CYCLIC BETA-GLUCANS OF MEMBERS OF THE FAMILY RHIZOBIACEAE

Cyclic beta-glucans are low-molecular-weight cell surface carbohydrate s that are found almost exclusively in bacteria of the Rhizobiaceae fa mily. These glucans are major cellular constituents, and under certain culture conditions their levels may reach up to 20% of the total cell ular dry weight. In Agrobacterium and Rhizobium species, these molecul es contain between 17 and 40 glucose residues linked solely by beta-(1 ,2) glycosidic bonds. In Bradyrhizobium species, the cyclic beta-gluca ns are smaller (10 to 13 glucose residues) and contain glucose linked by both beta-(1,6) and beta-(1,3) glycosidic bonds. In some rhizobial strains the cyclic beta-glucans are unsubstituted, whereas in other rh izobia these molecules may become highly substituted with moieties suc h as sn-1-phosphoglycerol. To date, two genetic loci specifically asso ciated with cyclic beta-glucan biosynthesis have been identified in Rh izobium (ndvA and ndvB) and Agrobacterium (chvA and chvB) species. Mut ants with mutations at these loci have been shown to be impaired in th eir ability to grow in hypoosmotic media, have numerous alterations in their cell surface properties, and are also impaired in their ability to infect plants. The present review will examine the structure and o ccurrence of the cyclic beta-glucans in a variety of species of the Rh izobiaceae. The possible functions of these unique molecules in the fr ee-living bacteria as well as during plant infection will be discussed .