Location of a second O-acetyl group in xanthan gum by the reductive-cleavage method

Resorbable membrane design: In vitro characterization of silver doped-hydroxyapatite-reinforced XG/PEI semi-IPN composite

In this study, the production and characterization of silver-doped hydroxyapatite (AgHA) reinforced Xanthan gum (XG) and Polyethyleneimine (PEI) reinforced semi-interpenetrating polymer network (IPN) biocomposite, known to be used as bone cover material for therapeutic purposes in bone tissue, were performed. XG/PEI IPN films containing 2AgHA nanoparticles were produced by simultaneous condensation and ionic gelation. Characteristics of 2AgHA-XG/PEI nanocomposite film were evaluated by structural, morphological (SEM, XRD, FT-IR, TGA, TM, and Raman) and biological activity analysis (degradation, MTT, genotoxicity, and antimicrobial activity) techniques. In the physicochemical characterization, it was determined that 2AgHA nanoparticles were homogeneously dispersed in the XG/PEI-IPN membrane at high concentration and the thermal and mechanical stability of the formed film were high. The nanocomposites showed high antibacterial activity against Acinetobacter Baumannii (A.Baumannii), Staphylococcus aureus (S.aureus), and Streptococcus mutans (S.mutans). L929 exhibited good biocompatibility for fibroblast cells and was determined to support the formation of MCC cells. It was shown that a resorbable 2AgHA-XG/PEI composite material was obtained with a high degradation rate and 64% loss of mass at the end of the 7th day. Physico-chemically developed biocompatible and biodegradable XG-2AgHA/PEI nanocomposite semi-IPN films possessed an important potential for the treatment of defects in bone tissue as an easily applicable bone cover. Besides, it was noted that 2AgHA-XG/PEI biocomposite could increase cell viability, especially in dental-bone treatments for coating, filling, and occlusion.

Efficiency of purification methods on the recovery of exopolysaccharides from fermentation media

De-Man Rogosa and Sharpe (MRS) is a complex medium commonly used to obtain exopolysaccharides (EPS) from lactic acid bacteria. However, the various nutrients (carbon and nitrogen sources) of media and supplements added to enhance the bacterial growth and EPS production, may interfere with the purification of the extracts resulting in an over-estimation of the EPS and erroneous structural assignments. The efficiency of trichloroacetic acid (TCA)/pronase and 5-sulfosalicylic acid - SSA methods was evaluated. In addition, the interference of the major MRS broth components as well as lactose was evaluated using xanthan gum as model control EPS. It was concluded that MRS medium is a major source of interfering compounds in the quantification of EPS, mainly glucose-rich material and to a lesser extent mannoproteins from yeast extract. This effect was found to be potentiated by the presence of lactose. TCA/pronase method was shown to be more efficient in eliminating interferents.

Xanthomonas citri ssp. citri requires the outer membrane porin OprB for maximal virulence and biofilm formation

Xanthomonas citri ssp. citri (Xcc) causes canker disease in citrus, and biofilm formation is critical for the disease cycle. OprB (Outer membrane protein B) has been shown previously to be more abundant in Xcc biofilms compared with the planktonic state. In this work, we showed that the loss of OprB in an oprB mutant abolishes bacterial biofilm formation and adherence to the host, and also compromises virulence and efficient epiphytic survival of the bacteria. Moreover, the oprB mutant is impaired in bacterial stress resistance. OprB belongs to a family of carbohydrate transport proteins, and the uptake of glucose is decreased in the mutant strain, indicating that OprB transports glucose. Loss of OprB leads to increased production of xanthan exopolysaccharide, and the carbohydrate intermediates of xanthan biosynthesis are also elevated in the mutant. The xanthan produced by the mutant has a higher viscosity and, unlike wild-type xanthan, completely lacks pyruvylation. Overall, these results suggest that Xcc reprogrammes its carbon metabolism when it senses a shortage of glucose input. The participation of OprB in the process of biofilm formation and virulence, as well as in metabolic changes to redirect the carbon flux, is discussed. Our results demonstrate the importance of environmental nutrient supply and glucose uptake via OprB for Xcc virulence.

