Clustering of mutations blocking synthesis of xanthan gum by Xanthomonas campestris

Bacterial extracellular biopolymers: Eco-diversification, biosynthesis, technological development and commercial applications

Synthetic polymers have been widely thriving as mega industries at a commercial scale in various commercial sectors over the last few decades. The extensive use of synthetic polymers has caused several negative repercussions on the health of humans and the environment. Recently, biopolymers have gained more attention among scientists of different disciplines by their potential therapeutic and commercial applications. Biopolymers are chain-like repeating units of molecules isolated from green sources. They are self-degradable, biocompatible, and non-toxic in nature. Recently, eco-friendly biopolymers such as extracellular polymeric substances (EPSs) have received much attention for their wide applications in the fields of emulsification, flocculation, preservatives, wastewater treatment, nanomaterial functionalization, drug delivery, cosmetics, glycomics, medicinal chemistry, and purification technology. The dynamicity of applications has raised the industrial and consumer demands to cater to the needs of mankind. This review deals with current insights and highlights on database surveys, potential sources, classification, extremophilic EPSs, bioprospecting, patents, microenvironment stability, biosynthesis, and genetic advances for production of high valued ecofriendly polymers. The importance of high valued EPSs in commercial and industrial applications in the global market economy is also summarized. This review concludes with future perspectives and commercial applications for the well-being of humanity.

Bacterial exopolysaccharides: Chemical structures, gene clusters and genetic engineering

In recent decades, the composition, structure, biosynthesis, and function of bacterial extracellular polysaccharides (EPS) have been extensively studied. EPS are synthesized through different biosynthetic pathways. The genes responsible for EPS synthesis are usually clustered on the genome or large plasmids of bacteria. Generally, different EPS synthesis gene clusters direct the synthesis of EPS with different chemical structures and biological activities. A better understanding of the gene functions involved in EPS biosynthesis is critical for the production of EPS with special biological activities. Genetic engineering methods are usually used to study EPS synthesis related genes. This review organizes the available information on EPS, including their structures, synthesis of related genes, and highlights the research progress of modifying EPS gene clusters through gene-editing methods.

Novel Endotype Xanthanase from Xanthan-Degrading Microbacterium sp. Strain XT11

Under general aqueous conditions, xanthan appears in an ordered conformation, which makes its backbone largely resistant to degradation by known cellulases. Therefore, the xanthan degradation mechanism is still unclear because of the lack of an efficient hydrolase. Here, we report the catalytic properties of MiXen, a xanthan-degrading enzyme identified from the genus Microbacterium MiXen is a 952-amino-acid protein that is unique to strain XT11. Both the sequence and structural features suggested that MiXen belongs to a new branch of the GH9 family and has a multimodular structure in which a catalytic (α/α)₆ barrel is flanked by an N-terminal Ig-like domain and by a C-terminal domain that has very few homologues in sequence databases and functions as a carbohydrate-binding module (CBM). Based on circular dichroism, shear-dependent viscosity, and reducing sugar and gel permeation chromatography analysis, we demonstrated that recombinant MiXen efficiently and randomly cleaved glucosidic bonds within the highly ordered xanthan substrate. A MiXen mutant free of the C-terminal CBM domain partially lost its xanthan-hydrolyzing ability because of decreased affinity toward xanthan, indicating the CBM domain assisted MiXen in hydrolyzing highly ordered xanthan via recognizing and binding to the substrate. Furthermore, side chain substituents and the terminal mannosyl residue significantly influenced the activity of MiXen via the formation of barriers to enzymolysis. Overall, the results of this study provide insight into the hydrolysis mechanism and enzymatic properties of a novel endotype xanthanase that will benefit future applications.IMPORTANCE This work characterized a novel endotype xanthanase, MiXen, and elucidated that the C-terminal carbohydrate-binding module of MiXen could drastically enhance the hydrolysis activity of the enzyme toward highly ordered xanthan. Both the sequence and structural analysis demonstrated that the catalytic domain and carbohydrate-binding module of MiXen belong to the novel branch of the GH9 family and CBMs, respectively. This xanthan cleaver can help further reveal the enzymolysis mechanism of xanthan and provide an efficient tool for the production of molecular modified xanthan with new physicochemical and physiological functions.

