Type IV secretion systems and their effectors in bacterial pathogenesis

Type IV secretion systems (T4SSs) are membrane-associated transporter complexes used by various bacteria to deliver substrate molecules to a wide range of target cells. T4SSs are involved in horizontal DNA transfer to other bacteria and eukaryotic cells, in DNA uptake from or release into the extracellular milieu, in toxin secretion and in the injection of virulence factors into eukaryotic host target cells by several mammalian pathogens. Rapid progress has been made towards defining the structures and functions of T4SSs, identifying the translocated effector molecules and elucidating the mechanisms by which the effectors subvert eukaryotic cellular processes during infection. These findings have had an important impact on our understanding of how these pathogens manipulate host cell functions to trigger bacterial uptake, facilitate intracellular growth and suppress defence mechanisms, thus facilitating bacterial colonization and disease development.

Structural determination of the Nod factors produced byRhizobium gallicumbv. gallicum R602

Rhizobium gallicum is a fast-growing bacterium found in European, Australian and African soils; it was first isolated in France. It is a microsymbiont which is able to nodulate plants of the genus Phaseolus. Rhizobium gallicum bv. gallicum R602 produces four extracellular signal molecules consisting of a linear backbone of N-acetyl glucosamine, bearing on the nonreducing terminal residue an N-methyl group and different N-acyl substituents. The four acyloligosaccharides terminate with a sulfated N-acetylglucosaminitol. This unit may be also acetylated. These structures were determined using carbohydrate and methylation analysis, mass spectrometric analysis and one-dimensional- and two-dimensional-nuclear magnetic resonance experiments. This work establishes the common structure that a lipochito-oligosaccharide must have so that the Rhizobium that produces and excretes it is able to nodulate plants of Phaseolus vulgaris. The substituents common to all the molecules are an N-methyl group and a C(18:1) fatty acid on the nonreducing terminal residue.

High temperature-induced changes in exopolysaccharides, lipopolysaccharides and protein profile of heat-resistant mutants of Rhizobium sp. (Cajanus)

A thermosensitive wild-type strain (PP201) of Rhizobium sp. (Cajanus) and its 14 heat-resistant mutants were characterized biochemically with regard to their cell surface (exopolysaccharides (EPSs) and lipopolysaccharides (LPSs)) properties and protein profile. Differences were observed between the parent strain and the mutants in all these parameters under high temperature conditions. At normal temperature (30 degrees C), only half of the mutant strains produced higher amounts of EPSs than the parent strain, but at 43 degrees C, all the mutants produced higher quantities of EPS. The LPS electrophoretic pattern of the parent strain PP201 and the heat-resistant mutants was almost identical at 30 degrees C. At 43 degrees C, the parent strain did not produce LPS but the mutants produced both kinds of LPSs. The protein electrophoretic pattern showed that the parent strain PP201 formed very few proteins at high temperature, whereas the mutants formed additional new proteins. A heat shock protein (Hsp) of 63-74 kDa was overproduced in all mutant strains.

Preparation of a lipid a derivative that contains a 27-hydroxyoctacosanoic acid moiety

[structure: see text] A general synthetic strategy for long-chain omega-1 hydroxy fatty acids has been developed, which employs as a key reaction step a cross metathesis between omega-unsaturated ester and 3-butene-2-ol. The resulting lipids were used for the preparation of lipid A derivatives of Rhizobium sin-1, which have the ability to inhibit the E. coli LPS-dependent synthesis of tumor necrosis factor by human monocytes.

[Lipopolysaccharides of rhizobiaceae: structure and biosynthesis]

The lipopolysaccharides (LPS) are major components of the outer membrane of Gram negative bacteria and, because of their location, are important mediators in the interaction between these bacteria and their environment and other organisms. The alpha-Proteobacterial family Rhizobiaceae includes the rhizobia and agrobacteria, microorganisms which establish symbiotic or parasitic relationships with plants. Mutants deficient in LPS biosynthesis show anomalous interactions with their hosts. The agronomical relevance of the relationship between rhizobia and agrobacteria with plants has promoted a large number of studies on the LPS from these bacteria. The complete structures of one or several domains of LPS from Rhizobiaceae have been determined in the last years. Additionally, several metabolic steps in the biosynthesis of these molecules have been elucidated. This review aims at the description of the more recent findings on the structure and biosynthesis of LPS in Rhizobium, Sinorhizobium and Agrobacterium.