Xanthan Pyruvilation Is Essential for the Virulence of Xanthomonas campestris pv. campestris

Xanthan, the main exopolysaccharide (EPS) synthesized by Xanthomonas spp., contributes to bacterial stress tolerance and enhances attachment to plant surfaces by helping in biofilm formation. Therefore, xanthan is essential for successful colonization and growth in planta and has also been proposed to be involved in the promotion of pathogenesis by calcium ion chelation and, hence, in the suppression of the plant defense responses in which this cation acts as a signal. The aim of this work was to study the relationship between xanthan structure and its role as a virulence factor. We analyzed four Xanthomonas campestris pv. campestris mutants that synthesize structural variants of xanthan. We found that the lack of acetyl groups that decorate the internal mannose residues, ketal-pyruvate groups, and external mannose residues affects bacterial adhesion and biofilm architecture. In addition, the mutants that synthesized EPS without pyruvilation or without the external mannose residues did not develop disease symptoms in Arabidopsis thaliana. We also observed that the presence of the external mannose residues and, hence, pyruvilation is required for xanthan to suppress callose deposition as well as to interfere with stomatal defense. In conclusion, pyruvilation of xanthan seems to be essential for Xanthomonas campestris pv. campestris virulence.

Challenges and perspectives in combinatorial assembly of novel exopolysaccharide biosynthesis pathways

Because of their rheological properties various microbial polysaccharides are applied as thickeners and viscosifiers both in food and non-food industries. A broad variety of microorganisms secrete structurally diverse exopolysaccharides (EPS) that contribute to their surface attachment, protection against abiotic or biotic stress factors, and nutrient gathering. Theoretically, a massive number of EPS structures are possible through variations in monosaccharide sequences, condensation linkages and non-sugar decorations. Given the already-high diversity of EPS structures, taken together with the principal of combinatorial biosynthetic pathways, microbial polysaccharides are an attractive class of macromolecules with which to generate novel structures via synthetic biology approaches. However, previous manipulations primarily focused on increasing polysaccharide yield, with structural modifications restricted to removal of side chains or non-sugar decorations. This article outlines the biosynthetic pathways of the bacterial heteroexopolysaccharides xanthan and succinoglycan, which are used as thickening and stabilizing agents in food and non-food industries. Challenges and perspectives of combining synthetic biology approaches with directed evolution to overcome obstacles in assembly of novel EPS biosynthesis pathways are discussed.

A mutation in the Xanthomonas oryzae pv. oryzae wxoD gene affects xanthan production and chemotaxis

Xanthomonas oryzae pv. oryzae causes bacterial blight in rice (Oryza sativa L.). The effect of a mutation in the wxoD gene, that encodes a putative O-antigen acetylase, on xanthan production as well as bacterial chemotaxis was investigated. The mutation increased xanthan production by 52 %. The mutant strain was non-motile on semi-solid agar swarm plates. In addition, several genes involved in chemotaxis, including the cheW, cheV, cheR, and cheD genes, were down-regulated by a mutation in the wxoD gene. Thus, the mutation in the wxoD gene affects xanthan production as well as bacterial chemotaxis. However, the wxoD gene is not essential for the virulence of X. oryzae.

Comparison of xanthans by the relative abundance of its six constituent repeating units

Five xanthans were hydrolyzed to their repeating units using cellulases. Hydrophilic interaction chromatography with online electrospray ionization ion trap mass spectrometry and evaporative light scattering detection was used to analyze the oligomers released. It was concluded that six different pentamer repeating units (RUs) exists within a xanthan sample. The most abundant RU shows acetylation on the inner mannose and pyruvylation on the outer mannose. The second most abundant RU shows acetylation on both the inner and the outer mannose. It becomes clear that more variations in the xanthan structure exist than generally recognized. Comparison of five different xanthan samples revealed that, although the molecular composition of xanthan samples can be exactly the same, the ratio in which the RUs occur can differ significantly. It is, therefore, concluded that xanthan samples should be characterized for both, their molecular composition and the relative abundance of the RUs present.

