Structural investigation of the extracellular polysaccharides metabolised by S19, a Xanthomonas type bacterium and by the unicellular red alga, Rhodella maculata

Abstract

Structural studies on the extracellular polysaccharide elaborated by the bacterium, Xanthomonas S19 have been made. The component monosaccharides, D-glucuronic acid, D-glucose, D-galactose, and D mannose were characterised and their approximate molar ratios of 1.6 : 3 : 1 : 1 determined. Attempted fractionation by a number of methods was unsuccessful and it is concluded that it is a single polydisperse acidic heteropolysaccharide. Partial hydrolysis studies confirmed this conclusion and indicated a repeating structure of 13 residues including all the above monosaccharides. Acetate esters were detected and confirmed and account for a degree of acetylation equal to 2. They were shown to be located on C-2 and/or C-3 of the glucose residues. The linkages of the sugar units were determined by methylation studies: the glucose is linked as non-reducing end-group and also as 1,2- and 1,4-linked units; the galactose units are solely 1,3-linked; the glucuronic acid residues are 1,4-linked; a major proportion of the mannose units are 1,2,4-linked and the rest 1,2 linked. The similarities and differences between this polysaccharide and those from other Xanthomonas species are discussed. The extracellular mucilage metabolised by the microscopic unicellular red alga, Ehodella maculata, has been investigated. The constituent sugars xylose, glucuronic acid (both major),3-O-methyl xylose, rhamnose, galactose and glucose have each been characterised. This is the first time 3-O-methylxylose has been found as a constituent of a natural polysaccharide. Protein to the extent of 16-26% is also present and 15 amino acids have been identified. All attempts to fractionate the material into more than a single polysaccharide were unsuccessful and gel electrophoresis showed only one band for polysaccharide and this was free from protein. Half ester sulphate groups are present to about 16.5% of the carbohydrate and tentative evidence is advanced that these may be linked to the glucuronic acid. Methylation and periodate oxidation studies proved that the xylose units are 1,4- and 1,3-linked, some of the 1,4-linked units carrying methoxyl at C-3- The glucuronic acid occurs as 1,3-linked units. Rhamnose is present as end-group together with 1,3 and 1,4-linked galactose, and 1,2- and 1,4-linked glucose. Branch points are occupied by 1,3,4- and 1,2,4-linked galactose and/or glucose units. The difference of this heteropolysaccharide from the characteristic galactans of the red algae is discussed.