[Infrared spectra analysis of chromium cation biosorbed by biosorbent ZL5-2]

Biosorption of Cr( VI) by the novel biosorbent ZL5-2 produced by Agrobacterium. sp was studied, and infrared spectra of native, absorbing and desorbing ZL5-2 were compared. The initial pH was important for biosorption. The optimum effect on Cr(VI) biosorption was acquired within initial pH 0.5-1.5, then with the pH increasing, the effect on Cr6+ biosorption was reduced. The biosorption for Cr(VI) was a quick process, only within 10 min 65.4% Cr(VI) was biosorbed, and the biosorption was in equilibrium after 60 min, then all the Cr(VI) was biosorbed after 80 min. The absorbed Cr(VI) could be recovered by desorption, and the desorption rate reached 13.6%-67.9%. The absorbance peak around 3 400 cm(-1), which reflected OH stretching vibration, moved about 8 cm(-1), and its absorbance decreased; the absorbance peak around 2 900 cm(-1) decreased, which reflected C-H stretching vibration; and the absorbance peak around 1 600 cm(-1), which reflected amide I group stretching vibration, moved about 13 cm(-1), and its absorbance decreased. After desorption, the absorbance peaks which reflected OH and C-H were increased to the initial degree, and the absorbance peak which reflected NH and amide I group did not increase. Therefore, authors presumed that the process of biosorption was reversible adsorption and irreversible adsorption simultaneously, however the reversible adsorption was more important in the biosorption.

Positive charge is an important feature of the C-terminal transport signal of the VirB/D4-translocated proteins of Agrobacterium

Several human pathogens and the plant pathogen Agrobacterium tumefaciens use a type IV secretion system for translocation of effector proteins into host cells. How effector proteins are selected for transport is unknown, but a C-terminal transport signal is present in the proteins translocated by the A. tumefaciens VirB/D4 type IV secretion system. We characterized this signal in the virulence protein VirF by alanine scanning and further site-directed mutagenesis. The Cre recombinase was used as a reporter to measure the translocation efficiency of Cre-Vir fusions from A. tumefaciens to Arabidopsis. The data unambiguously showed that positive charge is an essential characteristic of the C-terminal transport signal. We increased the sensitivity of this translocation assay by modifying the Cre-induced readout in host cells from kanamycin resistance to GFP expression. This improvement allowed us to detect translocation of the VirD2 relaxase protein in the absence of transferred DNA, showing that attachment to the transferred DNA is not essential for transport by the VirB/D4 system. We also found another translocated effector protein, namely the VirD5 protein encoded by the tumor-inducing plasmid. According to secondary structure predictions, the C termini of all VirB/D4-translocated proteins identified so far are unstructured; however, they contain a characteristic hydropathic profile. Based on sequence alignments and mutational analysis of VirF, we conclude that the C-terminal transport signal for recruitment and translocation of effector proteins by the A. tumefaciens VirB/D4 system is hydrophilic and has a net positive charge with a consensus motif of R-X(7)-R-X-R-X-R-X-X(n)>.

Diversity in a promiscuous group of rhizobia from three Sesbania spp. colonizing ecologically distinct habitats of the semi-arid Delhi region

Sesbania-rhizobia associations have immense significance in soil amelioration programs for diverse habitats. Diversity in symbiotic properties, LPS profiles, Sym plasmid and rhizobiophage sensitivity of 28 root- and stem-nodulating bacterial isolates of three Sesbania species (S. sesban, S. aegyptica and S. rostrata) inhabiting six ecologically distinct sites of semi-arid Delhi region was analyzed. The isolates were highly promiscuous among the symbiotic partners (Sesbania spp.). The root nodules formed by all the isolates were morphologically similar but they differed in their symbiotic efficiency and effectiveness. 16S rDNA sequence analyses revealed that root nodule isolates of sesbanias belong to diverse rhizobial taxa (Sinorhizobium saheli, S. meliloti, Rhizobium huautlense) whereas stem-nodule isolates were strictly Azorhizobium caulinodans. Sinorhizobium spp. seem to dominate as microsymbiont partner of Sesbania in the Delhi region. The genetic diversity revealed by cluster analyses based on NPC-PCR reflects sorting of isolates across the ecological gradient. Parallel diversity was also observed in the grouping based on LPS profiles and sym plasmid (NPC-PCR). Segregation of different rhizobial taxa into distinct types/clusters based on LPS and NPC-PCR analyses suggest its significance in the circumscription of the taxa. However, subtypes and subclusters showed their sorting across the ecological gradients. Sesbania rhizobia showed extremely high specificity to rhizobiophages. Enormous diversity in LPS profiles and high specificity of rhizobiophages might be the result of environmental selection pressures operating in ecologically distinct habitats. The ability of sesbanias to enter into effective symbioses with different rhizobial taxa and colonize diverse habitats with various biotic and abiotic stresses appears to contribute to its wide ecological amplitude.