Dynamic protein phosphorylation during the growth of Xanthomonas campestris pv. campestris B100 revealed by a gel-based proteomics approach

Xanthomonas campestris pv. campestris (Xcc) synthesizes huge amounts of the exopolysaccharide xanthan and is a plant pathogen affecting Brassicaceae, among them the model plant Arabidopsis thaliana. Xanthan is produced as a thickening agent at industrial scale by fermentation of Xcc. In an approach based on 2D gel electrophoresis, protein samples from different growth phases were characterized to initialize analysis of the Xanthomonas phosphoproteome. The 2D gels were stained with Pro-Q Diamond phosphoprotein stain to identify putatively phosphorylated proteins. Spots of putatively phosphorylated proteins were excised from the gel and analyzed by mass spectrometry. Three proteins were confirmed to be phosphorylated, the phosphoglucomutase/phosphomannomutase XanA that is important for xanthan and lipopolysaccharide biosynthesis, the phosphoenolpyruvate synthase PspA that is involved in gluconeogenesis, and an anti-sigma factor antagonist RsbR that was so far uncharacterized in xanthomonads. The growth phase in which the samples were collected had an influence on protein phosphorylation in Xcc, particular distinct in case of RsbR, which was phosphorylated during the transition from the late exponential growth phase to the stationary phase.

Relationship between glucose catabolism and xanthan production in Xanthomonas oryzae pv. oryzae

Two genes involved in central carbon metabolism were inactivated to modulate intracellular glucose 6-phosphate and to evaluate its effects on xanthan production in wild-type Xanthomonas oryzae pv. oryzae. Upon the inactivation of the phosphogluconate dehydratase gene (edd), intracellular glucose 6-phosphate increased from 0.05 to 1.17 mmol/g (dry cell wt). This was accompanied by increased xanthan production of up to 2.55 g/l (culture medium). In contrast, inactivation of 6-phosphogluconate dehydrogenase gene (gndA) did not influence intracellular glucose 6-phosphate nor xanthan production. The intracellular availability of glucose 6-phosphate is proposed as a rate-limiting factor in xanthan production, and it may be possible to increases production of xanthan by modulating the activities of enzymes in central carbon metabolism.

Polysaccharide Lyases: Recent Developments as Biotechnological Tools

Polysaccharide lyases, which are polysaccharide cleavage enzymes, act mainly on anionic polysaccharides. Produced by prokaryote and eukaryote organisms, these enzymes degrade (1,4) glycosidic bond by a beta elimination mechanism and have unsaturated oligosaccharides as major products. New polysaccharides are cleaved only by their specific polysaccharide lyases. From anionic polysaccharides controlled degradations, various biotechnological applications were investigated. This review catalogues the degradation of bacterial, plant and animal polysaccharides (neutral and anionic) by this family of carbohydrate acting enzymes.

Mutational analysis of the gum gene cluster required for xanthan biosynthesis in Xanthomonas oryzae pv oryzae

Genome sequence analysis of Xanthomonas oryzae pv. oryzae has revealed a cluster of 12 ORFs that are closely related to the gum gene cluster of Xanthomonas campestris pv. campestris. The gum gene cluster of X. oryzae encodes proteins involved in xanthan production; however, there is little experimental evidence supporting this. In this study, biochemical analyses of xanthan produced by a defined set of X. oryzae gum mutant strains allowed us to preliminarily assign functions to most of the gum gene products: biosynthesis of the pentasaccharide repeating unit for GumD, GumM, GumH, GumK, and GumI, xanthan polymerization and transport for GumB, GumC, GumE, and GumJ, and modification of the pentasaccharide repeating unit for GumF, GumG, and GumL. In addition, we found that the exopolysaccharides are essential but not specific for the virulence of X. oryzae.

The genome of Xanthomonas campestris pv. campestris B100 and its use for the reconstruction of metabolic pathways involved in xanthan biosynthesis

The complete genome sequence of the Xanthomonas campestris pv. campestris strain B100 was established. It consisted of a chromosome of 5,079,003bp, with 4471 protein-coding genes and 62 RNA genes. Comparative genomics showed that the genes required for the synthesis of xanthan and xanthan precursors were highly conserved among three sequenced X. campestris pv. campestris genomes, but differed noticeably when compared to the remaining four Xanthomonas genomes available. For the xanthan biosynthesis genes gumB and gumK earlier translational starts were proposed, while gumI and gumL turned out to be unique with no homologues beyond the Xanthomonas genomes sequenced. From the genomic data the biosynthesis pathways for the production of the exopolysaccharide xanthan could be elucidated. The first step of this process is the uptake of sugars serving as carbon and energy sources wherefore genes for 15 carbohydrate import systems could be identified. Metabolic pathways playing a role for xanthan biosynthesis could be deduced from the annotated genome. These reconstructed pathways concerned the storage and metabolization of the imported sugars. The recognized sugar utilization pathways included the Entner-Doudoroff and the pentose phosphate pathway as well as the Embden-Meyerhof pathway (glycolysis). The reconstruction indicated that the nucleotide sugar precursors for xanthan can be converted from intermediates of the pentose phosphate pathway, some of which are also intermediates of glycolysis or the Entner-Doudoroff pathway. Xanthan biosynthesis requires in particular the nucleotide sugars UDP-glucose, UDP-glucuronate, and GDP-mannose, from which xanthan repeat units are built under the control of the gum genes. The updated genome annotation data allowed reconsidering and refining the mechanistic model for xanthan biosynthesis.

