title: ExoR is genetically coupled to the ExoS-ChvI two-component system and located in the periplasm of Sinorhizobium meliloti.
abstract: Sinorhizobium meliloti enters into a symbiotic relationship with legume host plants, providing fixed nitrogen in exchange for carbon and amino acids. In S. meliloti, exoR and the exoS-chvI two-component system regulate the biosynthesis of succinoglycan, an exopolysaccharide important for host invasion. It was previously reported that a loss-of-function mutation in exoR and a gain-of-function mutation in exoS cause overproduction of succinoglycan and loss of motility, indicating that ExoR negatively regulates and ExoS-ChvI positively regulates downstream genes. However, a relationship between exoR and exoS-chvI has never been clearly established. By identification and detailed characterization of suppressor strains, we provide genetic evidence that exoR and exoS-chvI control many similar phenotypes. These include succinoglycan production, symbiosis, motility, and previously uncharacterized prototrophy and biofilm formation, all of which are co-ordinately restored by suppressors. We further demonstrate that ExoR is located in the periplasm, suggesting that it functions to regulate downstream genes in a novel manner. In pathogenic bacteria closely related to S. meliloti, exoS-chvI homologues are required for virulence and the regulation of cell envelope composition. Our data suggest that periplasmically localized ExoR and ExoS-ChvI function together in a unique and critical regulatory system associated with both free-living and symbiotic states of S. meliloti.
authors.affiliation: Department of Medicine, Division of Infectious Diseases, University of California, 513 Parnassus Avenue, San Francisco, CA 94143-0654, USA.
authors.forename: Derek H
authors.initials: DH
authors.lastname: Wells
authors.forename: Esther J
authors.initials: EJ
authors.lastname: Chen
authors.forename: Robert F
authors.initials: RF
authors.lastname: Fisher
authors.forename: Sharon R
authors.initials: SR
authors.lastname: Long
mesh_terms: D000114:Acetylene / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D018441:Biofilms / Q000254:growth & development; D002934:Cinnamates / Q000494:pharmacology; D024881:Drug Resistance, Bacterial; D005407:Flagella / Q000378:metabolism / Q000648:ultrastructure; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects; D006921:Hygromycin B / Q000031:analogs & derivatives / Q000494:pharmacology; D046529:Microscopy, Electron, Transmission; D008954:Models, Biological; D009586:Nitrogen Fixation; D019897:Periplasm / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D010766:Phosphorylation / Q000187:drug effects; D011135:Polysaccharides, Bacterial / Q000096:biosynthesis; D053204:Root Nodules, Plant / Q000254:growth & development / Q000378:metabolism / Q000382:microbiology; D016962:Sinorhizobium meliloti / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D002934:Cinnamates; C107451:ExoR protein, Rhizobium leguminosarum; D011135:Polysaccharides, Bacterial; D014157:Transcription Factors; C083719:chvI protein, Bacteria; D006921:Hygromycin B; C026273:hygromycin A; C056779:succinoglycan; D000114:Acetylene