Why boron?

It is now more than 80 years since boron was convincingly demonstrated to be essential for normal growth of higher plants. However, its biochemical role is not well understood at the moment. Several recent reviews propose that B is implicated in three main processes: keeping cell wall structure, maintaining membrane function, and supporting metabolic activities. However, in the absence of conclusive evidence, the primary role of boron in plants remains elusive. Besides plants, growth of specific bacteria, such as heterocystous cyanobacteria and the recently reported actinomycetes of the genus Frankia, requires B, particularly for the stability of the envelopes that control the access of the nitrogenase-poisoning oxygen when they grow under N2-fixing conditions. Likewise, a role for B for animal embryogenesis and other developmental processes is being established. Finally, a new feature of the role of boron comes from signaling mechanisms for communication among bacteria and among legumes and rhizobia leading to N2-fixing symbiosis, and it is possible that new roles for B, based on its special chemistry and its interaction with Ca would appear in the world of signal transduction pathways. In conclusion, the diversity of roles played by B might indicate that either the micronutrient is involved in numerous processes or that its deficiency has a pleiotropic effect. The arising question is why such an element? Since all of the roles clearly established for B are related to its capacity to form diester bridges between cis-hydroxyl-containing molecules, we propose that the main reason for B essentiality is the stabilization of molecules with cis-diol groups turning them effective, irrespectively of their function.

Gas chromatography-mass spectrometry analysis of indoleacetic acid and tryptophan following aqueous chloroformate derivatisation of Rhizobium exudates

A new method for preparing alkyl esters of indole-3-acetic acid (IAA) in aqueous solution is adapted from the chloroformate method originally described by Husek for the analysis of amino acids. This method has the significant advantage of avoiding the generation and use of diazomethane, and is done in aqueous solution without the need to dry the sample with concomitant non-specific losses of IAA. The effectiveness of this method is demonstrated by its use in an isotope dilution gas chromatography-mass spectrometry (GC-MS) assay of IAA and L-tryptophan (Trp) in the culture supernatant of a series of Sinorhizobium meliloti and Rhizobium leguminosarum bv. trifolii strains that can interact with rice to either enhance or inhibit rice plant growth. We were testing the hypothesis that the rice growth inhibition was related to the biosynthesis of IAA. It was found that S. meliloti and Rhizobium strains produced high amounts of IAA in Trp supplemented BIII minimal medium compared to BIII media. All the strains produced more than the minimum amount of IAA required to inhibit rice growth and thus IAA is not the major inhibitory factor of rice seedling growth from S. meliloti and Rhizobium strains.

Rhizobial lipopolysaccharide as the receptor in lectin-Rhizobium interaction

Rhizobial specificity was examined on the basis of interaction between legume lectins (peanut, pea and soybean) and different rhizobial species (various bradyrhizobia specific for peanut, P 14-93 and SB16). Legume lectins showed higher affinity towards host-specific Rhizobium and lipopolysaccharides (LPS) isolated from those particular rhizobia. Two LPS mutants of peanut-specific Bradyrhizobiumn sp. (Arachis) strain GN17 were isolated by Tn5 mutagenesis. These mutants (GN17M1 and GN17M2) were characterized by their higher hydrophobicity with respect to the parent cells. The hexose content in exopolysaccharides (EPS) and LPS of the mutants was found reduced significantly, whereas 2-keto-3-deoxyoctulosonic acid (Kdo) and uronic acid in LPS were less by 20-times and thrice, respectively in the mutants. Glucose was the major sugar in LPS from all the strains. However, glucosamine appeared only in the mutants. Spectrofluorimetric analysis showed that LPS from GN17M1 mutant interacted most significantly with peanut root agglutinin or lectin (PRA II). The results indicate that LPS on the surface of rhizobial cells is the possible receptor for lectin.

