alpha-L-rhamnosidase of Sphingomonas sp. R1 producing an unusual exopolysaccharide of sphingan

A soil bacterium with alpha-L-rhamnosidase was isolated from a cumulative mixed culture containing a polysaccharide of gellan as a carbon source and identified to be Sphingomonas paucimobilis, known as a potent producer of gellan. The isolate (designated Sphingomonas sp. R1) produced an unusual exopolysaccharide of sphingan (denoted HWR1) distinct from gellan. The rhamnose in gellan was replaced with mannose in HWR1. The bacterium had a peculiar cell surface covered with many complicated plaits. alpha-L-Rhamnosidase purified from Sphingomonas sp. R1 grown in the presence of naringin was a monomer with a molecular mass of 110 kDa and most active at pH 8.0 and 50 degrees C. The enzyme required divalent metal ions for the activity and released L-rhamnose from various rhamnosyl glycosides.