Periplasmic glucans of Pseudomonas syringae pv. syringae

We report the initial characterization of glucans present in the periplasmic space of Pseudomonas syringae pv. syringae (strain R32). These compounds were found to be neutral, unsubstituted, and composed solely of glucose. Their size ranges from 6 to 13 glucose units/mol. Linkage studies and nuclear magnetic resonance analyses demonstrated that the glucans are linked by beta-1,2 and beta-1,6 glycosidic bonds. In contrast to the periplasmic glucans found in other plant pathogenic bacteria, the glucans of P. syringae pv. syringae are not cyclic but are highly branched structures. Acetolysis studies demonstrated that the backbone consists of beta-1,2-linked glucose units to which the branches are attached by beta-1,6 linkages. These periplasmic glucans were more abundant when the osmolarity of the growth medium was lower. Thus, P. syringae pv. syringae appears to synthesize periplasmic glucans in response to the osmolarity of the medium. The structural characteristics of these glucans are very similar to the membrane-derived oligosaccharides of Escherichia coli, apart from the neutral character, which contrasts with the highly anionic E. coli membrane-derived oligosaccharides.