Identification of new internal promoters of the Xanthomonas oryzae pathovar oryzae gum gene cluster

The Xanthomonas oryzae pathovar oryzae gum gene cluster is composed of 14 ORFs designated, in sequence, as gumB to M, XOO3167, and gumN. The gum gene cluster constitutes an operon expressed from multiple promoters located upstream of gumB and gumG, respectively. To identify new promoters responsible for the expression of the gum gene cluster, we have conducted a computer-assisted promoter search and identified previously unreported promoter-like sequences upstream of the gumH and gumM genes, respectively. Moreover, the ability of these putative promoters to stimulate the transcription of the downstream genes was demonstrated by RT-PCR analyses using the mutant strains carrying an insertion of the rrnB transcriptional terminator into the gumG, gumH, and gumL gene, respectively.

Synergistic interactions between the genetically modified bacterial polysaccharide P2 and carob or konjac mannan

Rheological studies have confirmed that the bacterial polysaccharide P2, a genetically modified variant of the Acetobacter xylinum polysaccharide acetan, undergoes synergistic gelation with either of the plant polysaccharides carob or konjac mannan. X-ray fibre diffraction data shows that P2 can form a 5-fold helical structure of pitch 4.7nm and an axial rise per disaccharide repeat of 0.92nm. Optical rotation data demonstrate that P2 undergoes a coil-helix transition in solution and that deacylation enhances the stability of the helical structure in solution. Studies made on mixtures prepared at different temperatures and ionic strengths suggest that denaturation of the P2 helix favours interaction and gelation. Deacetylation of P2 enhances gelation. X-ray diffraction data for oriented fibres prepared from deacetylated P2-konjac mannan mixed films reveal a 6-fold helical structure of pitch 5.54nm with an axial rise per disaccharide repeat also of 0.92nm. This mixed helix provides direct evidence for binding between the two polysaccharides. P2 contains two sites of acetylation: one on the backbone and one on the sidechain. The former site of acetylation inhibits helix formation for P2. It is suggested that this site of acetylation also inhibits formation of the mixed helix, explaining the enhanced gelation of mixtures on deacetylation.

Overexpression, purification, and biochemical characterization of GumC, an enzyme involved in the biosynthesis of exopolysaccharide by Xylella fastidiosa

GumC is one of nine enzymes involved in the biosynthesis of fastidian gum, an exopolysaccharide produced by Xylella fastidiosa that may be linked directly to the pathogenicity of the microorganism. GumC may be responsible for gum polymerization or secretion through the membrane of X. fastidiosa. To perform structure and functions studies, we developed an expression system for the production of GumC as a fusion protein with maltose binding protein (MBP) using pMAL-c2x vector. The GumC-MBP fusion protein was expressed as a 94 kDa protein, which strongly reacts with anti-MBP antibodies. GumC-MBP was isolated by affinity chromatography through an amylose column and used to produce antibodies against the fusion protein. After the enzymatic cleavage of MBP, GumC was purified on a Q Sepharose Fast Flow column. GumC showed a molecular weight corresponding to the expected one (52 kDa) and its N-terminal sequence was identical to that deduced from the DNA. The shape of the circular dichroism spectrum was compatible with a folded protein that contains alpha-helical regions in its structure. Therefore, in this study we describe, for the first time, the production of GumC recombinant protein.

