Structural studies of a phosphocholine substituted beta-(1,3);(1,6) macrocyclic glucan from Bradyrhizobium japonicum USDA 110

Configured for the Human Gut Microbiota: Molecular Mechanisms of Dietary β-Glucan Utilization

The β-glucans are a disparate group of structurally diverse polysaccharides, whose members are widespread in human diets as components of the cell walls of plants, algae, and fungi (including yeasts), and as bacterial exopolysaccharides. Individual β-glucans from these sources have long been associated with positive effects on human health through metabolic and immunological effects. Remarkably, the β-configured glucosidic linkages that define these polysaccharides render them inaccessible to the limited repertoire of digestive enzymes encoded by the human genome. As a result, the various β-glucans become fodder for the human gut microbiota (HGM) in the lower gastrointestinal tract, where they influence community composition and metabolic output, including fermentation to short chain fatty acids (SCFAs). Only recently, however, have the specific molecular systems that enable the utilization of β-glucans by select members of the HGM been fully elucidated by combined genetic, biochemical, and structural biological approaches. In the context of β-glucan structures and their effects on human nutrition and health, we summarize here the functional characterization of individual polysaccharide utilization loci (PULs) responsible for the saccharification of mixed-linkage β(1→3)/β(1→4)-glucans, β(1→6)-glucans, β(1→3)-glucans, β(1→2)-glucans, and xyloglucans in symbiotic human gut bacteria. These exemplar PULs serve as well-defined biomarkers for the prediction of β-glucan metabolic capability in individual bacterial taxa and across the global human population.

Microbial cyclic β-(1→3),(1→6)-glucans as potential drug carriers: Interaction studies between cyclic β-glucans isolated from Bradyrhizobium japonicum and betulinic acid

Betulinic acid (BA), a pentacyclic triterpenoid, is a very promising therapeutic drug with varied medicinal properties but it has low water solubility and consequentially low bioavailability. Cyclic β-(1→3),(1→6)-glucans (CBG), microbial cyclooligosaccharides produced by Bradyrhizobium japonicum ATCC 10324 having a cavity structure and good solubility in water have been tested for their ability to encapsulate betulinic acid and drug-binding interactions of CBG and BA were studied. First, in silico approach was employed to study the scope of any interaction between the CBG and BA. Then, the cyclic glucan-betulinic acid complexes were prepared in three compositions of 1:1, 1:2 and 1:3 CBG:BA. The complexes were analysed using UV-VIS spectroscopy, IR spectroscopy, powder XRD, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to confirm the computational results and consequently the encapsulation efficiency was found to be 9.53%.

Osmoregulated Periplasmic Glucans

Among all the systems developed by enterobacteria to face osmotic stress, only osmoregulated periplasmic glucans (OPGs) were found to be modulated during osmotic fluxes. First detected in 1973 by E.P. Kennedy's group in a study of phospholipid turnover in Escherichia coli, OPGs have been shown across alpha, beta, and gamma subdivisions of the proteobacteria. Discovery of OPG-like compounds in the epsilon subdivision strongly suggested that the presence of periplasmic glucans is essential for almost all proteobacteria. This article offers an overview of the different classes of OPGs. Then, the biosynthesis of OPGs and their regulation in E. coli and other species are discussed. Finally, the biological role of OPGs is developed. Beyond structural function, OPGs are involved in pathogenicity, in particular, by playing a role in signal transduction pathways. Recently, OPG synthesis proteins have been suggested to control cell division and growth rate.

Modification of linear (β1→3)-linked gluco-oligosaccharides with a novel recombinant β-glucosyltransferase (trans-β-glucosidase) enzyme from Bradyrhizobium diazoefficiens

Recently, we have shown that glycoside hydrolases enzymes of family GH17 from proteobacteria (genera Pseudomonas, Azotobacter) catalyze elongation transfer reactions with laminari-oligosaccharides generating (β1→3) linkages preferably and to a lesser extent (β1→6) or (β1→4) linkages. In the present study, the cloning and characterization of the gene encoding the structurally very similar GH17 domain of the NdvB enzyme from Bradyrhizobium diazoefficiens, designated Glt20, as well as its catalytic properties are described. The Glt20 enzyme was strikingly different from the previously investigated bacterial GH17 enzymes, both regarding substrate specificity and product formation. The Azotobacter and Pseudomonas enzymes cleaved the donor laminari-oligosaccharide substrates three or four moieties from the non-reducing end, generating linear oligosaccharides. In contrast, the Glt20 enzyme cleaved donor laminari-oligosaccharide substrates two glucose moieties from the reducing end, releasing laminaribiose and transferring the remainder to laminari-oligosaccharide acceptor substrates creating only (β1→3)(β1→6) branching points. This enables Glt20 to transfer larger oligosaccharide chains than the other type of bacterial enzymes previously described, and helps explain the biologically significant formation of cyclic β-glucans in B. diazoefficiens.

