A NOVEL CYCLIC BETA-1,2-GLUCAN MUTANT OF RHIZOBIUM-MELILOTI

The periplasmic cyclic beta-1,2-glucans produced by bacteria within th e Rhizobiaceae family provide functions during hypo-osmotic adaptation and plant infection. In Rhizobium meliloti, these molecules are highl y modified with phosphoglycerol and succinyl substituents, and it is p ossible that the anionic character of these glucans is important for t heir functions. In the present study, we have used a thin-layer chroma tographic screening method to identify a novel R. meliloti mutant spec ifically blocked in its ability to transfer phosphoglycerol substituen ts to the cyclic beta-1,2-glucan backbone. Further analysis revealed t hat the cyclic glucans produced by this mutant contained elevated leve ls of succinyl substituents. As a result, the overall anionic charge o n the cyclic beta-1,2-glucans was found to be similar to that of wild- type cells, Despite this difference in cyclic beta-1,2-glucan structur e, the mutant was shown to effectively nodulate alfalfa and to grow as well as wild-type cells in hypo-osmotic media.