Biosynthesis of a substituted cellulose from a mutant strain of Xanthomonas campestris

In Xanthomonas campestris the genes involved in polysaccharide (xanthan) biosynthesis are located in a gene cluster (gum) of 16 kb. A Tn5 insertion mutant with a reduced slimy phenotype has been characterized. This mutant failed to produce the pentasaccharide repeating-unit of xanthan. Only three sugars were transferred to the prenyl phosphate intermediate. Several lines of evidence suggested that the lipid-associated saccharide was the trisaccharide reducing end of the pentasaccharide from the wild-type strain. This trisaccharide was built up from UDP-Glc and GDP-Man, and a glucose residue was at the reducing end, linked to an allylic prenol through a diphosphate bridge. Results from one- or two-stage reactions showed that the trisaccharide-P-P-polyprenol was the precursor of the polymer. This new polymer, a polytrisaccharide, was detected also in vivo. The transposon responsible for the mutation was located within gumK gene. Therefore, this gene encodes for the glycosyltransferase IV, which catalyses the transfer of glucuronic acid to the lipid-linked beta-D-Manp-(1-->3)-beta-D-Glcp-(1-->4)-beta-D-Glcp trisaccharide. A recombinant plasmid with the whole gum cluster restored the wild type phenotype.

Fastidian gum: the Xylella fastidiosa exopolysaccharide possibly involved in bacterial pathogenicity

The Gram-negative bacterium Xylella fastidiosa was the first plant pathogen to be completely sequenced. This species causes several economically important plant diseases, including citrus variegated chlorosis (CVC). Analysis of the genomic sequence of X. fastidiosa revealed a 12 kb DNA fragment containing an operon closely related to the gum operon of Xanthomonas campestris. The presence of all genes involved in the synthesis of sugar precursors, existence of exopolysaccharide (EPS) production regulators in the genome, and the absence of three of the X. campestris gum genes suggested that X. fastidiosa is able to synthesize an EPS different from that of xanthan gum. This novel EPS probably consists of polymerized tetrasaccharide repeating units assembled by the sequential addition of glucose-1-phosphate, glucose, mannose and glucuronic acid on a polyprenol phosphate carrier.

Location of O-acetyl groups in S-657 using the reductive-cleavage method

A two-step procedure is described in this paper to identify the position of O-acetyl groups in S-657 polysaccharide. Reductive-cleavage experiments performed on the fully methylated (base-catalyzed) polysaccharide, followed by acetylation of anhydroalditols, identified individual sugar residues and their position of linkage. In a second experiment, the polysaccharide was methylated under neutral conditions leaving native acetate groups intact. Reductive cleavage of the neutral methylated polysaccharide using CF3SO3SiMe3 as a catalyst, followed by acetylation in situ, identified sugar residues containing native acetate groups and established their position of substitution. Using this two-step procedure of analysis, S-657 polysaccharide is shown to contain O-acetyl groups on the 2-position and the 2,6-positions of 3-linked glucopyranosyl residues.

Role of galactomannan composition on the binary gel formation with xanthan

The influence of the galactomannan characteristic ratios (M/G) on the temperature of gelation (Tg) and the gel strength of mixtures of galactomannan with xanthan is reported. Two galactomannans were investigated: one highly substituted from the seeds of Mimosa scabrella (M/G = 11), and the other, less substituted, from the endosperm of Schizolobium parahybae, with (M/G = 30) [Ganter JLMS, Zawadzki-Baggio SF, Leitner SC, Sierakowski MR, Reicher F. J Carbohydr Chem 1993;12:753]. The xanthan:galactomannan systems (4:2 g l(-1), in 5 mM NaCl) showed a temperature of gel formation (Tg) of 24 degrees C for that of S. parahybae [Bresolin TMB, Milas M, Rinaudo M and Ganter JLMS. Int J Biol Macromol 1998;23:263] and 20 degrees C for the galactomannan of M. scabrella, determined by viscoelastic measurements and microcalorimetry. A Tg of 40-50 degrees C was found by Shatwell et al. [Shatwell KP, Sutherland IW, Ross-Murphy SB, Dea ICM. Carbohydr Polym 1991;14:29] for locust bean gum-LBG (M/G = 43). Lundin and Hermansson [Lundin L, Hermansson AM. Carbohydr Polym 1995;26:129] reported a difference of 13 degrees C for Tg of two LBG samples with M/G = 3 (40 degrees C) and 5 (53 degrees C), in mixtures with xanthan. It appears that the more substituted galactomannans have lower temperatures of gelation in the presence of xanthan. The mechanism of gelation depends also on the M/G ratio. For the lower values it involves only disordered xanthan chains in contrast to M/G ratios higher than 3. In addition, the presence of the galactomannan from M. scabrella increased slightly the temperature of the conformational change (Tm) of xanthan probably due to the ionic strength contribution of proteins (3.9%) present in the galactomannan. On the other hand, the galactomannans from S. parahybae, with 1.5% of proteins and M. scabrella, with 2.4% of protein, did not show this effect, the Tm of xanthan alone or in a mixture being practically unchanged.