Bacterial Molecular Signals in the Sinorhizobium fredii-Soybean Symbiosis

Sinorhizobium (Ensifer) fredii (S. fredii) is a rhizobial species exhibiting a remarkably broad nodulation host-range. Thus, S. fredii is able to effectively nodulate dozens of different legumes, including plants forming determinate nodules, such as the important crops soybean and cowpea, and plants forming indeterminate nodules, such as Glycyrrhiza uralensis and pigeon-pea. This capacity of adaptation to different symbioses makes the study of the molecular signals produced by S. fredii strains of increasing interest since it allows the analysis of their symbiotic role in different types of nodule. In this review, we analyze in depth different S. fredii molecules that act as signals in symbiosis, including nodulation factors, different surface polysaccharides (exopolysaccharides, lipopolysaccharides, cyclic glucans, and K-antigen capsular polysaccharides), and effectors delivered to the interior of the host cells through a symbiotic type 3 secretion system.

An overview of the metabolic differences between Bradyrhizobium japonicum 110 bacteria and differentiated bacteroids from soybean (Glycine max) root nodules: an in vitro 13C- and 31P-nuclear magnetic resonance spectroscopy study

Bradyrhizobium japonicum is a symbiotic nitrogen-fixing soil bacteria that induce root nodules formation in legume soybean (Glycine max.). Using (13)C- and (31)P-nuclear magnetic resonance (NMR) spectroscopy, we have analysed the metabolite profiles of cultivated B. japonicum cells and bacteroids isolated from soybean nodules. Our results revealed some quantitative and qualitative differences between the metabolite profiles of bacteroids and their vegetative state. This includes in bacteroids a huge accumulation of soluble carbohydrates such as trehalose, glutamate, myo-inositol and homospermidine as well as Pi, nucleotide pools and intermediates of the primary carbon metabolism. Using this novel approach, these data show that most of the compounds detected in bacteroids reflect the metabolic adaptation of rhizobia to the surrounding microenvironment with its host plant cells.

The effects of beta-glucan on human immune and cancer cells

Non-prescriptional use of medicinal herbs among cancer patients is common around the world. The alleged anti-cancer effects of most herbal extracts are mainly based on studies derived from in vitro or in vivo animal experiments. The current information suggests that these herbal extracts exert their biological effect either through cytotoxic or immunomodulatory mechanisms. One of the active compounds responsible for the immune effects of herbal products is in the form of complex polysaccharides known as beta-glucans. beta-glucans are ubiquitously found in both bacterial or fungal cell walls and have been implicated in the initiation of anti-microbial immune response. Based on in vitro studies, beta-glucans act on several immune receptors including Dectin-1, complement receptor (CR3) and TLR-2/6 and trigger a group of immune cells including macrophages, neutrophils, monocytes, natural killer cells and dendritic cells. As a consequence, both innate and adaptive response can be modulated by beta-glucans and they can also enhance opsonic and non-opsonic phagocytosis. In animal studies, after oral administration, the specific backbone 1-->3 linear beta-glycosidic chain of beta-glucans cannot be digested. Most beta-glucans enter the proximal small intestine and some are captured by the macrophages. They are internalized and fragmented within the cells, then transported by the macrophages to the marrow and endothelial reticular system. The small beta-glucans fragments are eventually released by the macrophages and taken up by other immune cells leading to various immune responses. However, beta-glucans of different sizes and branching patterns may have significantly variable immune potency. Careful selection of appropriate beta-glucans is essential if we wish to investigate the effects of beta-glucans clinically. So far, no good quality clinical trial data is available on assessing the effectiveness of purified beta-glucans among cancer patients. Future effort should direct at performing well-designed clinical trials to verify the actual clinical efficacy of beta-glucans or beta-glucans containing compounds.

Global consequences of phosphatidylcholine reduction in Bradyrhizobium japonicum

Phosphatidylcholine (PC) is the major phospholipid in eukaryotic membranes. In contrast, it is found in only a limited number of bacteria including members of the Rhizobiales. Here, PC is required for pathogenic and symbiotic plant-microbe interactions, as shown for Agrobacterium tumefaciens and Bradyrhizobium japonicum, respectively. Two different phospholipid N-methyltransferases, PmtA and PmtX1, convert phosphatidylethanolamine (PE) to PC by three consecutive methylation reactions in B. japonicum. PmtA mainly catalyzes the first methylation reaction converting PE to monomethyl PE, which then serves as substrate for PmtX1 performing the last two methylation reactions. Disruption of the pmtA gene results in a significantly reduced PC content causing a defect in symbiosis with the soybean host. A genome-wide survey for differentially expressed genes in the pmtA mutant with a custom-made Affymetrix gene chip revealed that PC reduction affects transcription of a strictly confined set of genes. Among the 11 up regulated genes were pmtX3 and pmtX4, which code for isoenzymes of PmtA. The expression of two typical two-component systems, a MarR-like regulator and two proteins of a RND-type (resistance nodulation cell division) efflux system were differentially expressed in the pmtA mutant. Our data suggests that a decrease in the PC content of B. japonicum membranes induces a rather specific transcriptional response involving three different transcriptional regulators all involved in the regulatory fine-tuning of a RND-type transport system.