Phenotypic Switching Affecting Chemotaxis, Xanthan Production, and Virulence in Xanthomonas campestris

The chemotaxis towards sucrose and yeast extract of nine strains of Xanthomonas campestris representing pathovars campestris, armoraciae, translucens, vesicatoria, and pelargonii was analyzed by using swarm plates. Unexpectedly, each of these strains formed small or reduced swarms typical of nonmotile or nonchemotactic bacteria. With time, however, chemotactic cells appeared on the swarm plates as blebs of bacteria. These cells were strongly chemotactic and were concomitantly deficient in exopolysaccharide production. The switch from the wild type (exopolysaccharide producing and nonchemotactic) to the swarmer type (exopolysaccharide deficient and chemotactic) appeared irreversible ex planta in bacteriological medium. However, in radish leaves swarmer-type strains of X. campestris pv. campestris were able to revert to the wild type. Swarmer-type derivatives of two X. campestris pv. campestris wild-type isolates showed reduced virulence and growth in the host plants cauliflower and radish. However, exocellular complementation of X. campestris pv. campestris Hrp (nonpathogenic) mutant was achieved by coinoculation with a swarmer-type strain.

An oxidoreductase is involved in cercosporin degradation by the bacterium Xanthomonas campestris pv. zinniae

The polyketide toxin cercosporin plays a key role in pathogenesis by fungal species of the genus Cercospora. The bacterium Xanthomonas campestris pv. zinniae is able to rapidly degrade this toxin. Growth of X. campestris pv. zinniae strains in cercosporin-containing medium leads to the breakdown of cercosporin and to the formation of xanosporic acid, a nontoxic breakdown product. Five non-cercosporin-degrading mutants of a strain that rapidly degrades cercosporin (XCZ-3) were generated by ethyl methanesulfonate mutagenesis and were then transformed with a genomic library from the wild-type strain. All five mutants were complemented with the same genomic clone, which encoded a putative transcriptional regulator and an oxidoreductase. Simultaneous expression of these two genes was necessary to complement the mutant phenotype. Sequence analysis of the mutants showed that all five mutants had point mutations in the oxidoreductase gene and no mutations in the regulator. Quantitative reverse transcription-PCR (RT-PCR) showed that the expression of both of these genes in the wild-type strain is upregulated after exposure to cercosporin. Both the oxidoreductase and transcriptional regulator genes were transformed into three non-cercosporin-degrading bacteria to determine if they are sufficient for cercosporin degradation. Quantitative RT-PCR analysis confirmed that the oxidoreductase was expressed in all transconjugants. However, none of the transconjugants were able to degrade cercosporin, suggesting that additional factors are required for cercosporin degradation. Further study of cercosporin degradation in X. campestris pv. zinniae may allow for the engineering of Cercospora-resistant plants by using a suite of genes.

The molecular genetics of virulence of Xanthomonas campestris

Bacteria belonging to the genus Xanthomonas are important pathogens of many plants, and their virulence appears to be due primarily to secreted and surface compounds that could increase host nutrient loss, or avoid or suppress unfavorable conditions in the host. Type II and III secretory pathways are essential for virulence. Some individual extracellular enzymes (type II-secretion dependent) affect final bacterial population levels, whereas some avirulence gene products (type III-secretion dependent) affect virulence by altering host metabolism. Avr proteins, probably secreted via a pilus, can also be recognized by host resistance gene products. Virulence is also associated with bacterial surface polysaccharides, which may help to avoid host defense responses, and regulatory gene systems, which can control virulence gene expression.

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

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

Novel genomic locus with atypical G+C content that is required for extracellular polysaccharide production and virulence in Xanthomonas oryzae pv. oryzae

Three exopolysaccharide (EPS)- and virulence-deficient mutants of Xanthomonas oryzae pv. oryzae, the causal agent of bacterial leaf blight of rice, were isolated by Tn5 mutagenesis. These insertions are not located within the gum gene cluster. A 40-kb cosmid clone that restored EPS production and virulence to all three mutants was isolated, and the three transposon insertions were localized to contiguous 4.3- and 3.5-kb EcoRI fragments that are included in this clone. Sequence data indicate that two of the transposon insertions are in genes that encode a putative sugar nucleotide epimerase and a putative glycosyl transferase, respectively; the third insertion is located between the glycosyl transferase gene and a novel open reading frame (ORF). A 5.5-kb genomic region in which these three ORFs are located has a G+C content of 5-1.7%, quite different from the G+C content of approximately 65.0% that is typical of X. oryzae pv. oryzae. Homologues of this locus have not yet been reported in any other xanthomonad.