Three-dimensional reconstruction of Agrobacterium VirE2 protein with single-stranded DNA

Agrobacterium tumefaciens infects plant cells by a unique mechanism involving an interkingdom genetic transfer. A single-stranded DNA substrate is transported across the two cell walls along with the bacterial virulence proteins VirD2 and VirE2. A single VirD2 molecule covalently binds to the 5'-end of the single-stranded DNA, while the VirE2 protein binds stoichiometrically along the length of the DNA, without sequence specificity. An earlier transmission/scanning transmission electron microscopy study indicated a solenoidal ("telephone coil") organization of the VirE2-DNA complex. Here we report a three-dimensional reconstruction of this complex using electron microscopy and single-particle image-processing methods. We find a hollow helical structure of 15.7-nm outer diameter, with a helical rise of 51.5 nm and 4.25 VirE2 proteins/turn. The inner face of the protein units contains a continuous wall and an inward protruding shelf. These structures appear to accommodate the DNA binding. Such a quaternary arrangement naturally sequesters the DNA from cytoplasmic nucleases and suggests a mechanism for its nuclear import by decoration with host cell factors. Coexisting with the helices, we also found VirE2 tetrameric ring structures. A two-dimensional average of the latter confirms the major features of the three-dimensional reconstruction.

The Biliverdin Chromophore Binds Covalently to a Conserved Cysteine Residue in the N-Terminus ofAgrobacteriumPhytochrome Agp1†

Phytochromes are widely distributed biliprotein photoreceptors. Typically, the chromophore becomes covalently linked to the protein during an autocatalytic lyase reaction. Plant and cyanobacterial phytochromes incorporate bilins with a ring A ethylidene side chain, whereas other bacterial phytochromes utilize biliverdin as chromophore, which has a vinyl ring A side chain. For Agrobacterium phytochrome Agp1, site-directed mutagenesis provided evidence that biliverdin is bound to cysteine 20. This cysteine is highly conserved within bacterial homologues, but its role as attachment site has as yet not been proven. We therefore performed mass spectrometry studies on proteolytic holopeptide fragments. For that purpose, an Agp1 expression vector was re-engineered to produce a protein with an N-terminal affinity tag. Following proteolysis, the chromophore co-purified with a ca. 5 kDa fragment during affinity chromatography, showing that the attachment site is located close to the N-terminus. Mass spectrometry analyses performed with the purified chromopeptide confirmed the role of the cysteine 20 as biliverdin attachment site. We also analyzed the role of the highly conserved histidine 250 by site-directed mutagenesis. The homologous amino acid plays an important but yet undefined role in plant phytochromes and has been proposed as chromophore attachment site of Deinococcus phytochrome. We found that in Agp1, this amino acid is dispensable for covalent attachment, but required for tight chromophore-protein interaction.

Cyclosophoraose as a catalytic carbohydrate for methanolysis

A novel catalytic methanolysis can be induced by a natural cyclooligosaccharide, a cyclosophoraose (cyclic-(1-->2)-beta-D-glucan, Cys), which is a member of a family of unbranched cyclooligosaccharides produced as intra- or extraoligosaccharides by soil microorganisms of the genus, Rhizobium. Cys catalyzed the methanolysis for 5(4H)-oxazolones and various phospholipids. Cys enhanced the methanolysis reaction about 9200-fold for a benzylidene oxazolone or 250-fold for dipalmitoylphosphatidylcholine comparing with control. In this study, we describe that natural cyclosophoraoses isolated from the Rhizobium species function as catalytic carbohydrates for the methanolysis.

A catalogue of molecular, physiological and symbiotic properties of soybean-nodulating rhizobial strains from different soybean cropping areas of China

We have analysed 198 fast-growing soybean-nodulating rhizobial strains from four different regions of China for the following characteristics: generation time; number of plasmids; lipopolysaccharide (LPS), nodulation factors (LCOs) and PCR profiles; acidification of growth medium; capacity to grow at acid, neutral, and alkaline pH; growth on LC medium; growth at 28 and 37 degrees C; melanin production capacity; Congo red absorption and symbiotic characteristics. These unbiased analyses of a total subset of strains isolated from specific soybean-cropping areas (an approach which could be called "strainomics") can be used to answer various biological questions. We illustrate this by a comparison of the molecular characteristics of five strains with interesting symbiotic properties. From this comparison we conclude, for instance, that differences in the efficiency of nitrogen fixation or competitiveness for nodulation of these strains are not apparently related to differences in Nod factor structure.