Enantioseparation of some chiral flavanones using microbial cyclic beta-(1-->3),(1-->6)-glucans as novel chiral additives in capillary electrophoresis

Cyclic beta-(1-->3),(1-->6)-glucans, microbial cyclooligosaccharides produced by Bradyrhizobium japonicum USDA 110, were used as novel chiral additives for the enantiomeric separation of some flavanones such as eriodictyol, homoeriodictyol, hesperetin, naringenin, and isosakuranetin in capillary electrophoresis (CE). Among the flavanones, eriodictyol was separated with the highest resolution (R(s) 5.66) and selectivity factor (alpha 1.18) when 20mM cyclic beta-(1-->3),(1-->6)-glucans were added to the background electrolyte (BGE) at pH 8.3.

The Brucella abortus cyclic beta-1,2-glucan virulence factor is substituted with O-ester-linked succinyl residues

Brucella periplasmic cyclic beta-1,2-glucan plays an important role during bacterium-host interaction. Nuclear magnetic resonance spectrometry analysis, thin-layer chromatography, and DEAE-Sephadex chromatography were used to characterize Brucella abortus cyclic glucan. In the present study, we report that a fraction of B. abortus cyclic beta-1,2-glucan is substituted with succinyl residues, which confer anionic character on the cyclic beta-1,2-glucan. The oligosaccharide backbone is substituted at C-6 positions with an average of two succinyl residues per glucan molecule. This O-ester-linked succinyl residue is the only substituent of Brucella cyclic glucan. A B. abortus open reading frame (BAB1_1718) homologous to Rhodobacter sphaeroides glucan succinyltransferase (OpgC) was identified as the gene encoding the enzyme responsible for cyclic glucan modification. This gene was named cgm for cyclic glucan modifier and is highly conserved in Brucella melitensis and Brucella suis. Nucleotide sequencing revealed that B. abortus cgm consists of a 1,182-bp open reading frame coding for a predicted membrane protein of 393 amino acid residues (42.7 kDa) 39% identical to Rhodobacter sphaeroides succinyltransferase. cgm null mutants in B. abortus strains 2308 and S19 produced neutral glucans without succinyl residues, confirming the identity of this protein as the cyclic-glucan succinyltransferase enzyme. In this study, we demonstrate that succinyl substituents of cyclic beta-1,2-glucan of B. abortus are necessary for hypo-osmotic adaptation. On the other hand, intracellular multiplication and mouse spleen colonization are not affected in cgm mutants, indicating that cyclic-beta-1,2-glucan succinylation is not required for virulence and suggesting that no low-osmotic stress conditions must be overcome during infection.

Curdlan and other bacterial (1→3)-β-d-glucans

Three structural classes of (1-->3)-beta-D-glucans are encountered in some important soil-dwelling, plant-associated or human pathogenic bacteria. Linear (1-->3)-beta-glucans and side-chain-branched (1-->3,1-->2)-beta-glucans are major constituents of capsular materials, with roles in bacterial aggregation, virulence and carbohydrate storage. Cyclic (1-->3,1-->6)-beta-glucans are predominantly periplasmic, serving in osmotic adaptation. Curdlan, the linear (1-->3)-beta-glucan from Agrobacterium, has unique rheological and thermal gelling properties, with applications in the food industry and other sectors. This review includes information on the structure, properties and molecular genetics of the bacterial (1-->3)-beta-glucans, together with an overview of the physiology and biotechnology of curdlan production and applications of this biopolymer and its derivatives.

Structural analyses of novel glycerophosphorylated α-cyclosophorohexadecaoses isolated from X. campestris pv. campestris

Novel periplasmic anionic cyclic glucans produced by Xanthomonas campestris pv. campestris were isolated by trichloroacetic acid treatment and various chromatographic techniques. No report has been made on the presence of substituted cyclic glucans of the Xanthomonas species. We show, for the first time, that X. campestris pv. campestris produces the anionic cyclic glucans with phosphoglycerol residues, the presence of which can be predicted by analyzing the sequence database with the aid of the NCBI RefSeq database. To analyze the structure of isolated anionic cyclic glucans analyses, we used NMR spectroscopy, matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOFMS) and electrospray-ionization mass spectrometry (ESIMS). The results suggest that the novel anionic forms of the cyclic glucans of X. campestris pv. campestris are glycerophosphorylated alpha-cyclosophorohexadecaose with one or two phosphoglycerol substituents at the C-6 positions of the glucose residues.