Direct incorporation of glucosamine and N-acetylglucosamine into exopolymers by Gluconacetobacter xylinus (=Acetobacter xylinum) ATCC 10245: production of chitosan-cellulose and chitin-cellulose exopolymers

Gluconacetobacter xylinus (=Acetobacter xylinum) ATCC 10245 incorporated 2-amino-2-deoxy-D-glucose (glucosamine) and 2-acetamido-2-deoxy-D-glucose (N-acetylglucosamine), but not 3-O-methyl-D-glucose or 2-deoxy-D-glucose into exopolymers. Incorporation was confirmed by gas chromatography with and without mass spectrometry, Fourier transform infrared, and 1H nuclear magnetic resonance. The average molar percentage of glucosamine and N-acetylglucosamine in the exopolymers was about 18%.

Improvement in bioreactor productivities using free radicals: HOCl-induced overproduction of xanthan gum from Xanthomonas campestris and its mechanism

Free-radical induction has been employed as a novel strategy to improve bioreactor productivity and, more specifically, the quality and productivity of xanthan gum from Xanthomonas campestris cultures. A 210% increase in xanthan yield and a 20% increase in viscosity (quality) resulted from HOCl (oxidant) treatment. The acetate mass fraction in xanthan gum decreased by 42% and its pyruvate mass fraction increased by 63% as a result of HOCl treatment. The growth rate was almost unaffected by HOCl treatment. A hypothesis to explain the mechanism of xanthan gum overproduction by free-radical induction has been formulated. The significant aspects of the hypothesis, such as SoxS protein binding to the promoter region of the gum gene and the consequent increase in mRNA concentrations, have been experimentally verified.

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

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

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

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

Assignment of biochemical functions to glycosyl transferase genes which are essential for biosynthesis of exopolysaccharides in Sphingomonas strain S88 and Rhizobium leguminosarum

Glycosyl transferases which recognize identical substrates (nucleotide-sugars and lipid-linked carbohydrates) can substitute for one another in bacterial polysaccharide biosynthesis, even if the enzymes originate in different genera of bacteria. This substitution can be used to identify the substrate specificities of uncharacterized transferase genes. The spsK gene of Sphingomonas strain S88 and the pssDE genes of Rhizobium leguminosarum were identified as encoding glucuronosyl-(B1-->4)-glucosyl transferases based on reciprocal genetic complementation of mutations in the spsK gene and the pssDE genes by segments of cloned DNA and by the SpsK-dependent incorporation of radioactive glucose (Glc) and glucuronic acid (GlcA) into lipid-linked disaccharides in EDTA-permeabilized cells. By contrast, glycosyl transferases which form alternative sugar linkages to the same substrate caused inhibition of polysaccharide synthesis or were deleterious or lethal in a foreign host. The negative effects also suggested specific substrate requirements: we propose that spsL codes for a glucosyl-(beta1-->4)-glucuronosyl transferase in Sphingomonas and that pssC codes for a glucuronosyl-(beta1-->4)-glucuronosyl transferase in R. leguminosarum. Finally, the complementation results indicate the order of attachment of sphingan main-chain sugars to the C55-isoprenylphosphate carrier as -Glc-GlcA-Glc-isoprenylpyrophosphate.

