Two classes of Rhizobium meliloti infection mutants differ in exopolysaccharide production and in coinoculation properties with nodulation mutants

Rhizobial Exopolysaccharides: Genetic Regulation of Their Synthesis and Relevance in Symbiosis with Legumes

Rhizobia are soil proteobacteria able to engage in a nitrogen-fixing symbiotic interaction with legumes that involves the rhizobial infection of roots and the bacterial invasion of new organs formed by the plant in response to the presence of appropriate bacterial partners. This interaction relies on a complex molecular dialogue between both symbionts. Bacterial N-acetyl-glucosamine oligomers called Nod factors are indispensable in most cases for early steps of the symbiotic interaction. In addition, different rhizobial surface polysaccharides, such as exopolysaccharides (EPS), may also be symbiotically relevant. EPS are acidic polysaccharides located out of the cell with little or no cell association that carry out important roles both in free-life and in symbiosis. EPS production is very complexly modulated and, frequently, co-regulated with Nod factors, but the type of co-regulation varies depending on the rhizobial strain. Many studies point out a signalling role for EPS-derived oligosaccharides in root infection and nodule invasion but, in certain symbiotic couples, EPS can be dispensable for a successful interaction. In summary, the complex regulation of the production of rhizobial EPS varies in different rhizobia, and the relevance of this polysaccharide in symbiosis with legumes depends on the specific interacting couple.

Exopolysaccharide Characterization of Rhizobium favelukesii LPU83 and Its Role in the Symbiosis With Alfalfa

One of the greatest inputs of available nitrogen into the biosphere occurs through the biological N2-fixation to ammonium as result of the symbiosis between rhizobia and leguminous plants. These interactions allow increased crop yields on nitrogen-poor soils. Exopolysaccharides (EPS) are key components for the establishment of an effective symbiosis between alfalfa and Ensifer meliloti, as bacteria that lack EPS are unable to infect the host plants. Rhizobium favelukesii LPU83 is an acid-tolerant rhizobia strain capable of nodulating alfalfa but inefficient to fix nitrogen. Aiming to identify the molecular determinants that allow R. favelukesii to infect plants, we studied its EPS biosynthesis. LPU83 produces an EPS I identical to the one present in E. meliloti, but the organization of the genes involved in its synthesis is different. The main gene cluster needed for the synthesis of EPS I in E. meliloti, is split into three different sections in R. favelukesii, which probably arose by a recent event of horizontal gene transfer. A R. favelukesii strain devoided of all the genes needed for the synthesis of EPS I is still able to infect and nodulate alfalfa, suggesting that attention should be directed to other molecules involved in the development of the symbiosis.

Sinorhizobium fredii HH103 Invades Lotus burttii by Crack Entry in a Nod Factor-and Surface Polysaccharide-Dependent Manner

Sinorhizobium fredii HH103-Rif^r, a broad host range rhizobial strain, induces nitrogen-fixing nodules in Lotus burttii but ineffective nodules in L. japonicus. Confocal microscopy studies showed that Mesorhizobium loti MAFF303099 and S. fredii HH103-Rif^r invade L. burttii roots through infection threads or epidermal cracks, respectively. Infection threads in root hairs were not observed in L. burttii plants inoculated with S. fredii HH103-Rif^r. A S. fredii HH103-Rif^r nodA mutant failed to nodulate L. burttii, demonstrating that Nod factors are strictly necessary for this crack-entry mode, and a noeL mutant was also severely impaired in L. burttii nodulation, indicating that the presence of fucosyl residues in the Nod factor is symbiotically relevant. However, significant symbiotic impacts due to the absence of methylation or to acetylation of the fucosyl residue were not detected. In contrast S. fredii HH103-Rif^r mutants showing lipopolysaccharide alterations had reduced symbiotic capacity, while mutants affected in production of either exopolysaccharides, capsular polysaccharides, or both were not impaired in nodulation. Mutants unable to produce cyclic glucans and purine or pyrimidine auxotrophic mutants formed ineffective nodules with L. burttii. Flagellin-dependent bacterial mobility was not required for crack infection, since HH103-Rif^r fla mutants nodulated L. burttii. None of the S. fredii HH103-Rif^r surface-polysaccharide mutants gained effective nodulation with L. japonicus.

The presence of nodules on legume root systems can alter phenotypic plasticity in response to internal nitrogen independent of nitrogen fixation

All higher plants show developmental plasticity in response to the availability of nitrogen (N) in the soil. In legumes, N starvation causes the formation of root nodules, where symbiotic rhizobacteria fix atmospheric N2 for the host in exchange for fixed carbon (C) from the shoot. Here, we tested whether plastic responses to internal [N] of legumes are altered by their symbionts. Glasshouse experiments compared root phenotypes of three legumes, Medicago truncatula, Medicago sativa and Trifolium subterraneum, inoculated with their compatible symbiont partners and grown under four nitrate levels. In addition, six strains of rhizobia, differing in their ability to fix N2 in M. truncatula, were compared to test if plastic responses to internal [N] were dependent on the rhizobia or N2 -fixing capability of the nodules. We found that the presence of rhizobia affected phenotypic plasticity of the legumes to internal [N], particularly in root length and root mass ratio (RMR), in a plant species-dependent way. While root length responses of M. truncatula to internal [N] were dependent on the ability of rhizobial symbionts to fix N2 , RMR response to internal [N] was dependent only on initiation of nodules, irrespective of N2 -fixing ability of the rhizobia strains.

Sinorhizobium meliloti low molecular mass phosphotyrosine phosphatase SMc02309 modifies activity of the UDP-glucose pyrophosphorylase ExoN involved in succinoglycan biosynthesis

In Gram-negative bacteria, tyrosine phosphorylation has been shown to play a role in the control of exopolysaccharide (EPS) production. This study demonstrated that the chromosomal ORF SMc02309 from Sinorhizobium meliloti 2011 encodes a protein with significant sequence similarity to low molecular mass protein-tyrosine phosphatases (LMW-PTPs), such as the Escherichia coli Wzb. Unlike other well-characterized EPS biosynthesis gene clusters, which contain neighbouring LMW-PTPs and kinase, the S. meliloti succinoglycan (EPS I) gene cluster located on megaplasmid pSymB does not encode a phosphatase. Biochemical assays revealed that the SMc02309 protein hydrolyses p-nitrophenyl phosphate (p-NPP) with kinetic parameters similar to other bacterial LMW-PTPs. Furthermore, we show evidence that SMc02309 is not the LMW-PTP of the bacterial tyrosine-kinase (BY-kinase) ExoP. Nevertheless, ExoN, a UDP-glucose pyrophosphorylase involved in the first stages of EPS I biosynthesis, is phosphorylated at tyrosine residues and constitutes an endogenous substrate of the SMc02309 protein. Additionally, we show that the UDP-glucose pyrophosphorylase activity is modulated by SMc02309-mediated tyrosine dephosphorylation. Moreover, a mutation in the SMc02309 gene decreases EPS I production and delays nodulation on Medicago sativa roots.

