Increasing the yield and viscosity of exopolysaccharides secreted by Sphingomonas by augmentation of chromosomal genes with multiple copies of cloned biosynthetic genes

Certain bacteria of the Sphingomonas genus secrete structurally related capsular polysaccharides. Due to their unique properties, three (gellan, welan and rhamsan) are produced commercially by submerged fermentation and are used as modifiers of aqueous rheology and as gelling agents. However, conversion of glucose into these polysaccharides is relatively inefficient. To identify general methods for increasing the productivity of Sphingomonas, we augmented the normal chromosomal copy of the phosphoglucomutase gene (pgm) and the cluster of genes (sps) required for assembly of the carbohydrate repeat unit for strain S7 with multiple copies of plasmids carrying these genes. Although a sixfold increase in Pgm activity only lead to a small percentage increase in conversion of glucose to the S-7 polysaccharide, multiple sps genes caused a nearly 20% increase in the yield from glucose and an even larger increase in culture viscosity. The increased viscosity was accompanied by a change in the sugar composition of the secreted polymer. Journal of Industrial Microbiology & Biotechnology (2000) 25, 49–57. Received 18 February 2000/ Accepted in revised form 29 April 2000