Fused RolA protein enhances beta-glucoronidase activity 50-fold: implication for RolA mechanism of action

We report that the plant oncoprotein RolA from Agrobacterium rhizogenes acts to stabilize beta-glucoronidase (Gus) when the two proteins are expressed as a fusion protein in transformed tobacco. The observed 50-fold increase of Gus activity was shown to be related to protein accumulation, with no significant changes in mRNA abundance, kinetic properties of the enzyme and thermostability. The entire RolA sequence is essential to achieve the full effect since both the N-terminal region, spanning a putative reverse signal-anchor and nuclear targeting sequences, or the contiguous C-terminal portion were shown to increase Gus activity only 10-fold. A possible interference of RolA in the protein degradation pathway regulated by auxin is discussed.

Synthesis and biological evaluation of Rhizobium sin-1 lipid A derivatives

A highly convergent strategy for the synthesis of several derivatives of the lipid A of Rhizobium sin-1 has been developed. The approach employed the advanced intermediate 3-O-acetyl-6-O-(3-O-acetyl-4,6-O-benzylidene-2-deoxy-2-phthalimido-beta-d-glucopyrano-syl)-2-azido-4-O-benzyl-2-deoxy-1-thio-alpha-d-glucopyranoside (5), which is protected in such a way that the anomeric center, the C-2 and C-2' amino groups, and the C-3 and C-3' hydroxyls can be selectively functionalized. The synthetic strategy was used for the preparation of 2-deoxy-6-O-[2-deoxy-3-O-[(R)-3-hydroxy-hexadecanoyl]-2-[(R)-3-octacosanoyloxy-hexadecan]amido-beta-d-glucopyranosyl]-2-[(R)-3-hydroxy-hexadecan]amido-3-O-[(R)-3-hydroxy-hexadecanoyl]-alpha-d-glucopyranose (11) and 2-deoxy-6-O-[2-deoxy-3-O-[(R)-3-hydroxy-hexadecanoyl]-2-[(R)-3-octacosanoyloxy-hexadecan]amido-beta-d-glucopyranosyl]-2-[(R)-3-hydroxy-hexadecan]amido-3-O-[(R)-3-hydroxy-hexadecanoyl]-d-glucono-1,5-lactone (13), which contain an unusual octacosanoic acid moiety and differ in the oxidation state of the anomeric center. The results of biological studies indicate that 11 and 13 lack the proinflammatory effects of Escherichia coli lipopolysaccharides (LPS). Furthermore, 13 emulated the ability of heterogeneous R. sin-1 LPS to antagonize enteric LPS, providing evidence for the critical role of the gluconolactone moiety of R. sin-1 LPS in mediating this antagonistic effect. Compound 13 is the first example of a lipid A derivative that is devoid of phosphate but possesses antagonistic properties, making it an attractive lead compound for development of a drug to use in the treatment of Gram-negative septicemia.

Enantioseparation using cyclosophoraoses as a novel chiral additive in capillary electrophoresis

Cyclosophoraoses, cyclic beta-(1-->2)-D-glucans produced by Rhizobium meliloti 2011, were used as a novel chiral additive for the separation of terbutaline, amethopterin, thyroxine and N-acetylphenylalanine enantiomers in aqueous capillary electrophoresis (CE). Enantioseparation took place in the normal- or reversed-polarity mode when a high concentration of neutral (60 mM) or anionic (40 mM) cyclosophoraoses was added to the background electrolyte (BGE).

The polar lipid composition of Mesorhizobium ciceri

The extractable lipid composition of Mesorhizobium ciceri strain HAMBI 1750 grown in a phosphate sufficient medium (79CA) is reported. Cardiolipin (CL-27% of total lipids), phosphatidylglycerol (PG-18%), phosphatidylethanolamine (PE-1%), phosphatidylcholine (PC-30%) and two methylated derivatives of PE, i.e. phosphatidyl-N, N-dimethylethanolamine (DMPE-1%) and phosphatidyl-N-monomethylethanolamine (MMPE-1%), were found to make up the phospholipids of the analysed bacteria. Nonphosphorus, ornithine-containing lipid (OL-10%) was also detected. Polar groups of phospholipids were predominantly acylated with cis-11,12-methyleneoctadecanoyl (lactobacillic) residues, whereas the ornithine lipid contained mainly 3-hexadecanoyloxy-11,12-methyleneoctadecanoic acid bound to the alpha-amino group.