New mobilizable vectors suitable for gene replacement in gram-negative bacteria and their use in mapping of the 3' end of the Xanthomonas campestris pv. campestris gum operon

We describe useful vectors to select double-crossover events directly in site-directed marker exchange mutagenesis in gram-negative bacteria. These vectors contain the gusA marker gene, providing colorimetric screens to identify bacteria harboring those sequences. The applicability of these vectors was shown by mapping the 3' end of the Xanthomonas campestris gum operon, involved in biosynthesis of xanthan.

Xanthan-galactomannan interactions as related to xanthan conformations

The influence of xanthan conformation on the physicochemical behaviour of their mixtures with galactomannan from Schizolobium parahybae mannose:galactose ratio (M/G=3), was studied by viscoelastic measurements, differential scanning calorimetry (DSC) and chiroptical (circular dichroism) methods. The results suggested a more effective interaction of the galactomannan with disordered xanthan segments, which are more abundant in low salt concentrations but are still present in lower proportion at temperatures lower than the temperature of xanthan conformational transition (Tm). The dependence of ellipticity with temperature in a circular dichroism (CD) spectra suggested an ordering of the xanthan chains induced by galactomannan at the temperature of gel formation (Tg approximately 25 degrees C), under conditions where xanthan alone exhibits a disordered conformation. The lower Tg value found (approximately 25 degrees C) compared with that (60 degrees C) usually described in the literature is certainly related to the M/G ratio and the galactosyl unit distribution along the mannan main chain.

Evidence for intermolecular binding between deacetylated acetan and the glucomannan konjac mannan

Binary mixtures of deacetylated acetan and konjac mannan form thermoreversible gels under conditions for which the individual components do not gel. Such synergistic behaviour is normally attributed to intermolecular binding between the two polysaccharides. X-ray diffraction data obtained from oriented fibres prepared from deacetylated acetan-konjac mannan gels provides direct evidence for intermolecular binding between the two polysaccharides. The novel heterotypic junction zones appear to be six-fold helices with a pitch of 5.6 +/- 0.1 nm.

Evidence for a role for the gumB and gumC gene products in the formation of xanthan from its pentasaccharide repeating unit by Xanthomonas campestris

The biosynthesis of the extracellular polysaccharide xanthan in Xanthomonas campestris pv. campestris is directed by a cluster of 12 genes, gumB-gumM. Several xanthan-deficient mutants of the wild-type strain 8004 have previously been described which carry Tn5 insertions in this region of the chromosome. Here it is shown that the transposon insertion in one of these mutants, strain 8397, is located 15 bp upstream of the translational start site of the gumB gene. EDTA-treated cells of strain 8397 were able to synthesize the lipid-linked pentasaccharide repeating unit of xanthan from the three nucleotide sugar donors (UDP-glucose, GDP-mannose and UDP-glucuronic acid) but were unable to polymerize the pentasaccharide into mature xanthan. A subclone of the gum gene cluster carrying gumB and gumC restored xanthan production to strain 8397 to levels approximately 28% of the wild-type. In contrast, subclones carrying gumB or gumC alone were not effective. These results are discussed with reference to previous speculations, based on computer analysis, that gumB and gumC are both involved in the translocation of xanthan across the bacterial membranes.