The DivJ, CbrA and PleC system controls DivK phosphorylation and symbiosis in Sinorhizobium meliloti

Sinorhizobium meliloti is a soil bacterium that invades the root nodules it induces on Medicago sativa, whereupon it undergoes an alteration of its cell cycle and differentiates into nitrogen-fixing, elongated and polyploid bacteroid with higher membrane permeability. In Caulobacter crescentus, a related alphaproteobacterium, the principal cell cycle regulator, CtrA, is inhibited by the phosphorylated response regulator DivK. The phosphorylation of DivK depends on the histidine kinase DivJ, while PleC is the principal phosphatase for DivK. Despite the importance of the DivJ in C. crescentus, the mechanistic role of this kinase has never been elucidated in other Alphaproteobacteria. We show here that the histidine kinases DivJ together with CbrA and PleC participate in a complex phosphorylation system of the essential response regulator DivK in S. meliloti. In particular, DivJ and CbrA are involved in DivK phosphorylation and in turn CtrA inactivation, thereby controlling correct cell cycle progression and the integrity of the cell envelope. In contrast, the essential PleC presumably acts as a phosphatase of DivK. Interestingly, we found that a DivJ mutant is able to elicit nodules and enter plant cells, but fails to establish an effective symbiosis suggesting that proper envelope and/or low CtrA levels are required for symbiosis.

Production of succinoglycan polymer in Sinorhizobium meliloti is affected by SMb21506 and requires the N-terminal domain of ExoP

The protein tyrosine kinase ExoP, consisting of an N-terminal periplasmic and a C-terminal cytoplasmic domain, is important for polymerization of the exopolysaccharide succinoglycan (EPS I) in Sinorhizobium meliloti. We analyzed the contribution of the ExoP paralogs ExoP2 and SMb21506 to the production of the high molecular weight (HMW) form of EPS I. ExoP2, though not contributing to EPS I or lipopolysaccharide biosynthesis, showed increased expression at high osmolarity and was expressed in Medicago sativa nodules, suggesting an involvement in the synthesis of an as-yet-unidentified polysaccharide. Furthermore, a mutation in SMb21506 affected the production of HMW EPS I, particularly in the absence of the C-terminal ExoP domain. High salinity induced the production of HMW EPS I by the wild type and mutants whereas high osmolarity had the opposite effect. It was shown that ExoP localizes at the inner membrane of S. meliloti cells. Tyrosine phosphorylation of the C-terminal domain was strongly increased by amino acid substitutions in the polysaccharide co-polymerase motif (formerly proline-rich motif) located in the N-terminal domain, suggesting that this phosphorylation could be modulated by conformational changes of the N-terminal domain. Moreover, deletion of a coiled-coil motif present in the N-terminal domain abolished phosphorylation and EPS I production and, consequently, the ability to nodulate M. sativa.

The ExpR/Sin quorum-sensing system controls succinoglycan production in Sinorhizobium meliloti

Sinorhizobium meliloti is a gram-negative soil bacterium capable of forming a symbiotic nitrogen-fixing relationship with its plant host, Medicago sativa. Various bacterially produced factors are essential for successful nodulation. For example, at least one of two exopolysaccharides produced by S. meliloti (succinoglycan or EPS II) is required for nodule invasion. Both of these polymers are produced in high- and low-molecular-weight (HMW and LMW, respectively) fractions; however, only the LMW forms of either succinoglycan or EPS II are active in nodule invasion. The production of LMW succinoglycan can be generated by direct synthesis or through the depolymerization of HMW products by the action of two specific endoglycanases, ExsH and ExoK. Here, we show that the ExpR/Sin quorum-sensing system in S. meliloti is involved in the regulation of genes responsible for succinoglycan biosynthesis as well as in the production of LMW succinoglycan. Therefore, quorum sensing, which has been shown to regulate the production of EPS II, also plays an important role in succinoglycan biosynthesis.

Metabolite profiles of nodulated alfalfa plants indicate that distinct stages of nodule organogenesis are accompanied by global physiological adaptations

An effective symbiosis between Sinorhizobium meliloti and its host plant Medicago sativa is dependent on a balanced physiological interaction enabling the microsymbiont to fix atmospheric nitrogen. Maintenance of the symbiotic interaction is regulated by still poorly understood control mechanisms. A first step toward a better understanding of nodule metabolism was the determination of characteristic metabolites for alfalfa root nodules. Furthermore, nodules arrested at different developmental stages were analyzed in order to address metabolic changes induced during the progression of nodule formation. Metabolite profiles of bacteroid-free pseudonodule extracts indicated that early nodule developmental processes are accompanied by photosynthate translocation but no massive organic acid formation. To determine metabolic adaptations induced by the presence of nonfixing bacteroids, nodules induced by mutant S. meliloti strains lacking the nitrogenase protein were analyzed. The bacteroids are unable to provide ammonium to the host plant, which is metabolically reflected by reduced levels of characteristic amino acids involved in ammonium fixation. Elevated levels of starch and sugars in Fix(-) nodules provide strong evidence that plant sanctions preventing a transformation from a symbiotic to a potentially parasitic interaction are not strictly realized via photosynthate supply. Instead, metabolic and gene expression data indicate that alfalfa plants react to nitrogen-fixation-deficient bacteroids with a decreased organic acid synthesis and an early induction of senescence. Noneffective symbiotic interactions resulting from plants nodulated by mutant rhizobia also are reflected in characteristic metabolic changes in leaves. These are typical for nitrogen deficiency, but also highlight metabolites potentially involved in sensing the N status.

Lipopolysaccharides as a communication signal for progression of legume endosymbiosis

Establishment of a successful symbiosis between rhizobia and legumes results from an elaborate molecular dialogue between both partners. Bacterial nodulation (Nod) factors are indispensable for initiating plant responses, whereas bacterial surface polysaccharides are important for infection progression and nodule development. The mutant ORS571-oac2 of Azorhizobium caulinodans, affected in its surface polysaccharides, provokes a defective interaction with its host Sesbania rostrata. ORS571-oac2 induced structures with retarded development and continued generation of infection centers and organ primordia, leading to multilobed ineffective nodules. Bacterial development throughout the interaction occurred without major defects. A functional bidirectional complementation was obtained upon coinfection of ORS571-oac2 and a Nod factor-deficient mutant, indicating that the Fix- phenotype of ORS571-oac2-induced nodules resulted from the absence of a positive signal from ORS571-oac2. Indeed, the Fix- phenotype could be complemented by coinoculation of ORS571-oac2 with lipopolysaccharides (LPSs) purified from A. caulinodans. Our data show that Nod factors and LPSs are consecutive signals in symbiosis. Nod factors act first to trigger the onset of the nodulation and invasion program; LPSs inform the plant to proceed with the symbiotic interaction and to develop a functional fixation zone.