Phosphomannose isomerase of Xanthomonas campestris: a zinc activated enzyme

Phosphomannose isomerase (pmi, EC 5.3.1.8) was purified to homogeneity from a wild strain of Xanthomonas campestris. The apparent molecular weight as determined by SDS-PAGE and Sephadex G-100 Superfine was found to be 58 kDa. The purified enzyme showed a single band on acrylamide gel electrophocusing with pI = 5.25. The optimum pH was 7.0 and the Km for D-mannose-6-phosphate was 2 mM. Pmi can be activated by bivalent cations with the order of Co2+>Zn2+>Mn2+>Ni2+>Ca2+. Addition of low concentration of ZnCl2 (2 x 10[-7] M) in the growth medium resulted in the enhancement of pmi activity to around 2.5 x fold. The half life of pmi, as it was measured by the addition of chloramphenicol, was 110 min, whereas in the medium supplemented with ZnCl2 was 270 min. Chemical modification experiments implied the existence of one histidyl residue located at or near the active site.

Exopolymers from curdlan production: incorporation of glucose-related sugars by Agrobacterium sp. strain ATCC 31749

Three different exopolymers were purified from cultures of Agrobacterium sp. strain ATCC 31749 grown in a mineral salts medium containing 2% glucose at 30 °C for 5 days under aerobic culture conditions. These exopolymers were curdlan (extracellular, homo-β-(1-3)-glucan, water insoluble at neutral pH), a water-soluble noncurdlan-type exopolymer A (WSNCE-A), and a water-soluble noncurdlan-type exopolymer B (WSNCE-B). Curdlan, WSNCE-A, and WSNCE-B composed by weight 61, 27, and 12%, respectively, of the exopolymer produced from glucose. Compositions of all polymers were confirmed by gas chromatography (GC) and gas chromatography – mass spectrometry (GC–MS). The WSNCE-A is composed of glucose and galactose with lower contents of rhamnose. The WSNCE-B consists of glucose and mannose. To biosynthesize modified biopolymers, glucose-related sugars including 3-O-methyl-D-glucose, 2-amino-2-deoxy-D-glucose, and 2-acetamido-2-deoxy-D-glucose (N-acetylglucosamine) were fed separately as the sole carbon source. Using 3-O-methyl-D-glucose, 8 – 12 mol% of the curdlan repeats were 3-O-methyl-D-glucose based on GC and 1H-nuclear magnetic resonance spectrometry. N-Acetylglucosamine was incorporated into WSNCE-A at 10 mol% based on the GC–MS but was not found in curdlan or WSNCE-B.Key words: Agrobacterium sp., curdlan, exopolymer, 3-O-methyl-D-glucose, 2-acetamido-2-deoxy-D-glucose.

Linkage of genes essential for synthesis of a polysaccharide capsule in Sphingomonas strain S88

Several structurally related capsular polysaccharides that are secreted by members of the genus Sphingomonas are being developed as aqueous rheological control agents for diverse industrial and food applications. They include gellan (S-60), welan (S-130), rhamsan (S-194), S-657, S-88, S-198, S-7, and NW-11. We refer to these polysaccharides as sphingans, after the genus name. This paper characterizes the first gene cluster isolated from a Sphingomonas species (S88) that is required for capsule synthesis. Overlapping DNA segments which spanned about 50 kbp of S88 DNA restored the synthesis of sphingan S-88 in capsule-negative mutants. The mutations were mapped into functional complementation groups, and the contiguous nucleotide sequence for the 29-kbp cluster was determined. The genetic complementation map and the DNA sequences were interpreted as an extended multicistronic locus containing genes essential for the assembly and secretion of polysaccharide S-88. Many of the deduced amino acid sequences were similar to gene products from other polysaccharide-secreting bacteria such as Rhizobium meliloti (succinoglycan), Xanthomonas campestris (xanthan gum), and Salmonella enterica (O antigen). The S88 locus contained a four-gene operon for the biosynthesis of dTDP-L-rhamnose, an essential precursor for the sphingans. Unexpectedly, there were also two genes for secretion of a lytic or toxin-like protein nested within the polysaccharide cluster. The conservation and linkage of genes that code for a defensive capsule and genes for secretion of an offensive lysin or toxin suggest a heretofore unknown pathogenic life history for Sphingomonas strain S88.

