Biotechnology of the Bacterial Gellan Gum: Genes and Enzymes of the Biosynthetic Pathway.

Bacterial exopolysaccharides (EPS) are a diverse and remarkably versatile class of materials that have potential applications in virtually all sectors of modern industry and economy. Currently, many biopolymers are still in the developmental stage, but important applications are beginning to emerge in the areas of food production and biomedicine. A few bacterial EPS can directly replace synthetically derived material in traditional applications, whereas others possess unique properties that can open up a range of new commercial opportunities. This is the case of the commercial important Sphingomonas elodea exopolysaccharide, gellan gum, one of the few bacterial gum with gelling properties. In its native form, gellan is a linear anionic heteropolysaccharide based on a tetrasaccharide repeat unit composed of 2 molecules of D-glucose, 1 of L-rhamnose and 1 of D-glucuronic acid. The native gellan is partially esterified with acyl substituents (1 mole of glycerate and 0.5 mol of acetate) per repeat unit. The significant changes in rheology observed upon deacylation of gellan are essentially due to the glycerate substituents. The potential for using gellan or gellan-like gums in industrial applications is determined by their physical properties. Metabolic engineering may be used as a tool to produce altered polysaccharides and/or to increase gellan production. The eventual success of this approach requires a detailed understanding of the molecular biology, biochemistry and physiology of its biosynthesis. Gellan biosynthesis starts with the intracellular formation of the nucleotide-sugar precursors, UDP-glucose, UDP-glucuronic acid and dTDP-Lrhamnose, whose pathway was elucidated. The synthesis of the sugar precursors is followed by the formation of the repeat unit, by sequential transfer of the sugar donors to an activated lipid carrier by committed glycosyltransferases, followed by gellan polymerization and export. Most of these gellan specific processes are catalysed by enzymes encoded in the gel cluster of genes. The identification of genes and the elucidation of crucial steps in the pathway, indicate that possibilities now exist for trying exerting control over gellan production, by modifying the expression of any of the individual genes or of groups of genes. [ABSTRACT FROM AUTHOR]