A novel genetic locus outside the symbiotic island is required for effective symbiosis of Bradyrhizobium japonicum with soybean Glycine max

In order to investigate the symbiotic interaction between soybean and Bradyrhizobium japonicum, TnphoA mutagenesis of the microsymbiont was performed. Mutant strain 2-10 was found to induce a strongly reduced number of ineffective nodules. Ultrastructural analysis of the soybean nodule central tissue revealed the presence of numerous starch granules and vacuoles in the infected cells. In addition, the number of symbiosomes was extremely low, indicating an impaired interaction between the plant and invading bacteria. Cloning and sequencing of the mutated DNA region uncovered four open reading frames (ORFs) lacking any data base similarities. ORFs srrA1 and srrA2, the 2-10 TnphoA insertion site, are encoded in the same reading frame. A 35-kDa expression product in Escherichia coli indicated the presence of a common protein, called SrrA (symbiotically relevant region) in B. japonicum 110spc4, encoded by combined srrA1 and srrA2 genes. The analysis of gene disruption mutants revealed that srrB and srrC were also required for effective symbiosis with soybeans. Further downstream the gene for a putative inner membrane protein (pipA) of unknown function was encoded on the opposite strand. Primer extension studies led to the conclusion that the organization of genes differed from the RhizoBase annotation in this particular region of B. japonicum USDA110.

Four promoters subject to regulation by ExoR and PhoB direct transcription of the Sinorhizobium melilotiexoYFQ operon involved in the biosynthesis of succinoglycan

Succinoglycan (EPS I), the main acidic exopolysaccharide of Sinorhizobium meliloti, is required for the initiation and elongation of infection threads during nodulation of the host plant alfalfa. The gene products of the exoYFQ operon are involved in the first step of succinoglycan biosynthesis as well as in the polymerisation of subunits to the high-molecular-mass form of this exopolysaccharide. One promoter region that directs transcription of exoX and two promoter regions that drive transcription of exoY were mapped in the exoX-exoY intergenic region. The distal exoY promoter region containing three putative -10 promoter elements was active under standard growth conditions and was subject to ExoR-dependent regulation. Although this promoter region was stimulated in a phoB mutant, no PHO box-like sequences were found, suggesting an indirect regulatory effect of PhoB. The proximal promoter contains a PHO box-like sequence in the putative -35 region and was affected by low and high phosphate concentrations dependent on PhoB. In the case of deleted upstream regions, this promoter was also controlled by ExoR. An additional promoter displaying activity in exoR, mucR and phoB mutants under standard conditions was identified upstream of exoF. The putative -35 promoter element of this promoter is covered by a second PHO box-like sequence.

A Rab-related small GTP binding protein is predominantly expressed in root nodules of Medicago sativa

Rab-related small GTP-binding proteins are known to be involved in the regulation of the vesicular transport system in eucaryotic cells. In this paper we report the isolation of the cDNA clone MS- rab11f from Medicago sativa (alfalfa) root nodules using a combination of RT-PCR and SSCP analysis. MS- rab11f shows high homology to the Rab-related cDNA clone LJ- rab11f from Lotus japonicus root nodules. The MS-Rab11F protein expressed in Escherichia coli was found to bind GTP, confirming that the isolated cDNA indeed codes for a small GTP-binding protein. Expression analysis by RT-PCR demonstrated that MS- rab11f is preferentially expressed in root nodules of alfalfa. Using the cDNA-sequence of MS-rab11f, a peptide-specific antibody was generated. Western blot analysis with this antibody revealed that two Rab11F isoforms, designated MS-Rab11FA and MS-Rab11FB, are found in M. sativa root nodules.

Identification and characterization of a Brucella abortus ATP-binding cassette transporter homolog to Rhizobium meliloti ExsA and its role in virulence and protection in mice

Brucella abortus is a facultative intracellular bacterial pathogen that causes abortion in domestic animals and undulant fever in humans. The mechanism of virulence of Brucella spp. is not fully understood yet. Furthermore, genes that allow Brucella to reach the intracellular niche and to interact with host cells need to be identified. Using the genomic survey sequence (GSS) approach, we identified the gene encoding an ATP-binding cassette (ABC) transporter of B. abortus strain S2308. The deduced amino acid sequence encoded by this gene exhibited 69 and 67% identity with the sequences of the ABC transporters encoded by the exsA genes of Rhizobium meliloti and Mesorhizobium loti, respectively. Additionally, B. abortus ExsA, like R. meliloti and M. loti ExsA, possesses ATP-binding motifs and the ABC signature domain features of a typical ABC transporter. Furthermore, ortholog group analysis placed B. abortus ExsA in ortholog group 6 of ABC transporters more likely to be involved in bacterial pathogenesis. In R. meliloti, ExsA is an exopolysaccharide transporter essential for alfalfa root nodule invasion and establishment of infection. To test the role of ExsA in Brucella pathogenesis, an exsA deletion mutant was constructed. Replacement of the wild-type exsA by recombination was demonstrated by Southern blot analysis of Brucella genomic DNA. Decreased survival in mice of the Brucella DeltaexsA mutant compared to the survival of parental strain S2308 demonstrated that ExsA is critical for full bacterial virulence. Additionally, the B. abortus exsA deletion mutant was used as a live vaccine. Challenge experiments revealed that the exsA mutant strain induced superior protective immunity in BALB/c mice compared to the protective immunity induced by strain S19 or RB51.

Early symbiotic responses induced by Sinorhizobium meliloti iIvC mutants in alfalfa

A mutation in the ilvC gene of Sinorhizobium meliloti 1021 determines a symbiotically defective phenotype. ilvC mutants obtained from different S. meliloti wild-type strains are able to induce root hair deformation on alfalfa roots and show variable activation of the common nodulation genes nodABC. All of these mutants are noninfective. The presence of extra copies of nodD3-syrM in an IlvC- background does not promote nod expression but allows the detection of low levels of Nod factor production. The sulphation of the Nod factor metabolites, however, is not affected. Furthermore, IlvC- strains induce a specific pattern of starch accumulation on alfalfa roots as well as of early nodulin expression. Hence, the pleiotropic action of the ilvC gene in S. meliloti may reveal novel complexities involved in the symbiotic interaction.

