Isolation and characterization of periplasmic cyclic beta-glucans of Azorhizobium caulinodans.

Oligoglucose molecules isolated from Azorhizobium caulinodans were characterized by compositional analysis, Smith degradation, matrix-assisted laser desorption/ionization time of flight mass spectrometry, and (1)H and (13)C nuclear magnetic resonance analysis. A. caulinodans produced nonbranched and unsubstituted cyclic glucans composed solely of glucose, with the degree of polymerization ranging from 10 to 13. A major fraction of the periplasmic glucans contains 11 glucose residues within rings. The glucose residues are linked by beta-(1,3) and beta-(1,6) glycosidic bonds. These molecules seem to be quite similar to the periplasmic beta-(1,3);(1,6)-glucans synthesized by the Bradyrhizobium strain and are substantially different from the cyclic beta-(1,2)-glucans produced by Agrobacterium and Sinorhizobium species. Azorhizobial cyclic glucan synthesis is not osmoregulated. The response to the osmotic stress in Azorhizobium can be regulated similarly to Brucella spp. It is probable that the biosynthesis of beta-glucans is subject to the feedback control mechanism.