Mechanism of bacitracin resistance in gram-negative bacteria that synthesize exopolysaccharides

Four representative species from three genera of gram-negative bacteria that secrete exopolysaccharides acquired resistance to the antibiotic bacitracin by stopping synthesis of the exopolysaccharide. Xanthomonas campestris, Sphingomonas strains S-88 and NW11, and Escherichia coli K-12 secrete xanthan gum, sphingans S-88 and NW11, and colanic acid, respectively. The gumD gene in X. campestris is required to attach glucose-P to C55-isoprenyl phosphate, the first step in the assembly of xanthan. A recombinant plasmid carrying the gumD gene of X. campestris restored polysaccharide synthesis to bacitracin-resistant exopolysaccharide-negative mutants of X. campestris and Sphingomonas strains. Similarly, a newly cloned gene (spsB) from strain S-88 restored xanthan synthesis to the same X. campestris mutants. However, the intergeneric complementation did not extend to mutants of E. coli that were both resistant to bacitracin and nonproducers of colanic acid. The genetic results also suggest mechanisms for assembling the sphingans which have commercial potential as gelling and viscosifying agents.

A 3.9-kb DNA region of Xanthomonas campestris pv. campestris that is necessary for lipopolysaccharide production encodes a set of enzymes involved in the synthesis of dTDP-rhamnose

By mutational analysis it was found that a 3.9-kb SmaI-XhoII DNA fragment of Xanthomonas campestris pv. campestris is involved in lipopolysaccharide (LPS) biosynthesis. LPS samples isolated from different mutants carrying mutations in the 3.9-kb SmaI-XhoII DNA fragment exhibited banding patterns in silver-stained sodium dodecyl sulfate-polyacrylamide gels markedly different from that of the wild-type LPS. Moreover, comparison of the monosaccharide composition obtained by high-performance anion-exchange chromatography with pulsed amperometric detection of LPS purified from wild-type Xanthomonas campestris pv. campestris B100 and from mutants with mutations in the 3.9-kb SmaI-XhoII DNA fragment revealed a lack of rhamnose moieties in the mutant LPS. Sequence analysis of this DNA fragment revealed four open reading frames (ORFs), designated ORF302, ORF183, ORF295, and ORF351. The deduced amino acid sequences of these ORFs showed a high degree of homology to the deduced amino acid sequences of the rfbC, rfbD, rfbA, and rfbB genes of Salmonella typhimurium LT2, which have been shown to encode a set of enzymes responsible for conversion of glucose 1-phosphate to dTDP-rhamnose.

The opsX locus of Xanthomonas campestris affects host range and biosynthesis of lipopolysaccharide and extracellular polysaccharide

Xanthomonas campestris pv. citrumelo strain 3048 is the causal agent of citrus bacterial leaf spot disease and has a wide host range that includes rutaceous and leguminous plants. A spontaneous prototrophic mutant of strain 3048 (strain M28) that had lost virulence on citrus but retained virulence on bean plants was recovered. Growth studies in planta showed that M28 cells died rapidly in citrus leaves but grew normally in bean leaves. In addition to the loss of citrus-specific virulence, M28 displayed the following mutant phenotypes in culture: decreased growth rate, reduction of the amount of exopolysaccharide (to ca. 25% of the amount in 3048), loss of capsules, and significant alterations of the two 3048 lipopolysaccharide (LPS) bands visualized by silver stain on polyacrylamide gels, consistent with a defect(s) in LPS assembly. A 38-kb DNA fragment from a 3048 total DNA library that complemented the mutant phenotypes of M28 was identified. The 38-kb fragment did not hybridize to two similarly sized fragments carrying different hrp (hypersensitive response and pathogenicity) genes cloned from 3048. Subcloning, DNA sequence analyses, and gene disruption experiments were used to identify a single gene, opsX (for outer-membrane polysaccharide), responsible for the mutant phenotypes of M28. At least one other gene downstream from opsX also affected the same phenotypes and may be part of a gene cluster. We report here the DNA sequence and transcriptional start site of opsX. A search of protein sequence data bases with the predicted 31.3-kDa OpsX sequence found strong similarity to Lsi-1 of Neisseria gonorrhoeae and RfaQ of Escherichia coli (both are involved in LPS core assembly). The host-specific virulence function of opsX appears to involve biosynthesis of the extracellular polysaccharide and a complete LPS. Both may be needed in normal amounts for protection from citrus, but not bean, defense compounds.

