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Gene SMb21071 of plasmid pSymB is required for osmoadaptation of Sinorhizobium meliloti 1021 and is implicated in modifications of cell surface polysaccharides structure in response to hyperosmotic stress

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Megaplasmid pSymB of the nitrogen-fixing symbiont Sinorhizobium meliloti, implicated in adaptation to hyperosmotic stress, contains 11 gene clusters that apparently encode surface polysaccharides. However, only 2 of these clusters, containing the exo and exp genes, have been associated with the synthesis of the acidic exopolysaccharides succinoglycan and galactoglucan, respectively. The functions of the other 9 clusters remain unsolved. The involvement of one of those regions, pSymB cluster 3, on surface polysaccharide synthesis and its possible implication in osmoadaptation were investigated. In silico analysis of cluster 3 showed that it putatively encodes for the synthesis and transport of a methylated surface polysaccharide. Mutants affected in this cluster were symbiotically effective but showed defects in growth under saline and nonsaline osmotic stress. The gene SMb21071, encoding a putative initiating glycosyltransferase, is transcriptionally induced under hyperosmotic conditions. Sodium dodecyl sulfate - polyacrylamide gel electrophoresis and silver staining showed that osmotic stresses changed the profiles of surface polysaccharides of wild-type and mutants strains in different ways. The overall results suggest that cluster 3 is important for growth under saline stress and essential for growth under nonsaline hyperosmotic stress, and it appears to be implicated in maintaining and (or) modifying surface polysaccharides in response to osmotic stress.

Le mégaplasmide pSymB du symbiote fixateur d’azote Sinorhizobium meliloti impliqué dans l’adaptation au stress hyperosmotique, contient 11 grappes de gènes qui codent apparemment des polysaccharides de surface. Cependant, seules 2 de ces grappes de gènes contenant les gènes exo et exp ont été associées à la synthèse des exopolysaccharides succinoglycane et galactoglucane, respectivement. La fonction des 9 autres grappes demeure inconnue. L’implication d’une de ces régions, pSymB grappe 3, dans la synthèse d’un polysaccharide de surface, et son implication possible dans l’adaptation osmotique, ont été examinées. Une analyse in silico de la grappe 3 a montré qu’elle code possiblement un système de synthèse et de transport d’un polysaccharide de surface méthylé. Des mutants de cette grappe étaient efficaces d’un point de vue symbiotique mais montraient des défauts de croissance sous des conditions de stress osmotiques salin et non salin. Le gène SMb21071, codant une glycosyltransférase d’initiation présumée, est induit au niveau transcriptionnel par des conditions hyperosmotiques. Une SDS-PAGE et une coloration à l’argent ont montré que les stress osmotiques changaient de façon différentielle le patron des polysaccharides de surface des souches sauvages et mutées. L’ensemble des résultats suggère que la grappe 3 est importante à la croissance sous des conditions de stress hyperosmotique non salin et qu’elle semble impliquée dans le maintien ou la modification des polysaccharides de surface en réponse à un stress osmotique.

Document Type: Research Article

Publication date: October 1, 2009

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  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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