Isolation and characterization of periplasmic cyclic β-glucans ofAzorhizobium caulinodans

Oligoglucose molecules isolated from Azorhizobium caulinodans were characterized by compositional analysis, Smith degradation, matrix-assisted laser desorption/ionization time of flight mass spectrometry, and (1)H and (13)C nuclear magnetic resonance analysis. A. caulinodans produced nonbranched and unsubstituted cyclic glucans composed solely of glucose, with the degree of polymerization ranging from 10 to 13. A major fraction of the periplasmic glucans contains 11 glucose residues within rings. The glucose residues are linked by beta-(1,3) and beta-(1,6) glycosidic bonds. These molecules seem to be quite similar to the periplasmic beta-(1,3);(1,6)-glucans synthesized by the Bradyrhizobium strain and are substantially different from the cyclic beta-(1,2)-glucans produced by Agrobacterium and Sinorhizobium species. Azorhizobial cyclic glucan synthesis is not osmoregulated. The response to the osmotic stress in Azorhizobium can be regulated similarly to Brucella spp. It is probable that the biosynthesis of beta-glucans is subject to the feedback control mechanism.

Biosynthesis of phosphatidylcholine in bacteria

Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes and can be synthesized by either of two pathways, the methylation pathway or the CDP-choline pathway. Many prokaryotes lack PC, but it can be found in significant amounts in membranes of rather diverse bacteria and based on genomic data, we estimate that more than 10% of all bacteria possess PC. Enzymatic methylation of phosphatidylethanolamine via the methylation pathway was thought to be the only biosynthetic pathway to yield PC in bacteria. However, a choline-dependent pathway for PC biosynthesis has been discovered in Sinorhizobium meliloti. In this pathway, PC synthase, condenses choline directly with CDP-diacylglyceride to form PC in one step. A number of symbiotic (Rhizobium leguminosarum, Mesorhizobium loti) and pathogenic (Agrobacterium tumefaciens, Brucella melitensis, Pseudomonas aeruginosa, Borrelia burgdorferi and Legionella pneumophila) bacteria seem to possess the PC synthase pathway and we suggest that the respective eukaryotic host functions as the provider of choline for this pathway. Pathogens entering their hosts through epithelia (Streptococcus pneumoniae, Haemophilus influenzae) require phosphocholine substitutions on their cell surface components that are biosynthetically also derived from choline supplied by the host. However, the incorporation of choline in these latter cases proceeds via choline phosphate and CDP-choline as intermediates. The occurrence of two intermediates in prokaryotes usually found as intermediates in the eukaryotic CDP-choline pathway for PC biosynthesis raises the question whether some bacteria might form PC via a CDP-choline pathway.

Characterization of ndvD, the third gene involved in the synthesis of cyclic beta-(1 --> 3),(1 --> 6)-D-glucans in Bradyrhizobium japonicum

Previously, we identified two genes in Bradyrhizobium japonicum (ndvB, ndvC) that are required for cyclic beta-(1 --> 3),(1 --> 6)-D-glucan synthesis and successful symbiotic interaction with soybean (Glycine max). In this study, we report a new open reading frame (ORF1) located in the intergenic region between ndvB and ndvC, which is essential for beta-glucan synthesis and effective nodulation of G. max. This new gene is designated ndvD (nodule development). The ndvD translation product has a predicted molecular mass of 26.4 kDa with one transmembrane domain. Genetic experiments involving gene deletion, Tn5 insertion, and gene complementation revealed that the mutation of ndvD generated pleiotropic phenotypes, including hypoosmotic sensitivity, reduced motility, and defects in conjugative gene transfer, in addition to symbiotic ineffectiveness. Although deficient in in vivo beta-glucan synthesis, membrane preparations from the ndvD mutant synthesized neutral beta-glucans in vitro. Therefore, ndvD does not appear to be a structural gene for beta-glucan synthesis. Our hypothesis for the mechanism of beta-(1 --> 3),(1 --> 6)-D-glucan synthesis is presented.