Alfalfa root nodule invasion efficiency is dependent on Sinorhizobium meliloti polysaccharides

The soil bacterium Sinorhizobium meliloti is capable of entering into a nitrogen-fixing symbiosis with Medicago sativa (alfalfa). Particular low-molecular-weight forms of certain polysaccharides produced by S. meliloti are crucial for establishing this symbiosis. Alfalfa nodule invasion by S. meliloti can be mediated by any one of three symbiotically important polysaccharides: succinoglycan, EPS II, or K antigen (also referred to as KPS). Using green fluorescent protein-labeled S. meliloti cells, we have shown that there are significant differences in the details and efficiencies of nodule invasion mediated by these polysaccharides. Succinoglycan is highly efficient in mediating both infection thread initiation and extension. However, EPS II is significantly less efficient than succinoglycan at mediating both invasion steps, and K antigen is significantly less efficient than succinoglycan at mediating infection thread extension. In the case of EPS II-mediated symbioses, the reduction in invasion efficiency results in stunted host plant growth relative to plants inoculated with succinoglycan or K-antigen-producing strains. Additionally, EPS II- and K-antigen-mediated infection threads are 8 to 10 times more likely to have aberrant morphologies than those mediated by succinoglycan. These data have important implications for understanding how S. meliloti polysaccharides are functioning in the plant-bacterium interaction, and models are discussed.

The ++Sinorhizobium meliloti lon protease is involved in regulating exopolysaccharide synthesis and is required for nodulation of alfalfa

While screening for Sinorhizobium meliloti Pho regulatory mutants, a transposon mutant was isolated that constitutively expressed higher levels of acid and alkaline phosphatase enzymes. This mutant was also found to form pseudonodules on alfalfa that were delayed in appearance relative to those formed by the wild-type strain, it contained few bacteroids, and it did not fix nitrogen. Sequence analysis of the transposon insertion site revealed the affected gene to have high homology to Lon proteases from a number of organisms. In minimal succinate medium, the mutant strain was found to grow more slowly, reach lower maximal optical density, and produce more extracellular polysaccharide (EPS) than the wild-type strain. The mutant fluoresced brightly on minimal succinate agar containing calcofluor (which binds to EPSI, a constitutively expressed succinoglycan), and gas chromotographic analysis of purified total EPS showed that the glucose-to-galactose ratio in the lon mutant total EPS was 5.0 +/- 0.2 (mean +/- standard error), whereas the glucose-to-galactose ratio in the wild-type strain was 7.1 +/- 0.5. These data suggested that in addition to EPSI, the lon mutant also constitutively synthesized EPSII, a galactoglucan which is the second major EPS known to be produced by S. meliloti, but typically is expressed only under conditions of phosphate limitation. (13)C nuclear magnetic resonance analysis showed no major differences between EPS purified from the mutant and wild-type strains. Normal growth, EPS production, and the symbiotic phenotype were restored in the mutant strain when the wild-type lon gene was present in trans. The results of this study suggest that the S. meliloti Lon protease is important for controlling turnover of a constitutively expressed protein(s) that, when unregulated, disrupts normal nodule formation and normal growth.

Structural characterization of the symbiotically important low-molecular-weight succinoglycan of Sinorhizobium meliloti

The production of succinoglycan by Sinorhizobium meliloti Rm1021 is required for successful nodule invasion by the bacterium of its host plant, alfalfa. Rm1021 produces succinoglycan, an acidic exopolysaccharide composed of an octasaccharide repeating unit modified with acetyl, succinyl, and pyruvyl moieties, in both low- and high-molecular-weight forms. Low-molecular-weight (LMW) succinoglycan, previously thought to consist of monomers, trimers, and tetramers of the repeating unit, has been reported as being capable of promoting the formation of nitrogen-fixing nodules by succinoglycan-deficient derivatives of strain Rm1021. We have determined that the three size classes of LMW succinoglycan species are in fact monomers, dimers, and trimers of the repeating unit and that the trimer is the species active in promoting nodule invasion. A detailed structural analysis of the components of LMW succinoglycan by using various chromatographic techniques, along with nuclear magnetic resonance analyses, has revealed that there is considerable heterogeneity within the LMW succinoglycan oligomers in terms of noncarbohydrate substitutions, and we have determined the structural basis of this heterogeneity.

A novel screening method for isolating exopolysaccharide-deficient mutants

A screening method based on differential staining of the wild type and exopolysaccharide-deficient mutants of Rhizobium (Sinorhizobium) meliloti by the lipophilic dye Sudan Black B is described. Mutants defective in the production of either succinoglycan or EPS II (galactoglucan) were isolated by using this method, which might also prove useful for isolating exopolysaccharide-defective derivatives of other bacteria.

The succinyl and acetyl modifications of succinoglycan influence susceptibility of succinoglycan to cleavage by the Rhizobium meliloti glycanases ExoK and ExsH

In Rhizobium meliloti (Sinorhizobium meliloti) cultures, the endo-1, 3-1,4-beta-glycanases ExoK and ExsH depolymerize nascent high-molecular-weight (HMW) succinoglycan to yield low-molecular-weight (LMW) succinoglycan. We report here that the succinyl and acetyl modifications of succinoglycan influence the susceptibility of succinoglycan to cleavage by these glycanases. It was previously shown that exoH mutants, which are blocked in the succinylation of succinoglycan, exhibit a defect in the production of LMW succinoglycan. We have determined that exoZ mutants, which are blocked in the acetylation of succinoglycan, exhibit an increase in production of LMW succinoglycan. For both wild-type and exoZ mutant strains, production of LMW succinoglycan is dependent on the exoK+ and exsH+ genes, implying that the ExoK and ExsH glycanases cleave HMW succinoglycan to yield LMW succinoglycan. By supplementing cultures of glycanase-deficient strains with exogenously added ExoK or ExsH, we have demonstrated directly that the absence of the acetyl group increases the susceptibility of succinoglycan to cleavage by ExoK and ExsH, that the absence of the succinyl group decreases the susceptibility of succinoglycan to cleavage, and that the succinyl effect outweighs the acetyl effect for succinoglycan lacking both modifications. Strikingly, nonsuccinylated succinoglycan actually can be cleaved by ExoK and ExsH to yield LMW succinoglycan, but only when the glycanases are added to cultures at greater than physiologically relevant concentrations. Thus, we conclude that the molecular weight distribution of succinoglycan in R. meliloti cultures is determined by both the levels of ExoK and ExsH glycanase expression and the susceptibility of succinoglycan to cleavage.

