucose instead of sucrose by the same strain. The discovery of this new polysaccharide has revealed that the bacterium is able to synthesize two different kinds of water-soluble polysaccharides.

Genetic analysis of fructan-hyperproducing strains of Streptococcus mutans

Fructan polymer, synthesized from sucrose by the extracellular fructosyltransferase of Streptococcus mutans, is thought to contribute to the progression of dental caries. It may serve as an extracellular storage polysaccharide facilitating survival and acid production. It may also have a role in adherence or accumulation of bacterial cells on the tooth surface. A number of clinical isolates of S. mutans which produce large, mucoid colonies on sucrose-containing agar as a result of increased production of fructan have been discovered. By using eight independent isolates, we sought to determine if such fructan-hyperproducing strains represented a genetically homogeneous group of organisms. Restriction fragment patterns of total cellular DNA were examined by using pulsed-field and conventional gel electrophoresis. Four genetic types which appeared to correlate with the serotype of the organism and the geographic site of isolation were evident. Southern blot analysis of several genetic loci for extracellular enzymes revealed some minor differences between the strains, but the basic genomic organizations of these loci were similar. To evaluate whether the excess fructan produced by these strains enhanced the virulence of these organisms in the oral cavity, it was of interest to create mutants deficient in fructosidase (FruA), the extracellular enzyme which degrades this polymer. The fruA gene was inactivated by allelic exchange in two fructan-hyperproducing strains as well as in S. mutans GS5, a strain which does not hyperproduce fructan. All of the fruA mutant strains were devoid of fructan hydrolase activity when levan was used as a substrate. However, the fructan-hyperproducing strains retained the ability to hydrolyze inulin, suggesting the presence of a second fructosidase with specificity for inulin in these strains.

Fructo-oligosaccharides from Urginea maritima

Fructo-oligosaccharides from red squill (Urginea maritima) were isolated by precipitation with methanol, GPC on Biogel P2/P4, and reversed-phase HPLC. Structures of the tri- and tetra-saccharides were verified by the reductive cleavage method. A tetrasaccharide that contained both (2-->1)- and (2-->6)-linked beta-D-Fru f residues was isolated. The higher fractions from GPC were analysed by the reductive cleavage method without prior purification by reversed-phase HPLC. The mode of biosynthesis of sinistrin is discussed.

The structure of the fructan sinistrin from Urginea maritima

The structure of sinistrin from red squill (Urginea maritima) was determined by methylation analysis and 13C NMR spectroscopy, using the fructans from Pucinella peisonis and quack-grass (Agropyron repens) as reference substances. Application of the reductive cleavage method showed that, of the beta-D-fructofuranosyl residues in sinistrin, 33% were 1-linked, 19% were 6-linked, 25% were terminal, and 19% were 1,6-linked. The average dp was 31 and, of the 3.24% of alpha-D-glucopyranosyl residues, 0.54% were terminal and 2.70% were 6-substituted. The fructan of quack grass was also highly branched with a (2-->6)-linked backbone, terminal alpha-D-glucopyranosyl residues, and a dp of approximately 45. The fructan from Pucinella peisonis was slightly branched, with a dp of approximately 10 and a (2-->6)-linked backbone.

Fructo-oligosaccharides from the stems of Triticum aestivum

Fructo-oligosaccharides, extracted with hot water from wheat stems before flowering, were fractionated by gel-permeation chromatography on Biogel P2. The tri-/tetra- and penta-saccharide fractions were purified by HPLC and analysed by the reductive cleavage method. The trisaccharides 1-kestose and 6-kestose, the branched tetrasaccharide bifurcose, and the (2-->6)-linked pentasaccharide were identified. The fractions of higher molecular weight were also investigated and confirmed the branched-levan structure of this fructan.

Characterization of the Streptococcus mutans GS-5 fruA gene encoding exo-beta-D-fructosidase

The complete nucleotide sequence (5,010 bp) of the fructanase gene (fruA) and flanking regions of the chromosome of Streptococcus mutans GS-5 was determined. The fruA gene appears to be the sole transcript arising from a proximal promoter. The presumed precursor of the secreted FruA protein consists of 1,423 amino acids, and it has an M(r) of 158,656 and a pI of 4.82. The N terminus of FruA has characteristics in common with signal peptides of gram-positive organisms. The C terminus consists of a serine- and threonine-rich region, followed by the peptide LPDTGD, 4 charged amino acids, 21 amino acids with a strongly hydrophobic character, and a charged pentapeptide tail, which are proposed to correspond to the wall-spanning region, the LPXTGX consensus sequence, and the membrane-spanning domains of surface-associated proteins of gram-positive cocci. The FruA protein has significant homology with the Bacillus subtilis levanase (SacC), the Bacteroides fragilis levanase (ScrL), yeast invertases, and a number of other beta-fructosidases but not with fructosyltransferase, glucosyltransferases, or glucan-binding proteins of oral streptococci. Genes with homology to fruA were detected in S. mutans serotype c, e, and f strains, Streptococcus rattus, Streptococcus salivarius, and Streptococcus sanguis. A deletion derivative of FruA lacking the C-terminal 437 amino acids was still functional and could hydrolyze beta-(2,6)- and beta-(2,1)-linked sugars, but with altered preference for substrates. The data begin to define functional domains of the FruA protein and potential regulatory sites for induction, repression, growth rate control, and posttranslational localization of this multifunctional enzyme.