title: CbrA is a stationary-phase regulator of cell surface physiology and legume symbiosis in Sinorhizobium meliloti.
abstract: Sinorhizobium meliloti produces an exopolysaccharide called succinoglycan that plays a critical role in promoting symbiosis with its host legume, alfalfa (Medicago sativa). We performed a transposon mutagenesis and screened for mutants with altered succinoglycan production and a defect in symbiosis. In this way, we identified a putative two-component histidine kinase associated with a PAS sensory domain, now designated CbrA (calcofluor-bright regulator A). The cbrA::Tn5 mutation causes overproduction of succinoglycan and results in increased accumulation of low-molecular-weight forms of this exopolysaccharide. Our results suggest the cbrA::Tn5 allele leads to this succinoglycan phenotype through increased expression of exo genes required for succinoglycan biosynthesis and modification. Interestingly, CbrA-dependent regulation of exo and exs genes is observed almost exclusively during stationary-phase growth. The cbrA::Tn5 mutant also has an apparent cell envelope defect, based on increased sensitivity to a number of toxic compounds, including the bile salt deoxycholate and the hydrophobic dye crystal violet. Growth of the cbrA mutant is also slowed under oxidative-stress conditions. The CbrA-regulated genes exsA and exsE encode putative inner membrane ABC transporters with a high degree of similarity to lipid exporters. ExsA is homologous to the Escherichia coli MsbA protein, which is required for lipopolysaccharide transport, while ExsE is a member of the eukaryotic family of ABCD/hALD peroxisomal membrane proteins involved in transport of very long-chain fatty acids, which are a unique component of the lipopolysaccharides of alphaproteobacteria. Thus, CbrA could play a role in regulating the lipopolysaccharide or lipoprotein components of the cell envelope.
authors.affiliation: Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Katherine E
authors.initials: KE
authors.lastname: Gibson
authors.forename: Gordon R
authors.initials: GR
authors.lastname: Campbell
authors.forename: Javier
authors.initials: J
authors.lastname: Lloret
authors.forename: Graham C
authors.initials: GC
authors.lastname: Walker
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D002473:Cell Wall / Q000378:metabolism / Q000502:physiology*; D004268:DNA-Binding Proteins; D005227:Fatty Acids / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D000071677:Histidine Kinase; D008070:Lipopolysaccharides / Q000378:metabolism; D000455:Medicago sativa / Q000254:growth & development / Q000382:microbiology / Q000502:physiology; D018517:Plant Roots / Q000382:microbiology; D011135:Polysaccharides, Bacterial / Q000378:metabolism; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D016962:Sinorhizobium meliloti / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D013559:Symbiosis; D015534:Trans-Activators
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; C095092:ExsA protein, bacteria; D005227:Fatty Acids; D008070:Lipopolysaccharides; D011135:Polysaccharides, Bacterial; D015534:Trans-Activators; C056779:succinoglycan; D011494:Protein Kinases; D000071677:Histidine Kinase