EFFECT OF PHOSPHATE LIMITATION ON SYNTHESIS OF PERIPLASMIC CYCLIC BETA-(1,2)-GLUCANS

Rhizobium meliloti and Agrobacterium tumefaciens synthesize periplasmi c cyclic beta-(1,2)-glucans during adaptation to hypoosmotic environme nts. It also appears that these glucans provide important functions du ring the interactions of these bacteria with plant hosts. A large frac tion of these glucans may become modified with anionic substituents su ch as phosphoglycerol or succinic acid; however, the role(s) of these substituents is unknown. In this study, we show that growth of these b acteria in phosphate-limited media leads to a dramatic reduction in th e levels of phosphoglycerol substituents present on the periplasmic cy clic beta-(1,2)-glucans. Under these growth conditions, R. meliloti 10 21 was found to synthesize anionic cyclic beta-(1,2)-glucans containin g only succinic acid substituents. Similar results were obtained with R. meliloti 7154 (an exoH mutant which lacks the ability to succinylat e its high-molecular-weight exopolysaccharide), revealing that succiny lation of the cyclic beta-(1,2)-glucans is mediated by an enzyme syste m distinct from that involved in the succinylation of exopolysaccharid e. In contrast, when A. tumefaciens C58 was grown in a phosphate-limit ed medium, it was found to synthesize only neutral cyclic beta-(1,2)-g lucans.