Structural characterization of the lipid A component of Sinorhizobium sp. NGR234 rough and smooth form lipopolysaccharide. Demonstration that the distal amide-linked acyloxyacyl residue containing the long chain fatty acid is conserved in rhizobium and Sinorhizobium sp

A broad-host-range endosymbiont, Sinorhizobium sp. NGR234 is a component of several legume-symbiont model systems; however, there is little structural information on the cell surface glycoconjugates. NGR234 cells in free-living culture produce a major rough lipopolysaccharide (LPS, lacking O-chain) and a minor smooth LPS (containing O-chain), and the structure of the lipid A components was investigated by chemical analyses, mass spectrometry, and NMR spectroscopy of the underivatized lipids A. The lipid A from rough LPS is heterogeneous and consists of six major bisphosphorylated species that differ in acylation. Pentaacyl species (52%) are acylated at positions 2, 3, 2', and 3', and tetraacyl species (46%) lack an acyl group at C-3 of the proximal glucosamine. In contrast to Rhizobium etli and Rhizobium leguminosarum, the NGR234 lipid A contains a bisphosphorylated beta-(1' --> 6)-glucosamine disaccharide, typical of enterobacterial lipid A. However, NGR234 lipid A retains the unusual acylation pattern of R. etli lipid A, including the presence of a distal, amide-linked acyloxyacyl residue containing a long chain fatty acid (LCFA) (e.g. 29-hydroxytriacontanoate) attached as the secondary fatty acid. As in R. etli, a 4-carbon fatty acid, beta-hydroxybutyrate, is esterified to (omega - 1) of the LCFA forming an acyloxyacyl residue at that location. The NGR234 lipid A lacks all other ester-linked acyloxyacyl residues and shows extensive heterogeneity of the amide-linked fatty acids. The N-acyl heterogeneity, including unsaturation, is localized mainly to the proximal glucosamine. The lipid A from smooth LPS contains unique triacyl species (20%) that lack ester-linked fatty acids but retain bisphosphorylation and the LCFA-acyloxyacyl moiety. The unusual structural features shared with R. etli/R. leguminosarum lipid A may be essential for symbiosis.

Nod factors activate both heterotrimeric and monomeric G-proteins in Vigna unguiculata (L.) Walp

Nod factors are lipo-chito-oligosaccharides secreted by rhizobia that initiate many responses in the root hairs of the legume hosts, culminating in deformed hairs. The heterotrimeric G-protein agonists mastoparan, Mas7, melittin, compound 48/80 and cholera toxin provoke root hair deformation, whereas the heterotrimeric G-protein antagonist pertussis toxin inhibits mastoparan and Nod factor NodNGR[S]- (from Rhizobiumsp. NGR234) induced root hair deformation. Another heterotrimeric G-protein antagonist, isotetrandrine, only inhibited root hair deformation provoked by mastoparan and melittin. These results support the notion that G-proteins are implicated in Nod factor signalling. To study the role of G-proteins at a biochemical level, we examined the GTP-binding profiles of root microsomal membrane fractions isolated from the nodulation competent zone of Vigna unguiculata(L.) Walp. GTP competitively bound to the microsomal membrane fractions labelled with [(35)S]GTPgammaS, yielding a two-site displacement curve with displacement constants ( K(i)) of 0.58 micro M and 0.16 mM. Competition with either ATP or GDP revealed a one-site displacement curve with K(i) of 4.4 and 29 micro M, respectively, whereas ADP and UTP were ineffective competitors. The GTP-binding profiles of microsomal membrane fractions isolated from roots pretreated with either NodNGR[S] or the four-sugar, N- N'- N"- N'"-tetracetylchitotetraose (TACT) backbone of Nod factors were significantly altered compared with control microsomal fractions. To identify candidate proteins, membrane proteins were separated by SDS-PAGE and electrotransferred to nitrocellulose. GTP overlay experiments revealed that membrane fractions isolated from roots pretreated with NodNGR[S] or TACT contained two proteins (28 kDa and 25 kDa) with a higher affinity for GTPgammaS than control membrane fractions. Western analysis demonstrated that membranes from the pretreated roots contained more of another protein (~55 kDa) recognised by Galpha(common) antisera. These results provide pharmacological and biochemical evidence supporting the contention that G-proteins are involved in Nod factor signalling and, importantly, implicate monomeric G-proteins in this process.

