Xanthan gum is a complex exopolysaccharide produced by the plant-pathogenic bacterium Xanthomonas campestris pv. campestris. It consists of D-glucosyl, D-mannosyl, and D-glucuronyl acid residues in a molar ratio of 2:2:1 and variable proportions of O-acetyl and pyruvyl residues. Because of its physical properties, it is widely used as a thickener or viscosifier in both food and non-food industries. Xanthan gum is also used as a stabilizer for a wide variety of suspensions, emulsions, and foams. This article outlines aspects of the biochemical assembly and genetic loci involved in its biosynthesis, including the synthesis of the sugar nucleotide substrates, the building and decoration of the pentasaccharide subunit, and the polymerization and secretion of the polymer. An overview of the applications and industrial production of xanthan is also covered.

Symbiotically essential genes have been identified in Rhizobium melilodi that are structurally and functionally related to chromosomal virulence (chv) genes of Agrobacterium tumefaciens. Homologous sequences also exist in the genomes of other fast-growing rhizobia including Rhizobium trifolii, Rhizobium leguminosarum, and Rhizobium phaseoli. In Agrobacterium, the chvA and chvB loci are known to be essential for oncogenic transformation of The family Rhizobiaceae has classically been considered to contain only two genera, Agrobacterium and Rhizobium. Agrobacterium species are plant pathogens that induce tumorous growths on a wide variety of dicotyledonous plants, while Rhizobium species are agriculturally beneficial plant symbionts that induce nitrogen-fixing nodules on the roots of legumes. We report here what is to our knowledge the first instance in which a group of symbiotically required Rhizobium genes has been shown to be structurally and functionally related to the Agrobacterium genes that are required for pathogenesis.Most of the genes necessary for virulence (vir) of Agrobacterium tumefaciens have been localized to a unique endogenous plasmid called the Ti plasmid (1). Mutations in these vir genes prevent oncogenic transformation, presumably by interfering with the successful transfer to the plant of another region on the Ti plasmid called T-DNA, that encodes enzymes involved in phytohormone production (2)(3)(4). In addition to plasmid-encoded vir genes, two closely linked virulence loci have been found in the chromosome of Agrobacterium (5). These loci, designated chvA and chvB, have the interesting feature that mutations at either locus interfere with the ability of Agrobacterium to bind to plant cells. Little is known about how the chv gene products function, but chv mutants show pleiotropic effects likely to be related to cell envelope changes (6, 7).The data presented here show that chvA and chvB are homologous to DNA sequences in the genomes of four different fast-growing Rhizobium species and that in the case ofRhizobium meliloti, the corresponding genes can functionally complement Agrobacterium chv mutants. R. meliloti mutants in the chv-equivalent loci are still able to induce nodule-like structures on alfalfa, but such nodules do not show normal bacterial invasion and differentiation. MATERIALS AND METHODSStrains and Plasmids. The following Rhizobium strains were used in this study: R. meliloti 102F34 (8), 1021 (9) and 41 (10); Rhizobium phaseoli 8002 (11) and its sym plasmidcured derivative 8400 (11); Rhizobium leguminosarum 128C53 (12) and its sym plasmid-cured derivative B151 (12); Rhizobium trifolii 162X68, from Nitragin (Milwaukee, WI), and RS 800 (13); Rhizobium japonicum USDA 110 (14). Agrobacterium strains have previously been described: A348 is A. tumefaciens C58 chromosome carrying pTiA6NC (15); Tn5 and Tn3HoHol insertion mutants were used for complementation studies (5). Escherichia coli strains were HB101 (pro, leu, thi, lacY, endoI, recA, hsdR, hsdM, str?) and HB1O1::Tn5....

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 gumhas 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.