This research was supported by National Institutes of Health grant A1072710 (E.I.S.). “
“Periplasmic cyclic β-1,2-glucans play a crucial role in symbiosis as well as in hypo-osmotic adaptation for rhizobia. These glucans are modified in many species by anionic substituents such as glycerophosphoryl and succinyl ones, but their role remains to be examined. In this work, the cgmA homolog is shown to be responsible for

glycerophosphorylation of cyclic β-1,2-glucans in Mesorhizobium loti. The mutation in cgmA converted most anionic glucans into neutral ones, leaving a small amount of succinylated ones. An additional mutation in opgC, which selleck chemical encodes a succinyltransferase homolog, abolished the residual succinyl substituents in the cgmA-mutant background. The double mutant in cgmA and opgC did not show any significant phenotypic differences from the wild type during both vegetative growth and symbiosis. It is concluded that the Trichostatin A anionic substituents make a minor contribution, if any, to the effectiveness of cyclic β-1,2-glucans in M. loti. Low-molecular-weight glucans are widely present in considerable amounts in the periplasm of Proteobacteria, although their backbone organizations are diverse among many bacterial families (Breedveld & Miller, 1994; Kennedy, 1996; Bohin, 2000). A subgroup of Alphaproteobacteria, including genera Agrobacterium, Brucella, Mesorhizobium, Rhizobium, and Sinorhizobium, possess β-1,2-linked

cyclic glucans consisting of 17–28 glucose residues. The ndvB/chvB/cgs and ndvA/chvA/cgt genes encode their synthase and exporter, respectively. Escherichia coli and some other Gammaproteobacteria have β-1,2-linked

linear glucans with branches connected by β-1,6-linkages, called membrane-derived oligosaccharides. These periplasmic glucans are commonly known to act as osmoprotectants: their presence makes a significant contribution to the maintenance of osmolarity of the periplasm (Kennedy, 1996). Sinorhizobium and Agrobacterium mutants in ndvB/chvB or ndvA/chvA are defective in growth and motility under low-osmolarity conditions (Cangelosi et al., 1990; Dylan et al., 1990a). Moreover, in the case of pathogenic or symbiotic bacteria, periplasmic glucans are crucial for the interaction with their eukaryotic PIK-5 hosts (Bohin, 2000; Mithöfer, 2002). Some residues of periplasmic glucans are modified by nonglycosidic substituents in many, but not all, bacteria; for example, phosphoglycerol for Agrobacterium tumefaciens and Sinorhizobium fredii (Miller et al., 1987; Crespo-Rivas et al., 2009); succinic acid for Brucella abortus (Roset et al., 2006); both of these for Sinorhizobium meliloti and Mesorhizobium loti (Miller et al., 1988; Kawaharada et al., 2008); and phosphoglycerol, phosphoethanolamine, and succinic acid for E. coli (van Golde et al., 1973; Kennedy et al., 1976). Phosphoglycerol and succinyl moieties confer a negative charge on glucan molecules, producing anionic fractions.