Xanthomonas campestris pv. campestris gum mutants: effects on xanthan biosynthesis and plant virulence

Xanthan is an industrially important exopolysaccharide produced by the phytopathogenic, gram-negative bacterium Xanthomonas campestris pv. campestris. It is composed of polymerized pentasaccharide repeating units which are assembled by the sequential addition of glucose-1-phosphate, glucose, mannose, glucuronic acid, and mannose on a polyprenol phosphate carrier (L. Ielpi, R. O. Couso, and M. A. Dankert, J. Bacteriol. 175:2490-2500, 1993). A cluster of 12 genes in a region designated xpsI or gum has been suggested to encode proteins involved in the synthesis and polymerization of the lipid intermediate. However, no experimental evidence supporting this suggestion has been published. In this work, from the biochemical analysis of a defined set of X. campestris gum mutants, we report experimental data for assigning functions to the products of the gum genes. We also show that the first step in the assembly of the lipid-linked intermediate is severely affected by the combination of certain gum and non-gum mutations. In addition, we provide evidence that the C-terminal domain of the gumD gene product is sufficient for its glucosyl-1-phosphate transferase activity. Finally, we found that alterations in the later stages of xanthan biosynthesis reduce the aggressiveness of X. campestris against the plant.

An investigation into the temperature dependence of the rheological synergy between xanthan gum and locust bean gum mixtures

The interaction between xanthan gum (XG) and locust bean gum (LBG) in water has been investigated using texture analysis, thermorheological analysis and high sensitivity differential scanning calorimetry. For the batches of XG and LBG used in the present study and at a total polymer concentration of 1% w/v, texture analysis indicated that the greatest synergy occurred at approximately 10% w/w XG, while the technique also allowed measurement of the increase in strength resulting from heating the mixes to 70 and 80 degrees C and cooling to room temperature. Thermorheological studies showed a maximum in viscosity for the xanthan gum gels at approximately 45 degrees C; this maximum became less pronounced as the concentration of XG decreased. High sensitivity differential scanning calorimetry indicated that XG undergoes two transitions at approximately 30 and 80 degrees C on heating, but on cycling a single transition is seen at approximately 74 degrees C. It was also noted that the transitions were diminished or were absent in the presence of LBG.

Structure and conformation of acetan polysaccharide

Acetan is an anionic bacterial polysaccharide. The chemical repeat unit consists of a cellobiose unit solubilised by attachment of a charged pentasaccharide sidechain to one of the glucose residues. The repeat unit contains two sites of acetylation. 1H and 13C NMR studies, coupled with both basic-methylation and mild-methylation studies, have shown that acetylation occurs at C6 on the (1,2)D-Man and the (1,34)D-Glc residues. A variety of techniques including NMR, optical rotation, circular dichroism and DSC show evidence for a thermoreversible conformational order (helix)-disorder (coil) transition for acetan in aqueous solution. The studies suggest that acetylation of the backbone does not prevent helix formation.

Isolation and nucleotide sequence of the GDP-mannose:cellobiosyl-diphosphopolyprenol alpha-mannosyltransferase gene from Acetobacter xylinum

A genetic locus from Acetobacter xylinum involved in acetan polysaccharide synthesis has been characterized. The chromosomal region was identified by screening a genomic library of A. xylinum in a Xanthomonas campestris mutant defective in xanthan polysaccharide synthesis. The A. xylinum cosmid clone can functionally complement a xanthan-negative mutant. The polymer produced by the recombinant strain was found to be indistinguishable from xanthan. Insertion mutagenesis and subcloning of the cosmid clone combined with complementation studies allowed the identification of a 2.3-kb fragment of A. xylinum chromosomal DNA. The nucleotide sequence of this fragment was analyzed and found to contain an open reading frame (aceA) of 1,182 bp encoding a protein of 43.2 kDa. Results from biochemical and genetic analyses strongly suggest that the aceA gene encodes the GDP-mannose:cellobiosyl-diphosphopolyprenol alpha-mannosyltransferase enzyme, which is responsible for the transfer of an alpha-mannosyl residue from GDP-Man to cellobiosyl-diphosphopolyprenol. A search for similarities with other known mannosyltransferases revealed that all bacterial alpha-mannosyltransferases have a short COOH-terminal amino acid sequence in common.

Promoter analysis of the Xanthomonas campestris pv. campestris gum operon directing biosynthesis of the xanthan polysaccharide

The Xanthomonas campestris gum gene cluster is composed of 12 genes designated gumB, -C, -D, -E, -F, -G, -H, -I, -J, -K, -L, and -M. The transcriptional organization of this gene cluster was analyzed by the construction of gum-lacZ transcriptional fusions in association with plasmid integration mutagenesis. This analysis, coupled with primer extension assays, indicated that the gum region was mainly expressed as an operon from a promoter located upstream of the first gene, gumB.