Genetic stability and xanthan gum production in Xanthomonas campestris pv. campestris NRRL B1459

A transposon (Tn5-SC) was constructed that can be used to quantify genetic deletions or amplifications. This transposon was used to evaluate the genomic stability of Xanthomonas campestris pv. campestris NRRL B1459 and we found that the genome of this bacterium is as stable as other Gram-negative bacteria or even more stable. Homologous recombination between plasmid sequences was determined in strain NRRL B1459 and was found to occur at a similar level to that reported for other Gram-negative bacteria. We report here that in X.c.c. NRRL B1459 there is no straightforward correlation between the occurrence of genetic rearrangements and frequency of homologous recombination. These data are discussed with respect to the reported instability of strain NRRL B1459 for xanthan gum production.

Sequential assembly and polymerization of the polyprenol-linked pentasaccharide repeating unit of the xanthan polysaccharide in Xanthomonas campestris

Lipid-linked intermediates are involved in the synthesis of the exopolysaccharide xanthan produced by the bacterium Xanthomonas campestris (L. Ielpi, R. O. Couso, and M. A. Dankert, FEBS Lett. 130:253-256, 1981). In this study, the stepwise assembly of the repeating pentasaccharide unit of xanthan is described. EDTA-treated X. campestris cells were used as both enzyme preparation and lipid-P acceptor, and UDP-Glc, GDP-Man, and UDP-glucuronic acid were used as sugar donors. A linear pentasaccharide unit is assembled on a polyprenol-P lipid carrier by the sequential addition of glucose-1-P, glucose, mannose, glucuronic acid, and mannose. The in vitro synthesis of pentasaccharide-P-P-polyprenol was also accompanied by the incorporation of radioactivity into a polymeric product, which was characterized as xanthan, on the basis of gel filtration and permethylation studies. Results from two-stage reactions showed that essentially pentasaccharide-P-P-polyprenol is polymerized. In addition, the direction of chain elongation has been studied by in vivo experiments. The polymerization of lipid-linked repeat units occurs by the successive transfer of the growing chain to a new pentasaccharide-P-P-polyprenol. The reaction involves C-1 of glucose at the reducing end of the polyprenol-linked growing chain and C-4 of glucose at the nonreducing position of the newly formed polyprenol-linked pentasaccharide, generating a branched polymer with a trisaccharide side chain.

Increase of xanthan production by cloning xps genes into wild-type Xanthomonas campestris

Previously, genomic banks of Xanthomonas campestris were constructed in Escherichia coli, using mobilizable broad-host-range cosmids as the vectors. Following conjugal transfer, genes involved in the biosynthesis of xanthan polysaccharide (XPS) were cloned by the ability to restore the mucoid phenotype to the non-mucoid mutants. In this study, all clones were transferred into the wild-type strain Xc17 to evaluate the effects of the cloned genes on XPS production. Most clones showed no significant effect; however, two plasmids, pP2401 and pP2201, caused 10 and 15% yield increases, respectively, compared with that of controls. While it was not clear how pP2201 caused the yield increase, the effect of pP2401 seemed to result from elevated phosphomannose isomerase activity. Since XPS synthesis in X. campestris is a very efficient process, only relatively small increases are to be expected; an enhancement of productivity by 10-15% is important to the commercial production of xanthan.

Genetics of xanthan production in Xanthomonas campestris: the xanA and xanB genes are involved in UDP-glucose and GDP-mannose biosynthesis

The nucleotide sequence of a 3.4-kb EcoRI-PstI DNA fragment of Xanthomonas campestris pv. campestris revealed two open reading frames, which were designated xanA and xanB. The genes xanA and xanB encode proteins of 448 amino acids (molecular weight of 48,919) and 466 amino acids (molecular weight of 50,873), respectively. These genes were identified by analyzing insertion mutants which were known to be involved in xanthan production. Specific tests for the activities of enzymes involved in the biosynthesis of UDP-glucose and GDP-mannose indicated that the xanA gene product was involved in the biosynthesis of both glucose 1-phosphate and mannose 1-phosphate. The deduced amino acid sequence of xanB showed a significant degree of homology (59%) to the phosphomannose isomerase of Pseudomonas aeruginosa, a key enzyme in the biosynthesis of alginate. Moreover, biochemical analysis and complementation experiments with the Escherichia coli manA fragment revealed that xanB encoded a bifunctional enzyme, phosphomannose isomerase-GDP-mannose pyrophosphorylase.