Analysis of phospholipids and ornithine-containing lipids from Mesorhizobium spp

Polar lipid compositions of seven strains belonging to the four species of the Mesorhizobium genus were described. The lipid patterns of Mesorhizobium strains were very similar. Only quantitative differences were observed. Diphosphatidylglycerol (DPG), phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and phosphatidylcholine (PC) were found to be the major phospholipids of the analysed bacteria. In addition, two methylated derivatives of PE were observed: phosphatidyl-N,N-dimethylethanolamine (DMPE) and phosphatidyl-N-monomethylethanolamine (MMPE). Polar head groups of those phospholipids were predominately acylated with lactobacillic (19:0 cyclopropane) acid. Ornithine-containing lipid (OL) was also identified. 3-hydroxy fatty acids found in the lipid preparations were derived exclusively from the ornithine lipid. 3-hydroxylactobacillic was the main acyl residue amide linked to the ornithine.

Osmoregulated periplasmic glucans of the free-living photosynthetic bacterium Rhodobacter sphaeroides

The osmoregulated periplasmic glucans (OPGs) produced by Rhodobacter sphaeroides, a free-living organism, were isolated by trichloracetic acid treatment and gel permeation chromatography. Compounds obtained were characterized by compositional analysis, matrix-assisted laser desorption ionization mass spectrometry and nuclear magnetic resonance. R. sphaeroides predominantly synthesizes a cyclic glucan containing 18 glucose residues that can be substituted by one to seven succinyl esters residues at the C6 position of some of the glucose residues, and by one or two acetyl residues. The glucans were subjected to a mild alkaline treatment in order to remove the succinyl and acetyl substituents, analyzed by MALDI mass spectrometry and purified by high-performance anion-exchange chromatography. Methylation analysis revealed that this glucan is linked by 17 1,2 glycosidic bonds and one 1,6 glycosidic bond. Homonuclear and (1)H/(13)C heteronuclear NMR experiments revealed the presence of a single alpha-1,6 glycosidic linkage, whereas all other glucose residues are beta-1,2 linked. The different anomeric proton signals allowed a complete sequence-specific assignment of the glucan. The structural characteristics of this glucan are very similar to the previously described OPGs of Ralstonia solanacearum and Xanthomonas campestris, except for its different size and the presence of substituents. Therefore, similar OPGs are synthesized by phytopathogenic as well as free-living bacteria, suggesting these compounds are intrinsic components of the Gram-negative bacterial envelope.

PROBING PLANT METABOLISM WITH NMR

Analytical methods for probing plant metabolism are taking on new significance in the era of functional genomics and metabolic engineering. Among the available methods, nuclear magnetic resonance (NMR) spectroscopy is a technique that can provide insights into the integration and regulation of plant metabolism through a combination of in vivo and in vitro measurements. Thus NMR can be used to identify, quantify, and localize metabolites, to define the intracellular environment, and to explore pathways and their operation. We review these applications and their significance from a metabolic perspective. Topics of current interest include applications of NMR to metabolic flux analysis, metabolite profiling, and metabolite imaging. These and other areas are discussed in relation to NMR investigations of intermediary carbon and nitrogen metabolism. We conclude that metabolic NMR has a continuing role to play in the development of a quantitative understanding of plant metabolism and in the characterization of metabolic phenotypes.

Osmoregulated periplasmic glucans of Erwinia chrysanthemi

We report the initial characterization of the osmoregulated periplasmic glucans (OPGs) of Erwinia chrysanthemi. OPGs are intrinsic components of the bacterial envelope necessary to the pathogenicity of this phytopathogenic enterobacterium (F. Page, S. Altabe, N. Hugouvieux-Cotte-Pattat, J.-M. Lacroix, J. Robert-Baudouy and J.-P. Bohin, J. Bacteriol. 183:0000-0000, 2001 [companion in this issue]). OPGs were isolated by trichloracetic acid treatment and gel permeation chromatography. The synthesis of these compounds appeared to be osmoregulated, since lower amounts of OPGs were produced when bacteria were grown in media of higher osmolarities. However, a large fraction of these OPGs were recovered in the culture medium. Then, these compounds were characterized by compositional analysis, high-performance anion-exchange chromatography, matrix-assisted laser desorption mass spectrometry, and (1)H and (13)C nuclear magnetic resonance analyses. OPGs produced by E. chrysanthemi are very heterogeneous at the level of both backbone structure and substitution of these structures. The degree of polymerization of the glucose units ranges from 5 to 12. The structures are branched, with a linear backbone consisting of beta-1,2-linked glucose units to which a variable number of branches, composed of one glucose residue, are attached by beta-1,6 linkages in a random way. This glucan backbone may be substituted by O-acetyl and O-succinyl ester-linked residues.