The Rhizobium meliloti ExoK and ExsH glycanases specifically depolymerize nascent succinoglycan chains

The Rhizobium meliloti ExoK and ExsH glycanases have been proposed to contribute to production of low molecular weight (LMW) succinoglycan by depolymerizing high molecular weight succinoglycan chains in R. meliloti cultures. We expressed and purified ExoK and ExsH and determined that neither enzyme can extensively cleave succinoglycan prepared from R. meliloti cultures, although neutral/heat treatment and acid/heat treatment convert succinoglycan to forms that can be cleaved efficiently by both enzymes. These results were somewhat surprising, given that the exoK+ and exsH+ genes play a crucial role in production of LMW succinoglycan in R. meliloti cultures. We demonstrated by Western blot analyses that R. meliloti expresses ExoK and ExsH, that both proteins can be detected extracellularly, and that ExsH secretion depends on the prsD+/prsE+ genes, consistent with previous predictions based on mutant analyses. Furthermore, we determined that the depolymerization activities associated with purified ExoK and ExsH are comparable with exoK+ and exsH+-dependent depolymerization activities expressed in R. meliloti cultures. We resolved the apparent contradiction between the results of our previous genetic analyses and depolymerization assays by determining that ExoK and ExsH can cleave high molecular weight succinoglycan that is being produced actively by R. meliloti, but not succinoglycan that has accumulated in cultures, to yield LMW succinoglycan. We propose that ExoK and ExsH dynamically regulate the molecular weight distribution of succinoglycan by cleaving nascent succinoglycan only during a limited period after its synthesis, perhaps before it undergoes a time-dependent change in its conformation or aggregation state.

Isolation and characterization of the UDP-glucose 4'-epimerase-encoding gene, galE, from Brucella abortus 2308

The UDP-glucose 4'-epimerase-encoding gene, galE or exoB, was isolated from Brucella abortus 2308 by complementation of an exoB mutant of Sinorhizobium meliloti. Confirmation of the identity was done by constructing an in-frame deletion of 660 bp with galE of the B. abortus genome by marker exchange. The resulting galE mutant lacked UDP-glucose 4'-epimerase activity. This activity was restored by in trans complementation with the intact gene. The B. abortus gal E mutant is not altered in colony morphology compared to wt 2308. The lack of UDP-glucose 4'-epimerase activity in the mutant and PCR analysis strongly suggest that only one copy of galE exists in B. abortus 2308. The galE sequence of B. abortus 2308 is more similar to galE from other animal-inhabiting bacteria than it is to exoB from the Sinorhizobium legume symbionts. We propose that galE in B. abortus evolved by lateral transfer from other animal-inhabiting bacteria rather than from a common ancestor of Brucella and Sinorhizobium.

Stationary-phase mutants of Sinorhizobium meliloti are impaired in stationary-phase survival or in recovery to logarithmic growth

A screening method was used to identify Sinorhizobium meliloti mutants which are affected in stationary-phase survival. Of 20,000 individual colonies mutagenized with transposon Tn5-B20, 10 mutant strains which showed poor or no survival in the stationary phase were identified. Analyses of expression patterns of the promoterless lacZ genes in the mutant strains revealed individual induction patterns. Most strains were induced in stationary phase as well as under carbon limitation and in pure H2O, but none of the mutants was induced under heat, alkali stress conditions, or low oxygen tension. Plant inoculation tests revealed that the symbiotic proficiency of the mutants was not affected. Two mutants, however, showed gene induction not only in the stationary phase under free-living conditions but also in the bacteroid state. A long-term starvation test was carried out to examine the ability of the 10 mutants to survive prolonged stationary-phase conditions. All mutants showed a clear decrease in the colony-forming ability under the chosen experimental conditions. Staining with green and red fluorescent nucleic acid stain showed that the mutants fell into two different classes. Seven mutants died during stationary phase; the three other mutants remained viable but did not resume growth after prolonged starvation. Five of the ten Tn5-B20 insertions were cloned from the genomes of the mutant strains. Nucleotide sequence analyses established that the transposon had inserted in five distinctive genes. Database searches revealed that four of the tagged loci corresponded to already characterized genes whose gene products are involved in important cellular processes such as amino acid metabolism or aerobic respiration.

The 32-kilobase exp gene cluster of Rhizobium meliloti directing the biosynthesis of galactoglucan: genetic organization and properties of the encoded gene products

Proteins directing the biosynthesis of galactoglucan (exopolysaccharide II) in Rhizobium meliloti Rm2011 are encoded by the exp genes. Sequence analysis of a 32-kb DNA fragment of megaplasmid 2 containing the exp gene cluster identified previously (J. Glazebrook and G. C. Walker, Cell 56:661-672, 1989) revealed the presence of 25 open reading frames. Homologies of the deduced exp gene products to proteins of known function suggested that the exp genes encoded four proteins involved in the biosynthesis of dTDP-glucose and dTDP-rhamnose, six glycosyltransferases, an ABC transporter complex homologous to the subfamily of peptide and protein export complexes, and a protein homologous to Rhizobium NodO proteins. In addition, homologies of three Exp proteins to transcriptional regulators, methyltransferases, and periplasmic binding proteins were found. The positions of 26 Tn5 insertions in the exp gene cluster were determined, thus allowing the previously described genetic map to be correlated with the sequence. Operon analysis revealed that the exp gene cluster consists of five complementation groups. In comparison to the wild-type background, all exp complementation groups were transcribed at a substantially elevated level in the regulatory mucR mutant.

Rhizobium meliloti exopolysaccharides: synthesis and symbiotic function

Bacterial exopolysaccharide (EPS) is required for establishment of the nitrogen-fixing symbiosis between Rhizobium meliloti and its host plant, Medicago sativa (alfalfa), but the precise role of EPS in this interaction is not well defined. Bacterial mutants which fail to produce EPS induce nodules on the roots of the host plant, but fail to invade these root nodules. Research conducted in our lab and others suggests that EPS plays a specific role in the R. meliloti-M. sativa symbiosis. A common theme emerging from these studies is that small quantities of low-molecular-weight (LMW) EPS are sufficient to mediate successful invasion by R. meliloti mutants which fail to produce EPS, implying that LMW EPS may act as a signaling molecule during this process.

Extension of the Rhizobium meliloti succinoglycan biosynthesis gene cluster: identification of the exsA gene encoding an ABC transporter protein, and the exsB gene which probably codes for a regulator of succinoglycan biosynthesis

Two new genes, designated exsA and exsB, were identified adjacent to the 24 kb exo gene cluster of Rhizobium meliloti, which is involved in succinoglycan (EPS I) biosynthesis. The derived amino acid sequence of ExsA displayed significant homologies to ATP binding cassette (ABC) transporter proteins. R. meliloti strains mutated in exsA were characterized by a decreased ratio of HMW to LMW EPS I, indicating a function for ExsA in EPS I biosynthesis. The R. meliloti NdvA protein, which is involved in the transport of cyclic beta-(1,2)-glucans, was identified as the closest homologue of ExsA. R. meliloti exsB mutants produced a three-fold increased amount of EPS I in comparison to the wild-type strain. In contrast, high copy number of exsB resulted in a decrease in the EPS I level to 20% of wild type, indicating that the exsB gene product can negatively influence EPS I biosynthesis. It was demonstrated that this influence is not due to transcriptional regulation of the exo genes by the exsB gene product. By plasmid integration it was shown that exsA and exsB represent monocistronic transcription units.

