Xanthan Biosynthesis by Xanthomonas Bacteria: An Overview of the Current Biochemical and Genomic Data
Anke Becker and Frank-Jörg Vorhölter
from: Microbial Production of Biopolymers and Polymer Precursors: Applications and Perspectives (Edited by: Bernd H. A. Rehm). Caister Academic Press, U.K. (2009)
Plant-pathogenic bacteria of the genus Xanthomonas are able to produce the acidic exopolysaccharide xanthan gum. Because of its physical properties, it is widely used as a viscosifer, thickener, emulsifier or stabilizer in both food and non-food industries. Xanthan consists of pentasaccharide repeat units composed 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. The xanthan polymer has a branched structure with a cellulose-like backbone. Synthesis originates from glucose as substrate for synthesis of the sugar nucleotides precursors UDP-glucose, UDP-glucuronate, and GDP-mannose that are required for building the pentasaccharide repeat unit. This links the synthesis of xanthan to the central carbohydrate metabolism. The repeat units are built up at undecaprenylphosphate lipid carriers that are anchored in the cytoplasmic membrane. Specific glycosyltransferases sequentially transfer the sugar moieties of the nucleotide sugar xanthan precursors to the lipid carriers. Acetyl and pyruvyl residues are added as non-carbohydrate decorations. Mature repeat units are polymerized and exported in a way resembling the Wzy-dependent polysaccharide synthesis mechanism of Enterobacteriaceae. Products of the gum gene cluster drive synthesis, polymerization, and export of the repeat unit. This review outlines aspects of the biosynthetzic pathway and genetic loci involved in xanthan biosynthesis, including the synthesis of the sugar nucleotide precursors, building of the repeat unit, as well as polymerization and export of the polymer. Comparative aspects based on recent genomic data of various Xanthomonas strains are also covered read more ...