Root nodulation and infection factors produced by rhizobial bacteria

Rhizobia are soil bacteria that can engage in a symbiosis with leguminous plants that produces nitrogen-fixing root nodules. This symbiosis is based on specific recognition of signal molecules, which are produced by both the bacterial and plant partners. In this review, recognition factors from the bacterial endosymbionts are discussed, with particular attention to secreted and cell surface glycans. Glycans that are discussed include the Nod factors, the extracellular polysaccharides, the lipopolysaccharides, the K-antigens, and the cyclic glucans. Recent advances in the understanding of the biosynthesis, secretion, and regulation of production of these glycans are reviewed, and their functions are compared with glycans produced by other bacteria, such as plant pathogens.

Synthesis of a low-molecular-weight form of exopolysaccharide by Bradyrhizobium japonicum USDA 110

A novel extracellular low-molecular-weight polysaccharide was detected as a contaminant within extracellular cyclic beta-1,6-beta-1,3-glucan preparations from Bradyrhizobium japonicum USDA 110 cultures. Compositional analysis, methylation analysis, and nuclear magnetic resonance analysis revealed that this low-molecular-weight polysaccharide was composed of the same pentasaccharide repeating unit previously described for the high-molecular-weight form of the exopolysaccharide (EPS) synthesized by B. japonicum strains. Mass spectrometry analysis indicated that the size of this low-molecular-weight form of EPS was consistent with a dimeric form of the pentasaccharide repeating unit.

Osmoregulated periplasmic glucans in Proteobacteria

Large amounts of osmoregulated periplasmic glucans (OPGs) are found in the periplasmic space of Proteobacteria. Four families of OPGs are described on the basis of structural features of the polyglucose backbone. Depending on the species considered, OPGs can be modified to various extent by a variety of substituents. Genes governing the backbone synthesis are identified in a limited number of species. They belong to three unrelated families. OPG synthesis is subject to osmoregulation and feedback control. Osmoregulation can occur at the level of gene expression and/or at the level of enzyme activity. Mutants defective in OPG synthesis have a highly pleiotropic phenotype, indicative of an overall alteration of their envelope properties. Mutants of this kind were obtained as attenuated or avirulent derivatives of plant or animals pathogen. Thus, OPGs appear to be important intrinsic components of the Gram-negative bacterial envelope, which can be essential in extreme conditions found in nature, and especially when bacteria must interact with an eukaryotic host.

The mdoC gene of Escherichia coli encodes a membrane protein that is required for succinylation of osmoregulated periplasmic glucans

Osmoregulated periplasmic glucans (OPGs) of Escherichia coli are anionic oligosaccharides that accumulate in the periplasmic space in response to low osmolarity of the medium. Their anionic character is provided by the substitution of the glucosidic backbone by phosphoglycerol originating from the membrane phospholipids and by succinyl residues from unknown origin. A phosphoglycerol-transferase-deficient mdoB mutant was subjected to Tn5 transposon mutagenesis, and putative mutant clones were screened for changes in the anionic character of OPGs by thin-layer chromatography. One mutant deficient in succinylation of OPGs was obtained, and the gene inactivated in this mutant was characterized and named mdoC. mdoC, which encodes a membrane-bound protein, is closely linked to the mdoGH operon necessary for the synthesis of the OPG backbone.

Glycine betaine: reserve form of choline in Penicillium fellutanum in low-sulfate medium

In spite of choline's importance in fungal metabolism, its sources in cytoplasm have not been fully established. 13C nuclear magnetic resonance analysis of mycelial extracts from day-5 Penicillium fellutanum cultures showed that, as well as choline-O-sulfate, intracellular glycine betaine is another reserve form of choline, depending on the availability of sulfate in the culture medium. These observations are discussed relative to the multiple roles of choline and its precursors in P. fellutanum.

Further studies of the role of cyclic beta-glucans in symbiosis. An NdvC mutant of Bradyrhizobium japonicum synthesizes cyclodecakis-(1-->3)-beta-glucosyl

The cyclic beta-(1-->3),beta-(1-->6)-D-glucan synthesis locus of Bradyrhizobium japonicum is composed of at least two genes, ndvB and ndvC. Mutation in either gene affects glucan synthesis, as well as the ability of the bacterium to establish a successful symbiotic interaction with the legume host soybean (Glycine max). B. japonicum strain AB-14 (ndvB::Tn5) does not synthesize beta-glucans, and strain AB-1 (ndvC::Tn5) synthesizes a cyclic beta-glucan lacking beta-(1-->6)-glycosidic bonds. We determined that the structure of the glucan synthesized by strain AB-1 is cyclodecakis-(1-->3)-beta-D-glucosyl, a cyclic beta-(1-->3)-linked decasaccharide in which one of the residues is substituted in the 6 position with beta-laminaribiose. Cyclodecakis-(1-->3)-beta-D-glucosyl did not suppress the fungal beta-glucan-induced plant defense response in soybean cotyledons and had much lower affinity for the putative membrane receptor protein than cyclic beta-(1-->3),beta-(1-->6)-glucans produced by wild-type B. japonicum. This is consistent with the hypothesis presented previously that the wild-type cyclic beta-glucans may function as suppressors of a host defense response.