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.

Conjugal transfer of recombinant plasmids into gellan gum-producing and non-producing variants of Pseudomonas elodea ATCC 31461

The conjugal transfer of recombinant plasmids into a gellan gum-producing, rifampicin-resistant strain of Pseudomonas elodea and into one of its non-producing variants, was studied in order to facilitate the cloning of gellan genes. The mobilization frequency of recombinant plasmids derived from pKT240, and of cosmid pJRD215 from Escherichia coli HB101 (hsdM), into the mucoid strain was below the values for the non-mucoid variant and decreased exponentially with plasmid size. Reducing the mating time on solid surfaces led to higher mobilization frequencies. Under optimal conditions, a gene bank (40-50 kb) constructed in the cosmid pJRD215 was efficiently mobilized into Gel- mutants during complementation experiments.

A Xanthomonas campestris pv. campestris protein similar to catabolite activation factor is involved in regulation of phytopathogenicity

A DNA fragment from Xanthomonas campestris pv. campestris that partially restored the carbohydrate fermentation pattern of a cya crp Escherichia coli strain was cloned and expressed in E. coli. The nucleotide sequence of this fragment revealed the presence of a 700-base-pair open reading frame that coded for a protein highly similar to the catabolite activation factor (CAP) of E. coli (accordingly named CLP for CAP-like protein). An X. campestris pv. campestris clp mutant was constructed by reverse genetics. This strain was not affected in the utilization of various carbon sources but had strongly reduced pathogenicity. Production of xanthan gum, pigment, and extracellular enzymes was either increased or decreased, suggesting that CLP plays a role in the regulation of phytopathogenicity.

Cloning and analysis of a 35.3-kilobase DNA region involved in exopolysaccharide production by Xanthomonas campestris pv. campestris

Cosmid clones able to restore exopolysaccharide production in possibly insertion sequence element-induced surface mutants of Xanthomonas campestris pv. campestris were isolated. By fragment-specific Tn5-lac mutagenesis of one of the cosmids, pXCB1002, a new DNA region which is involved in exopolysaccharide biosynthesis and which is organized into at least 12 complementation groups was identified.

Rhizobium leguminosarum exopolysaccharide mutants: biochemical and genetic analyses and symbiotic behavior on three hosts

Ten independently generated mutants of Rhizobium leguminosarum biovar phaseoli CFN42 isolated after Tn5 mutagenesis formed nonmucoid colonies on all agar media tested and lacked detectable production of the normal acidic exopolysaccharide in liquid culture. The mutants were classified into three groups. Three mutants harbored Tn5 insertions on a 3.6-kilobase-pair EcoRI fragment and were complemented to have normal exopolysaccharide production by cosmids that shared an EcoRI fragment of this size from the CFN42 genome. The Tn5 inserts of five other mutants appeared to be located on a second, slightly smaller EcoRI fragment. Attempts to complement mutants of this second group with cloned DNA were unsuccessful. The mutations of the other two mutants were located in apparently adjacent EcoRI fragments carried on two cosmids that complemented those two mutants. The latter two mutants also lacked O-antigen-containing lipopolysaccharides and induced underdeveloped nodules that lacked nitrogenase activity on bean plants. The other eight mutants had normal lipopolysaccharides and wild-type symbiotic proficiencies on bean plants. Mutants in each of these groups were mated with R. leguminosarum strains that nodulated peas (R. leguminosarum biovar viciae) or clovers (R. leguminosarum biovar trifolii). Transfer of the Tn5 mutations resulted in exopolysaccharide-deficient R. leguminosarum biovar viciae or R. leguminosarum biovar trifolii transconjugants that were symbiotically deficient in all cases. These results support earlier suggestions that successful symbiosis with peas or clovers requires that rhizobia be capable of acidic exopolysaccharide production, whereas symbiosis with beans does not have this requirement.

