Biosynthesis of cyclic beta-(1-3),beta-(1-6) glucan in Bradyrhizobium spp.

Inner membranes of Bradyrhizobium japonicum strain USDA 110 produced in vitro soluble and insoluble beta-(1-3),beta-(1-6) glucans. The reaction proceeded through a 90 kDa inner membrane intermediate protein; used UDP-glucose as sugar donor and required Mg2+. Gel chromatography of soluble glucans resolved a cyclic beta-(1-3) glucan with a degree of polymerization of eleven from a family of beta-(1-3),beta-(1-6) glucans with variable degree of polymerization higher than eleven. Bradyrhizobium strains BR4406 and BR8404 isolated from tree legume nodules in Southeast Brazil produce beta-(1-3), beta-(1-6) glucans very similar to that of B. japonicum. A 100 kDa protein was identified in these strains as intermediates in the synthesis of these glucans. Inner membranes of B. japonicum USDA110, B. japonicum I17, and Bradyrhizobium strains BR4406 and BR8404 incubated with UDP-glucose were unable to synthesize beta-(1-2) glucan and lacked the 235 kDa intermediate protein known to be involved in the synthesis of beta-(1-2) glucan in Agrobacterium tumefaciens, Rhizobium meliloti and Rhizobium loti.