Low-molecular-weight succinoglycan is predominantly produced by Rhizobium meliloti strains carrying a mutated ExoP protein characterized by a periplasmic N-terminal domain and a missing C-terminal domain

The membrane topology of the Rhizobium meliloti 2011 ExoP protein involved in polymerization and export of succinoglycan was analysed by translational fusions of lacZ and phoA reporter genes to the exoP gene. Based on this analysis, the ExoP protein could be divided into an N-terminal domain mainly located in the periplasmic space and a C-terminal domain located in the cytoplasm. Whereas the C-terminal domain of ExoP is characterized by a potential nucleotide-binding motif, the N-terminal ExoP domain contains the sequence motif 'PX2PX4SPKX11GXMXG', which is also present in proteins involved in the determination of O-antigen chain length. R. meliloti strains carrying mutated exoP* genes, exclusively encoding the N-terminal ExoP domain, produced a reduced amount of succinoglycan. This reduction could be suppressed by a mutation in the regulatory gene exoR. The ratio of low-molecular-weight to high-molecular-weight succinoglycan was significantly increased in the exoP* mutant strain. In the exoP*/exoR mutant strain only low-molecular-weight succinoglycan could be detected. Based on sequence homologies and similar hydropathic profiles, the N-terminal domain of ExoP was proposed to be a member of a protein family thought to be involved in polysaccharide chain-length determination.

Identification and analysis of the Rhizobium meliloti exoAMONP genes involved in exopolysaccharide biosynthesis and mapping of promoters located on the exoHKLAMONP fragment

Sequence analysis of a 7.494 kb DNA fragment from megaplasmid 2 of Rhizobium meliloti 2011 involved in exopolysaccharide I (EPS I) biosynthesis revealed the presence of five exo genes designated exoA, exoM, exoN, exoO, and exoP. ExoN was found to show strong homology to a UDP-glucose pyrophosphorylase from Acetobacter xylinum, whereas ExoO displayed weak homologies to the NodC proteins from R. meliloti and R. loti. Surprisingly, different mutations in exoP resulted in divergent phenotypes. One exoP mutant was able to establish an effective symbiosis with alfalfa, although no EPS I polymer could be detected. In contrast, other exoP mutations prevented the formation of an effective symbiosis. The transcriptional organization of the exoA-exoP gene region has been analysed in conjunction with the exoH, exoK and exoL genes. Using exo-lacZ transcription fusions in association with plasmid integration mutagenesis a strong promoter was identified upstream of exoH, which is able to direct transcription of the whole exoHKLAMONP gene cluster. A much weaker promoter upstream of exoL was found to be involved in the transcription of the exoLAMONP genes. In addition, weak promoters were identified upstream of exoK, exoA, exoN and exoP.

Family of glycosyl transferases needed for the synthesis of succinoglycan by Rhizobium meliloti

Rhizobium meliloti produces an acidic exopolysaccharide, termed succinoglycan or EPS I, that is important for invasion of the nodules that it elicits on its host, Medicago sativa. Succinoglycan is a high-molecular-weight polymer composed of repeating octasaccharide subunits. These subunits are synthesized on membrane-bound isoprenoid lipid carriers, beginning with a galactose residue followed by seven glucose residues, and modified by the addition of acetate, succinate, and pyruvate. Biochemical characterizations of lipid-linked succinoglycan biosynthetic intermediates from previously identified exo mutant strains have been carried out in our laboratory (T. L. Reuber and G. C. Walker, Cell 74:269-280, 1993) to determine where each mutation blocks the biosynthetic pathway. We have carried out a fine structure genetic analysis of a portion of the cluster of exo genes present on the second symbiotic megaplasmid of R. meliloti and have identified several new genes. In addition, the DNA sequence of 16 kb of the exo cluster was determined and the genetic map was correlated with the DNA sequence. In this paper we present the sequence of a family of glycosyl transferases required for the synthesis of succinoglycan and discuss their functions.

Biosynthesis of succinoglycan, a symbiotically important exopolysaccharide of Rhizobium meliloti

The exo genes of Rhizobium meliloti are needed for the synthesis of an acidic exopolysaccharide, succinoglycan. We have assigned biosynthetic roles to the products of the exo genes by characterizing succinoglycan biosynthetic intermediates from exo mutant strains. We propose a model of succinoglycan biosynthesis in which the products of the exoY and exoF genes function in the addition of the first sugar, galactose, to the lipid carrier; the products of the exoA, exoL, exoM, exoO, exoU, and exoW genes function in subsequent sugar additions; and the product of the exoV gene functions in the addition of pyruvate. The products of the exoP, exoQ, and exoT genes are required for polymerization of the octasaccharide subunits or transport of the completed polymer.

The acetyl substituent of succinoglycan is not necessary for alfalfa nodule invasion by Rhizobium meliloti Rm1021

Rhizobium meliloti Rm1021 requires a Calcofluor-binding exopolysaccharide, termed succinoglycan or EPS I, to invade alfalfa nodules. We have determined that a strain carrying a mutation in the exoZ locus produces succinoglycan that lacks the acetyl substituent. The exoZ mutant nodules alfalfa normally.

Characterization and symbiotic importance of acidic extracellular polysaccharides of Rhizobium sp. strain GRH2 isolated from acacia nodules

Rhizobium sp. wild-type strain GRH2 was originally isolated from root nodules of the leguminous tree Acacia cyanophylla and has a broad host range which includes herbaceous legumes, e.g., Trifolium spp. We examined the extracellular exopolysaccharides (EPSs) produced by strain GRH2 and found three independent glycosidic structures: a high-molecular-weight acidic heteropolysaccharide which is very similar to the acidic EPS produced by Rhizobium leguminosarum biovar trifolii ANU843, a low-molecular-weight native heterooligosaccharide resembling a dimer of the repeat unit of the high-molecular-weight EPS, and low-molecular-weight neutral beta (1,2)-glucans. A Tn5 insertion mutant derivative of GRH2 (exo-57) that fails to form acidic heteropolysaccharides was obtained. This Exo- mutant formed nitrogen-fixing nodules on Acacia plants but infected a smaller proportion of cells in the central zone of the nodules than did wild-type GRH2. In addition, the exo-57 mutant failed to nodulate several herbaceous legume hosts that are nodulated by wild-type strain GRH2.