Physicochemical characterization of an acetan variant secreted by Acetobacter xylinum strain CR1/4

Chemical mutagenesis has been used to produce mutants of Acetobacter xylinum NRRL B42 that are cellulose-negative and that produce variants of the acetan structure deficient in the side-chain sugar residues. The product of A. xylinum strain CR1/4 has been shown to possess a tetrasaccharide repeat unit with the side chain terminating in glucuronic acid. X-ray diffraction studies of oriented fibres suggest that the polysaccharide CR1/4 forms a fivefold helix with a pitch of 4.8 nm. Light-scattering studies on CR1/4 solutions suggest a molecular weight of 1.2 x 10(6) with radii of gyration values of 86 nm (aqueous solution) and 67 nm (0.1 M NaCl solution). The magnitude of the measured radii of gyration and the shape of the Holtzer plots suggest that CR1/4 can be described as a stiff coil. Preliminary differential scanning calorimetry data show melting behaviour consistent with order-disorder transitions of a charged helical structure. Rheological studies have revealed new synergistic interactions of CR1/4 with locust bean gum. Comparative studies of acetan and CR1/4 show that decreasing the length of the side chain enhances the solution viscosity.

Analysis of oligosaccharides containing 2-deoxy-alpha-D-arabino-hexosyl residues by the reductive-cleavage method

A mixture of oligosaccharides containing (1-->4)-linked 2-deoxy-alpha-D-arabino-hexosyl ("2-deoxyglucosyl") and (1-->4)-linked alpha-D-glucosyl residues (1) was analyzed by reduction, permethylation (perethylation), degradation to monomers, and GLC-MS. Degradation was performed either by hydrolysis with subsequent reduction, by methanolysis, or by reductive cleavage, always followed by acetylation. Reductive cleavage turned out to be the method of choice for the acid-labile 2-deoxy sugars. The main degradation product formed during acid hydrolysis of 2-deoxy-D-arabino-hexosyl residues yielded, after reduction and acetylation, (4R,S)-6-O-acetyl-2,3,5-trideoxyhexono-1,4-lactone (7). By methanolysis, in addition to the expected methyl glycosides, methyl 2,3,5-trideoxy-6-O-methyl-4-hexulosonate (12) is formed as a by-product. For determination of the distribution of chain lengths, the permethylated oligomers were separated by reversed-phase HPLC. For peak assignment, one isolated oligomer was investigated by FABMS and 1H NMR spectroscopy. The average degree of polymerization (dp) calculated from the HPLC chromatogram is in good agreement with the reductive-cleavage results.

Structure-function relationships in microbial exopolysaccharides

Sufficient well-characterized microbial exopolysaccharides are now available to permit extensive studies on the relationship between their chemical structure and their physical attributes. This is seen even in homopolysaccharides with relatively simple structures but is more marked when greater differences in structure exist, as are found in several heteropolysaccharides. The specific and sometimes unique properties have, in the case of several of these polymers, provided a range of commercial applications. The existence of "families" of structurally related polysaccharides also indicates the specific role played by certain structures and substituents; the characteristics of several of these microbial polysaccharide families will be discussed here. Thus, microbial exopolysaccharides frequently carry acyl groups which may profoundly affect their interactive properties although these groups often have relatively little effect on solution viscosity. Xanthan with or without acylation shows marked differences in synergistic gelling with plant gluco- and galacto-mannans, although the polysaccharides with different acylation patterns show similar viscosity. Similarly "gelrite" from the bacterium originally designated as Auromonas (Pseudomonas)elodea is of greater potential value after deacetylation, when it provides a valuable gelling agent, than it is as a viscosifier in the natural acylated form. The Klebsiella type 54 polysaccharide only forms gels when it, too, has been chemically deacetylated to give a structure equivalent to the Enterobacter XM6 polymer. Both these polysaccharides form gels due to the enhanced interaction with cations following deacylation and to the conformation adopted after removal of the acyl groups. Recent work in our laboratory suggests that deacetylation of certain bacterial alginates also significantly increases ion binding by these polysaccharides, making them more similar in their properties to algal alginates even although the alginates from some Pseudomonas species lack poly-L-guluronic acid sequences. The existence within families of polysaccharides of types in which monosaccharides are altered within a specific structure, or with varying side-chains, also gives an indication of the way in which such substituents affect the physical properties of the polymers in aqueous solution.