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Novel mixed-linkage β-glucan activated by c-di-GMP in Sinorhizobium meliloti

AuthorsPérez-Mendoza, Daniel; Rodríguez-Carvajal, Miguel A.; Romero-Jiménez, Lorena; Araujo Farias, Gabriela de; Lloret, Javier; Gallegos, María Trinidad ; Sanjuán, Juan
Cyclic diguanylate
Plant–microbe interactions
Issue Date2015
CitationProceedings of the National Academy of Sciences of the United States of America 112: E757- E765 (2015)
Abstract© 2015 PNAS. An artificial increase of cyclic diguanylate (c-di-GMP) levels in Sinorhizobium meliloti 8530, a bacterium that does not carry known cellulose synthesis genes, leads to overproduction of a substance that binds the dyes Congo red and calcofluor. Sugar composition and methylation analyses and NMR studies identified this compound as a linear mixed-linkage (1→3)(1→4)-β-D-glucan (ML β-glucan), not previously described in bacteria but resembling ML β-glucans found in plants and lichens. This unique polymer is hydrolyzed by the specific endoglucanase lichenase, but, unlike lichenan and barley glucan, it generates a disaccharidic →4)-β-DGlcp-(1→3)-β-D-Glcp-(1→ repeating unit. A two-gene operon bgsBA required for production of this ML β-glucan is conserved among several genera within the order Rhizobiales, where bgsA encodes a glycosyl transferase with domain resemblance and phylogenetic relationship to curdlan synthases and to bacterial cellulose synthases. ML β-glucan synthesis is subjected to both transcriptional and posttranslational regulation. bgsBA transcription is dependent on the exopolysaccharide/quorum sensing ExpR/SinI regulatory system, and posttranslational regulation seems to involve allosteric activation of the ML β-glucan synthase BgsA by c-di-GMP binding to its C-terminal domain. To our knowledge, this is the first report on a linear mixed-linkage (1→3)(1→4)-β-glucan produced by a bacterium. The S. meliloti ML β-glucan participates in bacterial aggregation and biofilm formation and is required for efficient attachment to the roots of a host plant, resembling the biological role of cellulose in other bacteria.
Identifiersdoi: 10.1073/pnas.1421748112
issn: 1091-6490
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