Osmoregulated periplasmic glucans of the free‐living photosynthetic bacterium Rhodobacter sphaeroides

Abstract

The osmoregulated periplasmic glucans (OPGs) produced by Rhodobacter sphaeroides, a free-living organism, were isolated by trichloracetic acid treatment and gel permeation chromatography. Compounds obtained were characterized by compositional analysis, matrix-assisted laser desorption ionization mass spectrometry and nuclear magnetic resonance. R. sphaeroides predominantly synthesizes a cyclic glucan containing 18 glucose residues that can be substituted by one to seven succinyl esters residues at the C 6 position of some of the glucose residues, and by one or two acetyl residues. The glucans were subjected to a mild alkaline treatment in order to remove the succinyl and acetyl substituents, analyzed by MALDI mass spectrometry and purified by high-performance anion-exchange chromatography. Methylation analysis revealed that this glucan is linked by 17 1,2 glycosidic bonds and one 1,6 glycosidic bond. Homonuclear and 1 H/ 13 C heteronuclear NMR experiments revealed the presence of a single a-1,6 glycosidic linkage, whereas all other glucose residues are b-1,2 linked. The different anomeric proton signals allowed a complete sequence-specific assignment of the glucan. The structural characteristics of this glucan are very similar to the previously described OPGs of Ralstonia solanacearum and Xanthomonas campestris, except for its different size and the presence of substituents. Therefore, similar OPGs are synthesized by phytopathogenic as well as free-living bacteria, suggesting these compounds are intrinsic components of the Gram-negative bacterial envelope.Keywords: periplasm; osmoregulation; cyclic glucans.Osmoregulated periplasmic glucans (OPGs) appear to be general constituents of the envelope of Gram-negative bacteria [1]. Their abundance in the periplasmic space is the greatest when the medium osmolarity is very low. Under these conditions, OPGs can represent between 5 and 10% of the cellular dry weight. These compounds play an important role in the interaction with specific plant hosts; in Sinorhizobium meliloti [2] and Bradyrhizobium japonicum [3] they are essential for nitrogen fixation; and in Agrobacterium tumefaciens [4], Pseudomonas syringae [5,6], and Erwinia chrysanthemi [7] for the development of plant disease. In this latter case, experiments in which OPG deficient mutants were coinoculated with wild-type bacteria have established that OPGs must be present in the periplasmic space of the bacteria to enable growth in the plant host [7]. However, beyond this functional homology, and the fact that glucose is the sole monosaccharide present, OPGs from various origins display an unexpected structural diversity. This variation occurs at two levels: the glucose backbone organization, and the absence or the presence of various substituents.Four families of OPGs can now be distinguished (a) OPGs of Escherichia coli, P. syringae and Erwinia chrysanthemi appears to range from six to 13 glucose residues [8,9]. The structure is highly branched, the backbone consisting of b-1,2 linked glucose units to which the...

Complete Genome Sequence of Nitrosospira multiformis , an Ammonia-Oxidizing Bacterium from the Soil Environment

“…Many of the transported molecules may travel further to the OM and the cell exterior. Periplasmic glucans are adaptive for bacteria that undergo hypo-osmotic stress (upshock after rainfall) but may have additional roles in virulence and motility (54,66). A gene encoding a putative cyclic beta-1,2-glucan synthetase was found (Nmul_A1183) containing five transmembrane helices and a glycosyltransferase motif.…”

The Brucella abortus Cyclic β-1,2-Glucan Virulence Factor Is Substituted with O-Ester-Linked Succinyl Residues

“…Cyclic glucans of Azospirillum brasilense (1) are substituted only with succinyl residues, and cyclic glucans of B. japonicum are substituted only with phosphocholine (35). Cyclic glucan from S. meliloti and Agrobacterium tumefaciens are substituted with both succinyl and phosphoglycerol residues (7), while the cyclic glucan of Rhodobacter sphaeroides (41) and linear glucans of Erwinia chrysanthemi (15) are substituted with succinyl and acetate residues. Osmoregulated periplasmic glucans (OPG) originating from Escherichia coli (linear glucans) are the only ones known to be substituted with three different substituents, phosphoglycerol, phosphoethanolamine, and succinyl residues (27).…”

Properties and current applications of bacterial cyclic β-glucans and their derivatives

“…The DP ranges varied with 13, 16, and 18 glucose units depending on the species, in Ralstonia [80][81][82], Xanthomonas [83,84], and Rhodobacter [85], respectively. In detail, the homogenous sizes such as DP 13 (a-cyclosophorotridecaoses) and 16 (a-cyclosophorohexadecaoses) are kept in the cyclic b-(1,2)-a-(1,6) glucans of Ralstonia and Xanthomonas strains, whereas Rhodobacter species produce cyclic b-(1,2)-a-(1,6) glucans containing 16,17,19,21,22,23, and 24 glucose residues as well as the major DP 18.…”