Analysis of the Rhizobium meliloti exoH/exoK/exoL fragment: ExoK shows homology to excreted endo-beta-1,3-1,4-glucanases and ExoH resembles membrane proteins

Nucleotide sequencing of a 4.15 kb DNA fragment from megaplasmid 2 of Rhizobium meliloti 2011 revealed the location of the genes exoH, exoK and exoL. The putative proteins encoded by these genes have molecular weights of 41, 30, and 44 kDa, respectively. The hydrophobicity profile of the ExoH amino acid sequence resembles that of transmembrane proteins. The predicted exoL gene product does not contain hydrophobic regions, indicating a cytoplasmic localization. The exoK gene product is characterized by a putative signal peptide and exhibits significant homology to endo-beta-1,3-1,4-glucanases of bacilli and Clostridium thermocellum. R. meliloti exoK mutants induced pink nodules and synthesized a reduced amount of exopolysaccharide (EPS). Colonies of this mutant showed a delay in the appearance of the Calcofluor white fluorescence. In addition, the formation of the characteristic halo was strongly delayed. R. meliloti exoL and exoH mutants induced pseudonodules. The exoH, but not the exoL mutant, synthesized an EPS that could be precipitated by cetyl pyridinium chloride (CPC) and also by ethanol. Plasmid integration mutagenesis revealed promoter regions preceding exoH, exoK and exoL.

The Rhizobium meliloti pmi gene encodes a new type of phosphomannose isomerase

Interspecific complementation of a Xanthomonas campestris pv. campestris phosphomannose isomerase (PMI) mutant was used to isolate a cosmid from a genomic library of Rhizobium meliloti 2011 carrying the pmi gene of this strain. Subcloning experiments localized the coding region to a 2.0-kb SalI-ClaI fragment. Nucleotide sequence analysis of this fragment indicated the presence of two open reading frames (ORFs), coding for 18- and 43-kDa polypeptides. The analysis of the gene function by gene disruption experiments showed that ORF2 codes for pmi. A comparison of the deduced amino acid sequence with the corresponding sequences of the Pseudomonas aeruginosa and Escherichia coli PMIs revealed no significant homology, indicating that the isolated gene encodes a new type of PMI. The construction of a pmi-deficient mutant of R. meliloti using the sacB-sacR cassette technique showed that the loss of PMI activity does not affect the symbiotic properties of this strain.

Molecular and symbiotic characterization of exopolysaccharide-deficient mutants of Rhizobium tropici strain CIAT899

We studied the symbiotic behaviour of 20 independent Tn5 mutants of Rhizobium tropici strain CIAT899 that were deficient in exopolysaccharide (EPS) production. The mutants produced non-mucoid colonies, were motile, grew in broth cultures at rates similar to those of the parent, and produced significantly less EPS than did CIAT899 in broth culture. A genomic library of strain CIAT899, constructed in pLA2917, was mobilized into all of the mutants, and cosmids that restored EPS production were identified. EcoRI restriction digests of the cosmids revealed nine unique inserts. Mutant complementation and hybridization analysis showed that the mutations affecting EPS production fell into six functional and physical linkage groups. On bean, the mutants were as efficient in nodulation and as effective in acetylene reduction as strain CIAT899, induced a severe interveinal chlorosis, and all but one were less competitive than CIAT899. On siratro, CIAT899 induced nodules that were ineffective in acetylene reduction, whereas the EPS-deficient mutants induced effective nodules. Microscopic examination of thin sections showed that nodules from both siratro and bean plants inoculated with either CIAT899 or an EPS-deficient mutant contained infected cells. These data indicate that EPS is not required for normal nodulation of bean by R. tropici, that it may contribute to competitiveness of R. tropici on bean, and that the loss of EPS production is accompanied by acquisition of the ability to reduce acetylene on siratro.

A Rhizobium meliloti lipopolysaccharide mutant altered in competitiveness for nodulation of alfalfa

A transposon Tn5-induced mutant of Rhizobium meliloti Rm2011, designated Rm6963, showed a rough colony morphology on rich and minimal media and an altered lipopolysaccharide (LPS). Major differences from the wild-type LPS were observed in (i) hexose and 2-keto-3-deoxyoctonate elution profiles of crude phenol extracts chromatographed in Sepharose CL-4B, (ii) silver-stained sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis patterns of crude and purified LPS fractions, and (iii) immunoreactivities otherwise present in purified LPS of the parental strain Rm2011. In addition, Rm6963 lost the ability to grow in Luria-Bertani medium containing the hydrophobic compounds sodium deoxycholate or SDS and showed a decrease in survival in TY medium supplemented with high calcium concentrations. The mutant also had altered symbiotic properties. Rm6963 formed nodules that fixed nitrogen but showed a delayed or even reduced ability to nodulate the primary root of alfalfa without showing changes in the position of nodule distribution profiles along the roots. Furthermore, 2 to 3 weeks after inoculation, plants nodulated by Rm6963 were smaller than control plants inoculated with wild-type bacteria in correlation with a transient decrease in nitrogen fixation. In most experiments, the plants recovered later by expressing a full nitrogen-fixing phenotype and developing an abnormally high number of small nodules in lateral roots after 1 month. Rm6963 was also deficient in the ability to compete for nodulation. In coinoculation experiments with equal bacterial numbers of both mutant and wild-type rhizobia, only the parent was recovered from the uppermost root nodules. A strain ratio of approximately 100 to 1 favoring the mutant was necessary to obtain an equal ratio (1:1) of nodule occupancy. These results show that alterations in Rm6963 which include LPS changes lead to an altered symbiotic phenotype during the association with alfalfa that affects the timing of nodule emergence, the progress of nitrogen fixation, and the strain competitiveness for nodulation.

Exogenous suppression of the symbiotic deficiencies of Rhizobium meliloti exo mutants

The acidic exopolysaccharide (EPS I) produced by Rhizobium meliloti during symbiosis with Medicago sativa has been shown to be required for the proper development of nitrogen-fixing nodules. Cloned DNA from the exo region of R. meliloti is shown to stimulate production of the low-molecular-weight form of this exopolysaccharide, and in this report we show that the symbiotic deficiencies of two exo mutants of R. meliloti, the exoA and exoH mutants, can be rescued by the addition of this low-molecular-weight material at the time of inoculation. For exoA and exoH mutants, rescue with a preparation containing low-molecular-weight exopolysaccharide induces the formation of nitrogen-fixing nodules which appear somewhat later and at a reduced efficiency compared with wild-type-induced nodules; however, microscopic analysis of these nodules reveals similar nodule morphology and the presence of large numbers of bacteroids in each.

