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Gellan lyases-novel polysaccharide lyases

Institute of Cell and Molecular Biology, Edinburgh University, West Mains Road, Edinburgh EH9 3JH, UK

Author for correspondence: Ian W. Sutherland. Tel: +44 31 650 5331. Fax: +44 31 650 5392. e-mail: I.W.Sutherland@castle.ed.ac.uk

ABSTRACT

A number of bacterial strains capable of degrading the bacterial exopolysaccharide gellan® have been isolated by standard enrichment procedures. They include several pink-pigmented Gram-negative rod-shaped bacteria. A red-pigmented Gram-positive bacillus earlier found to degrade the exopolysaccharide xanthan fromXanthomonas campestris also showed slight gellanase activity. All the Gram-negative bacteria are non-fermentative, motile and amylase-producing. The gellan degradation in each case is due to eliminase-type enzymes(lyases) which appear to be extracellular enzymes cleaving the sequence ... β-D-glucosyl-(14)-β-D-glucuronosyl... in the tetrasaccharide repeat unit of the substrate polysaccharides. Although in some isolates these enzymes appear to be exo-acting, it appears from the loss in viscosity of the alternative substrate deacetylated rhamsan that they are predominantly endoenzymes. The enzyme activity is inducible: it is almost absent from glucose-grown cells. Associated with the ‘gellanase’ activity, all the Gram-negative bacterial isolates possess intracellular -L-rhamnosidase and β-D-glucosidase activities apparently located in the periplasm. The enzymes are highly specific and fail to cause significant degradation of most of the other bacterial exopolysaccharides which have been shown to be structurally related to gellan. As well as acting on gellan, they exert similar degradative activity against the chemically deacylated form of polysaccharide S194 (rhamsan gum), which is effectively a gentiobiosylated form of gellan. The enzymes only have relatively slight activity against the natural, acylated gellan-like polysaccharides from the bacteria now designated as strains of Sphingomonas paucimobilis.

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