Genetic analysis of a cluster of genes required for synthesis of the calcofluor-binding exopolysaccharide of Rhizobium meliloti

Rhizobium meliloti produces an acidic, Calcofluor-binding exopolysaccharide which plays a role in nodulation of alfalfa plants by this bacterium. We constructed and mapped 102 transposon insertions in a 48-kilobase (kb) region previously shown to contain several exo genes. Mutations affecting production of the Calcofluor-binding exopolysaccharide were clustered in a 22-kb region and fell into 12 complementation groups. Strains carrying mutations in seven of the complementation groups (exoA, exoB, exoF, exoL, exoM, exoP, and exoQ) produced no Calcofluor-binding exopolysaccharide and induced non-nitrogen-fixing nodules on alfalfa. Mutants in an eighth complementation group, exoH (Leigh et al., Cell 51:579-587, 1987), produce an altered exopolysaccharide and also induce the formation of non-nitrogen-fixing nodules. Mutants in the remaining four complementation groups produced less Calcofluor-binding material than the wild type. Mutants carrying mutations in two of these complementation groups (exoK and exoN) formed apparently normal, nitrogen-fixing nodules, while mutants in the other two groups (exoG and exoJ) formed normal nodules less efficiently than the wild type.

Rhizobium meliloti mutants that overproduce the R. meliloti acidic calcofluor-binding exopolysaccharide

The acidic Calcofluor-binding exopolysaccharide of Rhizobium meliloti Rm1021 plays one or more critical roles in nodule invasion and possibly in nodule development. Two loci, exoR and exoS, that affect the regulation of synthesis of this exopolysaccharide were identified by screening for derivatives of strain Rm1021 that formed mucoid colonies that fluoresced extremely brightly under UV light when grown on medium containing Calcofluor. The exopolysaccharide produced in large quantities by the exoR95::Tn5 and exoS96::Tn5 strains was indistinguishable from that produced by the parental strain Rm1021, and its synthesis required the function of at least the exoA, exoB, and exoF genes. Both the exoR and exoS loci were located on the chromosome, and the exo96::Tn5 mutation was 84% linked to the trp-33 mutation by phi M12 transduction. Synthesis of the Calcofluor-binding exopolysaccharide by strain Rm1021 was greatly stimulated by starvation for ammonia. In contrast, the exoR95::Tn5 mutant produced high levels of exopolysaccharide regardless of the presence or absence of ammonia in the medium. The exoS96::Tn5 mutant produced elevated amounts of exopolysaccharide in the presence of ammonia, but higher amounts were observed after starvation for ammonia. The presence of either mutation increased the level of expression of exoF::TnphoA and exoP::TnphoA fusions (TnphoA is Tn5 IS50L::phoA). Analyses of results obtained when alfalfa seedlings were inoculated with the exoR95::Tn5 strain indicated that the mutant strain could not invade nodules. However, pseudorevertants that retained the original exoR95::Tn5 mutation but acquired unlinked suppressors so that they produced an approximately normal amount of exopolysaccharide were able to invade nodules and fix nitrogens. The exoS95::Tn5 strain formed Fix+ nodules, although some minor variability was observed.

Acquisition of new metabolic capabilities: multicopy suppression by cloned transaminase genes in Escherichia coli K-12

The four general transaminases of Escherichia coli K-12 have overlapping, but discrete, substrate specificities and participate in the final step in the synthesis of at least seven different amino acids. Through the use of strains that have mutations in one or more transaminase genes and carry a different wild-type (wt) gene on a multicopy plasmid, it was possible to detect instances in which an amplified wt gene suppressed nonallelic mutations. In these cases, overproduction of the enzyme permitted a broader range of substrates to be used at physiologically significant levels, either because a low catalytic efficiency (in the case analyzed here) or a low affinity of the enzyme towards the substrate prevented its effective utilization under normal conditions. Consequently, by compensating for a low catalytic reaction rate, enzyme overproduction circumvents the original lesion and restores biosynthetic activity to the mutant strain. The suppression of a mutation in one gene by amplified copies of a different wt gene is termed 'multicopy suppression'. This phenomenon is useful for detecting poorly expressed genes, for detecting duplicate genes, for identifying secondary functions of the products of known genes, and for elucidating the metabolic role of the product of the suppressed gene.