Location and cloning of the ketal pyruvate transferase gene of Xanthomonas campestris

Genes required for xanthan polysaccharide synthesis (xps) are clustered in a DNA region of 13.5 kb in the chromosome of Xanthomonas campestris. Plasmid pCHC3 containing a 12.4-kb insert of xps genes has been suggested to include a gene involved in the pyruvylation of xanthan gum (N.E. Harding, J.M. Cleary, D.K. Cabañas, I. G. Rosen, and K. S. Kang, J. Bacteriol. 169:2854-2861, 1987). An essential step toward understanding the biosynthesis of xanthan gum and to enable genetic manipulation of xanthan structure is the determination of the biochemical function encoded by the xps genes. On the basis of biochemical characterization of an X. campestris mutant which produces pyruvate-free xanthan gum, complementation studies, and heterologous expression, we have identified the gene coding for the ketal pyruvate transferase (kpt) enzyme. This gene was located on a 1.4-kb BamHI fragment of pCHC3 and cloned in the broad-host-range cloning vector pRK404. An X. campestris kpt mutant was constructed by mini-Mu(Tetr) mutagenesis of the cloned gene and then by recombination of the mutation into the chromosome of the wild-type strain.

The Rhizobium meliloti exoZl exoB fragment of megaplasmid 2: ExoB functions as a UDP-glucose 4-epimerase and ExoZ shows homology to NodX of Rhizobium leguminosarum biovar viciae strain TOM

A 2.6 kb ClaI-BamHI DNA fragment of megaplasmid 2 of Rhizobium meliloti 2011 was found to carry genes involved in exopolysaccharide synthesis and infection of alfalfa nodules. The analysis of the nucleotide sequence of this DNA fragment revealed the existence of two open reading frames (ORFs) running in opposite directions. Plasmid integration mutagenesis showed that these ORFs are organized as two monocistronic transcription units. One of the ORFs represents a new exo gene designated exoZ, which is involved in, but not essential for, the production of acidic exopolysaccharide. However, exoZ is not necessary for nodule formation with alfalfa. The ExoZ protein was found to show homology (23.3%) to the NodX protein of the R. leguminosarum biovar viciae strain TOM, known to be essential for nodulating the primitive Afghanistan pea. The second identified ORF corresponds to the exoB locus. The deduced amino acid sequence of the ExoB protein is homologous (39.6%) to that of the Escherichia coli GalE protein. In R. meliloti, exoB codes for a UDP-glucose 4-epimerase. A deficiency in the activity of this enzyme fully accounts for all the multiple carbohydrate defects that have been observed in exoB mutants.

The exoR gene of Rhizobium meliloti affects RNA levels of other exo genes but lacks homology to known transcriptional regulators

Rhizobium meliloti strains mutant in the exoR gene overproduce an exopolysaccharide called succinoglycan or EPS I. Protein fusions to several different exo genes required for EPS I biosynthesis are expressed at a higher level in an exoR strain than in a wild-type strain, showing that the overproduction of EPS I in exoR strains results at least in part from increased gene expression. This regulation is important to nodulation, since exoR mutants fail to invade alfalfa nodules unless secondary suppressor mutations that cause a decrease in EPS I production occur. Here, we show that an exoR strain contains higher levels of mRNA for other exo genes than does the wild-type parental strain. ExoR therefore most probably exerts its regulatory effect at the level of transcription. In addition, we have localized, subcloned, and sequenced the exoR gene. A newly constructed insertion allele of exoR has the same phenotype as the original mutant. The deduced sequence of ExoR is 268 amino acids long but does not show homology to other sequenced genes.

Heterologous exopolysaccharide production in Rhizobium sp. strain NGR234 and consequences for nodule development

Rhizobium sp. strain NGR234 produces large amounts of acidic exopolysaccharide. Mutants that fail to synthesize this exopolysaccharide are also unable to nodulate the host plant Leucaena leucocephala. A hybrid strain of Rhizobium sp. strain NGR234 containing exo genes from Rhizobium meliloti was constructed. The background genetics and nod genes of Rhizobium sp. strain NGR234 are retained, but the cluster of genes involved in exopolysaccharide biosynthesis was deleted. These exo genes were replaced with genes required for the synthesis of succinoglycan exopolysaccharide from R. meliloti. As a result of the genetic manipulation, the ability of these hybrids to synthesize exopolysaccharide was restored, but the structure was that of succinoglycan and not that of Rhizobium sp. strain NGR234. The replacement genes were contained on a cosmid which encoded the entire known R. meliloti exo gene cluster, with the exception of exoB. Cosmids containing smaller portions of this exo gene cluster did not restore exopolysaccharide production. The presence of succinoglycan was indicated by staining with the fluorescent dye Calcofluor, proton nuclear magnetic resonance spectroscopy, and monosaccharide analysis. Although an NGR234 exoY mutant containing the R. meliloti exo genes produced multimers of the succinoglycan repeat unit, as does the wild-type R. meliloti, the deletion mutant of Rhizobium sp. strain NGR234 containing the R. meliloti exo genes produced only the monomer. The deletion mutant therefore appeared to lack a function that affects the multiplicity of succinoglycan produced in the Rhizobium sp. strain NGR234 background. Although these hybrid strains produced succinoglycan, they were still able to induce the development of an organized nodule structure on L. leucocephala. The resulting nodules did not fix nitrogen, but they did contain infection threads and bacteroids within plant cells. This clearly demonstrated that a heterologous acidic exopolysaccharide structure was sufficient to enable nodule development to proceed beyond the developmental barrier imposed on mutants of Rhizobium sp. strain NGR234 that are unable to synthesize any acidic exopolysaccharide.

Exopolysaccharide mutants of Rhizobium loti are fully effective on a determinate nodulating host but are ineffective on an indeterminate nodulating host

By Tn5 mutagenesis of Rhizobium loti PN184 (NZP2037 str-1) and selection for nonfluorescence of colonies on Calcofluor agar, eight independently generated expolysaccharide (EPS) mutants (three smooth and five rough) were isolated. The parent strain, PN184, was found to produce an acidic EPS. This EPS was produced. with reduced O acetylation, by the smooth EPS mutants but not by the rough EPS mutants. Lipopolysaccharide was isolated from all mutants and was identical to that of PN184 as defined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. All mutants were resistant to lysis by R. loti bacteriophage phi 2037/1. Cosmids that complemented the mutations in the rough EPS mutants were isolated from a pLAFR1 gene library of NZP2037 by complementation of the nonfluorescent phenotype. The genes identified were shown to be unlinked and located on the chromosome. All mutants were fully effective when inoculated onto Lotus pedunculatus, a determinate nodulating host, but were ineffective, inducing the formation of very small nodules or tumorlike growths, when inoculated onto Leucaena leucocephala, an indeterminate nodulating host. These results, obtained in an isogenic Rhizobium background, support suggestions that acidic EPS is required for effective nodulation of indeterminate nodulating legumes but is not required for effective nodulation of determinate nodulating legumes.