Oligoglucoside elicitor-mediated activation of plant defense

Plants have acquired defense mechanisms to counteract potential pathogens. One such strategy involves inducible defense reactions that are activated by elicitors, signaling compounds of diverse nature. For one class of elicitors, oligoglucosides, recent developments in the characterization and isolation of an oligoclucan-binding protein, a putative elicitor receptor, and isolation of a cDNA that encodes the binding protein are discussed. Furthermore, the discovery of a role for calcium in the elicitation process is described. Finally, the identification of polymerase chain reaction products whose sequences indicate that they encode cytochrome P-450-dependent enzymes with possible roles in the formation of phytoalexins, antimicrobial plant defense compounds, is reported. These advances may lay the foundation for the first characterization of a receptor and subsequent signaling events in oligoglucan elicitor perception by higher plants.

Utilization of phosphocholine from extracellular complex polysaccharide as a source of cytoplasmic choline derivatives in Penicillium fellutanum

Penicillium fellutanum produces a phosphorylated, choline-containing extracellular polysaccharide, peptidophosphogalactomannan (pP(x)GM) [where x is the number of phosphodiester residues]). The 13C-methyl-labeled pP(x)GM ([methyl-13C]pP(x)GM) was prepared from the cultures supplemented with L-[methyl-13C]methionine and was used as a probe to monitor the fate of phosphocholine in this polymer. The addition of [methyl-13C]pP(x)GM to growing cultures in low-phosphate medium resulted in the disappearance within 5 days of [methyl-13C]phosphocholine and N,N'-dimethylphosphoethanolamine from the added [methyl-13C]pP(x)GM. Two 13C-methyl-enriched cytoplasmic solutes, choline-O-sulfate and glycine betaine, were found in mycelial extracts, suggesting that phosphocholine-containing extracellular pP(x)GM of P. fellutanum is a precursor of intracellular choline-O-sulfate and glycine betaine. The mycelia cultured in low-phosphate (2 mM) medium contained glycine betaine and 1.5-fold more choline-O-sulfate than those grown in high-phosphate (20 mM) medium. The high levels of extracellular nonspecific phosphocholine:phosphocholine hydrolase and acid phosphomonoesterase observed in the low-phosphate culture medium are likely related to the release of phosphocholine from pP(x)GM and hydrolysis of phosphocholine, respectively. These results suggest that extracellular pP(x)GM of P. fellutanum provides phosphate needed as the environment becomes depleted of this nutrient. Choline, in excess of that needed immediately, is stored in the cytoplasm in forms that can be reutilized.

Cyclolaminarinose. A new biologically active beta-(1-->3) cyclic glucan

A unique glucan has been isolated from a recombinant strain of a Rhizobium meliloti TY7, a cyclic beta-(1-->2) glucan mutant carrying a locus specifying beta-(1-->3; 1-->6) glucan synthesis from Bradyrhizobium japonicum USDA110. This compound, which appears to have considerable hydrophobic affinity, was separated from a perchloric acid cell extract by adsorption to a C-18 silica column. Unlike those cyclic glucans previously isolated from Rhizobium meliloti or Bradyrhizobium japonicum, this molecule contains neither phosphoglycerol nor phosphocholine substituents, respectively. 2D NMR, FAB mass spectrometric analysis and high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) confirmed that this glucan is a single, cyclic decasaccharide (cyclolaminarinose) in which one of the residues is substituted in its 6-position with beta-laminarabiose. This structural assignment was confirmed by mass spectral and NMR analyses of the product obtained from two consecutive Smith degradations. Unlike the complex 13C spectrum of the unoxidized material, the spectrum of this product consisted of only six resonances due to rapid time averaging of its symmetrical structure on the relatively slow NMR timescale. Synthesis of this newly described cyclic beta-glucan in the R. meliloti ndvB mutant restored the symbiotic and hypoosmotic adaptation characteristics of the R. meliloti wild type strain.

A water-soluble beta-D-glucan from Boletus erythropus

The main component of a water extract of Boletus erythropus fruiting bodies is a M(r) 10(6) glucan. The use of classical structural analysis and HMQC (heteronuclear multiple quantum coherence) NMR experiments indicates a (1-->3) linked beta-D-glucan structure with a single glucose residue attached to O-6 of the main chain and a branching frequency of 1/3.