Structural determination of the lipo-chitin oligosaccharide nodulation signals produced by Rhizobium giardinii bv. giardinii H152

Rhizobium giardinii bv. giardinii is a microsymbiont of plants of the genus Phaseolus and produces extracellular signal molecules that are able to induce deformation of root hairs and nodule organogenesis. We report here the structures of seven lipochitooligosaccharide (LCO) signal molecules secreted by R. giardinii bv. giardinii H152. Six of them are pentamers of GlcNAc carrying C 16:0, C 18:0, C 20:0 and C 18:1 fatty acyl chains on the non-reducing terminal residue. Four are sulfated at C-6 of the reducing terminal residue and one is acetylated in the same position. Six of them are N-methylated on the non-reducing GlcN residue and all the nodulation factors are carbamoylated on C-6 of the non-reducing terminal residue. The structures were determined using monosaccharide composition and methylation analyses, 1D- and 2D-NMR experiments and a range of mass spectrometric techniques. The position of the carbamoyl substituent on the non-reducing glucosamine residue was determined using a CID-MSMS experiment and an HMBC experiment.

Characterization of a novel lipid-A from Rhizobium species Sin-1. A unique lipid-A structure that is devoid of phosphate and has a glycosyl backbone consisting of glucosamine and 2-aminogluconic acid

The structure of the lipid-A from Rhizobium species Sin-1, a nitrogen-fixing Gram-negative bacterial symbiont of Sesbania, was determined by composition, nuclear magnetic resonance spectroscopic, and mass spectrometric analyses. The lipid-A preparation consisted of a mixture of structures due to differences in fatty acylation and in the glycosyl backbone. There were two different disaccharide backbones. One disaccharide consisted of a distal glucosaminosyl residue beta-linked to position 6 of a proximal 2-aminoglucono-1,5-lactonosyl residue, and in the second disaccharide, the proximal residue was 2-amino-2,3-dideoxy-d-erythro-hex-2-enono-1,5-lactone. For both disaccharides, the distal glucosamine was acylated at C-2' primarily with beta-hydroxypalmitate (beta-OHC16:0) which, in turn, was O-acylated with 27-hydroxyoctacosanoic acid. For some of the lipid-A molecules, the distal glucosaminosyl residue was also acylated at C-3' with beta-hydroxymyristate (beta-OHC14:0), whereas other molecules were devoid of this acyl substituent. Both the 2-aminoglucono-1,5-lactonosyl and 2-amino-2,3-dideoxy-d-erythro-hex-2-enono-1,5-lactonosyl residues were acylated at C-2, primarily with beta-OHC16:0. Minor amounts of lipid-A molecules contained beta-OHC14:0 at C-3 and/or beta-hydroxystearate (beta-OHC18:0) or beta-hydroxyoctadecenoate (beta-OHC18:1) as the C-2 and C-2' N-acyl substituents.

Molecular cloning and characterization of nodD genes from Rhizobium sp. SIN-1, a nitrogen-fixing symbiont of Sesbania and other tropical legumes

Rhizobium sp. SIN-1, a nitrogen-fixing symbiont of Sesbania aculeata and other tropical legumes, carries two copies of nodD, both on a sym plasmid. We have isolated these two nodD genes by screening a genomic library of Rhizobium sp. SIN-1 with a nodD probe from Sinorhizobium meliloti. Nucleotide sequence and the deduced amino acid sequence analysis indicated that the nodD genes of Rhizobium sp. SIN-1 are most closely related to those of R. tropici and Azorhziobium caulinodans. Rhizobium sp. SIN-1 nodD1 complemented a S. meliloti nodD1 D2 D3 negative mutant for nodulation on alfalfa, but failed to complement a nodD1 mutant of S. fredii USDA191 for soybean nodulation. A hybrid nodD gene, containing the N-terminus of S. fredii USDA191 nodD1 and the C-terminus of Rhizobium sp. SIN-1 nodD1, complemented the nodD1 negative mutant of USDA191 for nodulation on soybean.