title: Osmotically induced synthesis of the dipeptide N-acetylglutaminylglutamine amide is mediated by a new pathway conserved among bacteria.
abstract: The dipeptide N-acetylglutaminylglutamine amide (NAGGN) was discovered in the bacterium Sinorhizobium meliloti grown at high osmolarity, and subsequently shown to be synthesized and accumulated by a few osmotically challenged bacteria. However, its biosynthetic pathway remained unknown. Recently, two genes, which putatively encode a glutamine amidotransferase and an acetyltransferase and are up-regulated by osmotic stress, were identified in Pseudomonas aeruginosa. In this work, a locus carrying the orthologous genes in S. meliloti, asnO and ngg, was identified, and the genetic and molecular characterization of the NAGGN biosynthetic pathway is reported. By using NMR experiments, it was found that strains inactivated in asnO and ngg were unable to produce the dipeptide. Such inability has a deleterious effect on S. meliloti growth at high osmolarity, demonstrating the key role of NAGGN biosynthesis in cell osmoprotection. beta-Glucuronidase activity from transcriptional fusion revealed strong induction of asnO expression in cells grown in increased NaCl concentration, in good agreement with the NAGGN accumulation. The asnO-ngg cluster encodes a unique enzymatic machinery mediating nonribosomal peptide synthesis. This pathway first involves Ngg, a bifunctional enzyme that catalyzes the formation of the intermediate N-acetylglutaminylglutamine, and second AsnO, required for subsequent addition of an amide group and the conversion of N-acetylglutaminylglutamine into NAGGN. Interestingly, a strong conservation of the asnO-ngg cluster is observed in a large number of bacteria with different lifestyles, such as marine, symbiotic, and pathogenic bacteria, highlighting the ecological importance of NAGGN synthesis capability in osmoprotection and also potentially in bacteria host-cell interactions.
authors.affiliation: Laboratoire Interactions Biotiques et Santé Végétale Unité Mixte de Recherche, Centre National de la Recherche Scientifique 6243, Institut National de la Recherche Agronomique 301, Université de Nice-Sophia Antipolis, 06903 Sophia Antipolis, France.
authors.forename: Brice
authors.initials: B
authors.lastname: Sagot
authors.forename: Marc
authors.initials: M
authors.lastname: Gaysinski
authors.forename: Mohamed
authors.initials: M
authors.lastname: Mehiri
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Guigonis
authors.forename: Daniel
authors.initials: D
authors.lastname: Le Rudulier
authors.forename: Geneviève
authors.initials: G
authors.lastname: Alloing
mesh_terms: D001419:Bacteria / Q000235:genetics / Q000378:metabolism*; D004151:Dipeptides; D005966:Glucuronidase / Q000235:genetics / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D009842:Oligopeptides / Q000235:genetics / Q000378:metabolism; D009994:Osmolar Concentration; D011550:Pseudomonas aeruginosa / Q000235:genetics / Q000378:metabolism; D016962:Sinorhizobium meliloti / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D012965:Sodium Chloride / Q000378:metabolism / Q000494:pharmacology; D013559:Symbiosis / Q000187:drug effects / Q000235:genetics
chemical_list: D004151:Dipeptides; D009842:Oligopeptides; C060785:N-acetylglutaminylglutamine amide; D012965:Sodium Chloride; D005966:Glucuronidase