Beta-glucan synthesis in Bradyrhizobium japonicum: characterization of a new locus (ndvC) influencing beta-(1-->6) linkages

Bradyrhizobium japonicum synthesizes periplasmic cyclic beta-(1-->3),beta-(1-->6)-D-glucans during growth in hypoosmotic environments, and evidence is growing that these molecules may have a specific function during plant-microbe interactions in addition to osmoregulation. Site-directed Tn5 mutagenesis of the DNA region upstream of ndvB resulted in identification of a new gene (ndvC) involved in beta-(1--> 3), beta-(1-->6)-glucan synthesis and in nodule development. The predicted translation product was a polypeptide (ca. 62 kDa) with several transmembrane domains. It contained a sequence characteristic of a conserved nucleoside-sugar-binding motif found in many bacterial enzymes and had 51% similarity with a beta-glucanosyltransferase from Candida albicans. B. japonicum carrying a Tn5 insertion in ndvC resulted in synthesis of altered cyclic beta-glucans composed almost entirely of beta-(1--> 3)-glycosyl linkages. The mutant strain was only slightly sensitive to hypoosmotic growth conditions compared with the ndvB mutant, but it was severely impaired in symbiotic interactions with soybean (Glycine max). Nodulation was delayed by 8 to 10 days, and many small nodule-like structures apparently devoid of viable bacteria were formed. This finding suggests that the structure of the beta-glucan molecule is important for a successful symbiotic interaction, and beta-glucans may have a specific function in addition to their role in hypoosmotic adaptation.

Cell-associated glucans of Burkholderia solanacearum and Xanthomonas campestris pv. citri: a new family of periplasmic glucans

The cell-associated glucans produced by Burkholderia solanacearum and Xanthomonas campestris pv. citri were isolated by trichloroacetic acid treatment and gel permeation chromatography. The compounds obtained were characterized by compositional analysis, matrix-assisted laser desorption ionization mass spectrometry, and high-performance anion-exchange chromatography. B. solanacearum synthesizes only a neutral cyclic glucan containing 13 glucose residues, and X. campestris pv. citri synthesizes a neutral cyclic glucan containing 16 glucose residues. The two glucans were further purified by high-performance anion-exchange chromatography. Methylation analysis revealed that these glucans are linked by 1,2-glycosidic bonds and one 1,6-glycosidic bond. Our 600-MHz homonuclear and 1H-13C heteronuclear nuclear magnetic resonance experiments revealed the presence of a single alpha-1,6-glycosidic linkage, whereas all other glucose residues are beta-1,2 linked. The presence of this single alpha-1,6 linkage, however, induces such structural constraints in these cyclic glucans that all individual glucose residues could be distinguished. The different anomeric proton signals allowed complete sequence-specific assignment of both glucans. The structural characteristics of these glucans contrast with those of the previously described osmoregulated periplasmic glucans.

Elicitor-binding proteins and signal transduction in the activation of a phytoalexin defense response

Inducible plant defenses against potential pathogens are thought to be activated by signal compounds released during early stages of the infection process. In the incompatible interaction between soybean (Glycine max L.) and the oomycete Phytophthora megasperma f.sp. glycinea (= Phytophthora sojae) a rapid, localized phytoalexin response is activated at the level of transcription. The phytoalexin response is also stimulated in various soybean tissues, including cultured cells, following treatment with an elicitor derived from the cell walls of the fungus. The best characterized elicitors of P. megasperma for soybean are the branched (1→3)- and (1→6)-linked β-glucans, structural polysaccharides of the hyphal walls. The glucans are naturally released during the early stages of germination of the fungal cysts in a host-independent manner. Cyclic β-glucans of Bradyrhizobium japonicum USDA 110, a symbiont of soybean, arc not active in inducing phytoalexin production in soybean. When tested in combination, B. japonicum β-glucans inhibited stimulation of phytoalexin accumulation by the fungal glucans. Surface-localized glucan-binding proteins exist in soybean cells that display high affinity and specificity for the fungal β-glucans, including an elicitor-active hepta-β-glucoside fragment derived from the polysaccharide, suggesting that elicitor action involves a transmembrane signalling process. The main component of the soybean β-glucan binding sites appears to be a 70-kDa protein. Hepta-β-glucoside binding sites exist in several other legumes, such as bean (Phaseolus vulgaris L.), pea (Pisum sativum L.), and lupine (Lupinus albus L.). The signalling process initiated by the β-glucan elicitor, which leads to the activation of the phytoalexin defense response in soybean, involves changes in the permeability of the plasma membrane to Ca2+and H+. Chloride channel antagonists are more efficient than calcium channel antagonists in inhibiting both the phytoalexin response and the inducible ion fluxes. The results present evidence that the observed permeability changes of the plasma membrane are primary events in the transduction of the elicitor signal(s) by the challenged soybean cells. Key words: soybean (Glycine max), Phytophthora megasperma f.sp. glycinea, β-glucan elicitor, elicitor-binding proteins, phytoalexins, Ca2+.